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Welding Journal | January 2014

RWMA Q&A BY TOM SNOW Q: One of our resistance welding machine operators recently lost part of his finger when it was crushed between the electrodes of the machine. How can we protect our operators from similar injuries in the future? A: Forging forces ranging from several hundred to several thousand pounds are required to properly resistance weld metal together, so resistance spot and projection welding machines can be dangerous if care is not taken to protect the operator. For example, a welding machine operating at 600 lb of forging force with an electrode contact area of ¼ in. will produce more than 12,000 lb of force per square inch on a finger or anything else caught between them. Having witnessed an operator crush his finger while on a factory tour years ago, I certainly share your desire to avoid this unfortunate situation in the future. While not necessarily simple, the safest way to ensure operator safety on a resistance welding machine is to tool the machine to hold and clamp the part, which avoids the need for an operator to position and hold the part during the process — Fig. 1. In a welding machine with proper tooling and guards, the operator manually loads the parts and then completely clears out of the welding/pinch point area before the machine closes the tips. That being said, it is not always possible to accomplish. Following are outlined some other ideas that can help ensure operator safety. You did not specify whether the machine involved in the accident was a rocker arm-type spot welding machine or a vertical action press-type machine, so enhancing the safety of both is addressed. Rocker Arm Spot Welding Machine These machines are typically used to weld sheet metal parts that are held with both hands while the operator manipulates the part in the throat of the machine to access all the weld locations. Initiation of the machine should be with a shrouded foot switch, and a pinch point warning sign should be prominently displayed — Fig. 2. In addition, the operator should be instructed verbally and in writing to keep his hands away from the tips. Although using both hands to hold the part usually ensures that the operator’s fingers are not in the pinch point area, management can augment safety by selecting the right machine for the job and setting it up properly. A simple way to reduce the potential for a spot welding machine pinch point injury is to position the machine’s electrode tips so close together (typically about a ¼- in. gap) that a finger cannot get between the tips prior to the weld stroke. To overcome clearance problems caused by such a short working stroke, an optional feature for the welding machine called an adjustable and retractable stroke air cylinder can make it much easier to load bulkier parts, such as those with flanges or lips, into the welding machine’s throat prior to welding. When using retraction, the operator typically steps on a separate shrouded foot switch to activate the adjustable retraction stroke, also called “high lift,” which gives the operator extra clearance to load the part in the throat prior to reverting to a minimal stroke for welding. 22 JANUARY 2014 — continued on page 25


Welding Journal | January 2014
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