Comm cutting on a real lathe

[CustomCrawler]

My comm's are starting to show some wear, so I need to cut them. I don't have a comm lathe, but I do have a Sherline lathe. How should I hold the motor? Also, how much material should I take off?

[dabid]

I too have thought about this, just never figured out how to get the armature in the chuck. Maybe instead of trying to just have the shaft in the chuck (and worry about runout), chuck the fat part with one jaw for each pole (3 jaw chuck only, I guess). Then maybe don't crank it down really hard, and take off a few thousandths at a time. I know it could be made to work, just don't know if I want to sacrifice a good armature and comm to find out how much work it'll take!

[usmc1833]

you would need make a sleeve to to hold the arm true to the bit and you only need to take the smallest amount possible off

[CustomCrawler]

Thanks, I put it in the 3 jaw chuck and took only a little off each pass. It worked great, within a few passes, I had a nice uniform color. I love carbide tools.

[Ace]

I would use a sherline 1/8" mill collet and grab the main shaft. After all the shaft to comm runnout is what matters.

A 3 jaw would give about .002" to .006" runout... A 4-jaw would work too.

I also hear milk is a great lube for turning copper. Never tried it but its supposed to work well

[SPEED_RACER]

another trick to cut the com is to take a sharpie and blacken the com before cutting and as you cut you'll see the com come nice and clean

[BultacoJim]

Originally Posted by Ace

I would use a sherline 1/8" mill collet and grab the main shaft. After all the shaft to comm runnout is what matters.

A 3 jaw would give about .002" to .006" runout... A 4-jaw would work too.

I also hear milk is a great lube for turning copper. Never tried it but its supposed to work well

Ace’s method is how you should do it, if you want the concentricity to be correct.
You want the circle formed by the comm to share the same center as the circle made by the outer surface of the 1/8 shaft. (Is that confusing?)

http://lordmazak.tripod.com/geometric.htm

The method you used will make the comm surface a perfect circle, but its’ center will not be exactly the same as the shaft. Like Ace said, you are trying to make the shaft to comm distance the same, at each point on the circumference of the comm.

A comm lathe spins the motor on the axis of the shaft, and therefore the concentricity is correct.

If this is a slow running lathe motor in a crawler, your method will probably work Ok though.

Speed Racer’s tip is a good idea. A Sharpie or a commercial product like Dykem “painted” on the comm will keep you from taking too much material off.

Integy sells a diamond tool bit (expensive though) that cuts copper cleaner than HSS or carbide. Machining forces are 5 times less if you machine Cu with diamond tools as compared to HSS or carbide tools. Copper is extremely soft and ductile. It tries to pull the cutting tool into the workpiece.

We need a machining forum. I could ramble on forever.

By the way, if you use milk as a coolant/lube on your Sherline, you would give a whole new meaning to the phrase “Got Milk?”

[BENDER]

I like the idea of a machining forum BultacoJim.

We should at least start an official "RCC Machining Thread" somewhere, that way people could ask about, and compare, machining methods. I know there was a thread on machining going at one point, but I believe it was lost.

[Ace]

Machining forum...

Great, now I'll spend 8 hours a day here!