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There are published weld schedules that are for everybody to use, such as at: http://www.cmwinc.com/
On this page:http://www.cmwinc.com/welding-education.php
there are 5 tables at the bottom select the one you need. As you can see, the spot welding parameters (Force, current, and time) depend on thickness and: 1. Metal alloy 2. Coating 3. Electrode alloy, size, shape, and contact area 4. Thickness Ratio of the sheets In addition the set-up also depends on the type of welding: 1. Spot 2. Seam 3. Projection 4. Butt welding 5. Flash welding Also, the schedules depend on how you will perform the welds: 1. Production, automotive style (High value on strength, consistency). 2. Production, non-critical such as white goods (washing machine, refrigerators). 3. Prototype, emphasis on strength, no time or parts available to fine-tune specific schedules. 4. Aerospace, extreme emphasis on strength, inspection, record keeping. Each company makes their own set of schedules based on their selection of all the factors above. They also must make rules on how to apply the schedules, and how the parts must processed, such as: 1. Minimum weld spacing 2. Flange size 3. Fit-up 4. Allowable thicknesses and ratios between them So you see, it is difficult to give you schedule information for all combinations of everything. I do all my work in the automotive industry, most of my past was in high volume automotive body shop production, but for the last couple years I have been helping customers in prototype applications. I use GM's production guidelines when I have to set-up a production welder, but these are confidential GM property. If you are having trouble with a specific application I can advise, but I can't give away GM's schedules except to a company that is welding GM parts. They are specialized for their electrodes and metals, and strength requirements, which is not useful unless you are welding by all their standards. Please respond that you got this email, and tell me how I can help.
Dear Karthick M.,
You must avoid all pin holes in resistance spot welding, as it is prohibited in all weld acceptance specifications that I have ever seen. The few cases that I have seen are caused by low tip force, either during the whole weld, or failing near the end of the weld. Since I know nothing of your application, or even what units you use in your location, I will give an example of U.S. automotive welding. A typical weld of electroplated galvanized mild steel of 1mm to 1mm, written as "G1.0G/G1.0G" would require: (Actually this is for metal of 0.80mm to 1.09mm)
|GMT||Weld Force||K Amps||Pulses||Pulse Time||Cool Time||Tot Wld Time||Hold Time|
|mm||Lbs kN||kA||#||Cycles ms||Cycles ms||Cycles ms||Cycles ms|
|0.80-1.09||585 2.6||8.0||1||9 150||0 0||9 150||2 40|
Steppers are used to compensate for the current density reducing as the tips mushroom, reducing the heating action.
Chrysler publishes a chart of current density, showing 200,000 amps per square inch for a normal contact area of ¼' and 10,000 amps. When the tips grow to 3/16", the current density falls to 80,000 amps per square inch, and it quits welding. You will get a decreasing button size with tip wear, so you must add current. You cannot tolerate the shrinking button, as you are on the "slippery slope" of a chart that equates button size to weld current. Such as this one:
The independent variable is along the bottom, which illustrates our "Slippery Slope" best. As the tips mushroom, expressed as left-to-right, along the bottom, the results are a smaller button, which is expressed along the vertical axis on the left. This illustrates that the button is quickly lost as the tips mushroom, and this is due to the current density falling off to the point where it quits welding. So it is important to add current as the tips wear, to avoid the "Slippery Slope". Don't accept a smaller button as the tips wear, add current to stop this. You may find that you have to add current at the rate of one Amp per weld. This would give a current profile like this:
Of course these "Boosts" are added to the weld current so the actual current might go from 10,000 Amps to 14,000 Amps over the 4000 welds. Before Pertron introduced "Linear Steppers", we had "staircases" for steps, so we divided up however many steps the control allowed into even amounts. Square D had ten steps, so you would use that with 400 amps boost every 400 welds. With the Linear Stepper, you could set the whole thing in one "step" that simply boosted 4000 Amps over 4000 welds. This is the least confusing way to do it, but you miss some features if you do. One of the features is the "Near End of Stepper" warning that is announced by the controller while you are in the last step. For a Medar, this is step 5, Pertron used 4 steps. So with a Medar, we like to have this announcement for about ½ to 1 hour. If you make 500 welds in an hour, use 500 counts. Another feature is the ability to handle tip sticking while the tips "wear in" to give full contact area to the part. This generally takes about 50 welds on brand new tips. You might have to start at a reduced current of 500 amps to stop problems due to high current density if just the edge of the tip touches the part in the beginning. To handle this, you must reduce your "Base Heat" (What you have in the schedule) from 10kA to 9.5kA, then put 500 Amps boost in step 1 with a count of 50 welds. So now we have step 1 and step 5 set to give us the best results, what do we do with steps 2, 3 and 4? Whatever you want, they are usually divided up into whatever is left, and given the same boost rate as the rest, in this case 1 Amp per weld. The chart below shows that:
Now the stepper boosts and counts add to each other, so what you would enter into the control would look like this:
You can see that it could be simpler, but the control companies haven't done this. In fact, Medar made it even more complicated, because you can enter the "% boost" and the "Amps boost" for every step. That is what the seemingly unused zeroes are in the stepper programming. Don't worry about this now, save it for later. This gets even more complex, as Medar made the stepper work in conjunction with a tip dresser to tell the robot when to move over to the tip dresser, and when to stop for a new tip. We save this for way later…
I looked up "alimentation", which translates into "nourishment", so I think you mean the "feed" cable or power service.
So I will try to answer with that in mind. The kVA is the size of the transformer, as far as power handling capability. Your welder may not use the full capability of the transformer, so the service cable is usually sized for the job. So you must have the following information: 1. The current it carries i.e. it may carry 10,000 Amps for a typical spot weld) 2. The duty cycle of that current a) How many cycles it is carrying current in one minute of production (i.e. 20 welds of 12 cycles each = 240 cycles) b) Divide that by how many cycles there are in a minute (i.e. in the USA it is 3600) c) Use decimal form of duty cycle (not %) (i.e. the above is 240/3600 = 0.067 (also expressed at 6.7%) 3. The square root of that duty cycle (i.e. the above would = 0.258) 4. Multiply the current by the value in (3) (i.e. 10,000 x 0.258 = 2580 this is your equivalent Amps, continuous) 5. Refer to a sizing guide that shows length and continuous current (i.e. http://www.flexcable.com/images/uploads/downloads/AirCooledCableSizingGuide.pdf) And look up the proper cable (i.e. if you need 16", use 1200MCM. 6. Remember that Air cooled cable REQUIRE that each end be bolted to a water cooled connection block. 7. Also there is a different table for water cooled cables, kickless cables, and leaf shunts. Please tell me if this helped,
One place where many get confused is on the surface classification, and there are basically three surfaces:
The class A surface, where the surface is in plain view, like the side of a door, or the hood,
The class B surface, where an opening exists, like around the door opening or the trunk lip.
The structural surface, which is everything else.
So the welds are Class I, II, and III, and the surfaces are A, B, and "other".
First there are many types of welds, both resistance welds and plasma welds. I specialize in resistance welds, so I can pursue that, hoping that is the area you are asking about. Then there are many types of resistance welds, in order of popularity in the automotive industry: Spot welds, projection welds (nuts, studs, or embossments), seam welds (Overlapped and mashed), flash butt welds, and tube welds. There are many other types also, such as wire welding, hot upset staking, and more, but they are very rare in the automotive field. Each company has its own specification for welding, so I may need to know what company you are asking about. I know GM the best, but I am familiar with Chrysler, Ford, and some of the "Imports" (to USA) too. So to answer in general, I will refer to GM's specifications. Hoping you meant "Spot" welds, I shall continue with that: At GM, the spot welds are classified in specification GM4488M as: "Structural" welds, and "Processing" welds, and reads this way: 3.2.1 Structural Spot Welds. Structural spot welds are installed for performance of the product. All spot welds are structural, unless specifically noted as processing welds on the weld drawing. All structural spot welds shall be assigned to patterns. 3.2.2 Processing Welds. Processing welds are installed to facilitate in-process assembly, but are not required for structural performance of the product. Processing spot welds must be approved by Product Engineering and shown on the weld drawing. GM4488M is the basic specification for a structural weld, and then there are constraints placed on the finish for three classes of welds. This takes another specification to describe, GM----- (Hmm, can't find it right now…) GM4488M is a long document that spells out what is acceptable for: (with paragraph numbers) 4.1. size, 4.2. shape, 4.3. cracks, 4.4. holes, 4.5. missing welds, 4.6. edge welds, 4.7. location tolerance, 4.8 distortion, 4.9 indentation, 4.10. extra welds, 5. spot weld patterns, 6. spot welding classifications, 7. pattern conformance, 8. weld surface finishes, and repair requirements and procedures. Since you asked about "Classes" of welds, I have cut and pasted the section on "Spot Welding Classification": 8 WELD SURFACE FINISHES. 8.1 The following classifications are used to communicate the desired appearance of spot welds in a finished assembly. They do not modify any of the requirements in 4. The effect of metal finishing on spot weld performance should be considered when specifying Class I or Class II Weld Finishes. 8.2 CLASS I WELD FINISH. This classification covers sheet metal surfaces that must be free of markings, indentations, or imperfections so that they are not visible after painting. It is normally used for sheet metal surfaces that are visible from the exterior of the completed vehicle. This classification is not normally applied to GM-PCP weld patterns. Metal finishing will normally be required to achieve this finish. 8.3 CLASS II WELD FINISH. This classification covers a minimally marked welded surface. It is normally used for sheet metal surfaces that are occasionally visible in the completed vehicles, or where some weld marking is considered acceptable. Metal finishing is occasionally required to achieve this finish. 8.3.1 For Class II Weld Finish metal expulsion or excessive metal displacement (above normal surface) is not permitted. The degree of marking allowed will be defined by Product Engineering and normally controlled through approved visual samples. 8.4 CLASS III WELD FINISH. This classification covers normally marked surfaces where there is no need to define special requirements for surface finish. 8.4.1 For a Class III Weld Finish where metal expulsion or excessive metal displacement (above normal surface) are unacceptable for safety, appearance, fit up or other reasons, the drawing shall specify each weld with a flash free note. This will require removal of the expulsion or metal displacement when it occurs on the designated spot welds.
Do you want a list of all things that damage a kickless cable?
