In resistant welding of 050XLF steel parts, we are seeing arcing across parts via locating pins in fixture. Pins are steel, but isolated (using ceramics). We are seeing excessive pin wear and looking for possible alternative materials for the pins. Welding process utilizes a rotary 4-station welder. Any suggestions?

-- Lee Stevens (goatroper93@hotmail.com), June 07, 2001

Answers

Dear Lee,

It is an age-old problem that many companies have attempted to solve. There are many different types of machineable ceramics available as well as metallurgical coatings to insulate locator pins. You indicate that there is arcing in the area of the present ceramic locator pins. If the pins are truly insulated, my only thought would be that the pierced holes present burrs or extrusions that allow for arcing between parts at their interface in the area of the holes.

-- Bob Balla (bobballa@tjsnow.com), June 07, 2001.

Lee,

I tend to agree with Bob. If you are seeing arcing that generally means that the pins are not isolated.

You need to look at the whole picture. By that I mean close examination of the tool with parts in and guns clamped. Discount nothing. Please look very carefully at all possibilities.

Look for alternate paths through the tooling and the parts themselves.

Good Luck

-- Jeffrey A. Bertoia (jbertoia@slalomservices.com), June 12, 2001.

In my experience as a spot welding technician with two different tier 1&2 automotive suppliers, we have usually designed all or most of our fixtures from stainless steel, including locating pins. One weld point solidly grounded for safety. Concurrently welding transformers must be phased to present the same polarities on the 1/2 cycle. ANYTHING touching the welded part within 1-2 feet (depending on intensity of current, and weight of material (heavier material has less resistance to cross-conduction))of a weld MUST be insulated from ground. If in doubt, insulate it. Locating pins are usually mounted in blocks, and the blocks are insulated from the fixture base with fiberglass (G10 preferred, but can melt if it gets hot) nylon, or micarta in that order of material preference. Everything is insulated, block from base, dowel pins, allen bolts or other fasteners. I usually maintain 2-5 megohms insulation resistance as a minimum to keep down the "EDM" effect on the components. I've done 6 station rotary indexing machines, 25 station walking beam transfers, and individual weld stations all the same way (the big ones just have more crap, that's all). And as long as my setup people help to maintain the fixture insulation, we have'nt a problem from that angle. Tell your guys not to crank the allen bolts down into the insulators and cut through them. We only use the short end of an allen wrench to tighten the bolts where there are insulators. The only places I've used ceramics is for nut weld pins (retracting or not) and for part component (brackets) net plates where small size of parts, and mechanical wear precluded using insulators. Ceramics can be trouble because you often do not realize it is worn through until many parts later. My experience is all with single phase AC. Never got the chance to try 3 phase, DC, or medium frequency. Guess I'm deprived. :.) Hope this helps.

-- Joe Van Over (joev@gosrf.com), September 25, 2004.