Blanchard grinding the rotors

[dmcproud]

Is anyone familiar with the term "Blanchard grinding" the brake rotors.

A racing brake specialist that I know recommends NOT to have the rotors turned and to have them ground on a blanchard grinder.

Is this a common service and widely available??

He assures me that I would not be happy with the rotors turned.

BTW, he is very partial to Ferrodo brake pad compounds.

thanks

D

[NoLift911]

Not familiar with that - but unless you have pad deposits (reason behind doing this) on the rotor I would not turn nor grind. If the rotors are warped they should be replaced as removing more material will only exacerbate the problem down the road.

It is an extremely short term semi-fix. Unless these are new rotors also and just need to be touched up (no heat cycles yet).

-Jeff

[RickM]

The tolerances and uniformity are very good with Blanchard grinding. It's also faster than other forms of cutting. I believe this method is used used by the manufacturers as well.

Here's a rotor from my big brake conversion. I'm still working on the rotors

[NY65912]

Blanchard rotary surface grinding grinds down material on a rotary platen that is perfectly flat, with an abrasive wheel with coolant added. A slower process but cooler with no warpage. Flywheels are usually ground, not cut.

Cutting discs uses an actual "tool" either HSS (high speed steel or carbide on a lathe to cut (chip) away the unwanded metal.
Creates heat and is ok for street... solid discs IMHO.

[Grady Clay]

No, this isn’t common but very worthwhile. In the late ‘60s I found a machine shop with that capability. We devised fixtures to grind the rear rotors. The fronts are easy when mounted on the nabe (oops hubs).

Blanchard grinding both sides with one setting gives the rotors a far better setting (runout & thickness) that the Factory could ever produce. Of course this isn’t something that was ever done for production cars, including Porsche. The necessary tooling and set-up isn’t inexpensive. However, you can make the rotors better than the Factory could new.

Is it worth the effort? Well I think so. Not only does it give a more precise rotor but it allows the brakes to work at peak performance. Our cost was about 2X commercial turning. Not a big deal for perfect Porsche brakes. This process takes very little material off the rotor and can be repeatedly done to keep brakes near perfect.

Every track Porsche with fixed rotors should use this process. Even floating rotors benefit from Blanchard grinding. That attention to detail allows current 997 GT3 Cup cars to win races.

The similar effort is even more important with 356 drum brakes. Not only is careful machining necessary but you need to carefully torque the wheel on the drum for machining and maintain its orientation.

Best,
Grady

[jeff91C2T]

I picked up a set of used rotors (cheap) for my 965. They were pretty rough...ie deep groves and not true. Howerever no cracks from the holes. There's a local shop that turns cross drilled rotors reqularly so I thought I'd give it a try.

They came out looking new and I haven't had any issues. It's been a year w/ lots of autoX and some DE's.

[911pcars]

If resurfaced at all, rotors should be ground using a stone, not a cutting tool. Same with flywheels. I believe current rotor/drum refinishing equipment uses a stone. A cutting tool (usually carbide) will merely skip over any hard spots developed through localized heating/cooling cycles. In addition, unless the surface is finished off by random sanding, the resulting pattern is like a vinyl record groove rather than a smooth, randomly smooth surface.

On a Blanchard grinder, one surface is mounted onto the table and that mounting surface is assumed to be parallel to the surface to be ground. That would be the opposite friction surface unless there was some way of indexing the rotor so the friction surface is ground parallel with the mounting surface. In the case of new rotors, this is less of a problem. On a worn rotor, which side is considered true? In the case of floating-type rotors, it should be mounted on a hub that is chucked up in a lathe-like machine, then ground while spun.

Awhile back, some manufacturers were quite sensitve to customer complaints of pedal pulsations due to warped or unevenly worn/ground rotors. Some had on-the-car grinders so the existing hub was taken into consideration when regrinding the mounted rotor. Not sure what dealers do now; probably recommend replacement rather than regrinding.

Sherwood

[jeff91C2T]

Grinding certianly makes more sense then turning for rotors. At the time I had these done I was more interested how the holes would effect the process. I couldn't see any difference (for my untrained eye). But getting over a $1K worth of rotors for $165 sure was worth it!

[efhughes3]

If the rotors were really worn, wouldn't machining of either sort compromise the heat dispersing abilitiy of the rotors?

[Zeke]

Originally posted by 911pcars
Not sure what dealers do now; probably recommend replacement rather than regrinding.

Sherwood

That's pretty much how it goes around here.

I will not ever be able to forget the statement on one of the brake web sites that we've all seen at one time or another that the author "had never seen a warped rotor." All pulsation was due to uneven pad deposits.

I believe some of that, and I'll bet the grinding helps immensely. But, never seen a warped rotor? Come on, some must come out of spec laterally.

Now get this, when I got my car, the first thing I did was go thru the brakes. New M/C, new flex lines, pads and kits all around. I thought just to drive it, I'll use the old rusty rotors. So, I gets my belt sander out of my work truck and mount the rotors on a spare hub/strut in the vise. I sanded those puppies clean and flat(?) with some 50 grit. Run out was about .003 with a dial indicator. I bought some ATE rotors from a fellow Pelican along with pads and calipers for my 914 (it had 911 brakes on 4 corners) and I thought I'll slap all that on as soon as I get to doing any serious driving.

That was about 3 track days and a few thousand street miles with some Mulholland runs thrown in. I've got brakes that will lock the wheels with 205/50 stickies and not that much pedal pressure. Modulation is good, too. The ATE's and company sit waiting for their day. Go figure.