That could be a big list, and there are many obvious things, so I will list the non-obvious ones: 1. Overtemp. 2. Twisting a non-superflex type; Two types, Regular type cools easier because of better water contact, but shorts if twisted. Superflex type can handle axial twisting without shorting out, but doesn't cool as well. 3. Water flow in wrong direction, must flow in at lowest end, out at highest end, to purge bubbles. 4. Poor "dressing" on a robot causing more flex than needed. 5. Allowing ends to oxidize before installation. Always clean just before installation, or silver plate the surfaces. 6. Loose ends, always use "Supertanium" bolts, they have the same coefficient of expansion with temperature, as copper. Do not use just any old stainless, many are weak, such as "316". Being non-magnetic is not important. 7. Excessive broken strands, shorting across the cable, "leaking" current. Does this help?
First of all the robot should be checked for:
1. Alignment to the surface of the weld to be done, must be within 3 degrees of perpendicular. 2. Equalization, the gun must be free to move up and down at least 1/2" (13mm) from its weld position. 3. Repeatability, generally the robot is trusted to repeat position to 1mm, so check for how close the tips come to anything they can short to. Also don't even attempt to place a part on a pin for a nut welder without special processing to allow for the tight tolerances involved, or it will only fit some of the time. 4. Number of welds, it is a lot of strain to do any more than 20 welds in a minute. 5. Stepper how does it handle "end of stepper" from the weld control? Best is to finish the part, move to a window in the cell guard fence, and require a tip change. 6. Weld gun water cooling, does it flow more than 1/2 GPM with an in-line flow meter in each gun arm? (Do not use a bucket and a watch...) 7. Transformer size, will it overheat at maximum production using full stepper boost current? 8. Dressing of the hoses and cables, no tight bends, no dragging on the part. Is the primary side power wire easy to change? (Sometimes the plugs on the ends are harder to deal with than just terminating to clamp connectors) The secondary might be a kickless cable, is it easy to change? 9. Are all secondary copper connections silver plated? (Bare copper oxidizes to unusable in a few days...) 10. Are all secondary bolts grade 10 stainless? (Do not use "316" alloy, less strength than a cheap bolt) People do not realize we use stainless because its thermal expansion matches copper. Magnetics has nothing to do with it. 11. Nobody does this yet, but the water shutoff should be right at the gun, to prevent loss of water and an air-lock happening after tip change. 12. The air valve should also be right at the gun cylinder, as there is too much delay in squeeze time if a long length of line is between the valve and the cylinder. Measure squeeze time required to get to 90% of weld force. This may require explanation of how to do this, or you may know how. 13. Transformer capacity, will you have enough extra weld power to make up for line drops when at full stepper boost? You must know a lot about your power before you can answer this. Also are you able to weld above 50% for the lowest current weld and with no stepper boost. (Too much capacity is bad too...) 14. Force setting, does 80% air pressure give the optimum force? Too much capacity is bad, Too little is a disaster. 15. Tip size and alloy. I have my favorites, dome zirconium, 13 mm or larger. Your may vary. 16. Weld stroke over-travel, must be able to close gun with one tip off. Auto companies in the US do a Production Part Acceptance Procedure, which consists of a couple hours or more of actual production, where parts are checked for dimensional integrity, and weld integrity (completely torn down) Usually the teardown is done on parts 1,3,5,7,9,19,29,39,49,74,99 and every 25 thereafter, and the dimension check on 2,4,6,8,10,20,30,40,75,100, and every 25 parts thereafter (Sometimes every 50 or 100, depending on the customer) And failures must be corrected and (at the customer's choice) restart the test, or continue.TOP
Tip sticking is caused by excessive heat at the surface between the tip and the metal. It is most common with zinc coated steel, as the zinc alloys with the copper, forming brass, and brazes the tip to the part. It is rare with bare steel, so I will continue with my answer as if you are welding galvanized.
At the start of a weld, the resistance of the interface surfaces is high, and the resistance of the metal is low, because it is cold. When we pass current through the stackup, the heat is formed where the resistance is, unfortunately it is at the surfaces in the beginning. This is because the rough surfaces have only contact with each other in microscopic points and bumps. This heat will warm the metal, but also flattens the surfaces so they lose resistance, and therefore don't heat up much more as we continue welding.
When the metal becomes warm it raises in resistance, and as we continue passing current the heat is produced from this resistance. This change occurs about 30-50 milliseconds into the weld process. From this point on, most of the heat is generated within the metal, right where we want it. The copper tips will cool the surface so that the metal stays firm on the outside and helps to contain the growing "nugget" as we continue to heat it to the point where the two metals join. This point can just be the plastic state in bare metal, but must be the liquid state in galvanized metal, in order to mix the thin layer of zinc into the nugget where it won't harm the bond between the sheets. If we just heat to the plastic state, which is fine for bare metal, the layer of zinc will still be between the sheets, giving a "stuck weld" where the zinc was not moved aside and just solders the sheets together.
So the heat starts at the surface, and then changes to the inner part of the material, but if we start out with too much heat in the beginning, we may braze the tips to the part. So we must find what is causing excessive heat at the very start of the weld, some of those things are:
1. Low tip force, and this can exist on one side of a robot welding a clamped up part, so center the equalizer*, and make sure it allows free movement of the gun after it closes.
2. High current at the beginning of the weld, starting with an upslope helps a lot here.
3. Insufficient cooling, you said it was OK, but how did you determine this?
4. Small tips cause this too, because the mass of copper is actually what cools the surface, the water cooling then carries the heat away. "B" nose tips put the largest amount of copper right at the weld, without increasing the contact size, therefore the weld size.
5. Small contact area, which almost always happens with brand new tips, because:
a) They don't exactly meet right, but they do "break-in" over the next 25-50 welds.
b) They were installed with a hammer blow distorting the contact surface.
If using a stepper, you may lower the current by 500 Amps, then step that back up again in the next 50 welds to compensate for this.
Another thing that contributes to the tip sticking is the alloy of the copper. True zirconium tips work the best, as the 1/2% zirconium strengthens the copper without adding much resistance. Any other material in the alloy adds to the resistance, making the surface heat a bigger problem. Many people will sell you "Class 2" tips with zirconium when you ask for zirconium, but there are other ingredients, which are not always the same as "class 2" is a strength rating, not a chemical description. Chromium is a popular one, but it contributes to tip sticking.
One exception to the "added ingredients" problem is the titanium coated tip from Huys in Canada. Although this coating contributes considerably to the surface heat, it also absolutely sheds the molten zinc. This tip cannot be dressed, so many people avoid it because they want to have tip dressers, but it will solve the tip sticking problem.
If you must have a dresser, and there are reasons that make us put up with this troublesome addition to an already complex industrial process, then you must very carefully optimize the 5 items above.
Please tell me if this helps, because I provide this service for the joy of knowing I helped, and if you don't respond, then I was deprived of my reward. If you were closer, I would just come to your plant, then my reward would be instant, plus I get an additional reward when the invoice is paid!
I find the equalizer to be setup wrong on every gun I ever worked on.
It is supposed to let the gun move up and down when it is closed on the part, because all parts aren't not clamped up exactly the same, the tips wear, and the robot programmer doesn't always allow the same distance from the lower tip to the part on every weld.
Most people will go to the gun and try to move it up and down when they hear the equalizer may not be set right, then adjust the air regulator to the balancing cylinder until it does. This is wrong, they should have closed the gun first, which will have some means of holding it in place, until the gun closes, then it allows this movement.
When resistance welding, the metal is formed into an alloy. If there are certain materials contained in the metal, it will cause cracks and weaknesses. Carbon is the biggest problem, but there are seven materials that cause problems, and the combination of these ingredients in small amounts all adds up to strength problems. There is a formula for determining the weldability of metals containing any of these ingredients, and you can see it at
If you enter the amount of these ingredients as percentages by weight, this link will calculate the “Carbon Equivalency”, and if this total comes out to 0.5 or higher, the material is considered unweldable. The nickel coating would be one of these ingredients, and depending on the thickness, could easily cause the total to exceed 0.5%. Let’s say the nugget would have 2mm depth maximum, and the total thickness of the nickel coatings add up to 0.1 mm, then the amount of nickel alone (5% of the thickness) added to the common amount of carbon in mild steel (0.2%) applied to this formula puts the “Equivalency” over the 0.5% allowed. If the steel has any of the other ingredients it would push you way over the 0.5% limit.
When you exceed the 0.5% limit, it is simply unweldable and that is by any method, arc, MIG, resistance, laser, anything. This is because of the way the steel reacts to the heat of welding, and there is no way around it. Since your material may run right on the border of weldability, then you may have intermittent results.
The solution would be to remove the nickel plating where the weld spots must be placed. Sorry to give you the bad news, but that is the chemistry and that is how it reacts to welding heat.
For all electrical devices you must measure:
1. The line current (in Amps)
2. The line voltage (in Volts)
3. And multiply them for wattage (in Watts).
4. Then you must measure the time it is drawing current (in hours)
5. And multiply hours with wattage to get Watt-hours.
There are two difficult parts in getting the measurements:
1. Some devices are on for a very short time
2. Some welds use different currents.
With resistance welding, we read:
1. The secondary current with a Miyachi (or similar) "Weld checker", and divide it by the transformer turns ratio to get line Amps.
2. The voltage will be line voltage, measured with a volt meter. Remember if there are significant voltage drops during welding, then you must find a way to measure the line voltage during welding. Many weld controls have methods to read "lowest line volts during last weld", use that number.
3. Multiply these to get wattage during the weld.
4. The Miyachi also gives time (in cycles or milliseconds). Convert cycles to time by dividing by the line frequency. Then convert to hours.
5. Then multiply hours by Watts to get Watt-hours for that weld.
If there are multiple heat settings you must do this for each setting, then add them all up for one hour, and you will have the Watt-hours used in an hour of production.
I imagine you would do the same with laser welding, but I can't tell you the best way to get the readings.
The strength of the metal will be the strength of the tensile test, because the weld is stronger than the metal.
So you must find out the tensile strength of the metal. To do that, find the PSI from the material property sheets, which come from the steel producer.
Each steel producer will have different tensile strengths, as they process the steel differently. This is also known as UTS, Ultimate Tensile Strength.
This will be expressed as PSI or pounds per square inch. A 1/4" diameter weld will have more strength than a 1" wide sample of the sheet metal you are testing.
So divide the psi rating by the thickness, for example if the psi is 30,000 (Typical for mild steel) and your sheet metal is 0.100", then the UTS of a 1" wide sample is 3,000 pounds.
A 3/16" spot weld will be about the same strength, and will yield at about the same 3,000 pounds, because it will tear out of the steel just about the same.
Finding the thickness of a 12 gauge sample will be a problem as there are about five different gauge standards, so I would measure the thickness and not even bother describing the thickness in "gauge" as that is too unreliable of a measurement.
