Skimming brake discs on milling machine

[slim_boy_fat]

There are machines now which can skim discs while on the car (Wheeler Dealers demo'd one).

Did a good and has to be worth investigating?

Skim My Discs

Skim My Discs. 165 likes. Skim My Discs is a web site developed by Pro-Cut to help drivers find service centres where they can get the right brake expertise, and to explain the facts about brake...

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[eddie49]

This is far too embarrassing. I will definitely remember not to attach any photos of my "brake disc lathe".

 

[Morrisman]

I ‘skimmed’ a pair of Sherpa discs on my old Morris hot rod. They were horribly rusted and grooved, so armed with a new flap disc I gave the disc a manual spin (still on the stub axle) then carefully set the flap disc against it, and that caused it to keep on spinning.
The finished effect was quite impressive, looked like they had been properly ground.
Sort of.
If you squinted a bit.

They were such monster discs, on a 1000kg car, that they worked perfectly afterwards.

 

[MCKDAVID]

This is far too embarrassing. I will definitely remember not to attach any photos of my "brake disc lathe".View attachment 313018
View attachment 313019View attachment 313020

embarassing? why? nothing embarassing there, you got the job done, with what you had.... should be proud, not embarassed... would give that a double thumbs up, if there were any....


aye had to replace the discs on the fiat last month, 23 euros a side, plus 12 quid for the pads (ferrodo too).... 60 euros delivered all in for the front brakes... doubt that would even get the pads for the volkswagon...

 

[Brad93]

On a lathe sure.

Mill, no chance. You won't get them anywhere near flat on the hub/boss side unless you have a facing head with a single point tool.

With a facemill going back and forth or round the hub, few thousandths at best.

 

[Mikes]

There's a Garage in Ipswich that can skin them on the car as long as they are within manufacturer's tolerance. Think they are called Cattermoles (spelling might be wrong).

 

[8ob]

There's a Garage in Ipswich that can skin them on the car as long as they are within manufacturer's tolerance. Think they are called Cattermoles (spelling might be wrong).

Those on car brake grinders were very popular in Urmerika, never seen one over here.

Bob

 

[SAFC73]

BMW 535d.

I wouldn't mind uprating the brakes but seeing as it's just a commuter/ bus for ferrying the family around in, I doubt I'd get the full use out of them!

I replaced all 4 discs on my 330d for less than £225 don’t go to BMW for parts.

 

[Matchless]

BMW 535d.

I wouldn't mind uprating the brakes but seeing as it's just a commuter/ bus for ferrying the family around in, I doubt I'd get the full use out of them!

E60 twin turbo? what year?

 

[Rkweld]

If a mill is all you have and you dont want to get someone else to do it have a go. Fly cut them an see what you can achieve, it is important to get the hub face level and use logic and common sense.

If not maybe some of the nay sayers maybe able to offer you a good price to do them for you their better way

 

[Matchless]

Nice one @gaz1 that's fairly what I had in mind!


£350 for the front pair, £250 for the back. It feels like the rears but still... £250!

you would not feel much from a rear disc, how do you feel the judder? steering wheel, seat, whole car? also check the tyres for uneven wear in the form of raised blocks of tread around the outer 20% of the front tyres, sometimes you get an old three pence type effect every few inches, only notices when hard braking.

 

[Rkweld]

This is a common problem with some cars with tyres of certain tread types. Passats used to do it from experience. It could be a wheel bearing, there are a lot of possibilities to what it could be.

I think i would spend the £600 on an old fiesta with a full MOT if it was a commuter vehicle!

you would not feel much from a rear disc, how do you feel the judder? steering wheel, seat, whole car? also check the tyres for uneven wear in the form of raised blocks of tread around the outer 20% of the front tyres, sometimes you get an old three pence type effect every few inches, only notices when hard braking.

 

[Munkul]

I've had a couple of sets of performance brakes skimmed. It made no difference - first time they got warmed up properly they "warped". I read a lot about this as i was interested to know why.

There is one really important reason for this - it's the mechanics of how brakes work, and it's rarely an actual warping that you can measure as a geometric dimension.

Two phases to braking - low temperature abrasive friction, and high temperature adhesive friction. Normal driving is a mixture of the two.

In the low temp phase, about town etc, it's straightforward - your pads grab the disc, the material slips and abrades. The disc dissipates the heat quickly.

In the high temp phase - braking hard at good speeds, it's less straightforward. The disc and pads heat up and they glue themselves together and break apart, a million times a second (or whatever - extremely fast, anyway). The heat from this cycle is dissipated through the disc as it rotates, and the pad material is a fairly good thermal insulator.
This is why your pads last a long time, if they only worked on abrasive friction you'd put a new set of pads in every few thousand miles!

If you come to a stop with hot brakes and stand on them, you embed material in the discs due to the adhesion - and you also create hard spots in the disc with the heat under the pad which can't escape. Much easier to do this with an auto than a manual.
So... you drive on, you have warped discs. Drive around town and the material build-up will be abraded off the disc, and you might fool yourself that the discs are ok.

As soon as you brake hard and fast again, the pad material will build up in the same place as it did before due to adhesive friction - the hard spots pick it up more easily than the rest of the disc.

So... "skim the discs, they will be straight again" - yes sure, if you take enough material off them. Sometimes the hard spots will deflect the lathe tool and you'll machine a geometric warp INTO the disc, if you're unlucky!
If you get right underneath the hard spots, you'll take around 2mm off each side of the disc. No machinist wants to do this much, after all it "looks" straight before you take this much off.
Remember - you can't see the hard spots after the first skim, it all looks the same colour. So unless the machinist is listening for it and skimming at slow speed... again, machinists aren't usually metallurgists or brake specialists, so unlikely.
Most likely scenario is that they take most of the hardness out but not all of it.

