irwin@m.cs.uiuc.edu (04/12/90)
I am trying to restore an antique Seth Thomas electric mantle clock. Estimates of manufacture date are in the 1920s. It is a fine piece, beautiful finish and has a works about six inches in diameter. The works includes chimes. I got it through the estate of my grandmother. When I examined the works, I found several brass bearings worn in the front and back plates of the works. Also, I found that the shaft and bearings in the motor were badly worn. I first started on the works, took it apart and have managed to have replacement bearings turned and have installed them in the plates. Next, I took on the motor. In doing so, I had a mishap, which is why I am posting this. The motor is made up of two (my best discription) metal cups. These cups have slots cut in the sides, which form the pole pieces of the field part of the motor. If you position your two hands as if you were holding a grapefruit in each, then move your hands together such that the fingers interleave, it is somewhat what the motor frame looks like, except that the open side of the cups face the same direction and between them is a pole piece about 3/8" in diameter for a coil. An electrical coil is placed over this pole, the two cup pieces are placed on the shaft (pole) and the shaft is peened. The depth of the two cups are different, one shallow, one deep, such that when assembled, the ends of the fingers formed by the slots end up on an even plane. One end of the 3/8" pole that has the coil around it has a brass bearing in it and a motor shaft with a single bar to form the armature fits into the two assembled cups, and rotates within the cup of fingers when assembled. The opposite end of the motor shaft has a pinion gear on it and enough of the shaft protudes beyond the pinion to fit into a bearing in the works back plate. My problem was that one of the wires was broken off from the coil and could not be reached to get hold of the end of the coil without taking the assembly apart. I took a Dremel tool, with a thin disk grinder and ground away the peen on one end of the 3/8" pole where it was peened to the cup. There were about 5 peen marks, done with a machine. This worked well, and cleared the pole to be pressed from the cup. As I pressed it out, on one cup, next to the coil, there was a cicular magnet made of powdered iron (put in in paste form and allowed to harden after assembly) between the coil and cup. This was not visable until I pressed it apart and removed the coil. The magnet was only about 1/32" thick and as I pressed it apart, it cracked. Now, with the coil repaired, the motor will not run. I knew what I was seeing, that it was not just glue to take up space so that the coil would be tight between the two assemblies, when I saw it. I used an ohmmeter to verify that the substance was conductive and was actually made of powdered iron. Now, my need: I need a source of powdered iron (fine) with which I can make a ring of metal paste, to put back in place between the coil and cup as I assemble it. Where can one obtain such material? It would be simular to the material from which powdered iron cores are made in electronic applications. I have thought about a thin solid metal washer to take the place of the cracked magnetic ring, but am not sure that it would be as good. Al Irwin Univ of Ill Dept of Comp Sci Urbana, IL irwin@m.cs.uiuc.edu