Hope this helps,
There are many steps you must do to be able to specify the gun, but we need a lot more information about the application (we need the underlined values).
1. You must find the tip pressure required, and specify that this must be achieved at 80 psi. If I knew more about the metal, I could look that up, but I need to know if it is mild steel, HSLA, or AHSS. I need to know if it is coated, as galvanized is often used in automotive work, that makes a big difference in the weld schedule.
Probably the tip pressure would be between 470 pounds to 670 pounds, but if it is a manual gun or robot gun it would be between 670 and 950 pounds.
2. We must find the maximum current the gun must produce, to get this we need to decide on the tip contact area, because that makes a big difference on what the current should be. To weld 0.8 mm, usually the weld diameter required is 4 mm. Usually a 6 mm tip is used to insure we always have at least 4 mm. Depending on the details needed on the metal, as just discussed, the current required could be between 8 kA to 10 kA, and if a heat stepper is used with a max weld count of 4000 welds, an additional 3000 to 5300 more Amps would be needed, depending on the steel coating, if any.
3. Once we know more about the metal, we can look up the schedule and get the weld time, so that we can calculate the duty cycle. We need the ratio of time ON/Time OFF during production. This can vary from 3% to 18% depending on the weld schedule and the number of spots per minute. This will help us figure the transformer KVA rating, but we also need to know the transformer turns ratio.
4. To get the turns ratio, we must find the secondary voltage required to produce the maximum current required for our weld. This depends on the impedance of the gun, which is related to the throat area of the gun, and the cabling between the gun and the transformer. This can be as high as 24 Volts for a large gun and a kickless cable, and as low as 3 Volts for a small gun with an integral transformer. Once we know the secondary voltage, we can calculate the turns ratio as primary voltage divided by secondary voltage.
When we have the turns ratio, we use it to figure the primary current by taking the secondary current divided by the turns ratio.
5. Then we can figure transformer KVA, as it would be the equivalent constant thermal primary current (ECTC) times the primary voltage. the ECTC is the actual primary current multiplied by the square root of the duty cycle. This will give us the actual KVA, however transformers KVA ratings are at 50% duty cycle, so we must take our actual KVA and divide it by 0.7 to find the nameplate transformer KVA.
So, if you can supply the information that is underlined we can figure out everything for you.
Hope to hear back from you soon.
First of all the robot should be checked for:
1. Alignment to the surface of the weld to be done, must be within 3 degrees of perpendicular.
2. Equalization, the gun must be free to move up and down at least 1/2" (13mm) from its weld position. Of course, this is only when the gun is closed. The equalizer must be locked in position when the gun is open.
3. Repeatability, generally the robot is trusted to repeat position to 1mm, so check for how close the tips come to anything they can short to. Also don't even attempt to place a part on a pin for a nut welder without special processing to allow for the tight tolerances involved, or it will only fit some of the time.
4. Number of welds, it is a lot of strain to do any more than 20 welds in a minute.
5. Stepper, how does it handle "end of stepper" from the weld control? Best is to finish the part, move to a window in the cell guard fence, and require a tip change.
6. Weld gun water cooling, does it flow more than 1/2 GPM with an in-line flow meter in each gun arm? (Do not use a bucket and a watch...)
7. Transformer size, will it overheat at maximum production using full stepper boost current?
8. Dressing of the hoses and cables, no tight bends, no dragging on the part. Is the primary side power wire easy to change? (Sometimes the plugs on the ends are harder to deal with than just terminating to clamp connectors) The secondary might be a kickless cable, it is easy to change?
9. Are all secondary copper connections silver plated? (Bare copper oxidizes to unusable in a few days...)
10. Are all secondary bolts grade 10 stainless? (Do not use "316" alloy, less strength than a cheap bolt) People do not realize we use stainless because its thermal expansion matches copper. Magnetics has nothing to do with it.
11. Nobody does this yet, but the water shutoff should be right at the gun, to prevent loss of water and an air-lock happening after tip change.
12. The air valve should also be right at the gun cylinder, as there is too much delay in squeeze time if a long length of line is between the valve and the cylinder. Measure squeeze time required to get to 90% of weld force. This may require explanation of how to do this, or you may know how.
13. Transformer capacity, will you have enough extra weld power to make up for line drops when at full stepper boost? You must know a lot about your power before you can answer this. Also are you able to weld above 50% for the lowest current weld and with no stepper boost. (Too much capacity is bad too...)
14. Force setting, does 80% air pressure give the optimum force? Too much capacity is bad, Too little is a disaster.
15. Tip size and alloy. I have my favorites, dome zirconium, 13 mm or larger. Your may vary.
16. Weld stroke over-travel, must be able to close gun with one tip off.
Auto companies in the US do a Production Part Acceptance Procedure, which consists of a couple hours or more of actual production, where parts are checked for dimensional integrity, and weld integrity (completely torn down) Usually the teardown is done on parts 1,3,5,7,9,19,29,39,49,74,99 and every 25 thereafter, and the dimension check on 2,4,6,8,10,20,30,40,75,100, and every 25 parts thereafter (Sometimes every 50 or 100, depending on the customer)
And failures must be corrected and (at the customer's choice) restart the test, or continue.
Yes you can cure this, but it will cost you!
This relates to question 20 on my test, on my website. We will use the changing resistance of the weld stackup to regulate the heat for us.
First you have to have a reasonable weld schedule that would weld this without the fit-up problem, and if you tell me the two materials, I will get that for you.
Next, you use that schedule three times, with long cool times between. For instance let's say you were welding G1.2mmG/G1.0mmG. This means the 1.0mm Galvanized sheet would be the "GMT" or "Governing Metal Thickness", The new guys at GM call this "DT" now, "Determining Thickness".
We look this up and find the force, time and heat to be: 600#, 10,000 A, 13 Cycles. But I would start with 9000 or 9500 Amps because new tips concentrate the current more than tips that have 50 welds on them. I would imagine you are using a stepper, so boost 500 Amps in the first 50 welds, and you will get through the troublesome "tip break-in" period.
Anyway, on to the cure, since the first time you apply this heat the part won't get all of it, as the 2mm gap prevents current from flowing through the weld area, but it might heat it up and bring it together. At this point the resistance of the hot metal will be about 150 micro Ohms, because it is not welded, and because it is still hot. Let it cool for 13 cycles, and at that point the resistance will drop to 10 micro Ohms if it welded, or stay at 150 micro Ohms if it didn't.
Then you apply the same heat again, and this time will probably weld it, but because "probably" isn't good enough, we let it cool another 13 cycles, and apply the heat again. In each of these cases, after the cool time, the resistance will be high if it didn't weld, low if it did. If it did weld, during one of these heat times, the low resistance of the weld, after it cools, will prevent excessive heat to be formed on any following weld attempts. After all, this is called "Resistance Welding" for a reason, we need resistance to develop the heat when we pass current through the part.
So usually we have three "heats" each one of them a full weld schedule, and two "cools" each one as long as the heat time was. Then you put normal hold time after that, which is usually half the heat time. Actually it is a little more complicated to figure hold time, but just use half the heat time for now.
All of this was counting on you using a proper squeeze time, because if the gun hasn't stabilized its force, it doesn't weld the same each time. If in doubt put in 90 cycles, and work downward from that until you get sparking at the beginning of the weld.
Now here is the cost:
1. Your weld time will be 5 times as long, upsetting the production manager, who says you fix this by "just turning the heat up". Hi is usually big and burly and doesn't want anything to slow his line down, and offers to "make mashed ta-taytas outta yer head".
2. The tips will last less than 1/3 as long as they should. If you were getting 3000 welds, you won't even get 1000 now, and here comes that big production manager again to tell you how to weld.
3. You must tell me how this worked for you, that is part of the cost.
You have asked about almost every welding cosmetic problem there is.
So I will take the first one, and hope you will understand why I must break this up into multiple sections as each one of these requires a huge volume of information to cover all the situations that could exist.
1. Spot Burr, sometimes called "expulsion slivers":
This is caused by excessive heat at the surface, the causes are listed below in order of most likely to least likely:
A) The most likely cause is the weld force is too small on that surface. Note that it can happen on one side if the robot's equalizer is not letting the gun "float" after it closes on the part. This is very common, as the robot programmers do not check for the position that centers the equalizer's travel. Most just position the lower tip a few mm away from the surface, and then "teach" that spot. You should measure the amount of equalizer travel available, then position the lower tip away from the bottom by 1/2 that amount. Also you should close the gun and check to see that the equalizer air or springs, or counterbalance, is effective enough to allow the gun to be moved up and down without too much force.
For applications where the gun is turned over or working on its side, the equalizer will not be correct. This has never been looked at by the Engineers that design the equalizers, even though they have been doing that for years and years...
B) Next most likely, the gun force is too small, it is very hard to weld .65mm/.65mm with force below 480 pounds, or 2 kN The force required is higher for thicker material, hopefully you have a chart for this, but it goes up to as much as 5 kN
|DT mm||Weld Force|
|MIN WELD SIZE|
This will be a very long answer, because there is so much to specify to get from the metal thickness to the KVA size.
This will take us through all the known calculations that have ever been developed by man, except E=MC2
1. A 1mm/1mm stackup needs a 4mm diameter weld.
2. A 4mm weld is best done with a 6mm weld tip, to allow for production tolerance.
3. We must figure the current based on the material:
a) If this is in uncoated mild steel this will take 8,000 Amps for 9 cycles at (60 Hz)
b) If galvanized mild steel it will take 10,000 Amps for 13 cycles
4. Next we must figure the voltage required, based on the secondary design:
a) If this is a transformer/gun with a small throat area, it will be 3 to 4 Volts required.
b) If this is a "hard tool" with 16" cables it will be 12 Volts required.
c) If this is a "Portable gun" with a 10' kickless cable it will require 24 Volts
5. Then we can figure the INSTANTAINEOUS KVA Volts x Amps:
a) The amount could be 3V x 8kA (24KVA) to 24V x 10kA (240KVA)
b) Depends on material (3 above) and secondary voltage (4 above)
6. Be sure to allow for stepper boost required when figuring current:
a) A prototype shop will dress the tips every 100 welds, so won't use a stepper.
b) A production line may want to get 4000 weld per set of tips, so may boost 3000 Amps!
