Commutator Turning

[gpk9]

Looking for ideas on best way (tooling, speeds, feeds, etc) to turn a DC motor commutator on a lathe leaving a relatively smooth finish without buldozing copper into the grooves. Comms we work on average 12 inch diameter.

thanks,
George

[Geof]

Originally Posted by gpk9 Looking for ideas on best way (tooling, speeds, feeds, etc) to turn a DC motor commutator on a lathe leaving a relatively smooth finish without buldozing copper into the grooves. Comms we work on average 12 inch diameter. thanks, George

I think you will never fully avoid 'bulldozing' copper into the grooves.

The best results I have ever managed used HSS tooling with a top/side rake of 15 to 20 degrees, front clearance of about 2 degrees and a radius of .03" on the tip. You want the cutting tip working at an angle to the grooves so the chip is peeled off along the groove not chunked off across the whole tip width simultaneously.

Depth of cut no more than .02" for roughing and maybe .005" or less for final cut; feed around .005" or less for finishing. Speed maybe 150 fpm max. For lubricant use a tapping lubricant suitable for copper or other 'sticky' metals; alternatively try automatic transmission fluid.

[Mac T. Knife]

My experience is with .27 diameter coms,,,,, Have had good luck with carbide, and even better finish with diamond,,,,,, but they are small lathes. http://www.teamcobra.com/cobra/comlathes.htm

[gpk9]

Related questions: I'm an insert addict. My insert holder is zero front and side rake (i.e. flat front-to-back, and side-to-side).

1) Any thoughts on using TNMG4X1 positioned slightly below horizontal, or maybe TCMT4X1 positioned at horizontal to slightly above horizontal?

2) Is the negative 15 - 20 degree side-to-side rake angle a key factor? (If so, I'll have to figure out a way to mount the holder on a side angle.)

[Al_The_Man]

I followed this previous advice and it worked for me.http://cnczone.com/forums/showthread...utator+turning
Do you undercut the comms afterwards or do they have the automatic wear down ability?
Al.

[gpk9]

Our process is:
1) Undercut to 1.5 - 2.0 times groove width.
2) Turn face & cut/restore dust groove.
3) Backdrag grooves by hand to remove burrs and residual mica from groove sidewalls.
4) Initial polish with 120 grit emery cloth.
5) Knife segments by hand to bevel edges.
6) Final polish with 400 grit emery cloth followed by rubberized polishing block.

Our shop is pretty finicky on surface smoothness and runout. We allow no more than .002 runout on a comm, so we have to insure that the shaft is perfectly centered in the lathe chuck and tailstock. We indicate off the bearing journals and try to center it within .001, and zero if possible.

After the comm work (and all other armature repairs) we put the armature in a dynamic balancer and bring it down to under .1 mills @ 3600 rpm. That is near-perfect balance.

Our motor teardown/rebuild team foreman has been doing it for decades (centuries? since the dawn of time?) and is really tough to please. If the comm has any flaws, or if the comm "just doesn't sound right," he will attempt to smooth it with his own polishing block while the motor is running on the test stand. If he cannot "make it sound right" he will tear it down and send the armature back to us. BTW, they install new brushes on every DC motor, so we can't blame the brushes.

Keep in mind these are large motors we work on, where the customer is paying us to do the job right.

[JSCB]

gpk9,
I work for a commutator manufacturing company.
Most of the comms that we manufacture now are in the 6-inch down to .25-inch comms. We used to manufacture the larger comms, but everything is downsizing.
I will second what Geof suggested. If you try to turn sub flush mica comms with carbide inserts you will "bulldoze" copper into the sub fluch area. A diamond tool is the only way to turn sub flush comms. We have had a dozen or more top insert companies come in and say they can get an insert to work and they have never been able to do it.
JSCB

[gpk9]

Hi J,

Thanks for the info. Haven't tried the TCMT4X1 yet, so maybe you saved me some grief. Recommendations on a diamond tool? Not familiar with them.

thanks
George

[JSCB]

gpk9,
Sorry about the delay.
If you haven't done a search for commutator turning, you could try that first.
That usually pulls up several companies that manufactur diamond turning tools.
We have narrowed down to two companies (CITCO, and KITZEL) that we buy from, but there are several others that are just as good.
The main disadvantage with brazed turning tools is having to retouch the tools off. We have also found that the smallest chip on the cutting edge will leave a rough finish. Once they are chipped that have to be relapped. Diamonds can chip very easily, so you have to protect the cutting edge at all times.
Hope this helps.
JSCB