RWMA Q & A – May, 2018 – Welding Journal
Niels Johnson – Industry Sales Manager at Weld Systems Integrators, Inc. (WSI)
As seen in the May, 2018 issue of Welding Journal – © 2018 American Welding Society
Q: We manufacture sheet metal cabinets, making the spot welds with a single-sided, pogo-type spot welding gun. The cabinets are first placed on their backs on a copper table. Then, the machine operator pushes the gun to produce the welds where we need them. Recently, our customers have become more concerned with the cosmetic appearance of the welds, but when we turned down the heat to get a cleaner appearance, we noticed some of the welds are popping. We don’t have the time nor personnel for a lot of rework, so we need to make cleaner welds from the start; however, we can’t seem to balance the requirement for cleaner welds with the need for consistency. Also, as we’ve started to examine this part of our manufacturing process closely, we’ve realized our personnel have been rotating out of this cell a great deal over the years. What’s going on? Is it the welding equipment, employee turnover, or a combination of both that is causing these welding issues?
A: These are great questions. To start, let’s first discuss the three key components of resistance welding: current, time, and force. At the very basic level, current and force must be applied to the part over time to generate resistance and make a weld.
The current is supplied by the weld-ing transformer — in this case, an external transformer on a fixed or mov-able mounting — and connected to the welding gun by a set of long, heavy cables. The welding transformer takes low-amperage, high-voltage primary power, and transforms it to high-amperage, low-voltage secondary power.
Time is typically controlled by the welding control. In this case, an external control box is fixed to a beam somewhere away from the welding machine.
Force is supplied by a form of mechanical action, whether a pneumatic or hydraulic cylinder, a mechanical linkage to a foot pedal, or the machine operator’s body. With a single-sided push gun, a spring is set to allow a plunger to push the trigger once the minimum force has been exerted. This trigger allows the weld current to fire.
The force applied and the resistance generated have an inverse relation-ship. Lower force increases resistance, and higher force decreases resistance — Fig. 1. Resistance at the junction of the parts being welded (the “faying surfaces”) should decrease during the weld as the weld sets down because the materials are becoming one.
In a normal resistance welding schedule, the force should rise and plateau prior to beginning to fire the welding current. Figure 2 shows a single-impulse resistance welding schedule, where the force rises and remains consistent prior to, during, and after the passage of the welding current.
The goal in resistance welding is to use the highest possible heat for the shortest possible time. “Possible” is a critical word here because there may be reasons we cannot use very much heat; for example, force that is less than ideal. This may be due to a variety of things, such as long arms on a rocker-arm type spot welding ma-chine, or it can be because the force is supplied by a human machine opera-tor, as with a single-sided push gun spot welding machine. As discussed above, lower force increases resistance. If the resistance is too high, sparks and material expulsion from the weld are common results. The response is to decrease the welding current and increase the weld time. Changes in force directly affect the amount of cur-rent and time required to achieve consistent weld results.
There are often other things going on with force in applications like these, so let’s also examine how the direction of force affects each weld. Resistance spot welding is essentially clamping two parts together using electrodes. Similar to any clamping application, what holds your parts in place between the electrodes, or be-tween the welding gun electrode and your fixtures, is surface friction — in this case, between the materials being welded and the electrodes/fixtures.
In Fig. 3, there are blue and yellow arrows. The blue arrows represent the pogo gun reaching down into a cabinet. The yellow arrows represent the direction(s) of force being exerted. The gun on the left is exerting a clamping force straight down, perpendicular to the floor and tooling fixtures. The gun on the right is exerting a clamping force at a very slight angle, so the direction of force is not perpendicular to the fixtures and floor.
If the surface friction between the materials being welded is reduced or removed, the part will want to move in the direction that the force is being exerted — Fig. 4. Surface friction is reduced or removed when the phase of the metal changes (from solid to liquid) in the brief fraction of a second that the weld is forming. When your parts shift position during the weld, it’s referred to as skidding. This typically causes elongated (and therefore weaker) weld nuggets, or even no weld at all.
How do all of these mentioned factors apply to single-sided push gun welding machines? In many cases, the weld current shuts off if the trigger is released during the weld time, so if the machine operator pushes hard enough to trigger the weld current, but releases the force too quickly, the desired weld current will not be applied to the part for the complete time desired. Also, the pressure switch that is triggered when your operator pushes the electrode against the surface only makes certain that there is a minimum welding force present. It cannot measure or adapt for force exerted beyond that. Additionally, unlike a welding machine that supplies the force on its own, a single-sided push gun is affect-ed by the height, weight, and strength of the machine operator. This is be-cause the amount of force the machine operator exerts, and the direction of that force, is going to change depend-ing on all of these factors. This is not to mention changes in the amount of force an operator is able to exert at the beginning vs. at the end of a shift, or if he or she is sick, injured, or sore. The goal of resistance welding is to use resistance to generate heat to melt the materials and make the weld; however, the resistance needs to be controlled and repeatable to achieve consistent welding results. Changing force during the weld, and from weld to weld, changes resistance in an uncontrolled fashion.
Faced with all of these challenges, many companies using this type of welding machine attempt to overcome them by putting additional heat into the weld (increasing weld time). This solution fixes the inconsistent, elongated weld nuggets caused by inconsistent force applied in the wrong direction by creating a larger, elongated weld nugget. Longer weld times create a greater heat-affected zone (HAZ) and less cosmetically appealing welds. In essence, more heat is used to treat a symptom rather than the cause, and, therefore, masks the underlying cause of the inconsistency. This is why turn-ing down the heat to reduce the HAZ and improve the cosmetics of the welds is leading to weak/no-weld situations.
In summary, I believe that your issues with consistency are a result of both the type of welding machine you are using, and the results you are now expecting to achieve from that welding machine. As you noted, you are start-ing to have issues now that you are trying to improve the cosmetic appearance of your welds, which means you can’t use the figurative “crutch” of a longer weld time. This type of system may have been used to make these parts for years, and it was performing well. However, once the standard for quality changes, as it has in this case, the equipment is no longer capable of supplying the results you need. Regarding the employees repeatedly rotating out of that production cell, it wouldn’t be surprising if this operation was causing a sort of repetitive stress back and/or shoulder injury as well. Whether they realize it or are re-porting it, using their bodies to supply the weld force for each weld, not to mention lifting the gun into and out of each cabinet, as well as lifting the gun to move it from weld to weld with-in the cabinet, is a tough job to do all day.
The single-sided push welding gun has been a great tool, used for more than a century. It definitely has its place in some production lines, and in history. Small handheld units are used extensively in auto body repair, for ex-ample. However, large, heavy-push welding guns for reaching deep inside cabinets present some significant challenges, not just in weld quality, but in ergonomics and safety as well. Just one major shoulder or back injury (or likely two minor injuries) would pay off an investment in modern equipment. Fortunately, even without turn-ing to robotics, there are a number of solutions that can take a lot of the stress of lifting and pushing out of the job.
NIELS JOHNSON is Industry Sales Manager at Weld Systems Integrators Inc., Warrensville Height, Ohio, a Sustaining Member of AWS, and an active member of the RWMA and WEMCO. He is also Chairman of the Marketing Subcommittee of the RWMA. Send your comments and questions to Niels Johnson c/o Welding Journal, 8669 NW 36 St., #130, Miami, FL 33166-6672, or via email at firstname.lastname@example.org.
Electronic reprint with permission to Weld Systems Integrators, Inc. from Welding Journal, copyright of American Welding Society (AWS).
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