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May 21, 2018 22:06:57 GMT
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Hi all, Anyone know of any engineering companies or similar who can skim my discs off the car on a lathe? I work in Southend so the closer to there the better but anywhere in that part of Essex would be doable. Cheers  |
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May 22, 2018 20:12:03 GMT
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ChasR
RR Helper
motivation
Posts: 10,309
Club RR Member Number: 170
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May 23, 2018 16:37:55 GMT
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If your discs aren't too expensive to buy new I'd just buy a new set.
IME skimming can be very hit and miss. I've had discs previously warp despite being fine prior to skimming. A few others out there have too. The discs going thinner has been cited as one reason.
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May 24, 2018 22:09:05 GMT
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Skimming to a satisfactory standard of parallelism on a normal lathe is almost impossible.
You really need to skim both sides at the same time..
It the slight out of parallel that causes what people call warped discs.
The thick and thin areas cause high and low grip which causes uneven wear and therefore the unevenness grows over time.
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Good info.. to be honest the reason I considered skimming is just that the discs are pretty grotty and there is some pulsing through the pedal. I thought maybe a skim would be a quick and easy way to ensure a good uncontaminated surface but possibly it’s a bit extreme. I’ll try to sand them to good health and then go for new ones if I’m not happy after that.
I think I’ve found a place that can supply, waiting for a quote at the moment. Think they’re going to be £110+ though for a pair
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slater
Club Retro Rides Member
Posts: 6,390
Club RR Member Number: 78
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Skimming to a satisfactory standard of parallelism on a normal lathe is almost impossible. You really need to skim both sides at the same time.. It the slight out of parallel that causes what people call warped discs. The thick and thin areas cause high and low grip which causes uneven wear and therefore the unevenness grows over time. Sorry but that's complete BS. Any skilled machinist can do it. The issue is more to do with stresses in the material. You can machine them true enough but next time you get them hot they will probably just warp again. Once discs have been overheated they are as good as scrap imo. Not worth the hassle. |
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ChasR
RR Helper
motivation
Posts: 10,309
Club RR Member Number: 170
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May 25, 2018 10:20:09 GMT
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Skimming to a satisfactory standard of parallelism on a normal lathe is almost impossible. You really need to skim both sides at the same time.. It the slight out of parallel that causes what people call warped discs. The thick and thin areas cause high and low grip which causes uneven wear and therefore the unevenness grows over time. Sorry but that's complete BS. Any skilled machinist can do it. The issue is more to do with stresses in the material. You can machine them true enough but next time you get them hot they will probably just warp again. Once discs have been overheated they are as good as scrap imo. Not worth the hassle. This is exactly what happened to my RST. I skimmed them due to surface corrosion and wanting to not spend £80 on discs. A month after skimming they were shaking despite initially being fine. pardonthewait , that still sounds reasonable. IIRC my machine shop charged me about £30-40 anyway; I'll double check. If it makes you feel better the discs are around £280-360 a pair on my M3 depending on luck and the supplier. |
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820
South East
Posts: 793
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May 25, 2018 12:07:33 GMT
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Best to skim them on the car, most likely the run out is in the hub and skimmed off the car or new discs will do start to judder again in time.
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May 25, 2018 14:40:18 GMT
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Skimming to a satisfactory standard of parallelism on a normal lathe is almost impossible. You really need to skim both sides at the same time.. It the slight out of parallel that causes what people call warped discs. The thick and thin areas cause high and low grip which causes uneven wear and therefore the unevenness grows over time. Sorry but that's complete BS. Any skilled machinist can do it. The issue is more to do with stresses in the material. You can machine them true enough but next time you get them hot they will probably just warp again. Once discs have been overheated they are as good as scrap imo. Not worth the hassle. I'll be a bit more polite than you and simply disagree. I am a skilled machinist, I can skim discs, there are some on my car now that I did. However parallelism wasnt within few enough microns and thats what they need to be to prevent uneven wear, They were never warped before but had bad corrosion so its not a problem of previous warping and stresses in the material. After skimming they performed very well, but the judder appeared slowly and now is enough to make me just buy some discs instead of doing them again. I know you can get the internet to tell you almost anything so I apologise for doing it but a quick search on disc thickness variation found this as a reference of how parallel you need to get them. www.bendix.com.au/news/disc-thickness-variation-dtv-issue-14In there they say 0.03mm max thickness variation, which is quite hard to achieve if you are skimming the second side of a disc in a separate setup on a lathe, then machining it and looking for that little runout afterwards when cutting forces have been applied. Yes, any skilled machinist can skim a disc, and many of those discs will work afterwards, but their life will often be shorter than expected for various reasons including the difficulty getting parallelism. I would do it again, but not if new discs were available at a reasonable price. |
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MrSpeedy
East Midlands
www.vintagediesels.co.uk
Posts: 4,791 
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May 30, 2018 19:43:36 GMT
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Bendix would say that. They want you to buy new ones!