It's the ultimate hack job, that's for sure. But, my pattern on the rotors was kinda spiraling outward. That wore off long ago and the only reason to throw them out now is when they reach min thickness.

I gotta say, don't try this at home folks, YMMV.

[Dennis Kalma]

Couple of things. First of all, Porsche cross drilled rotors are most definitely ground....witness the attached photo from my Turbo brake upgrade...cross hatch grinding marks are visible.



Second, most likely the rotors just have pad material build up on it....I had a problem with vibration, was looking for Blanchard grinding, found a web site that suggested pad material was to blame and recommended burning it off. Did a bunch of high speed, sharp stops....problem went away.

Dennis

[Steve@Rennsport]

A few comments on this subject,.....


We always Blanchard grind rotors when the runout reaches .007 or more. This procedure is only for two-piece rotors where the hat can be separated from the rotor. Been doing this since 1976.

It makes little sense to grind an OEM rotor, its cheaper to replace them. I just grind real racing rotors (Brembo, AP, Alcon) that are more expensive and last a long longer,......

Full-floating rotors don't warp, they simply become grooved enough to make Blanchard grinding the best way to true the surfaces and regain full pad contact.

Most "warpage" complaints are related to brake pad deposit issues, but some early cars with 1-piece rotors can be overheated turning it into a "Belleville Washer",... In some cases, they can be trued a little bit, otherwise they are discarded for new ones to retain mass.

[dmcproud]

thanks to all of you, great info

I am re-thinking and may just get new rotors

D

[jeff91C2T]

Our host has front rotors for ~$420 (for the pair) for the 965. I was out the door for $30 (having two turned). Regarding the heat dispersion, ya…it’s not a move in the right direction. However relative to the rotor weight, the material lost is quite small. I can push the car hard and get a soft pedal w/ the stock system in 3 or 4 laps at our home track. Or I can drive “smarter” and save the car. Turns out smarter was improving my times more then hard braking.

For me it was more an economic decision then anything else. As an enthusiast on a budget…..turning (my) rotors worked out to be a viable alternative.

[Nine9six]

Originally posted by dmcproud
thanks to all of you, great info

I am re-thinking and may just get new rotors

D

For your application, new rotors would certainly be the easiest fix. For turbo rotors, which are $500+ a set, I would seek a machine shop who could blanchard grind the rotors.
One must be carefull when having a machine shop perform this operation, as the initial grind on one face of the rotor must be parallel to the mounting face. Then the other face of the rotor can be ground parallel to the initial grind.

[klaucke]

Can someone explain what exactly makes Blanchard grinding unique? Is a normal surface grinder used? I have a surface grinder that was given to me, but haven't yet found a use for it...

[Nine9six]

Without goint into a long dissertation...Its not that it is so unique...Its just a different method of grinding to achieve a specific surface finish requirement.

On a blanchard grinder, the part sits on a flat magnetic, round table that rotates. The head of the grinder has flat, cylindrical grind pads that also counter rotate to the table. This produces the cross hatched effect you see on new rotors, as depicted earlier on in this post.

Surface grinders have a magnetic table that moves in a back and forth motion, with a grinding wheel that makes small adjustable incremental steps across the surface of the part being gound.

Blanchard grinding is a much rougher grind process than surface grinding. It is desirable for finish requirements that are less than that obtained in surface grinding, while enabling parallelism requirements to approx. .001 - .0005"

Example: you could id grind the cylinder bores of your engine, but it is desireable to maintain a rougher x-hatch hone finish to retain oil along the cylinder walls for proper ring sealing.
The rougher finish is desireable on your rotors to assist brake pad bedding and a better pad to rotor bite.

[sammyg2]

Blanchard grinding is good, but overkill in this application.
Most shops just turn the rotors (machine) and this is not desirable because of the tendency of cast iron rotors to develop hard spots which result in run-out on the faces.

The most cost effective and reliable alternative is to use a tool post grinder on an engine lathe.
On parts that are too large to surface in-house we send them to Schafer grinding, but smaller parts (like rotors) that need a small amount of material removed get done with a tool post grinder. It is basically a small grinder that gets mounted on a lathe just like a regular cutting tool. We have many different stones, the use depends on the application.
The drawback to this is you have to make very small passes, usually around .0004" at a time. If you need to take alot of material off it would not be cost effective. In that case I would machine the rotor faces with a carbide insert, then make one or two clean-up passes with the tool post grinder.
Just about anyone with a lathe and a tool post grinder (even me ) can perform this operation and the cost should fall into the shop minimum charge. Even that might exceed the cost of new rotors tho...........

[RickM]

Is the tool post grinder self powered?

[Nine9six]

Originally posted by RickM
Is the tool post grinder self powered?

Yes,
Its a small motor with a grinding wheel. It gets mounted to a lathe tool post.
However I disagree with sammy's statement above, as you will not quickly achieve the surface finish desired, and it requires more time and setup operations than someone is likely to want to pay for. I am not saying it cant be done; I am saying that its not cost effective.

As an ex-journeyman machinist for 15 years, it is WAAAAAY easier and quicker to fab a locating jig for a blanchard grinder and grind the rotors, than it is to make a jig for the lathe, or perform the multiple setups required for the rotor faces to be parallel to each other within .001, and run true to the locating face of the rotor.
Also, you are working against gravity on a lathe, and with gravity on a blanchard grinder.
Nuff said.