7. Now we must consider duty cycle, or "how many welds in a minute?":
a) Add up the total cycles that current will pass in one minute.
b) Divide by the total cycles in one minute. (60/sec x 60 seconds=3600)
c) For example, 10 welds of 13 cycles each = 130 cycles. 130/3600=3.6 duty cycle
8. Now figure Equivalent Constant KVA:
a) Multiply KVA (found in 5 above, and allow for the stepper!)
by the square root of the duty cycle (7 above)
b) For example, 240KVA x SqRt(3.6)=240 x .19=45KVA
9. Transformers are rated at 50% duty cycle, so divide your KVA by .7 to = 50%:
a) 45KVA divided by .7= 65KVA.
And that is how you arrive at the KVA for the transformer.
BUT, I think you said; "Customer has 35KVA factory power" This could mean a lot of things.
So there are other factors:
10. Most likely this "35KVA" means the voltage multiplied by the current available.
11. It probably is the 3 phase power available, and the welder is a single phase load,
and the single phase power will only be .7 x 35KVA available, or 24KVA.
12. It will support a lot higher KVA than this because the welder is not a continuous load.
13. Also many factories do not have the right voltage, and often have a transformer installed
and the transformer must be selected to be a LOW IMPEDANCE transformer.
I really like the ARO guns that are marketed through ARO/Savair here in the US, but they said they use class 3 copper in the US, and class 2 elsewhere for the gun arms. Very important to avoid class 3 because of the beryllium dust caused by grinding and sanding. If class 2 works with its lower strength, I would recommend it.
Also MFDC is much better for materials that are affected by the high peak heat of every cycle, such as the new Advanced High Strength Steel, "AHSS".
If the material in the coating is harmful when included in the weld nugget, such as a Zinc (Galvanized) coating, then you must do something to minimize the amount that is melted into the center, “weld nugget”, area.
Many things can be done:
1. Use a thin even coating, such as Electro plated, Galvanneal (which is re-heated electro plated zinc) not G60, or even worse G90, two hot dipped standards that vary in thickness.
2. Use upslope to start your heat. Carefully applied, it can cause the zinc to melt and squeeze out before the steel reaches forging temperature.
3. Use impulse heat, allowing the nugget to reach molten temperature without melting through the surface. Note that uncoated steel is welded with lower heat, because you don’t have to reach melting temperature to fuse the metal together, just reaching the plastic state is acceptable. This is not acceptable for welding zinc coated materials, you must melt it and get it to mix together, or there will be a thin layer of zinc caught between, making a weak weld. Impulse heat uses 1 to 3 cycles of cool time, allowing the surface to cool without the center cooling much. Usually a weld of 16 cycles is broken into three or four impulses of 4 to 6 cycles with 2 cycles cool time between.
Makes a big difference in weld consistency.
This is very ambitious. Usually the welds are qualified in “patterns” where about 10% to 20% of the welds can be discrepant, but the Pattern is still acceptable, because of these extra welds. If you don’t do this, you will be constantly stopping production while you repair things, very inefficient.
As far as patterns, they must be 100% to insure the integrity of the assembly. If you were to judge by the individual spots, most assembly plants are between 95% and 98% acceptable welds. Trying to do better than that may be very difficult, and may not have a good reason to do this. Also you must never have a design that insists on a single weld, because the only way to insure 100% is to tear apart every weld.
This is a concept that suppliers in Mexico do not understand, they want 100% good welds because the US customer tells them that.
We must have 100% good parts, but a good part, if properly designed, can still be good with a missing weld. Many Automotive designers do not understand this important concept either. If you are going to run a plant, you must not try to do the impossible, or you will never get any parts shipped.
There are very few formal classes anywhere for resistance welding. Major universities in the US teach only arc welding and metallurgy in their degree programs. So you can have a degree in Welding Engineering, from Ohio State, LeTourneau University in Texas, or Ferris State in Michigan, and have no proficiency with resistance welding.
In 1976, I was made the Welding Engineer at Chevrolet Gear & Axle, in Detroit, when the real Welding Engineer passed away. My training was in Electronics, and I was working there as a Union Electrician, specialized in electronics, and I have been working exclusively in Automotive Resistance Welding ever since, 35 years.
I am surprised that this would change anything, as it does in arc welding, because there is no molten or vaporized material to flow from, or to, a tip in resistance welding. Usually the tip life on an automatic or a robot is almost 4000 welds, and it is limited by contamination of the zinc more than anything else. With steel welding, there isn’t much more life because the tips mushroom beyond usable diameter beyond 4000 welds or so.
Tip dressers are tried here, but they are not used right at all, this is another whole weld subject to go into.
I will help, and you will get knowledge that you can’t get in a university. I laugh because in the US they want a degreed Welding Engineer, then when he can’t handle resistance welding, they replace him with another one. They never figure out that the degree means he studied arc welding, and non-degree people like me might have used their time learning resistance welding. Unfortunately all these guys are self-taught, and that means they may not have a well rounded experience. I have been in Automotive Welding, and I am good with that, but there are many other applications, such as wire welding, soldering, tube welding, small motor armatures, etc. that I do not know about. Luckily I was involved in the growth of Pertron Controls Corp, and was exposed to some Aerospace Welding, as they were involved with many aircraft companies. I got them into Automotive Welding, and with their precision control, they gained many customers, and grew to be the largest supplier in the US. Square D Company bought them, in turn bought by Group Schneider from France.TOP
No, I would be too busy. It is better that just a few people find out about me, such as yourself.TOP
This is on the hem welders, right? If true, I must tell you much more about hem welders, so you can justify why you must get rid of them.
The places I have seen copper plates is in hem welding, around the folded lip of a door.
The outer skin is folded around the inner skin, making a 3-layer stack up. The outer surface is placed face-down on an insulated, water-cooled copper block, and a 6 mm diameter welding tip comes down from the top, touching the inside of the folded lip. Another gun is brought from the top to contact the inner skin, as a “ground” gun.
Current flows from the welding tip, through the folded lip, into the inner panel, then across the inner panel to the “ground” gun. No current flows through the outer side where the copper block exists. I see that Question #Q24 was about this, but needed this drawing:
Hem welding with simply a copper plate between one tip and the part is really hard to do, even worse than the hem welding I drew a picture of above, which the car companies have now given up on. The popular way to handle this hem is with heat cured epoxy, cured in a few places around the hem with induction heating coils. This stabilizes the part so it can be sent to the paint operation, where the hot electrostatic "ELPO" dip will finish curing the epoxy.
Now the problems with hem welding:
1. The heat is usually controlled by forcing a controlled current through two sheets of steel. The current is forced into a round column by the two tips of equal diameter where they contact the steel surface. Since the water-cooled tips also cool the surface, the area that reaches fusion temperature is halfway between the two tips. Also the "nugget" size is controlled by the diameter of this column of current, which in turn is controlled by the contact area of the tips.
The hem weld arrangement does not control this current path very well, and it is hard to predict where the hottest spot will be, but probably NOT at the junction of the two metals. This position would change, depending on the distribution of resistance within the current path.
2. The resistance of the weld depends partially on the force between the rough surfaces as they are forced together by the tips. If this resistance is not consistent, the heat, therefore the weld, will not be consistent. This pressure is greatly influenced by how tight the hem is folded over on itself. If more "open", the tips will not push the inner surfaces together as much as if it were more "closed". And the hemmer machine is constantly touched up by the machinists and set-up people, with total disregard for any affect it may have on the weld heat. They are not "bad guys", it is just that nobody realizes that it affects the weld at all.
3. Because the hem weld is so unreliable, a heat-cured epoxy has been added in the hem, applied with "sealer robots" just before the hemming operation. This further affects the resistance of the work, making the weld even more variable. Further, the inconsistencies of the bead size and control of where it goes is seldom considered a problem by the non-welding people, so often they don't even use more sophisticated sealer dispensing controls, making even further problems, including getting the sealer on the surface of the part.
4. Sealer on the surface of the part greatly accelerates the erosion and contamination of the tips, causing further problems with controlling the weld heat.
5. The hem area is held very tightly, and is very rigid, so when the tips close down on it, any imbalance in forces will be felt as more or less force on each side of the sheet. A proper gun equalizer will compensate for this, but very seldom the gun equalizer is properly designed, or even properly maintained and adjusted. Since this is true of most of the equalizers in the whole plant, we can also be sure it is true of the hem welders.
6. Testing of the hem welds is very difficult, as there is never an inspection station between the hem welder and the point where the robot places the part on the conveyor to the paint shop. Further, gently pry-checking this area will produce ugly deformations on the finish surface of the part, unlike all the other structural welds that can be gently pry-checked. So the welds can get out of tolerance without getting "caught" as quickly as the rest of the welds, making the problem a big one when the poor welds are finally discovered. We have a term: "Weld Spill" that is often used here, where bad welds were not contained and repaired before the parts left the body shop. All other welds are well inspected, and repaired or contained in some manner, and it is a rare occurrance for one to escape all this inspection.
So with all these things going against hem welding, is it any wonder why we have replaced it with epoxy?
It is pretty amazing how little power is used by a resistance welding machine. I once had a landlord in a small prototype shop I was a partner in that thought our share of electricity was much higher than my calculations. we never did settle our disagreement, and it led to the end of the prototype company.
Using your figures:
Transformer KVA: 100
Welding current: 9500 Amps
Secondary voltage 18 Volts
Weld time: 21 cycles
And adding an important ingredient:
Welds per day: 100 parts with 25 welds each = 2,500 welds
Then, using your formula:
Energy Consumption = Demand KVA X Weld time in cycles X energy cost/ (Frequency X Time X minutes X time seconds)
Demand KVA = (Sec.Volt X Sec.Current )/1000
So: Demand KVA = (18 Volts x 9500 Amps)/1000, which is 171 kVA
Energy consumption in weld = Demand KVA X Weld time in cycles X energy cost/ (Frequency X Time X minutes X time seconds)
Energy consumption in weld = 171 KVA X 21 cycles X $0.11 per kWH / (60 cps x 60 mins X 60 seconds)
Energy consumption in weld = $395.00/ (216000)
Energy consumption in weld = $0.001829 per weld (This is 5 welds for a penny)
So for a day's production of 2,500 welds, this would cost $4.57
So for a working month of 21 days, this would be $95.97 a month.
I have never done this, but it should be a matter of calculating the air used in a gun stroke, based on air volume, or piston area X stroke length. Of course area is 3.14 x radius squared. So, for a 4” cylinder with a 3” stroke:
The piston area = pi r (squared)
The piston area = 3.14 x (2” x 2”)= 12.56 Square inches
The volume = piston area x stroke = 12.56 x 3 = 37.68 Cubic Inches per weld.
Again I would allow extra capacity, because new guns are always added, and leaks seem to always occur.
