snip

In a number of messages (maybe in all of his "cutting
rotor" messages) jim beam emphasized the importance of
centering the disks when their surfaces are being
machined. This is what I was replying to - centering,
within limits, is irrelevant, but I used the wrong term
"coaxiality." Once again, important is the
"perpendicularity of axes and contact surfaces," and
anything that throws this off will cause problems.

In fact, there is the possibility that jim beam, too, is
talking about this when he writes about the importance of
centering: in a message at another list he wrote of the
importance of cleaning the mounting surfaces of the lathe
and of the disk.

snip

and

snip

Why bring up coaxiality, or lack thereof? It is
irrelevant as long as the contact surface with the pad
gets machined.

Relevant is that the machined surface and the mounting
surface are perpendicular to the axis, and it is therefor
important that the mounting surfaces are clean. This is
the area you have to concentrate on, not centering. No
wonder why you got "inconsistent results."

sorry - unless this disk has been operated well into red heat, you're
not going to get much change in microstructure.  you can however have
problems with a bad disk if it was not heat treated properly after
casting.  when operated hot, /that/ disk will distort.  more likely is
that there are simply different cooling rates on different parts of the
disk leading to local temporary distortion.  that shouldn't happen on a
disk of sufficient thickness and whose internal vanes have not lost too
much material through rust.

my experience is that the disk cutting process is far from perfect -
those cutting machines see heavy usage, and you can mount the same disk
up a dozen times and get 13 different centers.

A rotor that is "warped", not simply scored, is a very poor candidate for
resurfacing. The metal's properties have been changed by the level of heat
it has been exposed to, causing a permanent change in its structure. After
machining, once heat is applied, it will revert to its prior condition.

I went through this issue with my wife's Malibu, arguing with the Chevrolet
service manager every step of the way. They resurfaced the rotors 3 times to
cure pedal shudder, but it never worked because the rotors were warped. In
this case. it was a design flaw by GM - they made them too thin in the first
place, and paired them up with lousy pads. When I finally gave up dealing
with them and did the job myself, the rotors were ~0.003" thicker than the
bare minimum, suggesting that they were able to shave off a minimal amount
of metal each time they turned them. But so what? The rotors were junk the
first time they warped.

OTOH, a rotor that is merely scored can be effectively resurfaced and
returned to service, providing that not too much metal is removed. I've done
this on cars when the pad wore too far and "kissed" ther rotor. A competent
technician can do this.

Doug

Personally I have had very poor results in dealing with warped rotors
through turning them.  On my '03 Accord the rotors warped enough to
cause brake shudder.  Turning them helped for about 50 miles, when they
warped again.  New Brembo rotors cured the problem completely.   20,000
miles later the brakes are still working perfectly.

John

With all of the debate about turning rotors I thought I'd do a bit of
measuring and documenting while doing a brake job. Here is my results.

The car 1987(?) Honda accord; poor shape, 200,000K no brakes due to a
ruptured line. This soaked the pads and as per MAP standards required new
pads. Now disregarding the brake line (and the springs off in the right read
drum that he couldn't afford to repair) the car would appear as a prime
candidate for a DIY brake job in the front driveway. Both rotors had no
major grooves, very small ridge> I could picture this as a easy pad slap.

Now ASE standards have you measure several different angles to determine if
a re-cut is needed. We concentrate on the warpage factor. I don't feel like
buying a few hundred dollars more of measuring tools to tell me what one
pass on a lathe will tell me.

The right front rotor measured at .747 to start
   I took one pass at .002 on each side
   Final measurement was .742

The left front rotor measured at .697 on the outside edge and .703 at the
inside edge to start
   I took one pass at .002 on each side. the start of the cut was heavy, I
thought I'd have to do a second cut. Often we will see the O/S edge a
wee bit thinner than the inside edge, but when I returned to the lath one
cut was all that was needed.
   Final measurement was .696

I forgot to write it down, but discard was in the .660 range.

Most people who do brakes at home won't measure the rotors and it was
interesting to see the big difference in specs from the left to the right to
start. We thought there would be at least one new rotor needed, but specs
are specs.

You can see by the measurements that the lath action took off very little
metal, not enough to make a difference on heat dissipation, but the little
work guaranteed me that this car (the POS it was) had decent front brakes
when he left.
This is why I turn rotors every time. In the big picture, more metal is lost
in the time between brake jobs by the pads than on a simple clean up on a
lath.