I asked this question a while back but have lost the answer so I appologise for covering old ground again.

What is the smallest diamater a motor can be trued to.

I have a couple at 7.1mm and one a 6.9mm.

The motor at 6.9 is losing the leading edge of one of its segments and I have a feeling that if I try and true it again I'll lift a segment and damage the comm lathe bit, somthing I would like to avoid.

Any feed back would be appriciated. thanks

ive had a trinity monster stck armature go down to 6.66mm. From what ive read it seems to vary between the different brands (EPIC, YOKOMO and TOP).


why do you ask?

Because I don't that to have the com come apart in the lathe and damage the cutting bit.

I can't remember if the answer I got last time was 6.8 or 6.1.

If it's 6.8 then that would fit with what's happening with the motor that is at 6.9. If it's 6.1 then this particular motor has a problem but I can continue to true it.

If you have lifted a comm segment imho it won't be on the lathe. Ones I've seen, the segments lift because of too much heat or too much current.

Once they have lifted it can't be fixed; latheing it won't repair it. You need a new armature.

Haven't seen a lathe that can spin an arm hard enough to throw a comm segment...

Hee Hee, I know their was a reason my friends used to call me Conan.

I've lifted copper on the lathe that lifted in such a way that a big chunk would lift up and snot the bit a couple of times before the lathe comes to a stop.

I'm aware that once a comm lifts it's dead. I'm trying to avoid the above senario as well as make a decent judgement on a motors condition if I'm looking to sell it.

cheers

The minimum recommended comm size for a small diameter armature is .270". The minimum recommended diameter for a large diameter comm eg KR Reedy is .310". I don't know what this is in metric but I'm sure you can convert it

Note: As the comm gets smaller the torque decreases and the rpm increase

Don't think its a lathe error... its probably gotten loose on your last run. Just bad luck that it didn't get caught in the brushes instead of your lathe bit.

Just inspect the comm closely before loading the lathe to see if anything's loose... gotta do that every time. The one time that one doesn't... will be the bit-wrecker...!

WC - I've do it as well, arm was in great condition, cut it down too far and it caught a segment and tore it up. The comm I had it happen on was cut wayyyy down so I expected it to happen anyway. My carbide bit worked fine after it happened though....

6.85mm = .270inch

..hehe... guess the carbide guys can afford to be a bit more lax. I got a Hudy diamond in mine and I've been very careful with it... otherwise it'll be $200 a pop.

Thanks guys. Looks like 6.9 is a good place to stop skimming the comm at.

Why use a diamond tip.

Coxy.

that is a good question why use a diamond tip when a carbide tip can be bought from arround $12 and does as good a job as is needed.
What is the advantage of a diamond bit besides it has the ability to cut a comm out of round??????????????

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.

Don't need sharpening, main advantage as I'm a lazy git.

Hmmmmmm some of BJ's comments are a little suss
Just because you use a diamond tip, doesnt mean you are not going to have a round com.
You can make the com out of round just as easy with a carbide tip, he really should have a close look at the design of the tips cutting angle, then do some study on the subject.
He might surprise him self when he finds out this makes more difference as to how the tip will cut.