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If you can saw off some of the excess, you're going to be ahead but at first you'll have to sneak up on it to avoid chatter or worse, snagging the rotor and turning it into a very heavy Frisbee.
It's best to pick up the inside diameter. If your rotor indexes with countersunk screws then an adapter you can chuck up in your lathe then bolt the rotor to it will help make the job easier.
Definitely use carbide cutters and as slow as you can run the lathe until you have a continuous cut all the way around. Otherwise an "interrupted cut" is a good way to start a chatter that's sometimes hard to get smoothed back out. Cutting should be almost noiseless, just a little metallic hiss. If you're hearing squealing, you're hearing a harmonic so slow your speed down.
Others may tell you you can turn your work piece faster but if you ever have a part jump out of the chuck jaws, NOT being in its path at that moment may be the only thing that saves you.

We round out cylinder fins on some jobs and I start with our Monach at 60 rpm, .002" carriage travel per chuck rotation, .010" per pass until the fins are round. Very slow but that way I don't have to worry about snagging and breaking a fin or worse, having the cylinder get away from me.
 

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Discussion Starter · #22 ·
it does seem ya making a $20 job at a brake shop into a Nasa project,
First, im trying to learn something and do it myself, not just bring it somewhere unless i have to. Secondly, from what i read , motorcycle rotors are made of a harder heat treated steel than cars. Car rotors are much thicker which wouldnt warp as easily. Third, a machinist acquiantance sai the rotor is very hard and he would take care of it for me, and its perfect! I think ur misinformed about oriellys being able to do it but i have one near me and will ask and have a spare made
 

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I agree with learning to do stuff yourself. Later, even if you choose to take a part to have a mechanic or machinist do the work, you'll know and understand the procedure.
One thing that's invaluable is learning set-ups. Knowing 6 or 7 ways of holding and machining a part instead of 1 or 2 will open up the ability to do lots of jobs that otherwise might seem impossible.

One detail everybody slid past was checking your rotor to make sure it's not worn beyond minimums or has excessive runout. If not, find a machinist to walk you through the procedure or let you watch as he does the work. If it is worn beyond limits, it's a still good piece to practice machining on.
 

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"If it is worn beyond limits, it's a still good piece to practice machining on."

That's a good point, one of many in this topic. I am absolutely not complaining when I say it has attracted far more posts than the subject would have seemed to deserve. A whole lotta guys shared (nearly all) useful, (often) outside-the-box ideas, procedures, and knowledges in general. It is one version of what I think is the Ideal J.J. Tech Thread.
 

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Very true. Not much chance of warping either. Start with a nice piece of A2 and you should have a rotor that will last
 

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Discussion Starter · #27 ·
Do you have a face plate for your machine? One could bolt a rotor to a face plate with spacers between. The inner diameter could be cut larger much easier than cutting down the outer... Think lower surface speed of the cutting tool.

If you are cutting the inner diameter of a sprocket or a brake drum, yes the diameter does need to be concentric. Disc brake rotors are not so fussy.

I know many are shaking their heads about now...

The rotor doesn't need contact with the hub center for location, it's a non issue. Rotors don't see a radial load like a drum or sprocket. They simply need to be tight to prevent lateral run-out. If you used flat head mounting screws for the rotor, the screws will locate it by the countersunk mounting holes in the rotor anyway.

Still not convinced? Look at the 10" dual disc front rotors on every 77-83 Harley. The rotor bore on all of those are close to a quarter inch larger than the hub.

At times it pays to step back and get a slightly different view...
Thats pretty interesting! Seeing as how the bike has only rear brake and im very large i was worried about it not fitting snug to the hub but if that works on a sporty im sure its good for an old knuck that will never see 70mph
 

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Calipers are designed to float from side to side to 'find' the rotor and where they need to be to do their job. Not so with drum brakes on a fixed backing plate.
Once you get your inner and outer diameters where you want them to be, you should be in good shape. Just to have a dependable starting point, it might be a good idea to make sure the center hole is the right size and is where it should be. Once that's done, you can clamp the disc in your lathe by the center and dial in the O.D.
The outer diameter isn't super critical as long as it isn't so far off that it messes with the balance, or the high spot(s) could saw the caliper in two. A spinning disc doesn't have the centrifugal forces to deal with that a spinning brake drum does with lots of its weigh at its outer circumference.
Truing the outside too lets you know it was done and is concentric. Good practice for future machining jobs.
 

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I used half a scrap steel hub as a mandrel since my 14" lathe was too small to chuck the rotor and mostly because I didn't feel like installing a faceplate. Worked fine for boring late rotors for an AMF OD hub. However it makes more sense if ya don't have enough tool reach to visit your local machine shop who you REALLY should make friends with anyway. I used generic carbide insert tooling.
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