At 12:00 PM 8/14/01 -0400, lwdent wrote:
>OK, now that we have all pooled our ignorance, here is the straight
>scoop on drilled rotors and turning same, right from Brake Materials and
>Products.
>
>No problem in turning (cutting, dressing) any drilled rotor. If the
>drillings have a champher, as some do, you will loose the champher.
>
>To the hot dog machinist that wanted to do it in his lathe, let me
>stress that this cannot be done with proper accuracy.
>
>A brake lathe cuts both sides of the disk at once so that forces are
>equal on both sides and the disk will not flex and cause a taper. This
>will happen if you cut one side, the cut the other. The disk will be
>thinner on the outside with a taper on BOTH surfaces.
>
>I owned an automobile machine shop for many years. Just revivified all
>that with Scott Bethke at Brake Materials.
>
>Larry Dent
Larry, I'm the "ignorant hot dog machinist" that you refer to. I hope we
never meet on a track, as I find your attitude somewhat lacking in the
basics of politeness. I take offence when people - you in this case -
offend me.
To your points: an earlier poster had said something to the effect that
you needed a CNC lathe to cut a drilled disc, because the cutting tool
couldn't deal with the holes. I replied that you could do an interrupted
cut on a drilled brake rotor just fine, the holes weren't an issue. I also
said that I wouldn't actually do it by preference, and quoted a few
reasons. I don't, as a rule that I've never broken, run turned rotors on
anything high performance.
You're right - a brake lathe does cut both sides of the disc at the same
time. It does compensate for load to eliminate taper. For that reason and
others the right way to surface brake discs is to blanchard grind
them. Probably no one but the manufacturers do. You presume that I can't
make a tool holder to cut both sides of the disc at once in my lathe - I
assure you that I could, if I wanted to. I don't, for reasons stated
above. You presume that I couldn't fixture the disc on a face plate, which
would equalize the cutting forces by supporting the disc from behind rather
than from the center. You presume that I wouldn't hold the disc from the
outside, or whatever - that I couldn't fixture the disc to eliminate
taper. I think that I could cut a disc to within a thousandth of an inch
of taper over the two inches of rotor surface, and that closer than that
wouldn't make a damn bit of difference. The disc probably wears .001" in
the first race session and I bet that the pads in the calipers aren't
anywhere near that close to parallel. Finally, you presume that I wouldn't
finish with a tool post grinder, which when sparking out puts very
little - effectively no - load on the work piece and would completely
eliminate the taper issue that you think I'll have. If you were presuming
that I would hold the disc by the center, the taper would be such that the
disc would be thicker on the outer diameter, BTW - the opposite of your
prediction. I wonder if you ever did this on a engine lathe and measured
the difference? I think that a 9" dia. piece of 3/8" thick cast iron is
probably stiff enough to take a .005" cut without significant flexure.
You're right that a brake lathe is a good way to go, if you're going to do
this job. You're wrong that it's the only way to go, or that it might be
the best way to go. Brake lathes were invented to make turning discs quick
and fool-proof for a garage - not to replace a machine shop.
Brian
Brian Evans
Director, Canada
MCI Wholesale Internet Services
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