A weld lobe is a chart that you make from many coupon tests. You make weld coupons and record the cycle time and current. Then you tear them down and decide if they are Hot, Cold or OK, then enter each in a chart. The buzz-word people like to call this a “DOE” test, or “Design of Experiments”. They like their terms, I like the science.
The chart will then show the boundary of the acceptable welds, and you then decide where you want to operate, to stay away from the edges where welding produces “borderline” results.
If you look at my attached chart, which I have supplied in pdf format for easy viewing, and in Excel 2007 format in case you want to use it. This is an example of a weld lobe and I can explain how to build the chart, and how to use it.
Print this chart out so you can follow the step-by-step instructions
1. First you decide what is Cold, Hot, and OK. I use: Cold = undersize weld button when the coupon is peeled apart, OK = greater than minimum acceptable size, and there are charts for this that I can provide. Hot = expulsion occurred during the weld. Note that this example was for a projection weld, but the rules are similar, and the chart is the same. The chart will rename itself when you fill in the cell A37.
2. Select the proper tips, that have a contact size of at least the minimum button size required.
3. Then setup the proper force for the job, I can provide charts for this.
4. Next you condition the tips with 25 welds, this is very important for coated materials. Use a heat that is approximate for the job, and you can do this on just a single sheet.
5. Setup upslope if you are going to use it, to just enough heat to show a small dot of bare steel where the weld will form, but not enough to actually start welding it. You do not want to drive the zinc into the weld nugget, so too much heat is a bad thing. You may setup preheat if upslope is not available, use about 3-5 cycles at about 50% of the expected weld current, look for that dot.
6. Make a weld in a small coupon, record the current with an accurate weld current meter, along with the cycle time. In the case of an impulse weld, I use the standard GM schedules for the force, number of pulses, cycle time of each pulse and current of each pulse. I can provide the GM chart if you weld for GM or expect to in the future.
7. Peel the coupon apart, measure button size, length plus width, and divide by two. (Length and width are at a 90 degree axis)
8. Classify the weld, OK, Hot, or Cold. Note, if you got expulsion, it is Hot, don’t bother to peel it.
9. Then enter the cycle time in the first spot, cell B5 on the excel spreadsheet, the next three cells will also become that number. In my example I entered “5” in cell B5, and B6, B7 and B8 all became “5”.
10. Enter the weld current under the appropriate column, there are four columns for OK, three for Cold, and three for Hot, use whatever one you want. In my example I entered “14.0” in cell A5, because it was an OK weld.
11. Continue with different current levels, you can see in my example, I entered five OK welds, two Hot and two Cold ones.
12. Then I changed cycle time to 4 cycles, and entered 7 more welds.
13. Then 6 cycles, then 7 cycles, then 3 cycles.
As you fill in the area on the left, a chart is constructed on the right, that is your weld lobe. The spreadsheet also finds which cycle time gave the widest acceptable current range, and announces that is the cycle time to use, along with a current that is about 10% below the expulsion level. See this in cell A27, which is also reprinted at the top of the chart.
Many older weld controls do not allow heat entry in kAmps, so a handy chart is available at the right side, where you enter various percents and their resultant currents. I made three entries in cells Y5 through Z7, and it averages these and predicts what current you would get at 20% through 100% settings. This is handy if you are trying to achieve a certain current to closely define the edge of OK–Hot, or OK–Cold. It is important to find those edges accurately, as that determines your acceptable weld current range.
I also made a chart like this for air pressure and the resultant weld force, to optionally assist in setting force.
If you get a very small weld lobe, one that doesn’t have at least a 20% range of weld current at the best cycle time, then you have a problem. This application will not run well in production. You could try another weld force, and do it all again, you might find a better force than you started with. There are many factors in a welding machine that can cause unacceptable results too, so don’t be hampered by a poorly designed machine. This is a big subject, which would take me weeks to cover, hopefully you don’t need that. Basically if a new weld force doesn’t make it any better, you need expert help to review the job and the machine.
I see many applications that are outside of the normal way of welding parts. Usually the Corporate Welding Engineers will identify these cases, and put words in the Corporate Weld Spec that says “Don’t do this…” It is unfortunate that Designers don’t pay attention to these specs, some even think they are rude, ordering how to do things and ignoring new thinking. The Welding Engineer doesn’t mean to be rude, he is just recording a list of “Lessons Learned”. He just wants to make it easier to start and run a production line.
It is said that “if we don’t learn from history, we are bound to repeat it”.
Please tell me what you think of the chart, I made this because it appears to be the best way to setup your welder to assure trouble free production operation.
Usually saturation takes out the main fuses, because the transformer has no inductive reactance when saturation is reached. This means the (typical) primary resistance of about 0.01 Ohms will draw (I=E/R) 48,000 amps from a 480 Volt (here in US) line. The fuses don’t last very long. I have seen an application where the transformer just slipped into saturation at the very peak of the AC sine wave, but this is an unstable condition, and I don’t think it would repeat many times before going into deeper saturation.
Saturation is where the transformer’s core is completely filled with magnetic energy, and any increase in current flow does not cause an increase in the magnetic field. The growing magnetic field is what cause inductance, by cutting across the same windings that the current is flowing in, generating a reverse voltage, fighting the rise in current. In a large transformer this inductance is the limiting factor for the primary current, without it the current is uncontrolled.
Saturation usually occurs on one half cycle and not the other, because one of the most popular causes is imbalanced firing, driving the core more in one direction than the other. The core remembers this and will exhibit the same saturation even if the transformer has not been used in months. Most weld controls have a method of anti-saturation firing, the most popular and simplest one is patented by Square D company, and they are willing to license it. It consists of the first half cycle never exceeding 50% power, and the last half cycle fired the same way to offset any imbalance the first one caused. This is not perfect however, and impulse welding tends to drive the transformer core one way, because the flux has a small chance to reset from the last half cycle on each cool time, and each heat pulse before a cool time ends in the same polarity. This really requires a drawing to explain properly.
The safe range of a transformer needs to be defined by the manufacturer, as it is based on core material and size, and the windings around it. There are two types of core, the stacked core and the wound core, with the wound core being easier to saturate. If you can capture the waveform of the primary current during one of these events, on a storage ‘scope, saturation would show up as a much greater current in one direction than the other, and if just barely slipping into saturation, it shows up as a spike of amplitude on the very peak of the sine wave.
An upslope will degauss the transformer, and if you add that to the start of the heat, it might make the noise stop. If it doesn’t affect the noise, you probably just have something loose in the transformer. Old ones were not potted in epoxy, and this can happen, it is usually harmless. My bet is that is the case, because operating near the saturation point for any length of time is very likely to take out all the fuses.
If you can describe the noise as a loud grunt, it could be saturation, where as a loud rattle would be a loose cover or even lamination inside. True saturation is a very frightening sound, much different from a rattle.
Most of the energy used by a weld station is lost in "IR Losses" (heat) in the conductors carrying the weld current. This includes the transformer primary and secondary, the cables ("shunts"), the gun arms, shanks and tips.
This heat can build up, so most of this is water cooled. Even air-cooled "shunts" get a lot of their cooling from the water cooled pads you are supposed to provide for each end.
As a rough estimate, 90% of the energy put in the gun is lost in heat to the cooling system. There can be more in a portable gun with a kickless cable, less in a close-coupled trans gun. Some trans guns may have as low as 60% lost in heat.
So, once you have calculated the KVA requirement, using peak KVA and Duty Cycle, this is about what you should allow for in the capacity of the cooling system.
To do this for 150 guns is a matter of summing them all up.
Two reasons to do this:
1. Selecting new transformer for a gun,
2. Verifying the existing transformer is proper for the application it is on.
Let’s take #2 first, as this is less work:
1. Verify that the transformer produces enough current to allow for steppers (usually a boost of 4000 amps to make it to 4000 welds per tip change.
2. Verify that the transformer can supply “boosted” power at less than 90% settings, to allow for line voltage compensation.
a) The voltage deviation on the line is important here, and must be measured carefully, as you must determine the drop under worst case production.
b) The effectiveness of the control’s line voltage regulation must be considered, most modern controls will adjust for + or – 20%.
c) If you use “Current Regulation” Then you must determine how bad the secondariesget for the control to compensate for.
Some controls will indicate their % they used to achieve the current you asked for, my favorite is a Medar control that indicates “C-Factor”, which is what the total maximum current would be on the gun it just fired if the line voltage was at the normal voltage. (This can be a whole study in itself)
3. Verify the above by welding the actual part and allowing for the added boost, line voltage compensation, and current compensation for aging secondaries.
4. If the transformer can supply adequate current, then figure the Demand KVA by:
Nameplate Secondary voltage on the tap that meets your requirements for max current X The max current under worst case operation.
5. Then figure duty cycle by:
Divide the number of cycles of heat in 60 seconds of production by 3600 cycles (If you are 60 Hz line frequency)
6. Then figure the ECTC (Equivalent Constant Thermal Current”) by:
Multiply the square root of the Duty Cycle by the Demand kVA.
7. Then compare it to the transformer’s kVA rating by:
Multiply the nameplate rating by .7 (Because it is stated at 50% duty cycle, you need 100%)
Compare it to the requirement kVA calculated above.
OK, so now I can discuss the first reason:
How to find out the requirement of spot welding transformer rating for certain application during projects:
For sizing a transformer for a new application, you must first figure out what secondary voltage is required. The Roman Manufacturing Co. here in Michigan has a formula that they apply to answers you give to them. When you call they will ask you questions about the gun and cables to it to determine its impedance (resistance and reactance). I have only met one Engineer that could calculate the impedance himself, and I haven’t seen him in years now. He would consider the path for current flow, how long each conductor was and what the cross section of the copper was. That included the cables, the gun arm, the shanks, and the tips. I am sorry that I do not have that info to give, everyone I knows calls Roman and they figure it out. Of course they want to supply the transformer, so they tell you what part number you need.
Then the next thing is a repeat of the above discussion of figuring kVA rating needed to handle production welding. Remember that transformers are rated at 50% duty cycle, so you will have to divide the kVA requirement you need by 0.7 to equal the nameplate rating the manufacturer puts on his transformers.
