Comm cutting question

[FilthyPierre]

I needed to cut the comm on my group 20 armature (20x1) on the weekend, and yet again noticed that no matter how I cut, the comm always has a noticable effect that can probably be best described as "rifling" - that is, a spiral mark left on the comm by the cutting bit. It's not huge, and previously hasn't affected the motor's running after reinstalling the arm and new brushes. So far this has happened on 4 different arms. The lathe is an Eagle Mod4 lathe, and has a carbide bit. Of the 4 arms cut I've proably done about 24 or so separate passes/cuts. (The G20 comm was so worn it took about 16 cuts to get it trued - there's not a great deal of meat left on it now.....).
Anyway, I've never noticed this effect when I've had comms cut with diamond bits, so is it an artifact of the carbide bit, or could the bit be a little blunt ?
Any ideas?, especially from you motorheads like Gizzbob - am I worrying unnecessarily or should I go with a diamond bit ?
Thanks, FP

[Grizzbob]

Well, I'm not sure about that rifling effect(haven't seen that before), but it sounds like you done quite a few cuts with that carbide bit, so it may simply be dull. If you can afford a diamond one, then I definitely suggest getting it, I've been using one in my Cobra for a year & a half, & it still cuts like it was brand new..... :)

[combones]

I think the rifling is just artifacts from the carbide bit. It won't make it super shiny and smooth like the diamond bit can, however, I think once you run the motor, those less than diamond brushes will mar the surface of the comm instantly, regardless of cutting bit material on your lathe. http://www.plauder-smilies.com/person/rainfro.gif

[FilthyPierre]

Thanks. Grizz, I would have thought that I'd get more than 20 to 30 cuts from the carbide bit. I've heard that around 100 is the norm, but I can't speak from experience here, I'm just quoting what I've heard.
I will attempt to see if I can "borrow" a diamond bit - the manager of the LHS where I bought the lathe races at my club, and I know he has a diamond bit in his lathe, which is simialr to mine. I'll get him to put his bit in my lathe and make the cut himself... and we'll see what happens.....

[Oyster]

The rifling is usually caused by cutting too fast. You have to move down the comm very slow to get a nice cut. Sometimes people will make a nice slow cut, then return back to the stop quickly. This will cause a spiral up the comm. After you make a cut, back off and reset.

You don't need a diamond bit to make a smooth cut. Also, depending on the bit, you can also sharpen a bit and reuse it.

To avoid having to make deep cuts, be sure to cut the comm every few runs. If the brushes are still in good shape, you don't have to replace them. You can use the same brushes a few times (depending). It takes time to get a motor running right each time, but if done right it'll cost you less and less as you go.

- jon

[FilthyPierre]

Oyster, I thought it might be my technique, but I tried a number of different things, like slowing down, speeding up, varying the input voltage to the drive motor - none seemed to make any difference. I usually back off rather than winding the bit back up the comm after the cut. How slow is slow ? How long should it take to make the cut, from one end to the other ? 30 secs, 1 minute ?
It may be that the bit is a little blunt, and was from the start, although it's hard to tell just by look and feel. I will also attempt to get it sharpened somewhere.....

[smeg]

Hey Fithy
You sound like you are loaded so why are you bothering to com cut anyway?
JUST TOSS THE MOTOR after a few runs and buy another one you stingy, head banging Australian.
hugs and kisses
Smeg
Australian Off Road (http://www.users.bigpond.com/smeg666/rc)

[ 07-19-2001: Message edited by: smeg ]

[FilthyPierre]

On ya smeg. Loaded eh? I swear orificer I haven't had a drink this morning!

I don't like throwing money away unnecessarily, which is why I bought a lathe - to assist with the rebuildable aspect of my motors.

Head banger?!?!!? Just 'cause I like a bit o' Quo, Purple, Sabbath, 'tallica doesn't make me a head banger....oh wait a minute, yes it does. Oh well. Just remember, when you point a finger at someone else, 4 of 'em are pointing back at ya. :p

[smeg]

Gee, I can only see one finger, and it's pointing up!
And as they say it takes one to...

[ 07-19-2001: Message edited by: smeg ]

[Leet TC3]

It generally takes me 4-5 minutes to do a single comm cut. I really more that wheel slow, but I always get a nice, clean cut.

[FilthyPierre]

Leet, ok, I'll try it reeeeeaaaaalllll sssllllooooowwwww next time. 4-5 mins, geez, I would never have guessed.....

[EMAXX3]

Getting a good cut is easy. Make sure to use a "Sharpie" marker first. This lets you see where you are and also lubricates the comm and bit while cutting. Ease the bit slowly across the comm... 30 seconds or so. DON'T BACK OFF!!! Cut back along the comm without adjusting the depth. This gives you the shine. A new carbide bit costs me $3.14 at a local tool supplier. It costs $8.50 to get it sharpened. Try a new bit. Also... use some bearing oil on the comm after the first cut and each cut after. Relube!!What slave motor are you using. It should be a stock zero timed motor. A motor that is timed 24 degrees is too much. Use an old Traxxas Stinger or something. A four cell battery is best.
Oh here is another thought. The bit may be to low. It should hit just above the center line of the comm. Too high is better than too low.

Good Luck!!

Emaxx3

Good Luck.

[combones]

Isn't the Traxxas "stinker" a 20 turn?
http://www.plauder-smilies.com/person/pimpflash.gif

[Oyster]

I usually make a cut in about 30 seconds. Always get a nice smooth cut. Now, for the whole motor, I might be at the lathe for 5 mins. Keeping in mind I'm using a REAL Lathe made by atlas. I bet it weighs half a ton. Sure makes a nice cut though! :)

Check your bit. You can resharpen a carbide bit. Perhaps you have a piece hanging on it... ?

- jon

[Emaxx3]

Technically the Traxxas stinger is a 20 turn closed endbell non-rebuildable motor. It is a mod motor only by definition of the turns. It is a junk motor but it does work well on a lathe as a slave motor.

Emaxx3