I'd agree with Slater, it's easy enough to get within 2 or 3 thousands of an inch and you really won't notice that through the pedal (unless you're some kind of hypersensitive tap dancer)
I've skimmed loads in the past and never had an issue with pulsing on the pedal.
One thing I will say though is I do my own, so it costs me nothing. When I do them for others it's usually about £20 each so it's not always worth it really.
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May 30, 2018 21:15:23 GMT
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Fair enough Mr Speedy
I agree that you can get within a few thou, I know I did on mine, and at that you cant feel it at the pedal.
What makes me strongly believe that slight disc thickness variation is what causes pulsing to grow many miles later is that I don't believe that (for example) the 28mm thick vented discs on my car will ever warp.
People always use the term warped discs, I think its highly unlikely and in any case the disc moving from side to side wouldnt cause the pedal to push back, moving up and down. At least not at low speed, the caliper would just move side to side as its on sliders.
I go along with the theory that the up and down movement is DTV, and that starting with slight DTV means imperceptible but uneven friction, which causes uneven wear and steadily increasing DTV and hence increase in pedal pulsing.
The discs on my car didnt pulse at all after skimming and not for a long time after, then it became perceptible, and has slowly increased to the point where the pedal pulses badly enough to need me to take action.
Within months of skimming I drove the car and braked hard from 140mph many times without problems, so they were fine and now arent. (on a track)
I suppose the way to work out if the discs are warped, or if the thickness variation has grown since the skim is to simply measure them.
When I come to change them I'll remove the wheels, refit the bolts and clock the discs to check for runout, then mic them for thickness variation.
If they are running out and the thickness variation is similar to when I skimmed them then I'm completely wrong (as is the advice I've read places about the importance of controlling DTV), if the discs are still flat and true but have excessive DTV it suggests that the theory of variation growing due to uneven wear is correct (in this case at least).
I'm not looking to argue at all, I respect your opinions but I'd like to know what has happened in this case and I'm in a position to find out.
Hopefully I'll report back in a few weeks.
I'll happliy report that I'm wrong if thats what the facts say.
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Last Edit: May 30, 2018 21:17:22 GMT by chris y
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ChasR
RR Helper
motivation
Posts: 10,309
Club RR Member Number: 170
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May 30, 2018 23:03:16 GMT
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Bendix would say that. They want you to buy new ones! I'd agree with Slater, it's easy enough to get within 2 or 3 thousands of an inch and you really won't notice that through the pedal (unless you're some kind of hypersensitive tap dancer) I've skimmed loads in the past and never had an issue with pulsing on the pedal. One thing I will say though is I do my own, so it costs me nothing. When I do them for others it's usually about £20 each so it's not always worth it really. I must have gone to you! But while I can see you are playing the cynic my experience gave me different results. I can tell you however that I felt I wasted £30 on skimming my discs, not to mention the hassle; I was without a car for 4 days waiting for the discs to get skimmed (my other car was stored in a very dubious location in the next town away)! I've changed many discs but never had a vibration come back with new ones; I've always gone with decent brands however. Maybe I'm a tap dancer as you say. But doing 10,000 miles in car(s) and not having that previously would say otherwise  . I think between all of the 3 Mundeos I did 500k and never had vibrations anywhere near as bad as that RST; you can see it shaking the camera in the MotoFest video, and it was enough for me to take it easy on the pedal and a mate to comment on it as a passenger, despite me trying to conceal it. These drilled RST discs were not so. For 200 miles it was fine. After 10 minutes on track and road use it was terrible! Anything above 30 and the damn Escort was shaking again under braking. My then Clio 172 on the same sized discs but having spent a day at Cadwell on new discs did shake initially from pad deposits building up but it did go away on the drive back home. Even my M3 on factory drilled discs where the stock brakes are always cited as a weakness on track has been fine; I've done the Nurburgring, Oulton and Silverstone on the very same discs that are on it now. I'm sorry, but I'm with chris y and Bendix on this one . But how fast do you go in your cars? Or do you lack senses  . |
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Last Edit: May 30, 2018 23:09:45 GMT by ChasR
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. I think between all of the 3 Mundeos I did 500k and never had vibrations anywhere near as bad as that RST; you can see it shaking the camera in the MotoFest video, and it was enough for me to take it easy on the pedal and a mate to comment on it as a passenger, despite me trying to conceal it.
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