Posted by Sherman in Idaho [24.32.202.166] on Sunday, March 17, 2013 at 10:32:39 :
In Reply to: Re: mis-information on the Forum posted by gmharris [71.105.35.169] on Saturday, March 16, 2013 at 20:48:05 :
...on what we mean by "emory". It's one of those words that means different things to different people. To some, any kind of sandpaper is "emory". To some, only the black silicon carbide paper is "emory", and to some "emory" is crocus cloth. To some "emory" means very fine "sandpaper" regardless of the material used for the grit. I'm not sure how the conductiveness of the grit matters IF all residue is thoroughly washed and blown off.
Perhaps a better rule is to only use very fine "sandpaper" (600 grit or higher) and most importantly, only use it if absolutely necessary. An ordinary pencil eraser actually does a good job of safely "cleaning up" a commutator, and can't leave any harmful residue.
More importantly, it's rarely necessary to "sand" or "polish" a commutator. There are two reasons commutators arc and get pitted: worn-out brushes that are barely touching it, and shorted turns on the armature. This applies to motors as well as generators. Most of the time, when a commutator is arcing, it's because some of the turns have shorted out. Nothing you do to "clean up" the commutator is going to fix that.
If the damage is due to running with worn-out brushes, the pitting still be too deep to "sand" out, and you'll need to turn the commutator down on a lathe. An ordinary lathe will work, but you need to be sure to have the shaft perfectly centered, and because the commutator is soft copper, use a very sharp tool with a lot of hook and take light cuts. It's not so much necessary to cut below the level of all the pits (which might be way too deep) but just to cut it down to a smooth surface. Then polish it with your finest "emory" or crocus cloth while it's on the lathe.
The main thing to remember is that the usual dark or "dirty" condition of a commutator is not a problem. Copper will not stay shiny very long anyway, and a certain amount of graphite from the brushes will quickly coat the commutator even if you have it nicely polished. I think a lot of guys think it will do some good to "clean up" a dark-looking commutator, when actually they'd be better off leaving well enough alone, especially if they "clean" it using any sort of sandpaper coarser than "extra fine". A commutator only needs doctoring if it's pitted or has scratches deep enough to snag a fingernail. If it feels even just reasonably smooth, and the brushes are plenty long and aren't hanging up in their holders, and the commutator still sparks, the armature has some shorted turns and needs to be replaced. The resistance of the coils is too low to measure and detect shorted turns with an ohm-meter. An old-fashioned auto-electric shop would have a "growler" that they use for that purpose. Basically, if you think the problem is a dirty commutator that needs "cleaning up", it's not, and it's better to leave it alone than mess with it in any way.
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