Ksor,
Ken may be rambling, but he speaks truth. Skyline is right too, but he may have assumed that the bolts may need to carry the greatest part of the current. He may also have assumed you were using an electrode bolted to the arm "axially normal" to the electrode . . .
Your design is sound with SS bolts. Clean arms and U-pieces down to bare metal on contact surfaces, and use washers under both bolt heads and nuts to spread pressure. I don't know how well this will translate, but a cupped spring washer (Belleville washer) will maintain tension on the bolt even if the nut works loose somewhat. Your bolts will be loaded in (opposite) shear when the electrode is tightened in place, but the major electrical contact will be direct to the end of the arm. You might consider using headless hex socket setscrews to retain the electrodes, as they can end up flush with the U-piece. This will give you a bit more clearance to get into the corners. You may need to dress the "U-piece to arm" joint with a file if the bolt clearances are too great. You need to end up with a stable, positive joint square to the arm in order to maintain a stable current . First thought was to silver braze the joint, but that would anneal the arm . . .
A better joint would involve a heavier arm, with a split closed around the electrode to eliminate the "U-piece". Your solution is almost as good. Use a recognized dielectric grease as an assembly lube to avoid joint corrosion over time.
Usually, if radial setscrews are used to secure the electrodes, the electrodes themselves will have a very small step on them to keep them from sliding down inside the holders.
Take this with "a grain of salt". I see very few spot-welders in for repair, and most of those are for shorted transformers. The basic engineering is there . . .
DJD
