Page 23 - MetalForming July 2014

Class B and C welding schedules often find use when a metalformer uses a welding machine too small for the material thickness being welded. These schedules include lower force ranges and longer weld times, which result in lower weld-shear strength and more surface marking, due to a larger heat- affected zone.
Skilled-labor retirements and lay- offs during the past few years have reduced the tribal knowledge of many shops performing spot welding, but several sources offer books and training courses on the process. For example, the Resistance Welding Manufacturing Alliance (RWMA), a standing committee of the American Welding Society (AWS), sells an excellent manual on resistance welding and offers various technical papers on the subject. It also offers a two-day resistance-welding course each fall in conjunction with FABTECH.
Additionally, the AWS is about to roll out a formal certification program for the resistance-welding process,
which will no doubt spawn training courses designed to prepare applicants to take the Certified Resistance Welding Technician (CRWT) exam.
2) Select the right welding machine.
The most important thing to remember: Select a machine than can make optimum-strength Class A welds with about 25 percent of its available amperage and force range left in reserve.
Most companies are not equipped to properly size a machine on their own, so heed the advice of an experi- enced machine builder. A good sales engineer will quote a machine only after asking questions about the mate- rials to be welded and the speed at which the machine will operate.
Since an RSW machine’s KVA rating can be inflated by using a duty cycle of less than the RWMA standard of 50 percent, be sure to ask if the machine being quoted meets those standards.
Although rocker-arm RSW machines are the most common due to their rel- atively low purchase price, they apply weld force with a lever action—the tips often skid if the arms are not perfectly aligned. Therefore, a more-expensive vertical-action press-type machine often gets the call when the application requires an attractive, low-marking show surface.
A welding machine that’s too large can cause just as many problems as one that’s too small, especially when the air-cylinder diameter is so over- sized that it must be run on an air-line pressure below 40 lb. to achieve the desired weld force.
Unacceptable weld strength can result from inadequate air-cylinder fol- low-up action at the instant the sheet- metal reaches the molten state, when it needs to be properly forged.
To accommodate the new high- strength steels finding use in the auto- motive industry, manufacturers of RSW machines and controls have
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