Collision Repair Training | United States
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Current Events/News: Advantage Online: 2003 Archives
SQUEEZE-TYPE RESISTANCE SPOT WELDING: A VIABLE REPAIR METHOD January 13, 2003 -Squeeze-type resistance spot welding (STRSW), or simply "spot welding," cannot be called a new repair method. Yet we still receive numerous basic questions on the process. These questions are the focus of this article.Recommended ProcessDoes I-CAR recommend spot welding for collision repair?Yes. I-CAR has been involved in monitoring and testing STRSW for collision repair for many years. The development and quality of the welding equipment has improved during the years. If the proper equipment is used, STRSW is a viable repair method. Do vehicle makers recommend spot welding for collision repair?Currently, all vehicle makers except for a few recommend STRSW on their vehicles for collision repair. DaimlerChrysler is the most recent major OEM to approve and recommend STRSW for collision repair. Their recommendation is further explained in a publication which was the focus of an Advantage Online article in August 2002 (DaimlerChrysler Expands The Use Of Repair Spot Welding). STRSW And GMA (MIG) WeldingWill a spot welder replace the need of a GMA (MIG) welder in the collision repair facility?
How does STRSW compare to GMA (MIG) welding?STRSW for collision repair does offer some advantages compared to GMA (MIG) welding. Since STRSW uses a very short weld time, the heat-affected zone surrounding the weld nugget is minimal. Also, only minimal corrosion protection between the panels will burn off during the welding process. By applying less heat to the panel, galvanized coatings are less affected. There is also less risk for warpage of the panels due to overheating. STRSW is the same welding procedure used by the vehicle makers on the assembly lines. The repair welds closely duplicate the appearance of the original welds. No consumable materials, such as shielding gas or wire, are needed for STRSW. Weld electrode tips can be dressed several times before replacement and there’s no need to drill or punch holes in the metal before welding. Additionally, no, or very little, dressing of the weld is needed, however there are cleaning steps that need to be performed that differ from GMA (MIG) welding. Available PowerHow much current is required for spot welding equipment used for collision repair?A good spot weld depends on the amount of available current and applied pressure. Available current refers not only to the machine, but also the current available at the facility, and especially where the welder will be plugged in. Even the best equipment will not perform correctly if the source power is inadequate. Spot welding equipment can be a single-phase or three-phase input voltage. Three-phase power is generally much more efficient than single-phase power. Three-phase welders deliver a direct current (DC) output while single-phase welders deliver an alternating current (AC) output. Three-phase DC output delivers more energy per time, and with DC, there is less loss in the secondary current to the welding electrodes. The latest inverter technology spot welders are even more efficient since this equipment converts the 50/60 Hz current to 800-1000 Hz. It delivers a higher and more efficient weld current more quickly. The inverter technology also demands less power from the primary voltage. Pressure RequirementsWhat is electrode force?Electrode force, or squeeze pressure, is a very important ingredient for a structurally sound resistance spot weld. Too low of an electrode force results in too high resistance at the outside of the metals, which will heat up the area without getting the fusion (penetration) between the panels. There may be a pressure gauge on the machine, but that is not the pressure that is being applied at the electrode tips. A pressure multiplier compounds the force at the tips that can only be read with an external pressure gauge. The machine vendor may have a gauge to demonstrate the electrode force being applied by their machine. Machine RecommendationsWhat are the STRSW machine requirements for collision repair?The welding machine must be matched with the welding application. The determination needs to be made on the thickness and number of layers of metal to be welded, the length of the electrode arms required, and whether adhesives will be used in the joint, just to name a few of the many variables that exist. The Resistance Welder Manufacturers’ Association publishes a chart that can be used to reference STRSW operating parameters. This chart is included in WCS04. The weld parameters listed in this chart are becoming increasingly subjective with the introduction of new welding technology, but the figures that are given for required lap shear strength are still applicable today. The Society of Automotive Engineers (SAE) has been working on a document for acceptance criteria for collision repair spot welders. It is currently in draft form and should be available sometime in the near future. This document will provide reference on how to determine if a spot welder produces acceptable welds for various collision repair applications. This document will also include a duty cycle test that requires a series of welds to be produced within a specific time frame. Several of the welds made in the series are destructively tested to determine if the welder can consistently produce acceptable results in the collision repair environment. For any welder that is purchased, there should be clear instructions on the use of the machine. The equipment maker should also offer training and have adequate technical support available to answer questions as they arise. With some machines, dressing the electrode tips is a frequent operation, so a tip dresser must be available and tips should be easy to maintain. There should also be a variety of arm sets available to allow access to numerous weld joint locations on the vehicle. The long extension arms should be able to apply adequate clamping pressure to produce acceptable welds. To ensure that the welding equipment will work adequately in the collision repair facility, it may be best if the machine can be made available on a trial basis. The machine can then be evaluated in a variety of repair and test situations. This will allow the final determination to be made on whether the machine can consistently produce acceptable results in the actual facility where it will be used. I-CAR STRSW ProgramDoes I-CAR have a training program for resistance spot welding?Yes. There is an I-CAR Enhanced Delivery Program on spot welding. "Welding And Cutting Steel Program 4: Squeeze-Type Resistance Spot Welding" covers spot welding theory, equipment, machine adjustments, weld inspection, and weld bonding. Weld bonding is using spot welds in combination with adhesives, a process that DaimlerChrysler recommended exclusively until their expanded recommendation last year. ConclusionInvesting in a quality spot welder is a major investment. Spot welding relies solely on passing an electrical current through the panels to perform the weld. Make sure to invest in a welder that is powerful, reliable and has a good track record. There are several manufacturers available today. Spot welding is an effective welding process, approved by most vehicle makers. Invest in a welder that meets the demands and requirements. |
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