No simple answer to this question, as there’s so many factors to consider. Unless of course, your irresistibly intrigued to see if you can actually resurface your brakes in the home shop! That’s one of the main reason I went ahead and had a go…

You can click the pic above to see the video walk-through of the whole process. Essentially it involved making a fixture for the lathe so the brake discs could be mounted consistently and squarely without lots of fiddling round with the chuck.

This is a scrap of old steel pipe that I am using as the fixture mount – the bit that will be held in the three jaw chuck. Note the not entirely desirable stringy ‘chips’ – I guess the feed or speed or cutting angle, or something wasn’t quite dialled in there. It’s part of the fun when you are reusing scraps and you don’t really know what kind of steel it is I suppose.

After prepping that stub of pipe, I welded on a flat disk of steel, about 135mm or 5.3″ diameter (so it just fits inside the braking surface itself and on the wheel mounting) by 30mm or 1.2″ thick. I then drilled through this steel disc, so that I could weld in a section of M16 high tensile threaded rod. This will act as the clamp, to hold the brake rotor in place.

With all that done the fixture will inevitably be quite out of true from the welding distortion. It gets ’roundified’ while mounted in the lathe, and turned so that an exactly 62mm raised section, will locate against the central hole of the rotor, centring it. Above you can see the very fine finishing pass used to ensure the fixture is totally lined up with the lathe’s cross slide.

Note: the fixture doesn’t leave the chuck at all until all four discs have been nicely skimmed. If I were to re-chuck it, I would also take another fine finishing pass of the fixture.

From there, after cleaning the mounting surface of the brake disc, it is mounted on the new fixture. A beefy steel washer/spacer and M16 nut clamps it in place. Then after checking it with the dial indicator, to ensure it’s not so warped as to be a hopeless cause, it’s time to turn the brake rotor itself.

Turning the Disc Brake Rotors

When it comes to turning the rotors themselves, I used the slowest lathe speed, 50RPM, and a new sharp carbide inset to cut the brake surface. As disc brake rotors are cast iron, the dust from this process can be quite bad for the lathe ways. It is much finer than normal steel turning chips, and forms a really abrasive paste when it sticks to the lathe way oil. As you can see above I hooked up the vacuum to try and minimise the mess, by sucking the dust at source. This worked quite well, but there was still clean up to be done.

Once one side is flat, before flipping the disc we need to make sure the surface the wheel normally clamps to is absolutely parallel and coplanar with the inner axle-mounting surface and the brake surface we just turned. This can be off a touch from factory, and if it’s an old disc there will almost certainly be corrosion on this surface that needs cleaning off with the lightest of skims. Here, I’m not aiming to remove any good material, only high spots of corrosion.

Once done un-clamp, flip the disc over, and re-clamp the other way round. It then needs a good check with a dial indicator to confirm that the braking surface you just turned, is still running true. If so, we can go ahead and skim the remaining brake surface. Do a final check with the dial indicator to establish that all the faces are absolutely parallel and coplanar. If so hooray!

Rinse and repeat! Remember that disc brakes need to be done in pairs. Both the front or both the back. In my case I did all four rotors.

So is it worth bothering?

First thing to say here is that not all rotors can be turned. They have to have enough meat in them so that after the re-surfacing operation they are still above the specified minimum thickness. This wasn’t a problem with my discs. All four of them were significantly ridgy and uneven, such that I didn’t want to put new pads on without sorting them. If I had done, the pads would only have been making very minimal contact, and would have very quickly been unevenly worn.

Even at on-line parts suppliers, four new discs would have been quite pricey for our little VW caddy van (more than £200). So that’s something of an incentive! Once the fixture was made and setup the actual process for each rotor was quite quick (about 45mins).

When I posted this on the socials there was quite a lot of debate around the value of truing up old discs. Some people arguing it really wasn’t worth the time these days – many blaming the cheap cast iron from China. I guess it comes down to whether you going to get anything form the task. I saw it as a cool learning opportunity, and a chance to reuse some useless lumps of cast iron destined to the scrapyard (which may well equate to being shipped to China, melted, cast and machined, and re-shipped back here to the UK). I also think I have probably doubled the life of those discs, as after truing up they were well above minimum thickness.

So far I have done about 2000 miles on the rejuvenated discs, and the braking action feels smoother, more progressive and powerful than it ever has. Obviously I changed in for new pads when I did the discs. If your planing a similar thing I recommend you do watch the video, which has a few extra tips.

Whether it is worth paying a machine shop to do it for you, I’m not sure as the cost seems to vary wildly from place to place. On principal though, I’d always recommend it, as re-use rather than re-cycling is very much better in this instance. One note of caution though: if the disc has warped there is little point re-surfacing it, unless the underlying problem that cause it to warp is fixed (e.g. sticky caliper, stiff caliper bolts, old brake fluid with compromised boiling point, etc).

I hope you found some of that useful, let me know your experiences in the comments below :)

Posted: 06|26|18 at 1:58 pm. Categorically squeezed into: Mending.
Tags: disc brake, fix, re-surface brake discs, re-use disk brakes, skim disc brakes
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