Also alot of people will tell you that a Diamond bit is the best, this is not so, a carbide bit will cut just as good as a diamond one. Here is a quote directly from one of the greatest motor builders ever Big Jim Greenemeyer, "April 14, 2002
Sharpening Carbide Bits for Truing Comms
DIAMOND BITS VERSES CARBIDE
FOR YOUR PIT LATHE.
Choosing A Bit
First, the benefits of carbide supercede that of a diamond. Here's why. If you've never used a diamond tool bit before, you will probably chip it before you figure out how to set it up. Then you won't know it's chipped and you'll keep trying and trying and you'll look like Big Jim and Hank by the time you stop pulling your hair out. They chip very easily. Then you're out of a bit of money and have a whole lot of frustration ahead and behind you. Diamond bits are for experienced pit lathe users or machinists. I have been cutting toy motor comms for 30 years and I use carbide bits.
Diamonds can go bad if you take too big a cut, if you hit the tabs, even if it was cutting fine when you packed it away, a diamond bit can be bad the next time you take it out. Don't ask me why. I have even chipped a diamond just by using the wrong kind of brush to clear the chips away. I figured out later that the steel band around the small paint brush I was using must have hit the tip.
And no two diamond bits are the same as far as setup goes. Some bits need no shims. Others of the same brand and type will require 2 shims. Don't ask me why on that either. Carbide bits of the same brand always seem to setup the same. Another reason to use carbide is when it's dull, the finish looks like crap. A diamond will just start cutting out-of-round and you won't know it until your start putting your motors together because the finish looks fine.
Unless you are extremely experienced, stay away from diamond bits. Use carbide until you gain that experience. Then you can either buy a tool sharpener like I did to do the job right, keep honing the tip with a diamond file until it is so distorted it (you unintentionally change the angles a little each time if you won't work anymore then sharpen by hand), throw it away and start with a new $4 bit or use a diamond. Here's the tool sharpener I use. I've had it since 1980 and it still works fine. www.glendo.com
CHOOSING THE RIGHT CARBIDE BIT
The best place to buy carbide bits is McMaster-Carr www.McMaster.com
. They are cheap and provide excellent service and on-line ordering and support. Or if you have a machinist supply close to you, do that. Try the phone book.
To see if you need a left hand cutting or right hand cutting, see which way your lathe trues toward the tabs. But remember, your bit is installed upside-down so it will be backwards to what you have to order. Remember that most of the machining world cuts with bits right side up. So if your lathe cuts from right to left like my Integy Xipp or my old Cobra, you'll need a left hand bit. The old Twister lathes cut from right to left so they'll need a right hand bit.
On page 2327 of their on-line catalog, you'll see a bunch of different bits of different angles. Unless you have a tool bit sharpener, get the AR series, either R for right hand cutting or L for left hand cutting. Also get the "4" series for non-ferrous (no steel) cutting. And ALL pit lathes use a 1/4" shank. Don't get any other size even though it would fit in your holder. The point will be in the wrong place.
So if I were ordering tool bits for my Integy Xipp, I would want an AL4 bit #3367A131.
HAND SHARPENING YOUR CARBIDE TOOL BIT.
OK, the #1 rule when hand honing with a diamond file is, NEVER, EVER FILE THE TOP OF THE CARBIDE PIECE. If you get this part even slightly rounded, it will have to be sharpened with a tool sharpener because you can never cut it back far enough by hand to make it flat again. Even if you do, it will likely be at the wrong angle. There's no need to hone this part of the bit anyway.
The bit cuts from a sharp point formed by the convergence (coming together) of three angles on the bit. A new bit comes rounded on the point. It will work this way because it's uses the top sharp edge to cut with. But when this gets dull, you want to sharpen it to a point. When honing by hand, most of your work will be on the angled side (the part of the bit not 90 degrees from the commutator). However, you must lightly touch up the 90-degree side because of the burrs left from honing the angled side. Just a couple of strokes will do it (boy, I've heard that before, ha).
Try not to change any of the angles; file the bit with that in mind. If you have a good magnifying glass it's helpfully in inspecting the point during the sharpening process. Use the #400 grit file (also available from McCaster-Carr) if you have some material to cut away and then touch it up with the #600 file. It may take a little practice to get it cutting the way you like it but just think of all the advantages you'll have over a diamond and if you screw anything up, it's only 4 bucks! "