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Never thought I would need a comm lathe but decided to purchase a vintage one and begin to tru my old brushed motors. I found an old Trinity lathe for a good price and started the process of learning how to tru my old motors. I practiced on an old burned out arm and got a decent result but I wasnt completely satisfied and began to look closer at the actual lathe. The in/out feed would back out during the cut and result in a tapered comm. I tightened the setscrews on the block and fixed that but after a few more cuts, the e-clip on the threaded rod popped out of the grove and prevented the block from moving. After a closer look, it seems this had happened several times to the previous owner due to the scratches on the threads. I took out both the threaded rods on the in/out and cross feed and cut a deeper grove on the shaft and reassembled. Works better now. Also added a on/off toggle switch. The old bit seems like it is not cutting but rather rubbing out the comm as there is some chatter during the process. The tip looks to be a little rounded and not making first contact to the comm. Maybe needs sharpened or just buy a new bit. McMaster has 1/4" bits but not sure if I need the AR-4 or the AL-4.

Truing comms is a blast! I do them once in a while too, with the very eXtreme (=Early Hudy, sold by GM) lathe I had from last millenium...

Can't help you on your specific Earnie lathe issues, but the tricks I remember for a good cut setup are:
- nice clean bearings
- wrap the pulley on as circular a surface on the trued arm - not in the stack's grooves...
- with the comm turning away from you, you want the too tip to be sligtly above the centerline... so looking at it from the comm/left side on your pic below, you want the shaft to turn CCW (which should be the natural way if your slave johnson 540 is wired as expected), and the cutting edge to be above the horizontal "midlle" of the comm.
- try to use some kind of neoprene mat to lay the lathe on when you cut to dampen vibrations
- with a 540, and a pulley about half the diameter of the stack, I use about 4V of input voltage

The rest is all about having everything parallel and solidly fixed, taking small amounts of material at the time, and moving parts nice and slow and smooth. If you're still struggling with chatter, then your tool tip might be shot. I've used the very same diamond-tipped too I bought the lathe with 21 years ago and I've cut hundreds of comms with it... and it still performs like on day 1

Good luck,
Paul

Thanks for the tips! I tried several of them which resulted in a much better comm. I spun my first practice arm under a scope and found out the comm after cutting was not fully round. It had some wobble. I also managed to reshape the original carbide bit to allow the very tip to contact the comm. This allowed the bit to actually cut vs just rubbing out the comm. I also adjusted the pulley so it rides on the smooth part of the stacks. Cleaned the surface of the bearings as well. I adjusted the power supply to 4V. After tightening everything it was time to try it again on my Checkpoint money motor.
This is what it looks like before. Maybe several light passes will clean and tru it up. First pass cut like butter! Reshaping the bit really helped here. Second pass. Third pass trued it completely! This is actually pretty fun when eveything is dialed in. Not too shabby for such an old lathe! I inspected the comm under a scope, and the surface is much better now with zero run out. You are correct, cutting comms is a blast! This will certainly bring new life into my old brushed motor collection. I really miss the challenge of tweaking and tuning brushed motors for competition. Brushless has its place but I have more fun tinkering with brushed motors for some reason that I cannot explain!

That's awesome, glad you're getting the hang of it cutting comms is awesome when it works... has a hypnotizing effect on me

Now you have the basics nailed down (the cut above looks just right), some "advanced" tips. To note, a cut comm shouldn't be "shiny", but more of a satin finish - when the light hits it, it should show some minor light diffraction. Very hard to describe, think oil stains in a water puddle being hit by light to some extent.... Mirror finish is not good.

during the cut:
Looks like you're using a black sharpie already before you cut, which is perfect as it provides lube and shows up the areas that still aren't cut. that's perfect

when you're done cutting:
You want to remove possible sharp edges of the segments so they doen't hurt the brushes when the motor is in use, you can do it a few ways. This is my way, not necessarily the absolute best one but it's worked well with my old racing self in the past: run the tip of an allen wrench across the groove from stack to shaft edge, so you "break" the sharp bits down the groove if any. Any tool that has an angle works, a 60deg angle is perfect (hence the hex wrench) as it's sharp enough to ride across both segment edges, but not too pointy so it doesn't dig in the groove too deep. You need to apply low pressure but enough to push them down though. You need a super steady hand, too. Then run an x-acto blade in the groove, again, pushing the possible shavings "out". Unfortunately this might lift some of the shavings that have not totally detached from the comm edges back up, so you want to run that allen hex wrench one last time.
Then you want to shower the comm generously with motor spray with the comm pointing down, so it flushes metal shavings that might be stuck within segments., with gravity... then with a smooth, surgically clean, non scratching rag of some type you want to remove the motor spray residue.


Et voila, a new comm!


Honestly, I just LOVED cutting comms, but not under the time pressure of doing it between heats/main though... still do it at home with much pleasure

Enjoy

Paul

Thanks for the additional tips! I thought I would try another cut on an older Checkpoint arm.
This one was never cut it seems. After 1st pass. 2nd time. 3rd time. This one is going to take several more light passes before it levels out. Done! You are right, this is fun to do for some reason. I need to search for more motors to cut, hmmm......

Hey guys!

Thanks for this cool thread, I am gettin a comm lathe, anytime soon.

As I have never done this, but seen it done on other forums, I always loved that concept of reviving old motors!

Both of you gave some great tips, thanks for that.

From the picture I got from the lathe I am getting the diamond cutting the comm from your lathe seems very similar to mine. I was a bit worried as I have seen some that were much more pointier, like a needle tip almost.

That got me worried, so I was wondering if that tip cutting the comm can be change?

Thanks for the help and advice!

Hi-

I am using a carbide tip. I had to re-grind the tip when I got it and it really helps to have a magnifier handy to check the tip contact to the comm. I do not have a diamond tip yet but will acquire one when I get more practice. They are more delicate than carbide but they do result in a smoother cut. The tips are easily replaced, just make sure you get the right tip angle based on which lathe you are getting.

I agree it is fun to revive these old motors and get them tuned up.

This makes me want to get my lathe out and rebuild some motors, only problem is I don’t have any motors to rebuild, lol.

I bought the Hudy lathe and the attachment to cut coms on non removable endbell motors but never used that attachment, it's still in the bag.

Anyone have a brush cutter?