Skimming a brake disk.

[janvanruth]

what kind of lathe do you have?

 

[Skyboy]

Lood,

You do not need a mandrel. I have cut 100's of brake rotors on a conventional engine lathe, clamped in a 4-jaw chuck. Cut the car spindle side first, using the spindle surface to ensure zero radial runout. If necessary, you can skim the surface that fits against the spindle to ensure zero runout. For the second face, true the rotor with the indicator on the freshly machined side. .001" thickness runout or less is easy to achieve. I've been able to save some pretty bad rotors because a lathe offers more options to correct problems than a brake lathe.
I usually use hand ground carbide tools but if you have inserts for cast iron, go for it. It leaves a great finish.

The reason I started cutting my own rotors was because 40 yrs ago I took a set of rotors off my corvette to have them resurfaced at a "professional" auto shop. All four were ruined. Chatter, more material removed from one face than the other (convex face), undersized, not deburred, etc. You name it. The guy machining those rotors on a brake lathe is NOT a machinist. He is a person trained to put the part in the machine and turn it on. Keep that in mind.

 

[Bakkskive]

What skyboy said, but i try to use my regular chuck and try to face both sides in one setup so i dont need to indicate so much, a standard holder for the "front" and a thick boring bar set upside down for the "rear" with the spindle in reverse.

Or you can go your own way this is just what i found best in my shop with my tools and it works for me.

 

[bosephus]

my thoughts ,.. the rotors was already faced once , the chances of there being much left to face off are slim . but if there is enough left to face off you still have relatively thin rotors compared to when new ... thin rotors are always a problem , but its not a wear problem like most assume , it is purely a heat problem .

the biggest cause for complaints with brake pulsations with turned components is very rarely from the rather poor job of machining most of them get .
after all calipers float and will absorb a lot more run out then most expect .
nearly always the cause is from the rotor being heated up during brake pad break in .. or if the person who did the brakes did not break in and seat the brakes properly
they warp from overheating during normal use .

it takes a lot of heat to properly seat in brakes .. or if not seated properly they create more heat in normal use ... heat is the death of turned rotors .

 

[bmw rider]

my thoughts ,.. the rotors was already faced once , the chances of there being much left to face off are slim . but if there is enough left to face off you still have relatively thin rotors compared to when new ... thin rotors are always a problem , but its not a wear problem like most assume , it is purely a heat problem .

the biggest cause for complaints with brake pulsations with turned components is very rarely from the rather poor job of machining most of them get .
after all calipers float and will absorb a lot more run out then most expect .
nearly always the cause is from the rotor being heated up during brake pad break in .. or if the person who did the brakes did not break in and seat the brakes properly
they warp from overheating during normal use .

it takes a lot of heat to properly seat in brakes .. or if not seated properly they create more heat in normal use ... heat is the death of turned rotors .

Brake pulsation is never due to run out, it is always due to thickness variation. Excessive run out is also bad, but in a different way; it causes low pedal on initial application. Yes the caliper design will allow for some run out whether it is a floating mount set up or opposing pistons. But with excess run out when the brakes are released the wobble in the rotor pushes the pads apart further so there is space between them and the rotor. When the brakes are next applied, that additional clearance must be taken up first before any pad pressure can happen. In normal use it is the minute deflection of the square cut o-ring that draws the piston back so very slightly to release the pressure of the pad against the rotor. The pads will always drag very lightly on the rotor in normal conditions. It takes very little additional clearance to cause low pedal due to the large size of the caliper piston compared to the small diameter of the master cylinder.

Thickness variation causes pulsing because there are essentially thick parts of the rotor where the pads grab, then a thin spot where the pressure is reduced. If you apply the brakes hard enough, the friction may remain relatively constant as far as the braking action, but then you will feel the pulsing in the pedal as the pads are forced in and out on the rotor thus causing the caliper piston to oscillate and then transferring that action through to the master cylinder and the pedal.

Modern rotors seldom have much extra material for machining once let alone twice as you noted. Too often even a properly machined rotor that is still within the minimum thickness will still warp as the reduced mass decreases it's heat absorption capacity. Of further note on the minimum thickness, it is the minimum wear thickness, not the minimum machining thickness. There must still be sufficient material after machining to allow for wear in use so that the minimum thickness is not reached. This is a common error made by many, if you machine it to the minimum, it is now worn out without ever having been reused. Typically at least an additional .030" - .040" of thickness should be left for wear.

As for seating in of pads, that is less important in average cars with common brake materials. Some exotic pad and rotor materials used in performance application and or racing do need to be seated in properly. This process causes a transfer of friction material from the pad and embeds it to the rotor surface creating far greater friction properties.

 

[Lood]

@janvanruth: I have a Graziano SAG180 lathe

Thanks a lot for all the replies and really good advice! I like the idea of gauging the disk while still mounted on the car and my initial way of thinking was to mount the disk in the lathe, measure the runout, turn it true, set it up gauging against the true side and the skimming the opposite side. I'm sure this should work.
I took the disk to an "Engineering Works", not a shop that specializes in automotive brake disks. I'm very convinced that the guy who did the job didn't go through any trouble to measure much, but instead just turned the disk down until there were no sign of the original wear left.

I'll try it and give some feedback here on how it turned out. Thanks again for all the ideas and advice - much appreciated!

 

[janvanruth]

your graciano should be sturdy enough
make shure you take the biggest carbide tool you have and tighten everything well as chatter is likely to form
indexing in will not be easy but is essential for a good outcome

 

[Skyboy]

What a coincidence Lood, I have a Graziano Sag 14. Great machine and mine has cut a few brake rotors and drums.

FWIW, the min thickness on a brake rotor should be observed to prevent the piston(s) from extending to far out of the caliper bore. That is the only reason for that dimension.

 

[Superburban]

FWIW, the min thickness on a brake rotor should be observed to prevent the piston(s) from extending to far out of the caliper bore. That is the only reason for that dimension.

So if I make a spacer to fit between the inboard pad and caliper, I could run the rotors until the cooling fins are exposed.

I have seen different brand rotors for the same vehicle, have different marked min thicknesses. It is a fact that too thin rotors have a better chance of warping, then thicker rotors.

 

[george wilson]

Mt former journeyman messed up some discs trying to turn them in his lathe. And,he knows very well how to use a lathe. Turning the discs is not that expensive,and I recommend you get it done to avoid a lot of trouble on your own.