After all this, you'll put the discs back on the car, and if there are any hard spots left in the material, they'll pick up on the pad material the first time you brake really hard, and you're back to square one.

TL;DR - skimming discs is often a waste of time!

 

[Melvyn Best]

Not a great fan of skimmed discs myself but rather than guess if the problem is on the front or rear ask your local M.O.T test Center to test your brakes which will show up any fluctuations.
I remember back in the late 60s early 70s BMCs answer to brake judder on single piston brakes on 1300s was to place abrasive paper between the disc and the pads with the front end supported and drive the car while gently applying the brakes.
Some genuine Peugeot pads had an abrasive coating to light skim the discs while bedding in the pads

 

[fizzy]

I thought that price seemed steep. Went to the EBC brake shop online - yes crazy money even after market. WHY???
I just skimmed some discs on my Colchester Triumph - got them to within a tenth runout but more by luck than judgement.
A mandrel is the best way as minimutly said but I am sure it could be done on a mill.
I am lucky my Colchester will swing 24" in the gap and 15" over the bed.
It makes a hell of a mess though - I spent hours cleaning my precious lathe afterwards

 

[Munkul]

I thought that price seemed steep.

Story of your life

 

[PhillipM]

Those on car brake grinders were very popular in Urmerika, never seen one over here.

Bob

Cause they're so flexy they often put in as much varience from chatter as they take out.

And if the disc has actually warped from heat 9/10 it'll just come back as the cutters bounce over the carbide deposits that are formed from the heat as already explained above - most of the time skimming discs is just a sure fire way to make sure it comes back in a few thousand miles.

Like someone else said, I'd clean up the hubs, mounting faces and give the discs a good go around with scotchbrite first and then get the DTi on them - most of the time discs aren't warped, it's just rust or pad deposits.

 

[tom2207]

I've had a couple of sets of performance brakes skimmed. It made no difference - first time they got warmed up properly they "warped". I read a lot about this as i was interested to know why.

There is one really important reason for this - it's the mechanics of how brakes work, and it's rarely an actual warping that you can measure as a geometric dimension.

Two phases to braking - low temperature abrasive friction, and high temperature adhesive friction. Normal driving is a mixture of the two.

In the low temp phase, about town etc, it's straightforward - your pads grab the disc, the material slips and abrades. The disc dissipates the heat quickly.

In the high temp phase - braking hard at good speeds, it's less straightforward. The disc and pads heat up and they glue themselves together and break apart, a million times a second (or whatever - extremely fast, anyway). The heat from this cycle is dissipated through the disc as it rotates, and the pad material is a fairly good thermal insulator.
This is why your pads last a long time, if they only worked on abrasive friction you'd put a new set of pads in every few thousand miles!

If you come to a stop with hot brakes and stand on them, you embed material in the discs due to the adhesion - and you also create hard spots in the disc with the heat under the pad which can't escape. Much easier to do this with an auto than a manual.
So... you drive on, you have warped discs. Drive around town and the material build-up will be abraded off the disc, and you might fool yourself that the discs are ok.

As soon as you brake hard and fast again, the pad material will build up in the same place as it did before due to adhesive friction - the hard spots pick it up more easily than the rest of the disc.

So... "skim the discs, they will be straight again" - yes sure, if you take enough material off them. Sometimes the hard spots will deflect the lathe tool and you'll machine a geometric warp INTO the disc, if you're unlucky!
If you get right underneath the hard spots, you'll take around 2mm off each side of the disc. No machinist wants to do this much, after all it "looks" straight before you take this much off.
Remember - you can't see the hard spots after the first skim, it all looks the same colour. So unless the machinist is listening for it and skimming at slow speed... again, machinists aren't usually metallurgists or brake specialists, so unlikely.
Most likely scenario is that they take most of the hardness out but not all of it.

After all this, you'll put the discs back on the car, and if there are any hard spots left in the material, they'll pick up on the pad material the first time you brake really hard, and you're back to square one.

TL;DR - skimming discs is often a waste of time!

interesting read , many thanks

 

[Munkul]

interesting read , many thanks

I'm no expert, but there are plenty sources who can explain it better than I can

Here's one:

Brake Pads: Understanding Friction And Formulations

Friction is the force resisting the relative motion of elements sliding against each other. The brake pads push against a rotor and change energy into heat.

www.brakeandfrontend.com

And here's one from the brake experts Alcon themselves explaining the "warping" myth

The myth of warped brake discs

Have you ever heard someone say they had warped brake discs? Typically someone had the discs resurfaced to cure the problem and after a relatively short time the roughness or vibration came back. In almost every case, this problem is not caused by a warped discs, but that the brake pad...

alconkits.com

 

[fizzy]

I'm no expert, but there are plenty sources who can explain it better than I can

Here's one:

Brake Pads: Understanding Friction And Formulations

Friction is the force resisting the relative motion of elements sliding against each other. The brake pads push against a rotor and change energy into heat.

www.brakeandfrontend.com

And here's one from the brake experts Alcon themselves explaining the "warping" myth

The myth of warped brake discs

Have you ever heard someone say they had warped brake discs? Typically someone had the discs resurfaced to cure the problem and after a relatively short time the roughness or vibration came back. In almost every case, this problem is not caused by a warped discs, but that the brake pad...

alconkits.com

I always thought warped discs were a myth as well.
Along with cylinder heads.
Obviously when you loosen all the bolts they will no doubt change shape slightly - but bolt them down again and surely they will become flat again?