Turning Rotors: a case study...

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snip
Actually, he is talking about himself. He does not understand the physical relationship between runout (I believe this is the name - I am no mechanic) and perpendicularity of axis and contact surfaces - he concentrated on irrelevant coaxiality!:

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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.
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karl wrote:

Another factor is also how the disk is actually cut. Not only is each axis perpendicular to each other, but the surfaces for the pads to be parallel. A poor cutting bit, taking off too much at a time, or a crossfeed that is too high can cause these kinds of problems.
I just went through all of this with Chrysler, as the Jeep Cherokee's from 2000-2003 are notorious for premature warping of the front rotors (TSB issued). They (Chrysler) went through the trouble of changing the calipers and pads to another design in accordance with the TSB recommendations, but only cut the rotors (which I argued against), and they were warped again within 4K miles. I gave up arguing with them and purchased and installed a set of high performance rotors and pads.
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doug wrote:

diffusion, the migration of carbon atoms in the iron matrix, happens well below red heat. but /significant/ diffusion, recrystallization, graphite flake/nodule growth, or other phase changes, doesn't. and if you're trying to argue that a brake disk is martensitic, you need to think again.
if your gm disk warps in 15k, you need to consider other factors. for honda, elastic distortion caused by incorrect wheel lug torquing has a huge influence. but if it's the disk alone, things like bad post-casting heat treatments, uneven material thickness, etc. can influence whether a disk stays true at high temperatures. the most likely item is cutting corners on heat treatment and reducing heat soak time.
my money's on incorrect lug torquing.

i was a vehicle mechanic for 5 years and also have a materials degree. is that good enough?
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Apparently not - it hasn't helped you to do the job correctly. Like I said before, there's one in every group. Although Dave Kelsen may once again take issue with that.
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doug wrote:

so why don't you make a technical rebuttal? share your superior knowledge.
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It took you 12 days to come up with your lame-ass text book reply. Yet you still don't get it - you're trying to argue a point that many people have already demonstrated to you that you are wrong about. I can't be bothered to repeat their - and my - valid examples and explanations. Perhaps if you took the time to review the threads in this post and you might begin to understand what I mean. I doubt it, but you never know. As I said, you ARE the one.

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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."
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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.
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karl wrote:

you need to read around a bit more. if the disk plane is not exactly perpendicular to the rotation axis, on a floating [single piston] caliper, you have pulsing in the hydraulics because of momentum differences due to the mass of the caliper vs. the piston. with a fixed caliper and 2 [or 4 or 6] pistons, the mass on each side is the same and there's little net effect.
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=============================================================TOPIC: Turning Rotors: a case study... http://groups.google.com/group/alt.autos.honda/browse_thread/thread/19594af6aa37ae67 =============================================================

In two messages you wrote that "centering" the disks is important, and if this is not done properly it will give "inconsistent results." This is what I was responding to. Can you read? The relevant excerpts are right here. (I had written "coaxiality" when I meant centricity. I have corrected this already.)
Correcting you, I wrote, "Relevant is that the machined surface and the mounting surface are perpendicular to the axis." Can you read? You do not "need to read around a bit more," it's right here.
Pulsing "because of momentum differences due to the mass of the caliper vs. the piston," and "the mass on each side is the same and there's little net effect." Rubbish!
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karl wrote: <snip>

really? why? i'd love to see your explanation.
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karl wrote: <snip>

jim beam wrote:

Karl - he's not worth the time or effort to try and discuss this with. But I think you already know this ;-)
doug
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Stephen H wrote:

I'm confused. You just told us that you took 0.004" of material off the rotor, but your initial and final thicknesses only vary by 0.001"?
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Because the rotor wasn't true-- the outside edge was thinner than the inside edge, so only 001 was removed from the outside.
--
Stephen W. Hansen
ASE Certified Master Automobile Technician
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Stephen H wrote:

That's what I suspected, i.e., a lack of parallelism. Thanks for confirming it.
Eric
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Thanks for "confirming" suspicion of lack of parallelism are superfluous - Stephen H had told us of it:

snip
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Stephen H wrote:

anyway I installed new front rotors on my taurus today, turning the old ones was $20 ea, new ones was $27...guess I could've saved $14........ Oh well, maybe next time..
A couple years ago my brother asked me to fix his brakes, said they just quit workin'. The rotors were wore down through the vents, mustaben steel on steel for at least 10,000 miles..... wonder if they were past the minimum thickness....
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User wrote:

Not worth the $14 "savings" IMO.
John
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Seen that before; and had customers drive out of the store like that too!
As for the Taurus, You did just fine Ford Taurus factory rotors were crap. Wouldn't hold a clean cut for 2 months. Almost all Taurus brake jobs would have pulsation when they came in and most cutting would help, but on a few they would come back. we learned new rotors were the only real solution. Even on My and my Mom's Taurus.
--
Stephen W. Hansen
ASE Certified Master Automobile Technician
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