Resistance Welding: Solutions
Roger B. Hirsch, is ‘Elihu Thompson Resistance Welding Award’ winner, President of Unitrol Electronics Inc. of Northbrook, Illinois, and Former Chair of the Resistance Welding Manufacturing Alliance (RWMA), a standing subcommittee of the AWS.
We started to install a new resistance welding machine that has a built-in circuit breaker. The circuit breaker is marked 250A. My electrician says that the circuit breaker on our power panel that feeds this welder has to be at least 250A. This seems very large for our 100KVA welder operating on 440V.
The problem is that you are working with two different types of circuit breakers.
The circuit breaker on your power panel that feeds this welding control is a THERMAL type. This type of circuit breaker is designed to trip (open) if the current going through it exceeds the circuit breaker setting for a relatively long period of time.
Typically a resistance welding machine uses a MAGNETIC ONLY high-speed trip style circuit breaker. This means that it will almost instantly trip if the current going through it exceeds the breaker setting. It is designed to protect the solid-state switch (SCR contactor or IGBT contactor) from a catastrophic high-current short.
Typical setting of the MAGNETIC ONLY circuit breaker is 3 times higher than the maximum expected continuous current from the resistance welder. This trip value is set by turning a dial on the magnetic trip circuit breaker. The 250A rating of this circuit breaker only says that this circuit breaker’s mechanism can sustain 250A continuously, but does not select what amperage will cause the circuit breaker to trip.
One good sizing chart for resistance welders is in the RWMA Bulletin 16 chart 1.7.15. This will select the appropriate fuse disconnect or panel circuit breaker to back up a welder regardless of the magnetic-only circuit breaker that is furnished with the welding control.
We just starting using our first Capacitive Discharge resistance welder. Does this type of welder require the same water cooling as our standard spot welders?
Capacitive Discharge resistance welders build a high-voltage charge in a capacitor bank and then dump this stored electrical energy through the welding electrodes very rapidly. Because of this, current conduction time is very short compared to normal resistance welding, there is little time to heat the actual electrodes. As a result, many companies do not use water to cool the electrodes. However the secondary conducting components should have some water cooling. And if you are using very high currents, having some water cooling will extend electrode life.
In addition, multiple Capacitive Discharge welds can’t be made as rapidly since the capacitor bank has to charge up between welds. This allows more time for any heat built up in the electrodes to dissipate.
One trick is to use an electrode with a very large surface area that allows heat to dissipate to the air more efficiently.
I just inherited a large welder that has a function called FORGE DELAY. What is this and when is it used.
Chances are that if you are not producing welds to AWS D17.1 Class A standards you will not need this welding function.
FORGE DELAY is a resistance welder function that provides a very high electrode force typically near or at the end of a welding sequence to forge the weld nugget. This is often used when welding some higher-strength aluminum alloys to minimize thermal crack in the nugget. This cracking is caused by the rapid thermal expansion and contraction in the molten center of the nugget being formed. The forging action pushes the developing cracks together under the higher force.
Forge Delay also helps to minimize or eliminate metal expulsion that occurs at the very end of the nugget development by pushing the cracks forming in the cooling molten material back into the nugget area. It ensures that the weld nugget cross section is virtually the entire nugget diameter to produce the maximum tensile shear strength of the weld.
Our welder has a pressure regulator marked Bucking Pressure What is this and how is it used?
|TOP PISTON DIAMETER
This question relates to the previous question of Forge Delay. The Bucking Pressure is pressure put on the return side of the air cylinder piston during the weld sequence. This is the air pressure at the RETURN PORT of the air cylinder.
During the weld, the force produced by the electrodes is:
Electrode Force = (top piston area X forward psi) – (bottom area of piston X return psi)
For example, on a welder with a 10” inside diameter cylinder that has a 1” diameter shaft,
TOP AREA OF PISTON= πX52 = 78.5in2
BOTTOM AREA = π X (5 – 1)2 = 50.3in2
With 50psi in the forward port and 20 psi in the return port:
ELECTRODE FORCE = (50 X 78.50) – (20 X 50.3) = 2,919 pounds
Near the end of the welding sequence, air on the return port is quickly exhausted. The electrode force now is:
FORGE FORCE = (50 X 78.50)= 3,925 pounds
HEAD WEIGHT: To either of these ELECTRODE FORCE numbers you have to add the dead weight of the moving welder ram. This is the weight produced between the electrodes when all air is removed from the welder. For larger welders this can be 100 to 200 pounds. The calculated value should then match the actual force between electrodes during the WELD and FORGE portions of the sequence.