This is often called “Hem Welding” when it is done around the edges of the door, after the outer has been folded around the inner, and it actually is a three-layer stack up. This was done with a special arrangement where the outer lays on the water cooled copper plate, and the weld tip is lowered on the hemmed lip of the outer, and a “dummy gun” is lowered nearby on the inner. The current passes from the weld tip on top, through the top layer, to the middle layer, and back through the dummy gun. The visible part of the outer skin never get current, and is protected from heat by the copper plate. I see your method passes current through this outer skin, so you will have an easier time doing the weld, but more maintenance keeping the copper plate clean. The US auto suppliers used to hem weld, but it was so troublesome, that they dropped all structural requirements of the weld, and added heat cured epoxy in the assembly. Now they have stopped welding this at all, and replaced the hem welders with induction coils that quickly cure the epoxy applied by robots just before hemming.TOP
I can explain how we used upslope to move the zinc coating aside on galvanized metal. We were very successful with this at Pertron, because we had a very well controlled slope function with our microprocessor welder. Now all controls do this well, so it can be dealt with the same for non-Pertron welders.
The problems with the zinc coating are:
1. The coating has a higher surface resistance than bare steel has so it causes undesirable surface heating.
2. The amount of this surface resistance varies wildly from one weld to another so gives non-uniform heating.
3. The unpredictable surface resistance may cause the heat to be higher on one side of the weld than the other, causing an offset nugget.
4. The zinc alloys with copper making brass, which is a high resistance and brittle coating.
5. The brass breaks out of the tip, leaving a concave surface, which is very bad for starting a weld in the middle of the tip contact area.
Up Slope helps with this, as it will allow you to gently heat the coating and it will move aside, giving you a lot less zinc between the tips. As you can imagine, this gives less contamination, forming less brass out of your copper tips.
Also it reduces the problems with the unpredictable resistance at the surface, so the weld heat that follows is always applied in a mor repeatable way, giving much more consistent results, weld-to-weld.
To setup this slope, get a coupon of what you are welding, and make a guess as to what your final welding current will be. Find a % heat setting that gives you that weld current, either by experimenting, or reading the last weld information after a weld. Different controls have different ways of displaying this. Then, using an upslope command, set it up for 5 cycles of upslope heat, from as low as you can go (Usually 20%) to whatever the % is that gives you the normal welding current. Do not follow it with a weld heat, you want to just test the slope, you do not want it to weld.
Tear the coupon apart, if it welded, you have too much heat, and you are probably driving the zinc into the tips. Cut the heat down in one of two ways:
1. Less cycle time on the upslope heat.
2. Lower the final heat on the upslope heat command. Some controls don't allow a setting for final or "ending" heat, they just slope up to the weld heat level, since you will have a weld heat next. If your control does this, then setting the upslope heat would have required one cycle of weld heat, so that the control knew where you wanted to end up. But I am sure you get the idea.
If it just snapped apart, check to see that there is a tiny dot of bare metal facing each other in the middle of the weld area. If this is any larger than 1/32" diameter, lower the heat until it is. You just want to prove that you can expose a bit of bare steel, you don't need to do any more than that.
If it just fell apart, you may have to add heat until you get the little dot mentioned above. To do this, just set the starting heat a little higher. Sometimes you may notice that the starting heat is just a couple thousand amps lower than the ending heat. If this does the job without allowing an actual weld, then that is what you want even if it looks a little silly to start that high.
I recommend that the upslope heat be done in the "%" mode rather in the current regulation mode. The reason for this is the surface resistance is unpredictable, and trying to regulate the current poses problems when this is the situation. This is particularly true where there may be sealer involved, and the current compensator may over compensate and even blow a hole in the part! Upslope reduces this chance, as does welding in voltage mode, but doing both is even better!
ANSWER-These cylinders are popular for welding, and solve a lot of problems with follow-up on projection welders.
They also allow guns to fit in tight spaces, as the cylinders can be very small in diameter, and still achieve plenty of welding force.
The controller requires more logic, as it has to operate the gun close valve, wait for it to complete its weld stroke, then apply the intensifier valve, wait a bit more for pressure stabilization, then weld.
However, problems can arise based on the fact that once the intensifier is activated, the cylinder won't move much more. If it is activated too soon, the cylinder may stop before it even gets the gun closed.
Once the intensifier is activated, the cylinder can only stroke another 1/4". So if the part is springy, that will give problems. I urge everyone to use the cylinder that has a 1/2" intensifier stroke, and this document will tell you why.
Click here to open or save a PDF file on how Air-over-Oil cylinders could cause a problem with bad part fitup.
Please retain my name and company on the page as you share it, because I support my family by sharing my knowledge of welding.
I get this one asked so many different ways, because the subject of "KVA" is so grossly misleading.
People ask this because KVA is used to describe the size of a welder.
After all, everything else is sized by KVA;
Even I.C. engines !
Soon some marketing novelist will describe software that way!
To determine the amount of current a welder can produce,
you must consider the secondary voltage and the secondary impedance of the welder.
Physical measurements and a lot of math,
A well trained observation,
Or an actual measured test, fired tip-to-tip.
Yes you can fire this way, if the tips are clean and come together tightly.
But, the KVA of the transformer won't help you determine maximum welding current.
Here is what KVA determines in a weld transformer:
The measure of its ability to process heat.   Wow that sure helped...NOT!
Whole lotta good that does us... But, we must pay attention to heat,
so let's say we have an assembly operation with 10,000 amp welds being made at the rate of 20 every minute.
---This is normal for an automotive assembly line.---
We use KVA to determine if the transformer will overheat.
KVA means "Kilo-Volt Amps" and is closely related to "real power", or "Watts".
But Watts is actual power, it is Volts times Amps in a purely resistive circuit.
Welders don't have a purely resistive circuit.
They have a lot of inductive reactance in their secondary, so we use "KVA"
This is just a way to avoid doing all the math required to express "Watts" accurately.
(To use "Watts" would require some complex phase-angle calculations, and this could hurt
our already over-taxed frontal lobe...)
We need to know how much heat is being generated in the transformer.
So we can take the current times the voltage, and get pretty close.
The voltage of the secondary, go look at the name tag on the transformer.
Oh, I forgot, they mount that down against the mounting plate, so it won't get damaged...
Pretty clever eh?
--- Tell your favorite build shop to stop doing that. ---
Well then, let's go to the welder verification sheet, where it lists all the parts to the welding secondary, you DO have this right?
No?, well then you need to get modern and do "WGV" on your NEXT start-up, too late for this one...
It is difficult to measure the secondary voltage because the current's huge magnetic field induces a voltage into the meter probes.
If you don't believe this, make a reading with both probes on the same point, you will get at least a few volts.
So call the manufacturer of the transformer and ask him. But he will need a part number, and will have to know what tap you are on.
Do you see why we needed the "WGV" ("Weld Gun Verification") sheet?
Let's say our transformer has a 12 volt secondary, this will give about 16 kA in a secondary about 20" long by 8" high.
If you have a tiny little trans-gun, it may be only need 3 to 8 volts.
If you have a big portable gun with a 10 foot kickless cable, it will need 24 volts.
Note at this point that the physical size of the secondary is what determines the voltage needed to do the job.
This is an important concept to remember...
But we are using 12 volts here, so 12 Volts times 10,000 Amps is 120,000 or 120 KVA.
So we're done right?
No, this is only the Peak KVA, during a weld. There is OFF time between welds.
So now we must figure the duty cycle. If the welder is on for 10% of the time, it produces less heat than for 100% of the time.
So we must figure out how long it carries current over an "averaging time" of one minute.
Our weld time, "10 cycles", means "10 cycles of a 60 cycles-per-second AC power source".
So one weld is 10 cycles, or 10/60th of a second. and twenty of these welds are performed each minute,
so 10 times 20 equals 200 cycles "ON" time every minute, and a minute is 3600 cycles.
So our Duty Cycle is 200 divided by 3600
= 0.055 or 5.5%
Now we can figure the equivalent heat, which is the square root of the duty cycle times the peak heat.
The square root of .055 is 0.236
0.236 times the 120 KVA is only 28 KVA
So we only needed a 28 KVA transformer to do this job!
Well... No, it's more complicated, welding transformers are rated at 50% duty cycle... No, I don't know why, maybe they sound bigger that way...
So lets scale this up to see what NAMEPLATE rating we need...
If we had a 100 KVA load at 50% duty cycle, that would be 70.7 KVA equivalent load.
So a 70 KVA transformer would get a "Nameplate Rating" of 100 KVA...
Why do they do this? I don't know, ask them, it makes my life difficult though...
So we need to take our requirement of 28 KVA and divide it by .707 to find the "Nameplate" size of the transformer we need.
28 divided by .707 is almost 40 KVA, so that is the transformer we need.
So will an 80 KVA transformer weld a part twice as big?
No, but it can do these welds at a much faster rate without overheating...
The 80 KVA "Nameplate Rated" transformer can handle 56 KVA at 100% duty cycle
and 56 KVA divided by the 120 KVA of each weld is 0.46, which is the square root of the duty cycle we can run at.
If we square 0.46 we get 0.21 or 21% duty cycle. This means we can do more welds in a minute.
How many in a minute?
Well, 3600 cycles in a minute, we can be "ON" for 21% of them or 756 cycles.
This is 75 welds, almost 4 times as much...What's up with that?
We are dealing with squares and square roots, so it won't be a linear relationship...
Well all this is fine, but it doesn't really answer my question...
The answer is that you can't tell a welder by its KVA. If actual current capability is what you are after
then you must judge the secondary voltage capability against the secondary impedance. That is not easy...
So if you are buying a machine, ask what the max secondary current is tip-to-tip.
Then on a medium sized secondary (20" x 8") that will be only a little higher than your actual weld current.
How much higher?
That depends on:
If it is a large secondary, then the inductance of the part in the gun will load down the welder more.
The resistance of the part will be a small faqctor.
If it is a DC machine, then there is no reactance, but the resistance of the part is
a bigger consideration. Aluminum may pass a lot of current, steel may not (it has higher resistance).
If it has a kickless cable, then the resistance of the part will mean almost nothing
compared to the huge resistance of the kickless cable.
If it is a little trans-gun, then the resistance of the part is a major factor.
So you just can't go by KVA. If you just have to know, contact me, and I will ask you a bunch of questions,
but we can estimate the machines performance pretty well.
There are two parts to the answer:
1. Calculating the current requirement and
2. Looking up the proper wire size for that current.
Let's say you know the current already, you need to consult the NEC Wire tables, currently they are:
This links you to the Houston Wire and Cable page, who did a great job of posting the NEC information:
Table 310.16 Conductors in Conduit
Table 310.17 Conductors in Free Air
But if you didn't have the current figure, then you must calculate it, here's how:
A. Find out the highest primary current the machine will be used for. If there is a stepper, include the stepper boost.
Typical values will be 8000 to 15000 amps for sheet metal, triple that for projections.
