View Full Version : Skimming commutators. What's the secret?

Alan Smith
10-27-2007, 01:05 PM
Is there a trick to stop smearing copper into the gaps when tidying up a ragged commutator on the lathe? I wondered about rubbing some candle wax on before starting to cut but would love to know if there is a correct way to go about this job.

John Stevenson
10-27-2007, 01:11 PM
HSS tool, small nose radius, honed so you can shave with it AND at least 30 degrees of top rake, then play with speed.
The book says flat out for copper but some comms are copper from hell.

My experiance says it's anything from 1200 revs down to 250, the trick is finding the sweet spot.
Deburr using a rule slid down the slots and a final polish with some very fine scotchbrite.


10-27-2007, 01:16 PM
Like John said,but I will add a VERY small nose radius,like .01".

I spin them up as fast as you feel comfortable with and use the finest feed you have along with a shower of WD-40 or mineral spirits.

I usually finish up with croucus cloth.It may still need to have the slots scraped clean.

Alan Smith
10-27-2007, 01:26 PM
Thanks chaps for your prompt and helpful replies.

10-27-2007, 06:57 PM
Take a hacksaw blade and grind off part of the set on the teeth.
Wrap some tape on it to form a handle and this will make a tool to clean the slots. I have the teeth facing the handle so I can just drag it toward me while cleaning out the slots. Every bit of the copper must be removed from the slots.

10-28-2007, 11:14 PM
When you've done as the above and undercut the mica, got some clearance, don't forget you gotta bevel the edges of the bars to keep the brushes from bouncing off them.

Last DC rotor I got to work on, rest went to Round Rock, Texas, they gave me a diamond point tool. Might have been more expensive 25 years ago than today.

About a 25 ton rotor, on a 14 foot BM.

Whether anyone ever scraped the mica and beveled, I don't know.

Ah, well.