Turning very worn commutator of Prototrak servo motor

SBAER · Mar 31, 2020

This motor actually still worked when I took it apart, but the rear bearing was pretty well seized up with carbon. The wear on the commutator is very deep. My plan is to try to fix it by taking very small cuts with a sharp tool. Any suggestions that might increase my chances of success would be appreciated. Replacement motors are expensive from SWI, the one I have is labeled as a Magnetek 4020d-155, which looks a lot like an Indiana General 4020 series (dates back to the 60's) with a shaft extension out the back for the encoded. Either way, not too many choices in the used motor market.

mountie · Mar 31, 2020

Martindale Electric Co makes equipment and tooling for just this purpose. Commutators are generally ground while refinishing. Badly grooved commutators may be turned and then ground. The mica insulation separating the copper bars then must be undercut to prevent damaging the brushes.

Please feel free to reach out to our applications staff for details and tips:
216-521-8567
Martindale, Circular Saw Blades, Rotary Burs, Undercutters, Abrasives, Grinders, Electrical Test Equipment, Hand Tools, Etchers: Martindale, Electric Motor Repair Tools, Metal Cutting Saw Blades

Also a quick search lead to this discussion: Commutator refinishing?

Gordon Heaton · Mar 31, 2020

Looks like there may not be a whole lot of copper left to turn! Can you tell by looking at the end how much is left?

Tyrone Shoelaces · Mar 31, 2020

I've re-machined a few comms over the years. Under cutting the insulation by hand can be really tiresome.

Regards Tyrone.

eKretz · Mar 31, 2020

Wow! That's about the deepest wear I've ever seen on a comm. You can turn it just fine, grinding isn't imperative. I've turned a gazillion with no issues. Make an effort to turn a smooth finish so you don't need to do much polishing. Getting embedded grit in there from polishing would be bad news. They make stones just for comms too. And yeah, don't forget to undercut the mica... that is a pain.

crossthread · Mar 31, 2020

many years ago I worked in a motor rebuild shop so I have done this quite a bit. Turn the comm down with a tool with a good radius on the nose and quite a bit of back rake since it is soft copper. While you have it in the lathe, spin it up good and use crocus cloth to polish the segments to a good shine. Mount a form tool like you would use to cut a narrow "O" ring groove but turned 90 degrees in your tool holder. Use the carriage feed to drag the tool down between the comm segments to take out the mica. You don't have to take out much. Just enough so that the brushes ride on copper and not mica. As the copper wears down you're going to be riding on the mica some anyway.

lathehand · Mar 31, 2020

Eurton Electric can replace your commutator: Electric Motor Repair, Rewinds, & Parts | Eurton Electric

markz528 · Mar 31, 2020

lathehand said:
Eurton Electric can replace your commutator: Electric Motor Repair, Rewinds, & Parts | Eurton Electric

That's what I would do. Replace the comm. Don't think that there is enough left to turn it. There are about 3 or 4 leading comm manufacturers in the States - ICC, Akard and TEMCO are 3 of them. I would use Akard - Knoxville Tn area. Not sure what it would cost. I suspect its a molded comm.

eKretz · Mar 31, 2020

Might be enough left to turn it, but it won't have a whole lot of room for wear after that, that's for sure.

moonlight machine · Mar 31, 2020

SBAER said:
This motor actually still worked when I took it apart, but the rear bearing was pretty well seized up with carbon. The wear on the commutator is very deep. My plan is to try to fix it by taking very small cuts with a sharp tool. Any suggestions that might increase my chances of success would be appreciated. Replacement motors are expensive from SWI, the one I have is labeled as a Magnetek 4020d-155, which looks a lot like an Indiana General 4020 series (dates back to the 60's) with a shaft extension out the back for the encoded. Either way, not too many choices in the used motor market.
View attachment 283400

That commutator needs to be replaced, it is too worn for turning. At sometime in the past the brushes were probably changed and the new ones were of the wrong composition, IE too hard for this commutator. get your new brushes from whoever replaces the commutator.

SBAER · Mar 31, 2020

Thanks everyone, I will look into replacing the commutator, the wear was surprising bad even though there was 1/4" left on the brushes.

bhigdog · Mar 31, 2020

Won't cost but a little time to try cleaning it up and maybe 20 minutes to scrape out the mica. Your choice............Bob

Phil in Montana · Mar 31, 2020

Move your brush holders out to just fit the comm thats still ok, if you try to turn the worn comm you will have it fall apart, it is set in vee notchs at the ends of the copper bars with mica and steel rings. ...Phil

rons · Apr 1, 2020

SBAER said:
View attachment 283400

The spring tension for the brushes might have been more than necessary. Way more.

markz528 · Apr 1, 2020

Phil in Montana said:
it is set in vee notchs at the ends of the copper bars with mica and steel rings. ...Phil

Hard to tell from the pictures, but I don't think so. My bet would be on a molded comm.

Search

Largest Manufacturing Technology Community on the Web

Search

Turning very worn commutator of Prototrak servo motor

SBAER

Hot Rolled

mountie

Aluminum

Gordon Heaton

Titanium

Tyrone Shoelaces

Diamond

eKretz

Diamond; Mod Squad

crossthread

Titanium

lathehand

Cast Iron

markz528

Hot Rolled

eKretz

Diamond; Mod Squad

moonlight machine

Diamond

SBAER

Hot Rolled

bhigdog

Stainless

Phil in Montana

Stainless

rons

Diamond

markz528

Hot Rolled

Similar threads