Undercutting motor commutators by hand

I'm planning on taking a skim pass to clean up an axial commutator on a motor of mine because the surface of the bars has gotten a bit scalloped. Before I commit to this course of action, I'd like to gather some advice on how to go about undercutting the mica on it without resorting to expensive or deeply involved solutions. Has anyone ever ground a hand tool to do this manually? Or to at least single-point it between centers in a lathe?

Use a dermal grinder and 409 slitting stones, then grind 2 hack saw blades to a right and a left hook tool to de-burr...Phil

Google isn't too helpful with '409 slitting stones'. Any more details you can give on these?

EDIT: Dremel EZ409. Got it.

i know a generator's commutator is undercut but are the commutator's on motor's undercut ? and if so why and being you will have it in a lathe why not use your lathe as a manual shapper grind a tool bit and as Alex would say preform some of the ol in out in out

I was under the impression that undercutting was not a "thing" any more.

Clarification needed.

I've undercut the mica on some starter motor armatures by hand. I took a piece of a broken hand hacksaw blade (I believe it was 18 teeth/inch) and ground the 'set' (angling to each side of the blade) off the teeth. It was nothing fancy, just using a grinding wheel and a belt sander to thin the blade to a little less than the width of the slots/mica. I ground a radius on the end of the blade to avoid a sharp corner. I used light pressure and made sure to hold the blade parallel to the sides of the commutator bars as well as on a radial with the shaft centerline.

After undercutting the mica, there were likely some small burrs on the edges of the copper commutator bars. I put the armature back in the lathe and dressed the commutator with a commutator dressing block (rubber with abrasive in it), and finished with a piece of canvas on a stick. If the burrs are not taken off after undercutting, there can be excessive arcing and damage to the brushes. Another item I learned from powerplant electrical maintenance people as well as a text on electrical machinery maintenance: do not use emery cloth to sand or polish a commutator. I was told to use 'flint paper' or similar sandpaper used for woodworking. Emery cloth has an aluminum oxide abrasive which (or so I was told) would imbed in the soft copper commutator bars and cause excessive arcing and rapid brush wear.

There were mica undercutting attachments for lathes which consisted of thin saw blades with no set to them. These blades were mounted on a fixture with a handlever, and the fixture was mounted on the side of the cross slide saddle in the manner of a follower rest. The handlever was used to raise and lower the blade, pressing down on the lever and cranking the carriage caused the blade to cut the mica. Smaller lathes such as Southbend and Atlas offered this type mica undercutter as an accessory.

For the few times I have had to refinish a commutator, using a hand hacksaw blade ground into a hand undercutting tool has worked well.

CalG said:

I was under the impression that undercutting was not a "thing" any more.

Clarification needed.

Click to expand...

Motor makers went-over to use of composite separator material that self-crumbles as it is exposed. "Real Mica" does that, too, BTW, if properly selected. Thin "plates" the natural form of Mica. And highly frangible.

That's WHY it was selected for the task in the FIRST place.

Accordingly, undercutting hasn't been "essential" on mainstream commutators for the better part of 90 years.

Single-point turning, either. Commutator stone is about ten bucks, brand-new.

Unless.. the poor thing has been so long neglected it has a CANYON worn into it.

Old habits, OTOH, last a long time.

Some of the above posts were lengthy and I'm remiss in not reading them all...but, when I did it a million years ago, we would take an old hacksaw blade, tape the end as a handle, snap off the other end so it was at a bevel and use it in a 'pull' fashion. Electrical repair shops had their fancy under-cutters, but the hacksaw blade method worked very well.

Stuart

atomarc said:

Some of the above posts were lengthy and I'm remiss in not reading them all...but, when I did it a million years ago, we would take an old hacksaw blade, tape the end as a handle, snap off the other end so it was at a bevel and use it in a 'pull' fashion. Electrical repair shops had their fancy under-cutters, but the hacksaw blade method worked very well.

Stuart

Click to expand...

Last one I made.. one also .

A) used a "tooth set" blade, never a "wave-set".

B) ground OFF the 'set' within the working zone.

C) put the fool thing aside once the Union Brother full-time "motormen" explained it was basically an obsolete tasking.

If spacer material did NOT wear flush AS IT GOES?

There would be NO FINE WAY to get 2,000 to 6,000 Power-On-Hours out of a set of brushes.

But we do.

Best way to undercut the "mica"?

Not at all.

