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Re: To cut or not to cut

To: "Michael D. Porter" <>
Subject: Re: To cut or not to cut
From: "B. Vibert" <>
Date: Tue, 22 Feb 2000 18:36:08 -0500

"Michael D. Porter" wrote:
> Douglas Shook wrote:
> >
> > re: cutting rotors
> > I described the situation regarding the rotors that were turned
> > on a lathe, and he explained that the problem with cutting rotors
> > on a normal lathe is that if you do not cut both sides at the
> > same time, and if there are high spots, the cutter can deflect
> > the rotor, leaving a high spot.
> >
> > Watching the knowledgeable local auto parts machinist cut the
> > rotors, he only skimmed enough to true them, and he did it in
> > multiple passes, taking about 15 minutes per rotor (he has an
> > older machine with a tired motor).
> This is quite true, but has more to do with hard spots, rather than high
> spots. The cutting tool is more likely to deflect at points where the
> rotor has work-hardened from heat (these areas are sometimes
> 0.010-0.015" deep and must be undercut, which requires more tool
> pressure). And, quite so, tool pressure on both sides of the rotor
> ensures that the cut is true on both sides in the direction of rotation.

I've tried turning rotors in a lathe, it's rarely worth the time and trouble as
the results are usually not very good for the reasons Doug mentioned above. If
you cut one side and then rechuck it is quite difficult to get it parallel with
the side you just faced. I've used very accurate dial indicators when
rechucking, even tried facing both sides without rechucking, and still had
I think the problem is not so much with warpage as with minutely varying
thickness in the rotor, probably from the harder spots in the rotors.
While a warped rotor will shudder somewhat, a varying thickness one is very,
very, much worse. This is why the real brake lathes face both sides at once.
Usually it's not worth the trouble to turn them as new ones are not really that
I have no experience with drum brakes.

Burl Vibert
Kingston, Ontario
1983 Porsche 911SC
1983 Audi Coupe
1987 Audi 5000CStq
1985 Kawasaki Ninja 900R
1997 Shetland Sheepdog

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