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Re: Turning brake rotor

To: "Elton Clark" <>,
Subject: Re: Turning brake rotor
From: "Phil Ethier" <>
Date: Sun, 20 Feb 2000 00:19:30 -0600
After I wrote this, I see that Mr. Clark reposted (several times in fact!) a
more complete version, which made some of the same points I did, so I hope
this does not look to argumentative in retrospect.

From: Elton Clark <>
To: Peter J. Thomas <>; Shop Talk
Date: Saturday, February 19, 2000 9:34 PM
Subject: Re: Turning brake rotor

>> Is there any way to turn a brake rotor at home?  I don't have any
>> specific metal working tools, but I do have some wood working and home
>> repair tools, drill press, bench and angle grinder, sliding table, etc.
>I'd find some other way to economize . . . a drum or rotor lathe has to be
>pretty hefty piece of gear to attain the accuracy required. . . you need to
>machine BOTH sides of the rotor  at
>the same time to keep the surfaces pararell

Not if you are a gifted machinist with a proper lathe.

Besides, keeping the surfaces parallel is not the pertinent metric.  If they
are slightly cone-shaped, even if the cones are not parallel to each other,
the brakes will still bed in OK.  The important metric is that if the
surface is slightly conical, the axis of the cone must agree with the axis
of the disk's rotation when the disc is installed in the car.

Put another way, the thickness need only be constant along all of each
circular path at right angles to the axis of rotation.

It is for this reason that perfectly concentric waves and grooves are not a
reason to turn a disc.  Only two conditions require turning:  1) Roughness
which eats pads rather than offers proper friction.  2)  Runout due to
improper manufacture or warpage which causes vibration or pulsing in the
operation of the brakes.  If neither of these conditions exist, the new pads
will bed in fine.

Therefore, if an indicator on the car shows no concentric  runout an any
point on either side of the disk, just change the pads and smile.  You may
make things worse by taking it apart and cutting the disc thinner.

A machine which cuts both sides at once will produce a perfectly unusable
disk if the orientation is not perfectly aligned with the axis of the disk
when in use.  To check for this, run an indicator on the disk after it has
been re-installed in the car.

Phil Ethier    Saint Paul  Minnesota  USA
1970 Lotus Europa, 1992 Saturn SL2, 1986 Chev Suburban

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