Topic: Turning motorcycle rotors (non-drilled)

[Mille44]

Is there a chance that an automotive shop could turn a motorcycle rotor?  I'm interested in cutting 1mm off of of each side of a pair of 7mm rotors as well as truing them at the same time. Has anyone done this before? Is there any issues with the calipers by making the rotors thinner?

[bistromath]

It's possible, but make sure the shop knows the rotor is stainless steel, NOT cast iron like every other brake rotor ever made! Stainless will destroy their tool if they try to turn it with the same tool at the same rate they're used to.

[I Zombie]

A standard automotive brake lathe will not work. I've tried, you need a Flywheel Grinder with the right adapters to mount em up. Hope that helps.

[HavocTurbo]

Key word there being GRINDER.

It's hard as all get out to machine a rotor down.

Grind it and it will be thinner and look brand spanking new.

[mycb750k6]

There's a guy on this forum that does it - He did mine see below.

[mycb750k6]

OH - it's speedracer741_@hotmail.com. He's in OR or WA , I forget. Just PM him.

[Mille44]

OH - it's speedracer741_@hotmail.com. He's in OR or WA , I forget. Just PM him.

thanks for the info.
Did he grind them or machine them? Correct me if I'm wrong but if they are ground, the irregularities such as warping would remain in the rotor and turning (machining) will correct any warping.
Do you remember, about what he charged for that service?

[mycb750k6]

No they must be ground and that's the main reason to do it - remove any warps. These are stainless and can't be machined. There's a youtube video on the process - pretty cool. He advertises on ebay but gives SOHC4 members a break. Email him. Blasts the center too at no charge if your paint is chipping. He does drilling too but I wanted to stay stock. Outstanding work.

[Steve F]

There's a guy on this forum that does it - He did mine see below.

Wow, those came out great!  How much did it cost per rotor with shipping?  Mine are getting a little ragged looking and have a slight warp in the rear rotor.  I'm a machinist with lots of equipment at my disposal, but not a grinder that can handle those. 
Steve F

[mycb750k6]

It was 40 bucks as I recall. And the good news is it's holding up pretty good under wear. I don't think he takes much off. Well within tolerance anyway.

[Markcb750]

Not a good Idea: Stainless can be machined but it requires a much more force then automotive cast rotors.
 

Stainless has a much higher yield point then cast, particularly the grades of cast used for automotive rotors. This requires the tooling/fixturing to be much stronger to hold the part accurately for precision machining.

Stainless also tends to work harden quickly as a chip is formed. Bend a paper clip a few times to experience what the chip is doing ahead of the cutting tool edge.  Because of this work hardening it is difficult to produce a smooth finish on stainless, it is also difficult to remove less then a few tenths of a mm on each side. This could result in a thin/ruined rotor.


Grinding is better, any shop with a rotary grinder ("Blanchard") could repair your rotor.  The part is held on the machine with a magnet.  Some stainless is not magnetic but the Honda rotor is.

Mine was ground and drilled by Portland Machine. 

My rotor has taken on an interesting pattern around the drilled holes, as can be see in the second photo below.

[mycb750k6]

Were your holes chauffeured? Looks like a high spot.

[flybox1]

 

My rotor has taken on an interesting pattern around the drilled holes, as can be see in the second photo below.

yeah...looks like your cross drilled holes were not chamfered with that wear pattern.  not a bad thing, really.  it just makes a sharp edge and can wear down your pads if any burrs arent dealt with after drilling.  they cut water better.  Hopefully they resurfaced(ground) AFTER they cross drilled. 

here's mine that Speedracer741 did.  holes were chamfered.  1700mi since he did the work, and they still look new.

havent seen him on the forum lately.  i hope he doesnt mind the referrals 

[Markcb750]

That's what I thought, but it has a nice little 0.25 mm relief, looks like it was done with a deburing tool, and I stoned the surface when I assembled. 


It does not cause any vibration or pulsing of the wheel or brake lever.


I did not measure with micrometer or against a granite flat.

[754]

A bit of misinformation here, will try to correct.

 Lighter cut on lathe is often harder to do than a heavier one. moreso here as the disc outer surface may be workhardened..

 A double cut tooling setup, probably eliminates most of the problems.. I built one for that, but have not yet tried it.

 To thin the rotors, they should be tunrned, then ground.
 
