Posted by Alan Bowes on May 14, 1998 at 15:59:02:
In Reply to: Undercut your Mica? posted by Dave Horvath on May 14, 1998 at 14:55:26:
The mica refers to the insulating material used to separate the bars on the commutator (the copper bars that the brushes contact). If you don't undercut them just a bit, they will begin to stick up past the surface of the commutator as the bars wear down. The brushes will tend to keep "trimming" off bits of mica as this happens, but the mica ridges and flakes will still reduce the effective contact between the brushes and the commutator.
IF you really want to do it yourself, you can undercut the mica by hand, as follows:
1) Find a hacksaw blade that is a bit wider than the mica-filled space between the commutator bars.
2) Grind the sides of the blade flat, so that there is no kerf or wavy edge. Ideally, after grinding, the blade thickness should be exactly the width of the spaces between the commutator bars. You now have a basic tool for the job.
3) Carefully file out the mica with the teeth of the blade, holding the cutting edge of the blade absolutely square against the mica to cut it down evenly. It helps to break the blade off short and hold it with two fingers, directly over the mica.
4) Make sure you don't go too deep. If you go much beyond 1/32 inch, the bars will begin to lose support. A deep groove will also tend to trap graphite particles rather than be somewhat self-cleaning. This can short out the commutator bars.
It really isn't very difficult. Just do it carefully. If you happen to have a small jeweler's file of the correct thickness, you could use that instead of a hacksaw blade.
If the surface of the commutator is still pretty smooth, just rotate it in a scotch-brite pad to dress it a bit. If it is uneven or deeply scored, you should take it to a local generator shop and have them turn (and undercut) the commutator for you. This is not very expensive and might be the best solution anyway.
Alan