We just built a welder to weld filter bag frames (Fig. 1). Twenty 18” diameter wires form the cage with rings spaced every 5”.The first joining sequence is to weld one end of each wire to the inside of a drawn sheetmetal end cap. I have no problem welding the rings, but securely welding the end cap is a hit-or-miss proposition.
We are using a welder with 10 welding cylinders that are in a radial positioning and we weld two opposite cylinders at a time with an expanded mandrel on the inside connected to one side of the welding transformer. Each cylinder welds two wires to the end cup with the same electrode. The inside expanding mandrel connects to the other side of the welding transformer for direct welds.
I looked at the drawing you sent, Fig. 2, showing the 10 welding guns and electrode layout that joins two wires per electrode. After discussing the electrode geometry shown in Fig. 2A, I found the first problem. If you look at Fig. 2A, you will see that the welding current path is not directly from end cup to wire but is through the surface of the metal back to the wire. Change the geometry of the electrodes to that shown in Fig. 2B so that the current path is directly from cup to wire and you will see a major improvement in weld strength and consistency.
Second, I was told that the firing sequence has pairs of welding guns located opposite of each other going forward and welding at the same time. For example in Fig. 2, one weld sequence uses guns SV1 and SV6 at the same time. By closing the electrodes from opposite sides of the part, there is no way to maintain full force between the inside of the end cup and the wire. The end cup will not yield. The answer here is to either close and weld one electrode at a time or weld two next to each other. This will allow the end cup to float during the weld sequence to keep full force between the end cup and the wires.
Our company just recently changed over from arc welding of a 16ga. CR sheetmetal tool box to spot welding. Our first 100KVA spot welding machine was delivered and we have not been able to get good welds. The welds either flash metal with deep holes in the weld area, or they fall apart. We made our own electrodes from copper bar in our tool room and think the shape is OK. Our welding schedule is;
Squeeze Time = 60 cycles
Weld Time = 49 cycles
Weld percent heat = 47%
Hold Time = 25 cycles
Electrode force = 350 pounds
You have two different problems.
First, electrodes have to be made from special resistance welding alloy copper. You are probably using C110 copper rod. This copper does not have the alloyed material needed to maintain shape under the high welding forces and high welding temperatures inherent in the resistance welding process. Purchase commercially-available electrodes made of RWMA Class 2 alloy for your cold rolled steel welding.
The next problem is that your weld time is way too long. You are trying to melt the parts together. But by having relatively low welding current from the low heat% for a relatively long time you will just end up heating not only the nugget area but also the metal around it. The resistance welding process depends on high current for the shortest possible time to create a concentrated melt zone and good forging together of the two parts. Also you need enough electrode force to keep the molten metal from blowing outfrom under the electrode face (expulsion) and from between the sheets.
There are many good welding charts you can use as a starting point. A good one can be found in the RWMA Edition 4 handbook. It has welding charts for many alloys and different thicknesses. In this case, the welding chart on page 7-6 of the handbook recommends the following values for 16 ga (.062”) low carbon steel to start your setup:
Electrode Force = 800 pounds
Weld Time = 14 cycles
Welding Current = 12,000A
This will only work if you use electrodes that match the shape shown in this chart.
If your welding control does not have the ability to set weld heat in amps and only has %heat settings, set the electrode force as shown, set the welding time as shown, and then start the weld heat% at about 50% and increase until the weld strength and appearance matches your company’s requirements.
The SQUEEZE TIME should be long enough to allow the electrodes to close and get up to full force.
The HOLD TIME should be set to about 3 cycles. Any more HOLD TIME just wastes production time and does not add to the weld strength or appearance.
Our company is international. We are located in Spain and just received five spot welding machines from one of our U.S. facilities. They are marked 100KVA, 220V, 1Ø, 60Hz. Our power is 230V 50Hz. Is there a problem using these welders here?
A transformer designed and rated for 60Hz line frequency will be greatly derated when operated on a 50Hz line frequency. It will limit the welding current you can safely get out of the transformer before it saturates and blows fuses or destroys the transformer.
If this is a pedestal welder with a machine-type transformer, you can probably get away with weld heat settings of up to 50%. But the normal advice is to replace the transformer with a 50Hz version. If the welder uses a fixture-type transformer I would not even consider using it on 50Hz power.
If the situation was reversed, you could use a 230V transformer designed for 50Hz on a 230V 60Hz line with no problem.
Interestingly, if a transformer was designed for operation on 460V 60Hz, you could use it on 385V 50Hz without any difficulty.
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