where to have rotors turned?

[John Brandt]

I have a bit of warpage, or at least glazing on my nearly new Zimmerman crossdrilled rotors for my Carrera 3.2

I am a little reluctant to take them to NAPA. What sort of place does this, and how much should I expect to pay?

[ken_xman]

you cant turn cross drilled rotors

[John Brandt]

thanks but i don't belive that

[RickM]

If the holes are chamfered you'll probably find someone to do it.

But some believe that turning any worn rotor is poor practice, let alone a warped one. Additionally, a warped rotor warped for a reason, they'lll most like warp again. Then there's the debate on the compromised integrity of cross drilled rotoers in the first place....but that's another thread.

Good luck.

[ken_xman]

hello. I dont sell rotors for a living.
Call your local auto parts store and ask them.

[John Brandt]

i honestly belive that they're heatglazed and not actually warped.

[Rot 911]

Nothing special about a 911 Zimmerman rotor. Zimmerman sells cross drilled rotors for a number of vehicles including my Jeep. Any reputable machine shop can turn them for you, you don't need anyone special. I'm with RickM on the problems with cross drilled rotors, but like he said, that is another thread.

[rbcsaver]

John,
See the Carroll Smith white paper on the fallicy of "warped" rotors. Are you using Padgid pads by any chance? Before going through a hastle and unneccessry expense, try switching to a stock pad to wipe them clean. regards,
Steve

[John Brandt]

not Pagid.

I'm familiar with the Smith theory and thats why i think that the rotors are glazed.
These are a ferodo ds2500 The same thing happened when I used a KFP magnum gold pad. thats when I changed my rotors. My break-in prodedure may be too intense.

I just rebuilt my calipers and have everything apart anyway so I figured i would have them lightly machined

[Grady Clay]

Guys,

In Denver we used to have a machine shop that had a “Blanchard?” grinding machine on a huge lathe that would grind both sides of the rotor at once. The advantage of grinding is you can take very light passes and remove minimal material and get a rotor perfectly straight. This device centered on the front bearings and was driven by an external “dog.” For the rear rotors, they made a custom hub for us out of a 911 front hub. I just looked through the Yellow Pages and didn’t recognize the name.

With grooved or cross-drilled rotors, you should always chamfer the edges.

I have even had the front hub-rotor assembly dynamically balanced (had minimal imbalance.) With new Factory Porsche parts this isn’t necessary.

I think that slightly used (seasoned) rotors done this way are even better than new. That is unusual with Porsche parts.

Best,
Grady

[911pcars]

It's been awhile since I've had to turn rotors so I don't know about the equipment used today.

Let's hope the current machines don't use a lathe tool (carbide tip usually). Rotors and drums should be ground with a stone as in Grady's post. A tool bit is not sharp enough to cut heat-hardened areas of a rotor. It'll merely skip over them. A grinding stone will remove everything and produce a nice, discontinuous friction surface. And take off the minimal amount. If it's just a glazed surface, try rubbing on it with an oscillating block sander.

You can verify they're warped (or not) by measuring them on the car with a dial indicator.

Sherwood

[Noel]

Does anyone know where to obtain a copy of the Carrol Smith whote paper? On line perhaps?

[Bill Verburg]

I agree that turning on a lath is not good

Ground or honed w/ something like 150 grit or equivalent bead hone.

Smiths article is available at Stoptecs site

Contrary to Mr. Smith rotors do warp.

[Noel]

Thanks Bill.

[Oldporsche]

Grady, et al.

Blanchard grinding is sort of like an end mill operation, except the grinding stone is in place of the actual milling bit.

I have "turned" the rotors on a lathe, (mine) using a tool post grinder. This is a separate grinder set on the cross feed tool post table of the lathe. You can use it to take very small increments of metal off the face of the rotor.

You could use a blanchard grinder to do a rotor. It would be a very big one. I believe that you would have to very accurately set it into the center as the blanchard grinders I've seen had magnetic tables. You flip the rotor upside down and do the other side.

The tool post grinder with using a mandrel set up to center on the wheel bearings would be my preference. It would be much easier to see what you were doing.

Either way would result in a very good surface to imbed the faces of the brake pucks.

I really don't like to turn the surface of rotors. The "hot spots" will cause the tool bit to skip. To overcome this the average brake turner will try to take off more material. This sometimes works. It can take too much material too. (Don't ask) The problem is that your local auto parts store "technician" does the work so cheaply, it hard to ignore them. This combined with the China manufactured brake parts, assumes that it shouldn't cost much to do a brake job.

If you have a set of rotors and would like to reface them, consult a machine shop. The difference in their charges and new rotors for your Porsche might be helpful in your decision.

Rotors can warp.

Rotors can have a litte warp in them as long as the surfaces of the rotor are parallel with each other. (You may get a little chatter) When the rotor faces are not parallel and portions of the rotor are a little "thicker" in spots. you will get a pulsation in the brake pedal.

Best of luck,
David Duffield

[Grady Clay]

David,

The tool I remember was mounted on a lathe bed, the lathe was something like 36”swing and 24” between the ways – BIG. It had a fixed center in the head-stock plate with a piece of spring steel to turn the hub and a live-center in the tail stock. It had two cup grinding wheels to surface both sides simultaneously. The wheels were powered by two large electric motors and would oscillate in-and-out with adjustable stroke. The lathe turned at very low speed. I think this thing was designed to grind huge rotors, say 24” and larger.

Needless to say we measured everything after grinding. That tool would produce axial run-out of less than 0.002 mm (yes mm) and variation in thickness of the order of the surface finish, beyond our measuring ability with 0.001 mm micrometers.

I’ll see if that tool is still in service here.


These tools are all over the US. Go find one.

Best,
Grady

[911pcars]

FWIW,
Honda used to (still does?) grind their rotors right on the car. Their rotor grinding machine bolts up to the wheel assy. Takes into consideration hub runout and any mounting variations.

Sherwood

[rbcsaver]

http://www.stoptech.com/whitepapers/warped_rotors_myth.htm

[radcon]

Rotors can definitely warp. I don't know what this guy is talking about, because I've seen rotors that were straight and true when they were new and after extensive use, they were definitely warped, as evidenced by the checking the runout of the rotor. Measurements before and after don't lie.

[John Brandt]

mine haven't seen extensive use. Just two track events and ~1000 miles of street driving. I emded up last night just re-mounting a set of Metalmaster pads that i had to see if they would scrape off any glazing.

need to torque everything down and bleed i'll let yoiu guys know what happens