What materials or techniques can I use to reduce this electrical effect when building spotwelding tables?

Question

I have been experimenting with building various support tables around press-type spot-welders. These are relatively simple fixures; think of a tabletop with legs under it and devices like sliding gibs, pins, clamps, rollers, etc mounted to the table top to guide a specific set of parts.

My problem is what material to use to build these tables. Being in a metal-working shop, I usually reach for things like angle iron, sheet metal, and Unistrut. This is fine for building most parts of the tables, but due to a phenomenon inherent in spot-welding, it is not smart to build steel pieces bridging over the copper electrodes and shunts. I cannot think of the name of the effect right now, but basically if you have a steel member passing through the magnetic field that surrounds the shunts and other parts that come off the secondaries of the transformer, it will reduce the effective current delivered at the ends of the electrodes. I think the current drop is proportional to the mass of the steel (and presumably the distance), but I am having trouble finding reliable information on this effect. If I knew its name, that would help.

I am exploring two solutions for building these tables. One would be if I had a formula modeling the mass and distance of the steel relative to the current, I could (maybe) fit the steel structural members in a position where I will not get (much) interference.

Another solution would be to keep steel members out of it and to build those parts out of another material. I considered aluminum angle, but I really do not know if this would behave like the steel in the induced magnetic field. My gut tells me it would not, but I really do not know. Another choice could be plastic bar and sheet although I am not eager to go there since that would cause more fasteners, countersinking, thicker material and/or more supports to get comparable strength.

Do any of you know of a useful formula or rule of thumb that could guide me as to steel placement? Do you know if aluminum would not have side-effects in this situation? Do you have other ideas for materials that are easy to work with? Another idea: Is there a shielding that would help in this situation?

Answer 1

The welder you are using forms a very large inductive loop when the contacts are welding. Any conductive materials you bring within that loop will form one half of a transformer, and they will "steal" some of the energy meant to go into the weld.

In order to break this effect you need to eliminate conductive paths within the magnetic field developed by the welding loop.

This can be simply choosing insulating materials (plastic, for instance). If you must use metal, then you can choose higher resistance metals. Silver, copper, gold are bad choices. Iron, steel, and stainless steel are a magnitude higher resistance than the above, but they are still pretty conductive.

So split the steel table up, and use non conductive fastening methods to prevent the currents from moving from piece to piece.

A single length of steel bar near the welding tip will have little effect. But four lengths of steel electrically connected looping around the welding tip (ie, the tip goes through the center) will be highly coupled to the magnetic filed in the welder, and sap much more energy. Insert a plastic shim on just one joint, though, and that metal loop becomes open and affects the welding process very little.

So if you can use a plastic table (and there are many very durable plastic tables that would work for most industrial situations) then that might be your best bet. If not, though, then simply placing electrical insulation in the table will prevent eddy currents that would alter the welder. Consider asking another question if you want a particular design or table requirement critiqued for adjustments to prevent these currents.