 Grinding is a slow process, a 3 thou cut is very big for a lot of grinders, would take many pa$$e$ of a grinder to thin a disc. Usually taking a thou or so per cut.

  Next not just a blanchard grinder, must be a rotary grinder, ie a flywheel grinder that will be used (if doing both sides).  The part will be clamped in center with cones or adaptors to grind outer face first, then fipped to do the back.
 I disagree that turning will not take out the warp, rotors are usually turned on a lathe.

Also for resto work, the original rotors were Turned NOT Ground.

[mycb750k6]

Just send them to speedracer and you'll be delighted. Many people here have.

[Markcb750]

A bit of misinformation here, will try to correct.

 Lighter cut on lathe is often harder to do than a heavier one. moreso here as the disc outer surface may be workhardened..

 A double cut tooling setup, probably eliminates most of the problems.. I built one for that, but have not yet tried it.

 To thin the rotors, they should be tunrned, then ground.
 
 Grinding is a slow process, a 3 thou cut is very big for a lot of grinders, would take many pa$$e$ of a grinder to thin a disc. Usually taking a thou or so per cut.

  Next not just a blanchard grinder, must be a rotary grinder, ie a flywheel grinder that will be used (if doing both sides).  The part will be clamped in center with cones or adaptors to grind outer face first, then fipped to do the back.
 I disagree that turning will not take out the warp, rotors are usually turned on a lathe.

Also for resto work, the original rotors were Turned NOT Ground.

a bit of misinformation here...


A rotor can be held with out centering cones, most general purpose grinders will be able to set the center of rotation close enough to miss the center casting with an indicator on the center bore.

Turning both sides will double the applied forces on the holding fixture, requiring it to be twice as strong as single sided cutting.  It will increase the likelihood that the operation produces a part with parallel surfaces.

If you want to drill your rotors grinding will allow the rotor to be drilled first. If you are turning You should drill after finishing as the lathe cutting tool will bounce as the turning tool crosses the drilled holes, resulting in surface irregularities.

Honda could turn the rotors because they had the blanks with out the mating hub and they could have the part annealed (stress relieved) just prior to machining.  Also for production work special lathe chucks would have held the part securely for good production rates and good part quality.


If you want an accurate restoration it might be possible anneal the part again.  also there are Ceramic insets for general purpose lathes. Which would help reduce the loads reflected into the cutting tool and its support.

It would be interesting to see how they would cut these rotors, cutting pre-drilled rotors would reduce the life of the ceramic insert significantly, lots of $$.


Again I am not saying the rotor cannot be turned, I am saying it is not as easy as an automotive rotor.  Who ever does it needs to be experienced turning stainless.

I have been working on a dual disk design and have solid models of the center hub and rotor just about done, I want to tell you the intersecting surfaces on that hub are damned difficult to draw...accurately anyway.  

[754]

Why would you hold the 2nd side in grinding (would require collar or parallels) magnetically when you simply bolt it to the centre hole, and get nearly zero runout with a proper collar

 This is not a stainless tooling problem, normal inserts can be used.. it is a vibration problem. Again it is not fixturing problem, it is a disc vibrating problem, one that should cancel itself out completely if cutting both sides at once.

 I am getting the feeling here, a lot of you have been fed a lot of bull$hit, simply to justify a high machining price..

 

[Markcb750]

Why would you hold the 2nd side in grinding (would require collar or parallels) magnetically when you simply bolt it to the centre hole, and get nearly zero runout with a proper collar

 This is not a stainless tooling problem, normal inserts can be used.. it is a vibration problem. Again it is not fixturing problem, it is a disc vibrating problem, one that should cancel itself out completely if cutting both sides at once.

 I am getting the feeling here, a lot of you have been fed a lot of bull$hit, simply to justify a high machining price..

 

No offense but why you hold the part on a collar and face grind it?.  You understand even a tiny error in the center bore would translate to a large error on the rotor face?  Also because the center bore is so thin it cannot support the rotor against cutting or grinding forces.

Have you ever attempted to machine a disk from the center bore?  Had one of my ME's routed a part that way we would have let him run the part; or let him listen to the operator #$%* at him.

Machining both sides does not cancel out the forces, it can come close to canceling out the force perpendicular to the face of the rotor IF the depth of cut is equal on both sides, a big "if" in any case.  The torque direction "radial"  is doubled by cutting two faces at once. The fixture would need to hold the part twice as stiffly to counter this. 


