20 JANUARY 2013 RWMA Q&A BY TOM SNOW Q: Our company needs extra spot welding capacity because production is increasing. We have an old spot welding machine in storage that has not run in years. Should I try to get it going or buy a new one? A: As with many things in life, “it all depends.” Spot welding machines are among the most durable of production machines and the “three Rs” of machine maintenance can easily be applied — repair, retrofit, or rebuild. That being said, some brands of spot welding machines are better than others and an old, light-duty machine may not be worth fixing. To begin finding the answer to your question, check to see if your machine is suitable for the anticipated welding application. As an example, if you need to weld two pieces of 12-gauge mild steel, a little 10-kVA foot-operated rocker arm machine is not going to get the job done properly. Charts are readily available listing various combinations of amperage (heat) and pressure (forging force) required for welding various material thickness combinations. Hopefully your machine has enough capacity to achieve a “Class A” weld, which optimizes strength and appearance by using proper force, high amperage, and short weld time. Also, if your application involves projection welding, such as resistance welding nuts or studs, an air-operated vertical action “press-type” welding machine with the proper diameter cylinder is the way to go. A rocker arm spot welding machine, although typically less expensive, applies weld force with a rocking action and will not “set down” the projections evenly. And speaking of rocker arm spot welding machines, be wary of installing longer arms if the existing arms are too short to reach all the welds on your deepest part. Because the spot welding electrode tips are typically on the “wrong” end of the air-operated fulcrum mechanism, increasing the arm length robs the machine of weld force capability. As an example, one representative heavy-duty rocker arm spot welding machine built to RWMA Size 3 specifications can produce 2250 lb of weld force at 80 lb/in.2 of incoming air pressure with an 18- in. throat depth, whereas extending the throat to 36 in. reduces the maximum available force to 1150 lb, a loss of more than a half-ton of forging capability. Likewise, as the throat depth or vertical shut height (gap) between arms is increased, the available welding amps at the tips will decrease due to the larger secondary loop. In other words, a spot welding machine with an 18-in. throat depth will produce significantly more amps than the same machine with 36-in. arms installed. The same rocker arm spot welding machine spec sheet referenced in our example shows that a 50-kVA machine with 18- in. arms installed produces 22,100 secondary amps at full power settings, whereas the same machine with a 36-in. throat produces 15,700 A. This could be the difference between making a good or bad weld. Once you’ve determined that your welding machine in storage is suitable for the application, examine its condition. Many older resistance spot welding machines are built better than new ones, so a heavy-duty resistance welding machine that’s 20–40 years old should not be ruled out. A “veteran” American-made spot welding machine built to RWMA specifications is often superior to some of the light-duty imported machines on the market today. However, if the welding machine is more than 10–15 years old, it would probably benefit from being retrofitted with a new control that includes all the latest features — Figs. 1, 2. Virtually all spot welding machine controls sold today have fully programmable functions, such as pulsation and upslope, that were once expensive options. If you’re trying to weld heavy material thicknesses and/or coated steel, those two options are often a big help in achieving good welds. Also, these days it’s advisable to convert spot welding machines from mechanical contactors or mercury-filled ignitrons to modern SCR contactors. Just be sure to dispose of the ignitrons properly and legally. And if you want to greatly improve the safety of the machine, a spot welding machine control is now available with a “soft touch” safety feature that senses if something nonmetallic is between the tips, such as a finger, and retracts the tips before weld force is applied. This protects your operator from serious finger injury and also includes the ability to dress your electrodes under low force. Once you’ve gotten a retrofit welding machine control on order, if needed, it’s time to address the condition of the machine itself. Resistance welding machines are relatively simple to fix if you understand the basics of electricity, pneumatics, and mechanics. Here are the systems to check as you inspection and repair the machine as Fig. 1 — Example of a heavy-duty presstype combination spot and projection welding machine suitable for rebuilding. Fig. 2 — The same press-type welding machined after being stripped and rebuilt to like-new specifications, including the installation of a new control with programmable functions. needed or go through the process to completely strip and rebuild the machine. Welding Transformer: The Heart of the Machine Spot welding machine transformers have no moving parts and often run for
Welding Journal | January 2013
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