Originally Posted by
bainbob
OK Frank, Maybe you need to take a closer look or go to a manufacturer's web site. If you've never seen shellac on laminations in a transformer you just weren't paying attention. First, I happen to be a design engineer (last 41 years). If you don't insulate the laminations (especially for a/c transformers) eddy currents WILL flow between them and ruin your transformers efficiency (hense high current draw). For a d/c electromagnet it's not as big a deal (no flux reversals). When it comes to what's inside an electromagnetic chuck, there is no copper band. It's WIRE. In fact it's pretty small gauge because they're designed to run on high voltage DC. Since they get their power from a wall outlet it starts as 120 volt a/c and after rectification it peaks at 170 volts DC. Obviously that's NOT going into a copper band. The gauge of the wire depends upon its length, which depends upon the chuck size. On some of them they use separate electromagnets, on others they use a common core and criss-cross as they transition to the next one. In all cases (talking electromagnetic, not permanent magnet chucks) the cores meet with the top plate, not the bottom. The whole thing is then potted to seal it up (precision grinding is almost always done wet - flood coolant at high volume). Unless you had a massive chuck there's no way it drew 6 amps. The 12x72 chuck on my Mattison only drew 4 amps. Check out a scrap yard crane. It only requires 5 amps to lift thousands of pounds. It's all about flux density. The wires to that crane magnet are only 16 gauge, and that's only to avoid the voltage drop in that long run. By the way, the separator plates are not stainless. They're either brass or lead (poured lead). The mild steel plates do not run to the steel chuck body. They stop well short of it on both sides. Lead and brass are both non-ferrous so they are not susceptible to magnetic fields. I have never heard of a chuck that uses both permanent and electro magnets (possible but doubtful because they're already pressed for space inside the chuck body and using an electromagnet to counteract the permanent magnet would surely weaken it over time. On a permanent magnet chuck they use a cam (on the handle) to pull the magnet pack away from the top plate (it doesn't take much separation to disengage it since flux falls off as the square of distance). Hopefully this helps explain my points. I am certainly not trying to insult you. What you did is pretty clever. I think I might have just bought some door latch electromagnets instead (easy) as they're very strong and have a nicely machined face with poles.
Bob Bain