B. Calculate the equivalent primary current, divide (A) by the transformer turns ratio. (The primary voltage divided by the secondary voltage)
Typical turns ratio values will be 100 for transguns, 40 for machines, and 20 for hanging repair guns with kickless cables.
C. Find the "Duty Cycle", divide the number of cycles of heat that would exist in one minute by 3600 (Which is the number of cycles in a minute)
Typical values will be 0.10 to 0.20 for robots, 0.05 for machines, 0.03 for hand-fed projection welders
D. Find the square root of the duty cycle (C)
Typical values will be 0.32 to 0.45 for robots, 0.22 for machines, 0.17 for hand-fed projection welders
E. Multiply the primary current (B) by the square root of the duty cycle (D)
Typical values will be one-half to one-tenth of the primary current, so don't be surprised to see that you can supply only 100 amps to your big projection welder. This is called the "Equivalent Constant Thermal Current" or "ECTC" and it equals the same power you would draw, but averaged out over a short period of time.
Now you can look up the wire size. I recommend running it in free air to avoid the inductive effect that a metal conduit adds. One of the least observed precautions I see in the field is that the power runs are not low inductance. At the last place I worked they had a "salvage welder" which repaired bad projection welds, that was fed with a long run of cable in a conduit. It would start out at a high current, then slope down to just about zero in ten cycles. No one could understand my explanation of the inductive power service. They never saw a low impedance weld bus, and never thought about the concept. It is probably still doing this today.
Nice to hear from you. Is your lab still in the ARO plant?
I'll have to drop by sometime...
Anyway, yes the fused area stops growing when the current stops. There is a video that GM had made (that Dave Kelly should be able to get for you) that shows the growth of a nugget during welding. The video is developed at a high rate, I think 420 frames per second, and the view is of a pair of tips and two coupons of about 1mm each, all sliced in half at the center of the weld area. This video shows both the growth of the nugget, and the conduction of current at the same time. It is easy to see that the growth of the nugget corresponds directly to the application of current. In fact, the growth rate speeds up with the peak of the AC current, and slows down to a stop as the current approaches zero, at the end of each half cycle. I was asked by Dr. Karagoulis of the GM Weld Council (Tech Center) to show this video to all the GM students that I held welding classes for. I am not allowed to copy or distribute this video, but if you are working on GM projects, I can show it to you. (as Dave Kelly can also)
The hold time would not affect the fused area, but it could affect the weld quality, in such a way as to appear to affect the "button" size revealed with a peel test ("Coach-peel"). It can do this in a number of ways:
1. If the part is springy, and the hold time is short (shorter than 1/2 the heat time) the weld may be disrupted when the part moves while still hot enough to be in the plastic state.
2. If the water cooled tips provide too much quenching, and you have quench-sensitive HSLA, the nugget will fracture as the steel returns to room temperature size (shrinks) while it cools through the plastic state too quickly. This is why there are no 1983 Corvettes, it took extra time to engineer the weld heat profile to avoid this on their new (at the time) heavily galvanized (G-90) HSLA frame material.
3. If another weld is done nearby and disturbs the part while this weld is just entering hold time. This is pure conjecture on my part, but I did study this relationship while working on what Medar calls "Thermal Force Feedback" with Mr. Ariel Stiebel at the Pertron lab, before he went to Medar because Square D purchased and closed up Pertron.
4. Another conjecture on my part would be changes that might occur in the rich chemistry of AHSS during different cooling rates after the weld. There will be much discovered about this over the next 10 years as the auto plants start getting AHSS to work with.
Anyway, if the coach-peel is used to reveal the nugget size, anything that affects nugget strength will show up here. A better test if you really wanted to dig into this relationship would be done ultrasonically, such as with GE Krautkramer equipment, which can identify nugget size before any peel test is made. The new ultrasonics can identify the difference between metal that has been heated and cooled from that that has not, by measuring the losses in the sound reflection as it passes through the grain structure of the metal.
Presently they identify the nugget size by controlling the diameter of the sound beam, and identifying if anything within that beam was not welded. So to actually read nugget size you would have to try increasingly larger probes until lack of fusion is indicated. The equipment is designed to simply tell if the fused diameter was larger than the probe, which it does a good job of. They make no claim to find cracking, as the cracks are usually "seen" on edge by this equipment, but we actually did get indications of cracking when we tested welds with this condition at a demo GE Krautkramer did at AZ Automotive. The GE people did not know we "set them up" with known bad welds, and were pleasantly surprised when we "fessed-up" to our little trick, but they still will not tell you it will find cracks unless they are perpendicular, like "de-lamination" cracks.
I hope this long-winded answer helps, please respond on your thoughts.
Pertron had a couple of data entry panels;
The "6000" model was networked to 16 weld controls, and
The Tim 3000, TIM 4000 and TIM 5000 had their own data entry panel, which was also the timer. The box it mounted on was the power unit, with a firing board, but no processor, or "timer" as it is usually called.
The Pertron on my website is an early model 4000, only 125 were built. After a re-layout inside, the TIM 4000 became Pertron's most popular model single phase AC welder. This was also called the "Paragon", as it was conceived to be the best of the controls that Pertron was manufacturing.
The TIM 3000 was special made for GM Indianapolis, and were large banks of single phase controllers in a common cabinet. The TIM 5000 was a three phase controller, sometimes called the "Alpha" by the engineers in Chatsworth that dreamed up this monstrosity. The circuit board inside had 4 processors on it and was almost big enough to make a coffee table out of.
You will have to know more than the model as just discussed, you will have to know the software for it. Pertron had over 250 versions of TIM 4000 controls, with all sorts of features and I/O operation. Having a description of the terminal strip inside the cabinet will help identify it, but a better ident is made from the tag riveted to the front of the door, you will need that. Another method, is to get the DEP to report its SW and Version number in the readout, which I believe it does on power-up.
Pertron controls ceased production 5 years after Square D bought Pertron in 1987. The manager of board repair, Paul Gutierrez, left Square D (with their blessings and support) in 1992 and started "Industrial Control Repair" to service these old controls. ICR would be your best bet at finding anything Pertron, and second best would be "Welders and Presses" In the Northern Detroit MI area. Here are some web sites for them, tell them I referred you. ICR is http://industrialcontrolrepair.com/ Office (586) 751-3335
Welders and Presses is www.welders-pressesinc.com If you find someone else that has this item, they probably are getting it from one of these sources for you.
One more source of info would be Fred Kowal, who is an Application Engineer at Square D Troy, MI (248 680 4444) who would check each version of software that Pertron sent to its customers. He took this on himself, to save us all the embarrassment of a customer discovering something really stupid in the software. The home office didn't necessarily want Fred doing this, but as far as I was concerned every control had to be checked by Fred before I would let the customer have it. I don't know how much Fred saved them, but I tell you this was a valuable service he performed for Pertron, whether they appreciated it or not. He was so good at this, that I'll bet he would remember a lot of details of operation, just if you gave him the numbers that were written across the tags on the Prom chips under the DEP's cover.
The Pertron was very easy to use, most people never referred to the manual, as the keyboard says most of what you need. A program usually has "Squeeze, Weld, and Hold" in it, and the setup mode usually just needs the nominal line voltage value entered in the right spot, and selection of Major-Minor or None for each fault. The terminal strip should be fairly standard, with sched select, initiate, control stop, and pressure switch inputs, and weld valve, minor error and major error outputs.
This is a well known problem with all digital displays, if you get a display of 12.3KA, displayed as just three digits, that means it could have been 12.3000 to 12.3999... The reading has to be cut off somewhere. This is over and above all other accuracy limitations any instrument may have.
A good way to explain how the last digits get eliminated, even if they are .9999 is to take two pieces of paper, on one, draw square waves from right to left across the width of the paper, put about 12 of them there, so it looks like:
Cut out a window in the other page big enough to see only ten of these pulses.
Lay this "window" over the pulses drawn on the first sheet, what do you see? You may see ten full pulses, or you may see only nine full pulses, because of the way the window is placed. This is how meters register digital values, they count all the full pulses and ignore any incomplete ones. This is just a very simple example of digital error.
As this is trying to illustrate, three windows displaying 10.1 can only be trusted + or - "0.1" so we cannot expect to hold the measurement to "0.1"
Adding extra windows does not help, unless the instrument has the greater accuracy to deal with those windows. Since a current checker is customarily + or - 3% and those windows could be used to display 99.9, you could be off + or - 3.0 right there!
A popular gauge has an extra window for the last digit which always displays "0", and people believe it to be accurate to the last digit! The eyes play tricks on people, especially if they are unaware of the facts. Engineers rule!
Question 20 does requires a lot of explanation, but you asked for it so I assume you will sit
through the answer.
1. When we weld metal, the resistance of the workpiece changes during the process. The average resistance is 100 micro ohms, and it is often presented as just that, but it's not that simple. First, when the tips first contact the part, and the force stabilizes, we have 200 micro ohms, due to the rough surfaces meeting each other in three places; upper tip to work, faying surface, and lower tip to work.
2. During the first 3 cycles the surfaces soften, after all they get most of the heat, because that is where the bulk of the resistance is at this time. When they soften, the surfaces flatten and contact each other better, lowering the contact resistance. At the end of the 3rd cycle the resistance will be 50 micro ohms.
3. As we continue adding heat, the resistance rises, as steel (and I think all the metals) has a positive coefficient of resistance with temperature. So as we add heat, the resistance rises up to around 100 micro ohms. Many resistance feedback systems look for the fall, then this rise in resistance, to signal that the weld was successful, but it is only a fair approximation, we can't tell for sure that the weld is good, so resistance feedback never amounted to anything but a lab experiment. Believe me, at Pertron we pushed the technology of resistance feedback to the very limit, finally building a control for GM Indy Stamping that could tell if it was bare or galvanized, then what category of weld it was turning out to be in galvanized. There were three categories (for galvanized) , some dropped resistance right away, some never dropped resistance, some had multiple fluctuations. We didn't understand why, but we did recognize that all three could be made into a good weld, if the heat was controlled properly with feedback and we did that. But the system required even more care, because any unusual variables would throw it out of control.
4. Then the heat is stopped, and the part cools. In roughly the same time as the heat time was, the resistance drops to 10 micro ohms.
So, knowing this, with a fitup problem of unknown amount, we can try a weld heat. It will either weld, or if it was a bad fitup, soften and bend together. We can tell which by reading the resistance after a bit of cool time. I like to use the same time as the heat time just to be sure. If the resistance is high, it didn't weld. If the resistance is low, it did.
