best way to remove scoring from Corsa floating brake disk

henrywillms

henrywillms

Hero member
Location
Canada
Calling all machinist, amateur machinist and would be machinists....The rear brake on my Corsa stuck on last fall, by the time I felt it dragging (long story) and found a place to safely pull off the road it had locked right up. As a result the disk is scored. It looks like I will have enough thickness left to refinish it. It is a floating disk and I have the oversize buttons that are still holding tight. I read on the forum that it is better to have it surface ground than put on a lathe to ensure it runs true. I have a buddy with a large lathe with a large diameter faceplate that we can bolt it to (by removing the disk from the carrier). Would this lathe method achieve satisfactory results or is surface grinding the way to go? 
 
Helo Henry, rear brake locking up, been there done that, not a good experience. Make sure to find why it dragged and stuck so it won't happen again. On my SFC100 the return spring on the rear master cylinder broke and the piston was not returning = disc drag, lots of sparks, broken disc and a hitch hike home to get a trailer.
I think you will find it is difficult to machine the disc on a lathe, partly because of flex and partly due to its being cast iron, although your machinist may be able to deal with those issues. I have had discs surface ground with excellent results. A good brake shop ought to be able to do it.
Dennis
 
I am not sure of the reason for the lock up. In trying to release the lockup I pulled up on the brake lever and at  that point I think is when I broke the cast stub the spring attaches to and lever arm stops on, on the inside of the footrest casting. But maybe it was broken before and that was the culprit.  The heat was enough to melt the rubber boots on the caliper pistons but the pistons and seals are all clean, work smooth and no leaks. The return spring itself is not broken, and appears to have tension, will know when I reassemble.  The fluid brake fluid was clean and the master cylinder still seems to be working. I will do my best to flush out the M/C since there are no rebuild kits available. We got 5cm of snow and up to 10cm more expected today so I have some time........   
 
Back in the day we used to use lathes to machine cast disc's however it was a dedicated brake lathe which would machine both sides at once therefore ensuring true and concentric and also had special tips for the job. Once you start machining the material away then you  cant put it back if you go to thin.

How bad is the scoring ? if not that bad have you thought of trying to clean it up by folding some emery cloth in half and then sandwiching the disc between it and working it whilst the disc is slowly turning.

Other option is a new disc I found a guy in Italy who is re manufacturing these 280mm floaters but with modern materials and they look the business and work oh such more better with the right pads than the original cast ones :D
 
Surface grinding is ok if you shim the disc to make sure it's sitting flat before powering up the magnet otherwise you'll pull it flat and once the magnet is released it'll go back to where it was. There's also a chance that it'll have got a hard 'skin' on it if you got it really hot and it might move after you've removed the hardness so skim both sides then check for flatness rather than assuming it's right.

I've always skimmed discs on a lathe using recessed soft jaws and never had a problem with the faces not being parallel or the discs not being flat but patience is the key.
 
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