BodyShop Business
Success with Single-Sided Resistance Spot Welding
1/1/2006

Despite having tried twice, experiencing failed welds both times, I knew I could save myself time installing a floor pan by using single-sided spot welding — if I could just get it right.

by March Taylor

I, like most in the industry, believe that squeeze-type resistance spot welding (STRSW) aftermarket technology has finally caught up to the OEs — with inverted three-phase welders producing factory-like welds that maintain the integrity of the vehicle.

But what about single-sided spot welding? Until recently, I’ve been a skeptic, especially after experiencing failed welds with previous attempts. Two reasons I tried again: First, I thoroughly read and understood the operations guide, realizing that in our previous attempts, we may have strayed from the instructions. Second, because conventional floor pan installation by MIG welding is sloppy at best. There’s much grinding and finish work required in order to make the new floor panel look acceptable.

After realizing the significant improvements that have been made by some spot welding manufacturers and disciplining ourselves to follow the rigid step-by-step operational process, we decided to give it a third try.

I selected technician Ray Ikeda for the installation of this trunk floor panel. Ray is notorious for being meticulous and one who follows instructions. If anyone could be successful, Ray would be that person. And then Ray would mentor our other 12 metal men, ensuring that the others followed our newly developed standard operating procedure (SOP).

The Fundamentals
Before we delve into what’s changed, we need to understand the fundamental principles of single-sided welding, keeping in mind that single-sided resistance welding started many years ago as a cosmetic weld to substitute oxygen/acetylene brazing for patching outer panel rust corrosion:

  • Single-sided welding must never be considered for any structural panels or for multiple panels exceeding two layers. Two layers only!

  • The new panel being attached must never exceed the thickness of 1.2 mm. (Some welders have been approved for 1.5 mm of panel thickness; these welders are equipped with a technology called “current control.”)

  • If there are panels of different thickness, weld from the thinner layered panel to the thick; never weld from the thick to the thin.

  • Place the “ground shoe” as close to the weld area as possible, and secure it to the host panel. Never attach the ground shoe to the same panel being welded.

  • Securely attach the entire surface of the “ground shoe” to the host panel, with the surface area of the host panel cleaned to bare metal.

  • There must be no contamination between the panels being welded; the mating surface must be thoroughly clean and free of grease or sealer. (With single-sided spot welding, most inverted welders will allow the e-coat to remain between the two layers without affecting the integrity of the weld nugget.)

  • The mating sides of the new and host panel must have a smooth, tight, flush fit between them; there should be no gaps or air pockets (as much as possible avoiding the factory spot-weld area).

  • Gun pressure, vertical push of 20-25 pounds is recommended, with an emphasis on not too much and not too little pressure. Too much and you run the risk of blowing a hole in the panel; too little and the weld may fail.

  • Before you start the single-sided welding process, it’s vital you do a test weld, mimicking the panel installation, and then destroy that test weld. This will confirm you’re ready to start the process and that the settings you’ve selected are correct.

  • After your first single-sided welds, do a visual check on the underside of the host panel; a burn mark “heat affect zone” should be visible, confirming weld penetration.

  • Good tip: Try to avoid welding over the same area as the original factory spot weld. (Want a template to locate the original spot weld area? Cut the identical section from the original floor panel that was removed.) If you weld over the same area as the original weld, there’s a high probability that the weld may explode or burn through.

When to Consider It
You should only consider single-sided welding when the repair conditions dictate a successful single-sided weld, and most importantly, when squeeze-gun-type welding isn’t accessible. (This would be when the squeeze-gun welding electrode arms cannot reach the weld area.)

The example shown in the photos throughout this article is a rear floor pan at the spare tire area being attached to the upper flange of the unibody rails and to the inner side of the wheel house. In this repair scenario, prior to beginning the single-sided welding process to the floor panel (to be done only in the areas where the squeeze-gun electrode arms could not reach), the technician fashioned sample test panels, panels with the identical thickness as the panels that are to be joined, duplicating the actual repair. He then tested the weld integrity by attempting to separate/destroy the test weld.

Once the technician was convinced the welds passed the “destruction test,” he proceeded to weld the floor pan using both singled-sided and doubled-sided resistance welding, along with MIG welding in applicable areas.

The cautionary testing steps taken to qualify single-sided welding reinforces the fact that single-sided resistance welds require less finish-preparation (grinding and filling) than the MIG welds. The resistance welds also offer a look that matches the factory, as the photos indicate. Once the technician knew the welds passed the “destruction test,” he proceeded to welding the floor pan using both singled-sided resistance and MIG welding, keeping in mind the single-sided resistance welds require less finish preparation (grinding and filling) than the MIG welds. The resistance welds also offer a look that matches the factory, as the photos indicate.

Got a Three-Phase Inverted Welder?
Now that we know single-sided resistance welding can be successfully done, what types of resistance welders have this capability? Research, both by the European and now the rest of the OEs (Asian and American), has documented that three-phase inverted welders come the closest to duplicating what’s done at the factory, both in strength and appearance.

Resistance welding that’s done with inverted three-phase welders produces welds that meet these OE standards — welds that look like factory welds and welds that can’t be separated. And yes, this even applies to single-sided spot welding.

But approved single-sided welds can only be accomplished when the recommended operational steps are adhered to and only on non-structural panels. As a technical advisor from a manufacturer of a European welder recently told me while at our facility updating our techs on both STRSW and single-sided welding: “Before you consider doing any single-sided welding, you must put on your ‘thinking hat’ and plan your strategy.”

If your welder isn’t an inverted welder, use caution when performing single-sided welds, being sure that you thoroughly test several sample welds prior to installing/spot welding the new floor panel. Doing so will ensure the quality of your single-sided weld.

If you own a collision shop that does heavier-type volume collision, a three-phase inverted welder is a must. If you don’t have an inverted welder and want to consider one, ask your welding equipment representative to do a shop demonstration. Most offer this service.

A word of advice: Don’t settle for the demonstration to be done with new untarnished metal coupons; request a real-world demonstration, a more realistic repair scenario with a discarded panel from a vehicle, or better yet, an actual repair. Push the welder to its limits, doing as many spot welds in the least amount of time; this will expose how well the welder responds to overheating and to the available voltage at the outlet.

And, as many others have regretfully discovered too late, don’t buy on price alone; your purchase decision should be based on the welder that fits your electrical power supply, along with the type of work you want to do.

A knowledgeable welding rep should help you determine the amount of voltage at your outlet, and this will help you determine if you have to upgrade your voltage to meet the welder’s requirements — very important before you buy.

Writer March Taylor owns Auto Body Hawaii in Kailua-Kona, Hawaii. Not your typical shop owner, Taylor works alongside his employees as a technician. This, he says, gives him “the opportunity to see things how they really are. I’m not disconnected from production or management.”



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