But we aren't reading resistance, so what can we do? Well, if it didn't weld the resistance will be 100-200 micro ohms, and we need to try again, and if it did weld the resistance will be 10 micro ohms, and we don't want a lot of heat. So we give it another weld heat. If the resistance is 100-200 micro ohms, this will generate heat and possibly weld together. If the resistance is 10 micro ohms, it doesn't get much heat because heat = I squared R, so it gets roughly one tenth of the heat because it has one tenth of the resistance..
Classically we give it three heats, the 3rd one will solve even the toughest of fitup problems reliably.
The saying: "There ain't no free lunch" applies here. Tip life will be 1/3rd of normal, and the cycle time is 5 times longer (for the weld-cool-weld-cool-weld sequence that now exists between squeeze and hold). But it does solve your fitup problem, by using the dynamic resistance change of the weld to regulate the heat automatically.TOP
This is a very large subject, and may take a long time to collect and organize. The American Welding Society (AWS) has organized these subjects into a series of books, that are available to members, and non-members. Their page www.aws.org/ and www.aws.org/publications.html would help you. You would want to look at the Welding Handbook.
The RWMA (Resistance Welders Manufacturing Association) www.rwma.org/ has also compiled literature of this type. It is an organization that you join as a company, but you can purchase their literature as an individual. At www.rwma.org/html/services.html you can order literature such as the Resistance Welding Manual.
I   specialize in resistance welding, so I can help there, but your request is for the entire welding field, a very large request.
For information you can get quickly on resistance welding, setup parameters are available from many US manufacturers of welding electrodes. Of course they are fine-tuned to their particular electrodes, as the characteristics of the electrodes has a large effect on the welding setup. See these at the CMW Inc. page www.cmwinc.com/ then click Welding, then Technical Information, and there is a rich page of information there, schedules, "dos and don'ts" (to guide you in good and bad practices) and more.
As far as the basic definition and purpose, I can do that for resistance welding, and relate it to other forms of welding:1. Definition: A method of fusing metal with a combination of pressure and electrically generated heat, applied with low resistance electrodes and high current control devices.
Don't be impatient if all that knowledge doesn't come to you instantly, it takes a lifetime to learn just one aspect of welding.TOP
Manufacturers of kickless cable will supply a chart of resistance readings for each type of cable you use. If the reading is over 120% for either side, you will be replacing the cable soon. Use a Micro Ohm Meter, see Tools
If you cannot pass current, and you suspect the kickless, a troubleshooting tip would be to check the weld controller's last data for:
Current is low and power factor is high = open primary cabling (common with trans-guns in robots)
Current is low, PF is low = primary is good, secondary is open. (could be a cable, or sometimes the gun is not closed)
If only some welds have above problems = intermittent cable along robot arm, depends on robot position to make/break connection. With a micro ohmmeter, power off, measure at SCR output lugs, must be less than 0.1 ohms. Measure at transformer output lugs, must be less than 0.001 ohms.TOP
This would be based on the life expectancy of each component.
Your maintenance records will be the best guide, but I would start with the following,
based on number of welds made. Note that a weld gun on an automotive assembly line
may make 160,000 welds in a week on a robot, or only 7000 on a dedicated "hard tool",
such as a framing line or press welder.
Measure resistance of all water cooled cables, replace at 20% increase from new.
Perform cooling check on tips (They must return to original temperature within 5 seconds)
Check tip force, being careful to avoid false reading from "impact" of gun closing.
(Do not use "latching" feature of weld force gauge).
Check schedules in use, investigate all changes from original setup standard.
(Keep a copy of all standard settings and readings at each weld station)
Measure air cooled cables, replace if 50% increase.
Measure all bolted connections, repair (silver plate) if greater than 10 micro ohms.
Measure water flow to each gun arm, must be =1.0 GPM (3.8 Liters per minute)
Rebuild straight-acting gun cylinders
Rebuild rocker gun weld cylinders
Measure water flow to transformer, cables, and jumpers.
Water flow to transformer =1.0GPM
SCR=1.2GPM (4.5LPM) but this I think is very excessive, SCRs generate 10-100 watts of heat.
The "Stepper" is used to keep current density the same, as the tips
get larger. The variables differ with each application, but the main
ones to consider are:
Q9-       1. How much larger do you want to allow before changing tips?
2. What current is required at that tip contact area?
3. What current is required when the tips are new?
4. Can your process tolerate the contamination that the tip has at that point?
5. What is the weld count at that point?
I   have experience in automotive welding of zinc coated sheet metal, in the 0.8mm to 2.5mm range. It is generally accepted that the tips will have too much contamination (Zinc + Copper = Brass) at the count of about 5000 welds. Testing a welder with tips in that condition, we find that we need about 5000 more amps. Now, this will have to be done with tips that were allowed to wear to this point, making reasonably good welds. You cannot move these tips into another welder, as they will not act the same due to critical duplication of alignment. Also, we must use the same welding schedule (Squeeze, Heat time, Hold time) as was used in the beginning, all we can change is current.
Here is an opportunity to find out if your "starting"
weld schedule is good enough to be used as your "ending" weld
To explain: The schedule you setup with new tips will probably be optimized for that condition, BUT, when the tips get larger, you will have to make a larger weld, this takes a longer heat time. Now the problem... The weld controller manufacturers offer the "stepper" option for this, but it is NOT GOOD ENOUGH! You need more heat TIME. So, what you must do, is find a heat schedule that will weld with old tips, and see if a lower-current version will work for new tips. It usually does, but it upsets people who want short cycle time for high speed assembly lines.
So the answers to the questions above
FOR THIS APPLICATION would be:
1. Ending tip size?=about 50% larger (contact area).
2. Ending heat?=about 16,000 amps.
3. Starting heat?=about 11,000 amps.
4. Is contamination OK?=Yes, but not if it goes any further.
5. Weld count=about 5000 welds.
Now that we have our information organized, it is easy
to see we need to change tips in 5000 welds, we need 5000 amps above
our starting current for extra transformer capacity.
Now we have one last question to answer: How much extra capacity do we need for regulation of weld bus voltage? Few people have actually measured the voltage dips on their weld bus, it takes a "line disturbance analyzer", such as the one made by Dranetz. Weld busses vary, they usually have drops of only 10%, but I have seen the bus drop so low all the relays would drop out when welding. The customer's cure for this was to power the welder controllers separate from the weld bus. These are drops of 40-50%!
You should make measurements if you are having welds
missing every now and then. Modern weld controllers often will give
you this information in "last weld data". You can trust this, and use
it as a substitute for the Dranetz test. If you have a reasonable
weld bus, allow for a 10% drop, so the transformer (in the above
case) would be able to put out 16,000 amps with 90% of the line
voltage. Roughly this would mean we want a capacity of about 18,000
amps. Now, be careful, too much capacity means you will be welding at
a low "phase angle" on the SCR firing point, when the tips are new.
This means the pulses of power at your AC line rate will have high
peaks, and long inter-cycle cooling time. If your starting heat is
below 60%, you will have difficulty avoiding expulsion. In this case,
we are at a starting point of 61%, which is acceptable. In cases
where the transformers will produce 25,000 amps, you may have more
trouble with weld flash and sticking tips.
As you can see, the numbers you need must be gained from experience in your type of welding, so go back and learn from what you already have working.
When expulsion occurs, the metal that was conducting the current
between the sheets is suddenly gone. The effect is to cause current
flow to be lower than usual for the rest of that half-cycle. The
current feedback reports that current was below the "target" level,
and the control responds with more current.
This can be observed in the single-cycle data, watching the measured current and the gain factor that the control applys for each new cycle of heat. These values are easily obtainable in the ADC control, because I specified that to Toshiba, who built the control.
Some controls limit the amount of heat gain allowed cycle-by-cycle, and this feature makes up for a lot of this. Medar does this, I suspect that WTC does, as I met their engineering group, and they really impressed me with the way they control the SCR to regulate current. I cannot divulge the slick things they do, just trust me....
ATek does something to limit this too, because that control will fault if the current is only off by one percent! I have personally measured this at a Harrison plant, and it really works! Another thing that works for them is that the president is an Ohio State Welding Engineer and he won't let you set-up the control wrong... Those controls are on the real tough applications, like gas tanks that weld through coated metal, and all the tough jobs at Delco Kettering.
Too much limiting of current adjustment will reduce the control's ability to make up for a really bad weld bus. But if it is that bad, you should fix it.
The tip temperature must never cause permanent softening of the
copper. Zirconium tips (usually the preferred alloy) soften at 930
degrees F, Cupal (D.S.C.) is 1475 F.
To measure your tip cooling, measure the temperature before welding a part, then again 5 seconds after the last weld, the temp should be the same. If it takes longer, you have a water flow problem. Note that one hot tip can cause the nugget to be "off center" (closer to the hot tip) which can cause a peel test to give a small button! Tip cooling is more important than most people realize.
Expulsion occurs when the expanding metal escapes the containment force. The metal expands about 10% as it becomes molten, and the forces rise as high as 30,000 psi. If the tip force cannot contain this, it gets away, and it does so violently. It causes cosmetic problems, and is very irritating to the workers. It causes a lot of maintenance problems with alignment pins and prox switches getting coated with slag.TOP
There are a number of reasons, first make sure your tip force is at the proper setting, as per your company standards. Make sure that the force is fully applied at the time of welding. Slow guns are a popular cause of expulsion. Check that the tips are aligned on axis, and on center. A 2-mm misalignment is too much, you must fix it. "Normalize" the gun to the part (make it fit square, not at an angle).TOP
Is the current being applied too rapidly? You can heat the part two ways, a lot of current for a short time, and a little current for a long time. The latter method is less likely to expel metal. Again, check your company standard settings.TOP
Refer to the RWMA (Resistance Welder Manufacturers Association) or the AWS (American Welding Society) they have provided this information since the beginning of time. The tip manufacturers will have information also, such as CMW or Nippert, now called Luvata . In some cases you may have to contact a representative, so don't wait for a crisis to find out you have no standards.TOP
Well, the LAST thing you should do is dream up your own schedule, the
liability is too high… Refer to your Welding Engineer, or
contact the Standards Group. Without your feedback, they cannot
correct for application problems. Don't be too proud to call for
help, it is the smart thing to do. Give me a call, maybe I can figure
a way to charge you for my time.
Hey, did I mention the part has to be clean? You can't weld dirty parts, sorry, but the voltage on a resistance welder is very low and it just doesn't fire through CRUD!
You must have good measuring equipment available. I stress available because if someone has it locked up so they can find it at calibration time (remember the ISO9000 guidelines…) then you might as well not have it.