Tuesday, 31 October 2000 17:00

Welding can make or break a job literally!

Written by Toby Chess
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Recently, I was asked to inspect a vehicle with representatives of the California?s Bureau of Automotive Repair. The '98 Toyota Corolla had been towed from the accident scene to a body shop where the owner was promised a free rental car told that the shop would cover her deductible. 

Sounds too good to be true? Fast forward six months: the car still is not finished, the shop never gave the owner a rental and yes, you guessed it, they cashed her draft. The insurance company towed the vehicle out and brought it to me for inspection.

The BAR got involved looking for fraud, but they couldn't find any, i.e., the shop performed all the operations that were specified in the estimate, but the quality of repairs was so bad that the vehicle is now a total loss. It seems there is no law that says you have to do good work, just so long as you "do" everything listed on the invoice.

Welding the least understood process

First, the car was not pulled properly, but that could have been corrected. The welding was another story. The person who welded the frame rail, apron and core support had no idea on how to MIG weld. I have seen beginning welding students do a better job.

Welding is probably the least understood of all the repair processes, yet it is the most critical process. So here's an overview of welding for owners, managers and estimators. We'll begin with some basic terms. The acronym GMAW stands for gas metal arc welding or, to you and me, MIG (metal inert gas). GMAW along with resistance spot welding are the only approved welding methods on today?s vehicle.

Shielding gas is used to protect the molten weld puddle for contamination from the surrounding air. The best gas mixture for welding mild steel and HSS is a mixture of 75% argon and 25% carbon dioxide. This mixture will produce a stable and smooth arc. Furthermore, it will give the tech a cleaner weld (less spatter). Finally, using this gas will improve the strength of the tensile and yield strength of the weld.

Stick-out refers to the length of the wire from the contact tip to the end of the wire. The best wire size is .023" wire and the stick-out length should be from ¼" to 3/8" with that wire size. Rule: every time you begin a new weld, cut off the weld ball at the end of the wire.

The three most common welds that are performed every day are the butt joint with a backer, lap joint, and plug weld. Each requires a different gun angle. Gun angle is the angle measured from the work surface to the nozzle.

It is important to know if your tech is using the proper gun angle because improper gun angle is the biggest reason for weld failure. Your techs should be using the following gun angles: plug weld is 90 degrees, lap joint at 45 degrees, and butt or fillet weld at 70-75 degrees.

Other common weld failures

Incomplete fusion is when a gap occurs between the base metal and weld metal. Here?s how to correct the problem.

1. Reduce your travel speed.
2. Increase the current.
3. Check gun angle.

Porosity is a void in the weld metal scattered in small clusters or along the entire length of the weld.

Here's how to correct the problem:

1. Keep welding gun nozzle clean – this includes using an anti-spatter spray and welding pliers.
2. Reduce stick out.
3. Decrease travel speed.
4. Decrease current.
5. Increase flow rate of gas.
6. If your tech is welding in an area that is drafty, use a welding screen. Wind will blow away the shielding gas.
7. Make sure the all-welding surfaces are clean.

Weld spatter are particles of metal that sit on surface and are not part of the weld. These metal particles take time to clean up, create poor welds and waste electrodes. Here is how this is corrected:

1 Decrease current.
2. Decrease travel speed.
3. Shorten the stick out.

Melt-thru occurs when the welding arc burns through the base metal. Melt-thru is corrected as follows:

1. Decrease current
2. Increase travel speed
3. Reduce the gap between the two pieces of weld metal on a butt joint with backer (rule of thumb—opening should be twice the thickness of weld metal).

The best way to make sure that your techs are welding to acceptable standards is to insist that they pass the I-CAR Welding Qualification test. You might think that the test is too expensive, but what price do you place on your customer's safety -- and your shop's reputation. For more detailed welding information, check out Helpful Hints to Basic Welding from ITW Hobart Brothers Company (888-HOBART9).

Toby Chess is director or education for Caliber Collision Centers. He can be reached at toby.chess@caliber.com.


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