Machining brake discs

[AndyStobbs]

My car has been off the road some time while I do an engine swap. Nearly ready to put the engine back in now, and thought I would attempt to clean up the disc faces in the lathe prior to refitting them. First of all I noticed how difficult they were to get a decent finish on, using a HSS tool it needed to be dead sharp and a cut of no less than 0.005"- any pointers people can add to this?

Having got the faces useable I set about playing at drilling (and eventually grooving) the discs, which got me wondering, what's the optimum set up for this, ie. Hole diameter, hole depth, ontop of or offset from the finning (these are vented discs).

Anyone shed any light? Thanks

 

[brightspark]

My car has been off the road some time while I do an engine swap. Nearly ready to put the engine back in now, and thought I would attempt to clean up the disc faces in the lathe prior to refitting them. First of all I noticed how difficult they were to get a decent finish on, using a HSS tool it needed to be dead sharp and a cut of no less than 0.005"- any pointers people can add to this?

Having got the faces useable I set about playing at drilling (and eventually grooving) the discs, which got me wondering, what's the optimum set up for this, ie. Hole diameter, hole depth, ontop of or offset from the finning (these are vented discs).

Anyone shed any light? Thanks

use carbide tipped tools on the cast

 

[shenion]

Have done a few on mine. Carbide or HSS work fine. You need clearance and rake angles for a decent finish. Carbide will stay sharp longer on crusty cast.

 

[Lawnmowerboy]

Yeah. Brake discs work harden. Cast is hard enough in the first place. I've skimmed flywheels where the clutch has been slipping and goes hard as nails. Ok once you get through it. Carbide and a large radius cornered insert. If you are machining cast look for aluminium inserts which are uncoated and are a far better grade of carbide and dont break down as quickly

 

[ukracer]

If you are machining cast look for aluminium inserts which are uncoated and are a far better grade of carbide and dont break down as quickly

Ah it took me a few mins. I guess you are saying (uncoated) carbide inserts that are designed for aluminum. For a minute I thought you were saying aluminum inserts. lol

 

[nickk]

u need to start your cut under the skin of the crusty broken /intermittent layer that is extra hard,also negative rake on the top of the tool,eg the top surface of the tip slopes down towards the material.

 

[Bill Edwards]

I seem to recall that cutting tools for cast iron want no rake at all.

 

[PhillipM]

The hard spots on the top are actually carbide inclusions from hot spots, so you need to get under them with a bit of feed and then take a finish pass on a fresh surface, as mentioned, you want carbide tips and as stiff a setup as you can manage - you'll also want something to support the backside of the disc surface if you have it chucked on the top hat, otherwise it'll ring like a bell and the surface will be terrible.

 

[Wedg1e]

Don't forget that cast iron dust is the kiss of death to machine slideways... it gets in where you can't see, mixes with the lube oil and forms a nice lapping compound!

I just skimmed the discs and drums of my Connect trying to eliminate some monster brake judder (some of it was probably due to a snapped drop-link that I didn't see first time - or the second ). I always think it's odd how freshly-machined cast iron has a fantastically grainy surface yet by the time brake pads have finished with it, it's almost like a mirror polish!

 

[hotrodder]

I really wouldn't drill them. It won't make them work better on a road car IMO but will make them noisier* and far more prone to cracking. A lot of theory behind the advantages of cross drilling (especially for road cars) is marketing BS from people with a vested interest in selling the things with the exception of some of the bigger OEM names which are simply producing what the market wants and even say as much describing them as 'sporty looking'. Many of the 'better' cross drilled brake discs are actually cast with the holes in place...

These aren't about machining per se, more about the sort of tolerances you need to maintain to avoid potentially ended up with "warped discs"...
http://www.stoptech.com/technical-support/technical-white-papers/-warped-brake-disc-and-other-myths
http://www.ate-brakes.com/www/downl...on-parts/download/bd_praesentation_pdf_en.pdf

Obviously the discs annular size effects things from a grooving POV, massive 2 piece discs with huge ally bells aside the simple way is two straight lines like



but that obviously doesn't look as pretty as all the fancy curved slots manufacturers use...

* the pattern does influence things some

 

[AndyStobbs]

HR, interesting reading, but really who are ATE tring to kid, that article reads like a load of BS. Lateral run out of 30microns, are they on this planet? The drive flange wont be that flat let alone the caliper mount being that square. Think they are bigging themselves up a lot there, we've all changed brake discs on old sheds of cars with rust everywhere and the lateral run out was probably in the order of tenths of millimetres with no adverse effects.

Can understand that drilling and grooving wont make any difference on my car, but it does seem to be the in thing at the moment.

 

[PhillipM]

I would never bother with drilling/drilled discs.

 

[Cyberprog]

Adding holes and grooves after casting will induce stress cracks IIRC

 

[AndyStobbs]

Seems to be common enough, and I've yet to have a car come in with a cracked brake disc...

 

[Lawnmowerboy]

Drilling and grooving is nearly always done after casting afaik. We do a lot of disc modifications for some fairly high up race teams . They're all done on a cnc mill. We've never had one crack yet. What warpage we do get is skimmed off

 

[PhillipM]

Seems to be common enough, and I've yet to have a car come in with a cracked brake disc...

I've seen at least half a dozen, if not more.

 

[Rocketman]

Usually the drilled and grooved brake disc's are a higher quality alloy.

I've been using brembo max discs for some years, they have a very light groove. They aren't particularly expensive but the main reason I buy them is corrosion resistance compared with standard disc material.

 

[AndyStobbs]

Thanks for all the input given

 

[Bill Edwards]

Personally I'd never consider drilling holes in a brake disk, what's the point? If the normal disks aren't OK for the use they're getting then you've got the wrong vehicle altogether, and drilling holes in a critical part that isn't designed for it (unlike the ones done by the manufacture) does not seem sensible. You may have never seen a disk crack, but you certainly don't want one to disintegrate when you go for that all important emergency stop at a critical moment.
Brake discs aren't something that should be altered lightly.

 

[brightspark]

[quite frankly i wouldnt bother skimming discs if they were readily available at a reasonable cost. the only time i would is if they were unavailable or very overpriced