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Brake Disk Machining

siksII
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siksII Grand Master

siksII

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I got my disks machined yesterday due to some vibration caused by some grooves worn into one disk.

Apart from the grooves on one of the disks they were in perfect condition without any shudder etc, and only a few months old.

They machined 0.75mm off one disk, and 0.77mm off the other disk. Since they have 2mm of life this means they have machined off approx 40% of the usable disk.

When you get your disks machined for whatever reason, what is the average amount of material machined off? Is this normal or am i justified in being really pissed off?

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https://www.sau.com.au/forums/topic/176881-brake-disk-machining/
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Beer Baron Grand Master

Beer Baron

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the will machine off whatever is the minimum in order to get a nice true surface. if your grooves were say .35mm thick (which is bugger all) then they would need to machine off double that.

siksII Grand Master

siksII

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the will machine off whatever is the minimum in order to get a nice true surface. if your grooves were say .35mm thick (which is bugger all) then they would need to machine off double that.

Yes of course they do, that is not what i want to know.

To be more specific, assuming there was no visible grooves, just the usual shudder that develops, how much is generally shaved off? And say you have to shave off 0.2mm off one side but only 0.1mm off the other side, would you just shave 0.2mm off each side to make the rotors even thickness, or shave off the minimum off each side and leave the rotors slightly different thicknesses?

B0oStEr Mentor

B0oStEr

Posted
Posted

Grooves don't cause a shudder by themselves.

It's most likely they had a bit of run out due to warping.

I had a pair of discs machined the other day that looked perfect but caused a shudder.

I lost 1mm thickness to clean up the run out but now the brakes pedal is smooth again.

grandmasterb Grand Master

grandmasterb

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Yes of course they do, that is not what i want to know.

To be more specific, assuming there was no visible grooves, just the usual shudder that develops, how much is generally shaved off? And say you have to shave off 0.2mm off one side but only 0.1mm off the other side, would you just shave 0.2mm off each side to make the rotors even thickness, or shave off the minimum off each side and leave the rotors slightly different thicknesses?

Hang on your first post said that you had grooves in your disks causing vibration hence why they were machined in the first place but that would generally be caused by warping as well...

If thats the case then they will generally machine each disk to the same drop so that you not only get even wear but also lifetime out of the set and not having to replace 1 or 2 at different times.

Did you talk to your mechanic before/after you picked them up and question why they were machined to the levels they were and also what tolerance would be left???

You can be lucky sometimes and have them machined exceeding the minimum thickness but its not really recommended and most places would probably refuse to do so.

If your brakes are set up correctly then you shouldn't have to worry about the rotor for a while so chances are your not even going to own the car by the time they have to be machined again due to wear and tear anyhow :domokun:

siksII Grand Master

siksII

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Hang on your first post said that you had grooves in your disks causing vibration hence why they were machined in the first place but that would generally be caused by warping as well...

If thats the case then they will generally machine each disk to the same drop so that you not only get even wear but also lifetime out of the set and not having to replace 1 or 2 at different times.

Did you talk to your mechanic before/after you picked them up and question why they were machined to the levels they were and also what tolerance would be left???

You can be lucky sometimes and have them machined exceeding the minimum thickness but its not really recommended and most places would probably refuse to do so.

If your brakes are set up correctly then you shouldn't have to worry about the rotor for a while so chances are your not even going to own the car by the time they have to be machined again due to wear and tear anyhow :domokun:

I should probably explain a bit more, they vibration is definitely only cause by the grooves, and not because the disk was warped/runout whatever.. The disks are only a few months old, while i was driving a few weeks ago the pad material tore off the backing plate after which the backing plate came in contact with the disk and created the grooves. As soon as it happened i stopped the car and got it towed home.

Before that incident, the disks were absolutely perfect, no vibration whatsoever, and as i said, nearly brand new.

When i dropped the disks off one was 29.95mm and the other which had the grooves was 29.74 as measured in the grooved area. When i picked them up the good one was 29.18 and the grooved one was 28.99.

So not only did they machine slightly more off the one that was perfect, they are different thicknesses. It seems they just picked approx 0.75mm out of their arse and decided to chop that much off both without any thought.

I specifically told them when i dropped them what happened, and made sure they understood that they should only take the absolute minimum off. When i picked them up i expressed my concerns on why so much had been taken off and they just like well thats how much was required which seems to me complete BS.

I notice the degradation in braking performance as the disk thins out and i go through brakes reasonably quick anyway so it is a concern for me ;)

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