Feelings can be misleading.

[I Zombie]

Again I am not saying the rotor cannot be turned, I am saying it is not as easy as an automotive rotor.  Who ever does it needs to be experienced turning stainless.

I work in a Napa store & I've tried to turn an already junk rotor. On a standard automotive brake lathe it can't be done. Now maybe on our flywheel grinder, with the right adapter, NP. But in NO way can I a good/correct finish from turning.

[754]

I will just report what I actually did, to mine.

 I had a previously thinned disc that had about 7 thou diff in thickness, ie it had runout on one side. And it had been run several years.

 I mounted it in my lathe on a stub that the disc fit on to with the mounting surface, that the rotor bolts to against to the hub, roughly the same size.

 I proceeded to take a light cut off both sides. now the side where I was cutting from approx 8 thou to 1 thou, proved to be a bit harder to do than the other. Finish was acceptable but not super nice. Left it as machined and ran it.
 
  I used either a standard grade carbide or HSS, not sure which.
 Not saying it is easy, but it is doable. I am sure I would have far less trouble doing a 20 or 30 thou cut on each side simultaeneously. My main concern would be chip control & ending the cut on the side where it runs into the carrier.

 Also I did not even bolt it to the stub, it merely slipped over and I used a bushing and the tailstock, to push the mounting surface of the disc against the stub.

[Markcb750]

One side was out 0.007, what was the run out on the other side?

What caused this out of parallelism?

What does "previously thinned" mean, did someone attempt to turn the disk in the past?

Did you chuck the disk on the outside of the rotor or did you clamp it through the center? 

 

[754]

I got the disc that way, and had run it, I knew it was out due to constant feedback thru lever.. it was between 5 and 6mm thick.

 It is a stock 750 rotor, that has been thinned, and redrilled to 4 bolt pattern to fit an early Kaw disc brake Morris mag.

 As  I tried to describe I turned a piece in my 16 inchlathe to roughly replicate the snout of the hub where it mounts.

 I did not clamp ID, I merely slipped it over the snout/stub I had turned.. no press fit.. then I used a bushing that pressed against the bolt circle area, with a plate or large washer over the end of that. The  revolving centre was pushed tightly against  said plate/washer (with a hole in it for the centre)..exerting pressure against the disc centre (bolt circle area), holding it against the shoulder on the stub.

 I realize I can probably hold it a bit stiffer, by bolting to the stub arbor, in fact I think using the recommended torque to hold it with the mounting bolts, is a good  method as it puts it in actual mounted conditions. I dont think a slight interference fit on disc ID would hurt either.

 Disc is now 5.2mm thick.
 I did have to pull a trick out of my sleeve to get it to cut properly.. I think it is now time to try my double toolholder, I thought up a methed to set up the 2 tools without building an adjusting setup.

 

[Markcb750]

It is a stock 750 rotor, that has been thinned, and redrilled to 4 bolt pattern to fit an early Kaw disc brake Morris mag.

 As  I tried to describe I turned a piece in my 16 inchlathe to roughly replicate the snout of the hub where it mounts.

Must look like one of those 100/105 mm mag wheels available on EBay for my Miata.

Great way to center the part, what lathe do you have, Best results are with a Monarch...(I worked for them for a while in the 80's and 90's  )

I realize I can probably hold it a bit stiffer, by bolting to the stub arbor, in fact I think using the recommended torque to hold it with the mounting bolts, is a good  method as it puts it in actual mounted conditions. I dont think a slight interference fit on disc ID would hurt either.

 Disc is now 5.2mm thick.

Bolting & torquing probably not necessary for the tolerance  required for good performance of the rotor.  Pressing against the face of the arbor with the tailstock is an interesting way to clamp the part.

Are you using a stock caliper as Honda recommends a min thickness of 5.5 mm?  Mine is 5.9mm after grinding.

I did have to pull a trick out of my sleeve to get it to cut properly.. I think it is now time to try my double toolholder, I thought up a methed to set up the 2 tools without building an adjusting setup.
 

Post a photo will you, I am sure it would help others.



I still believe for "one off" runs tossing the part on a rotary grinder is a viable solution.  Finding a shop with the right grinder is key although a good machinist can make up for poor equipment. 

[mycb750k6]

Like I said " Just send them to speedracer on this site and you'll be delighted. Many people here have." He's the expert.