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Details to look for during new and used machinery inspection:
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Welding Tips


Welding is the technique used in joining metallic parts usually through the use of heat. Resistance welding was invented in 1877 by Elihu Thomson, an electrical engineer and inventor whose discoveries in the field of alternating-current led to the development of successful alternating -current motors. The earlier type of resistance welding was accepted long before arc welding for the spot and seam joining of sheets. Spot, seam, and projection welding are resistance welding processes where the required heat for joining of metals is created at the interface by the electrical resistance of the joint. Welds are typically made in a short time of approximately 0.2 seconds using a low-voltage, high current power source with force applied to the joint through two electrodes, one on each side. Spot welds are made at systematic intervals on sheet metal that has an overlap. The strength of the joint depends on the number and size of the welds. Seam welding is a continuous process where the electric current is successively pulsed into the joint to form a series of overlapping spots or a continuous seam welder . This process is used to weld containers or structures where spot welding is insufficient. A projection weld is formed when one of the parts to be welded in the resistance machine has been dimpled or pressed to form a bulge that is melted down during the welding cycle. This process permits a number of determined spots to be welded at one time. All of these processes are capable of very high production rates with continuous quality control. Modern equipment in resistance welding includes complete feedback control systems to correct any weld that does not meet the desired specifications. 

Selecting The Right Welding Machine For Your Application?

Selecting the right spot welding machine can be a confusing experience for the average metal fabricator or welding engineer because the price differences for the various machines are difficult to understand and justify. Selecting the wrong spot welding machine can be an expensive mistake. Many recalls have been traced to improperly made spot welds. A properly made spot weld will not shear or break in the fused zone. Instead, the base metal will tear around the weld. In addition to strength, spot welding offers speed and cost effectiveness that no other metal -joining method can match. You can determine the right welding machine for your job by following the recommendations prescribed by the Resistance Welder Manufacturers' Association (RWMA). The RWMA has been dedicated to setting and improving industry standards, as well as education for over 50 years. It currently represents about 40 manufacturers of resistance welding machinery, electrodes and component supplies, including transformers and controls. Resistance spot welding machines have always been rated in KVA (a thermal rating), where the RWMA's rating of standard machines is based on a 50% duty cycle. However, not all resistance welding machine manufacturers hold to these standards. Another performance measure is the available short-circuit current of the machine. RWMA standards specify the short-circuit current based on the machine size, KVA rating and throat dimensions, along with the allowable duty cycle when operated under short-circuited conditions. In order to comply with this standard, a resistance welding machine must have adequate copper sections in the secondary loop, well-designed water cooling, and the KVA rating of the welding transformer based on a 50% duty cycle.

Weld force is the other part of the resistance welding equation. The spot welding machine is a sophisticated device used to heat and forge metal together, the force applied to the weld area is critical to the weld's strength. Today, most resistance welding machines are operated with air cylinders of various diameters that apply the weld force either directly (press-type) or through a lever mechanism (rocker arm types). The force produced by a press-type machine is easily computed from the diameter of its air cylinder and the air pressure applied. Rocker arm machines are entirely different. Extending the arm length (throat deep) of a rocker arm has a dramatic effect on the maximum welding force available since the welding force decreases in the same ratio as the throat depth increases. Therefore, a deep-throat rocker arm not specifically designed to have arms of that length can reduce the force and adversely affect weld strength. RWMA specifications specify the maximum force expected from each spot welding machine frame size for both rocker arm and press types. In today's market, automation is increasingly important to manufacturers using the resistance welding process, it is important to find a company that also builds special design machines to weld customers' parts at high production rates.



*This is one article in a series of How to Buy Metalworking Equipment. Each article showcases and explains a particular type of metalworking machine. They were originally published in the Metalworking Machinery Mailer published by the Tade Publishing Group.