Subject: Re: turning brake rotors?

From: [email-masked] (Barry Workman)
Date: Tue, 11 Apr 1995 06:49:37 GMT

[email-masked] (Ray Spinhirne) writes:
 
>Bob Powell ([email-masked]) wrote:
>: Given a lathe big enough to mount them, what's involved in "turning"
>: disc brake rotors?  Workholding tips?  Smoothness of finish surface
>: required?  Any non-obvious details?

>: I think a real "brake lathe" uses a mandrel.  By default I'd try
>: clamping each side in turn against a faceplate, probably using a
>: drawbar through the spindle since the rotor covers the entire slotted
>: area of the faceplate.  Alternately I could make a mandrel and spin it
>: between centers.  Either way the swing-over-carriage is not enough to
>: cut both sides in one mounting.

>: Also assuming it's important to get it right the first time, to remove
>: as little material as possible, while cutting deep enough to leave a
>: true surface as well as a finish surface.  So suggestion of feed, speed
>: and cutter material also appreciated.

>: thanks

>: Bob

>I did the 10 inch disks from my daughter's car on my 11 inch lathe.
>The particular model was easier than some because the disk
>were not part of the hub but simple bolted between the wheels and
>the hub.  I make a mandrel by press fitting a disk larger than the
>center of the brake disk on  to a 1.5 inch bar.  ( I used the low tech
>"hit it with the 8lb hammer" method to press the mandrel disk onto the
>bar.)  After the mandrel was together it was turned true between centers
>and the the brake disk was attached.  I had a 1.25 tap and die so the
>I simple threaded the end of the bar and made a nut to hold the disk.
>You may have to use a different arrangement depending upon the configuration
>of the brake disk.

>There was not room enough to move the carriage
>under the disk drum.  However I could turn both sides (one at a time)
>by removing the mandrel and placing the carriage first on one side
>and then the other.  The specs for this disk called for max runout
>of .003 (not too hard) but only .0008 for thickness change.  I don't
>think I could get this precise if I had to remount for the second
>cut.  

>The disk material itself is some sort of cast and it turns very
>easyly.  I took light cuts and finished up with the slowest feed available.
>Even so this produced a surface like a phonograph record and the
>brakes went "thump thump thump" as the pads moved (in or out ) radially
>on the drums for the first few stops.  I remembered later having
>seen something on one of those "fix you car yourself" TV shows that
>it was recommended to run a sander (or grinder?) lightly over the
>disk after turning to avoid this problem.  I don't think that with the
>very light cuts I made this was a real problem.  It actually took me
>longer to design and make the mandrel than it did to turn the disks.
> 
>Given that the dealer wanted $400 for the job and the replacement pads
>cost less that $100 for all 4 wheels I thing it was worth the weekend
>time I spent.  (Beside it was a good excuse to get my daughter to
>help me in the shop.)

>I think that if you are reasonably careful you could do a safe job 
>yourself.  The typical person I see working in the brake section of
>a repair shop doesn't exactly inspire confidence in having it
>done by a "professional".  A repair manual on the particular
>model involved would be a good investment.

>Have fun and good luck.

>Ray Spinhirne
>Director Computer Services
>St. Edward's University

The commercial "lathes" for disk truning has dual tools and an
even cut is made on both sides of the disc at the same time.  The
disc itself is mounted on a mandril with a slight taper as a forcing
cone if this is of any help....
-- 
    - Barry Workman

---

From: [email-masked] (Bob Powell)
Date: 12 Apr 1995 06:27:58 GMT

Thanks for the various follow-ups to my original query
about turning brake rotors on a home-shop lathe.

As for my subsequent experience ... I quickly determined the
rotors were already too thin to turn.  But more significantly,
it was apparent that the wheel hubs were warped and/or out
of round, and that may have caused some of the brake problems.

As an experiment, I made a pair of cone mandrels and chucked
the wheel hubs on the bearing cupss.  Was then able to indicate
on the hub flange and verify a slight warp.  Successfully
cut about .003" from both sides of the hub flange, and would
have put it all back together (with new rotors) but then 
decided the hubs were too trashed to be worth wasting the
$$ for new rotors and wheel bearings on.  Still, had a lot
of fun doing the setup for that, new territory for me.

BTW, pretty sure the hubs as well as the rotors were simple,
if high quality, cast iron.

Bob

---

From: [email-masked] (Lara J. Burnett)
Date: 11 Apr 1995 10:43:47 GMT

Ray Spinhirne ([email-masked]) wrote:
: Bob Powell ([email-masked]) wrote:
: : Given a lathe big enough to mount them, what's involved in "turning"
: : disc brake rotors?  Workholding tips?  Smoothness of finish surface
: : required?  Any non-obvious details?

: : I think a real "brake lathe" uses a mandrel.  By default I'd try
: : clamping each side in turn against a faceplate, probably using a
: : drawbar through the spindle since the rotor covers the entire slotted
: : area of the faceplate.  Alternately I could make a mandrel and spin it
: : between centers.  Either way the swing-over-carriage is not enough to
: : cut both sides in one mounting.

: : Also assuming it's important to get it right the first time, to remove
: : as little material as possible, while cutting deep enough to leave a
: : true surface as well as a finish surface.  So suggestion of feed, speed
: : and cutter material also appreciated.
I work in a machine shop and make the Amco brake lathe centering cones. 
Be sure to look on the disk and notice the minimum thickness on the 
rotor. You can cut off more than this but it could prove unsafe. You need 
a support on the back (a cup and something the angle of the outer race 
and one the same angle for the inner race and a rod threaded with a nut 
Amco uses 1 inch and an arbor spacer and a spring. If you like e-mail me 
and I can fax you a copy of how the parts fit together.