Big Jim's Guide to RC Motors - Comm Cutting


Question: When you cut the comm, are you supposed to run a ball point pen down the "cracks" of the comm to round the edges? Does this reduce arcing or does it cause other problems?
BJ's Answer: Running a ball-point pen down the slots of the comm is an old slot car trick done by the "fiddlers" amounst us. It is suppose to remove the burrs on the edges of the slot caused from cleaning the slots with an Exacto after cutting the comm. In actuality, these burrs are microscopic and are easily wiped away by the break-in procedure of the brushes. Many racers spend a lot of time doing this or getting the finish just right. To me, if it's round, that's all that matters. Other than getting ink on the comm, this procedure doesn't hurt anything though.

Question: When using a comm lathe what should the finished cut look like?
I've been told that it should have a mirror finish and I've been told it should have a finish like a CD satin like. I am sooo confused!!
BJ's Answer: I can understand your frustration with the lathe. The best way to describe how a comm should be is round. It really doesn't matter how the finish looks as long as it ends up that way.
The mirror finish that most racers try to achieve looks pretty but that's about all. This is usually made with a diamond with a rounded tip. This kind of tip pushes the metal away as much as it cuts it off, consequently you can have a nice shiny finish and it be out-of-round. This is not the most desirable for breaking in brushes on either.
The best comm finish is achieved with a very sharp tip so that it ends up looking very much like a CD, as you described. It should reflect colors when put in front of a strong light. However, this is ideal, but not necessary. I've been at a lot of races where my tip is getting old and the finish isn't as pretty as it once was but it still cuts round, so it works fine. It actually seems to work better if it's a little rougher. Brushes seat quicker.
When you cut a comm, you can test it for roundness by running the motor at about 4-5 volts and holding a pinion wrench or other small tool on the end of the brush where the shunt goes in and feel for vibration. It should feel as if it was not running at all. Most pit lathes are hard to set up just right. Diamond bits can cut very shiney and be cutting out-of-round. This is why I always use a C2 carbide bit. But you need a tool grinder to put the right edge on it. It also never chips when you go too deep and hit the tabs.
And it's cheap, about $4 a bit.
I use an old Cobra I've had for years when I'm in the field. Otherwise, I use a Unimat 3 with a brass collet dedicated to just cutting comms.
Lubrication is important too when cutting copper. Unless you have a very fancy set-up like Mike Reedy does for cutting comms, you'll need some sort of lube. Most racers know the Sharpie trick. There's something about the ink in that pen that works for lubicating a comm as you cut it. And it's clean and easy. Just cover the comm in black ink just before you cut. WD-40 also works well as does mineral spirits, or mix the two together.
Hope this helps in your quest for round comms, for that's the most important thing. Heck with the finish.

BJ's Comments: Here's something I do that might help. I don't always put new brushes in when I cut the comm. I find that after the brushes are deglazed they work just fine. Just cut the comm. This way, your brushes are already seated and the comm is really why it needs to be rebuilt anyway. If you really do need brushes, cut the comm and let the brushes break-in for a long time.