Use the 409 sliting stone (1 inch di) and a 402 arbor. works good.... the ez409 is too big...Phil

thermite....Shame on you undercutting is done on all large dc motors STILL, only starter motors are not undercut.. The hard copper brushes rub the mica away. Everything else is undercut with carbon brushes...Phil
....

Phil in Montana said:

thermite....Shame on you undercutting is done on all large dc motors STILL, only starter motors are not undercut.. The hard copper brushes rub the mica away. Everything else is undercut with carbon brushes...Phil
....

Click to expand...

"In YOUR shop" ... maybe.

As said.. old habits die hard. Easily as hard as Old Farts?



Not my problem. Even AS an "Old Fart".

If it actually WAS a problem?

The (composite) "carbon" (plus metals and ceramics. and, and, etc..) brushed motors would quit a few hundred service-hours in.. when the separators stood proud of the Copper .. and bounced the brushes out of contact.... rather than run 2,000 to 6,000 POH.

They do not quit!

They wear smoothly as a team as they go along. Just as they were designed to do.

JF deal with that.

As the motors - which could not possibly care LESS, about your "opinion" . or MINE ..do.

"real world thing" as it were.

"Undercutting" is sorta like spritzing on underarm deodorant after a bath or shower..

Considerate, but..

..makes no nevermind in a world where your next bath is scheduled 12 to 60 months into a hard-working future, does it!

It is my understanding that mica is harder than copper and will abrade carbon brushes. It will also cause them to chatter & lift off of the copper as it wears, causing arcing. Fuzzy, feathered mica left proud from skimming is also responsible for putting streaks into the brushes and commutator, concentrating the damage caused by arcing.

For perspective, this is an early-patent Century repulsion start motor made prior to 1915. It's been in active service compressing air for over 100 years. I'd like to give it the best possible chance at surviving another 100.

Just a Sparky said:

It is my understanding that mica is harder than copper and will abrade carbon brushes. It will also cause them to chatter & lift off of the copper as it wears, causing arcing. Fuzzy, feathered mica left proud from skimming is also responsible for putting streaks into the brushes and commutator, concentrating the damage caused by arcing.

For perspective, this is an early-patent Century repulsion start motor made prior to 1915. It's been in active service compressing air for over 100 years. I'd like to give it the best possible chance at surviving another 100.

Click to expand...

Even at THAT age it isn't a certainty that "real mica" was used.

But I take your point. No HARM in undercutting it. even if it has run 100 years with less than 100 under-cuts.. taking MY point.

As to streaking and blackening?

The "beloved" old-penny brown Copper surface is SLIPPERY.. very important to max brush life, actually.

. and.. per Reliance tests and "white papers"...forms and STAYS formed .. in a rather narrow load - current-flow- and resulting surface temperature range ... at around 215 to 250 F.. "IIRC".

So larger motors with mixed load cycles even have paralleled gangs of brushes some of which have disengagement "lifters" .. so as to assure enough current through the "active" ones to keep the surface sweet at low load.

If not operated at that favourable temp? Expect blackening.

Regardless of undercut.

2CW

Very good to know about the copper patina. In that case I might just do the sensible thing and leave well enough alone until an actual symptom presents that demands attention. Ain't broke, don't fix it and all that.

If your going to do it, follow phil in montana advice i have been in his shop he is not just talking the talk. if he says thats the way to do it then thats the way to do it.

A side benefit of undercutting the Mica....removing copper burrs formed by the turning operation.

Just a Sparky said:

Very good to know about the copper patina. In that case I might just do the sensible thing and leave well enough alone until an actual symptom presents that demands attention. Ain't broke, don't fix it and all that.

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DO check that the brush holders are properly ALIGNED, though.

10EE motors have suffered when some prior Pilgrimage - mayhap a clumsy bearing replacement - had bent a holder out of line such that the brushes no longer tracked and followed properly.

See to the health of the tampers as well. Helwig Carbon have pubs on those and make both stock and custom ones for those in-need.

For reference, here's what Audel's Handybook of Practical Electricity, ca. 1942 offers on the matter:



This sheds some light on why one of my other Centuries insists on flinging solder from it's commutator. Seems that one might be in need of undercutting.

I have an old (1958) Coles diesel electric crane .....16 ton yard crane ,self propelled by a big DC motor .......when I got it ,I did a bit of general maintenence on the electrics,and one of the 50kw DC generator brush holders was hanging by a thread,and the three ganged brushes were worn into a long wedge ,instead of square.....still worked OK ,but close to scoring the commutator .....that fixed ,the commutator was unmarked ,the mica still well undercut .