Dialing up a brake disc on a lathe, help

[dobbslc]

I'm trying to skim a small (200mm from the rear of a motorcycle) brake disc on my lathe, what's the best way to dial it in on a 3 jaw chuck to run true?
I'm not sure it's even possible to take a small skim off it but worth a try before splashing out £45 on a new one!
I have a dial gauge etc.
Thanks

 

[Pete.]

Not as easy to do as you'd think, they tend to sing and chatter like crazy that's why they have the two-sided tools that cut both sides together. If you have a faceplate, drill and tap it to suit the disk fixing holes and bolt it hard up against the faceplate. skim one side then turn it over and do the other.

 

[dobbslc]

Thanks Pete.
I've got a face plate but never used it, I think my one has slots in it so it might be fairly easy to fix the disc on.

 

[Pete.]

I suggest you clamp it by the bolt holes so it doesn't warp, or you might machine it true when it's clamped bent, then it will not be true when released.
It might still sing when you machine it because the wear face on the disc won't be touching the faceplate. Perhaps some packing like thick tape would help that. Use a sharp tool with a very small nose radius if it chatters.

 

[Robotstar5]

I'm surprised there's enough material left to skim a motorcycle disc as they normally only have 0.5 - 1.0mm before hitting the wear limit.

 

[outofthefire]

I'm pretty sure I read somewhere that brake disks aren't meant to be a 100% true

 

[brightspark]

I wouldn't even bother for 45 quid a new one . not worth the trouble

 

[dobbslc]

I wouldn't even bother for 45 quid a new one . not worth the trouble

Where's the fun in that......

 

[mike os]

As an exercise it's fine,,I suspect the disk will be below minimum thickness by the time you are done.

 

[dobbslc]

Ok just tried the face plate and it won't fit over the boss on the lathe, it's just a bit tight.
I don't want to machine the boss as the chuck then won't fit!
Tried it in the chuck and I am now deaf!
It was working ok though but I can't guarantee it was true on the chuck so it'll probably not run true on the bike.
Left it for now.

 

[brightspark]

Ok just tried the face plate and it won't fit over the boss on the lathe, it's just a bit tight.
I don't want to machine the boss as the chuck then won't fit!
Tried it in the chuck and I am now deaf!
It was working ok though but I can't guarantee it was true on the chuck so it'll probably not run true on the bike.
Left it for now.

that's the fun u can do without after years of making and breaking I choose whats worth making . some of the things ive made have taken days and got enjoyment out of doing where as I could have made 300 quid for the day and bought the article for £150

 

[langy]

I wouldn't even bother for 45 quid a new one . not worth the trouble

I was thinking the very same !!!

 

[Wendelspanswick]

I'm pretty sure I read somewhere that brake disks aren't meant to be a 100% true

They have to be 'wonky' otherwise the pad would be rubbing all the time, or I suppose there is enough play in the wheel bearing to push the pad back?

 

[Pete.]

Trying out something like this can be a valuable experience for when you come to machine something you cannot buy or is cost-prohibitive. It's not all about how much a part costs. I have made or repaired many things that were of almost negligible value.

 

[Gareth J]

£45 isn't too bad for a bike. I think mine are closer to £140!! All 3 are approaching the limit

 

[Shox Dr]

Wobbly discs are about as much use as mudflaps on a tortoise.

I guess no ones had the disc open the pads up after a tank slapper.

 

[Arc Tourist]

Consider jerry rigging a Dremel or ? to your tool post, using your cross slide to grind the disc.
Also, a piece of rubber innertube stretched (and trimmed) over a work piece can cut down squeal.

 

[brightspark]

Consider jerry rigging a Dremel or ? to your tool post, using your cross slide to grind the disc.
Also, a piece of rubber innertube stretched (and trimmed) over a work piece can cut down squeal.

for 45 quid wouldn't bother and for putting a toolpost grinder of any sort on a lathe id have to be desperate. it was bad enough when I was sandblasting something and the wind blew it on lathe took a lot of cleaning down before I could use it

 

[Arc Tourist]

If time is important, throwing money may be quicker to resolve an issue, but to me as a hobby machinist, it's about the challenge in finding a way to succeed, and the satisfaction of not being beaten.

 

[awemawson]

OK you can't do it on your face plate as the boss fouls, but for future work consider supergluing items on the face plate. Often best to lightly skim the face plate to ensure that it's properly true (and also clean), then the superglue. Take of by dusting with a propane torch. Only take light cuts or you'll heat it up enough for it to let go