"jim>> Anyway my point was to show how little metal comes off during a lathe
job.

My point is different;
       We do it for free with a brake job
       It takes about 10 minutes to do total

       We have never had an issue with machining them wrong. Its to simple.
But and idiot could play with the settings on the lath and screw it up...
Perhaps the people who turned your rotors in the past were idiots?

I agree with pressed on rotors... What a wonderful idea someone had...

       Take care,

i have two points:

1. it ain't cheap enough to be worth bothering with [unless it's a disk
that requires complete disassembly of the hub] AND

2. the results frequently aren't good enough to be worth the trouble.
in fact it often /causes/ problems.

i'd much rather live with a slightly scored disk than one that's been
machined wrong and is now ruined.

You may have had problems, and in a perfect world everyone would replace
parts with new when worn with high quality stuff, but many want it done
cheap.

Anyway my point was to show how little metal comes off during a lathe job.

ok, so now let's /cut/ a disk with a piece of dirt under the rotor.  now
we have a disk that is perfectly planar with respect to the dirt, not
the mounting.  and that causes brake judder.

well, i've never had a problem with a new disk.  i've had repeated
problems with skimmed disks.  you figure it out.  labor and a $5k
machine to futz about with a part that can be replaced with a flawless
new disk for $50-$60 in about 10 minutes?  forget it.

no kidding.

If this were the only factor in the performance of brakes than I would be
concerned. But by just removing the tire and rotor you can effect the
bearing to rotor play if a small piece of dirt were to fall between them. In
fact, how do you know your "new Honda replacement rotor is true to your old
wheel bearing?. We do have a 5000$ on car lathe, that is suppose to be "the
most accurate way to turn the brakes" but it broke again. (600$ was the last
repair bill) and having a brother-in law that is a machinist, we can see no
great advantage to turning it on the car of off, as long as the turn job is
true.

Wear of the rotor and warpage (runout) are the two biggest factors of a
brake job.

what does measuring disk thickness tell you about how accurately the
disk was kept axial with the bearing?

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.