redsunbeam
Part of things
I can't hear you .............
Posts: 136
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Jul 20, 2012 19:35:04 GMT
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If its ok to skim a brake disc, is it ok to reduce the outer diameter too? I've found some discs that fit lovely except they are 290mm dia and i need 280mm. Can i just machine 10mm off?
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tuner
Part of things
Posts: 62
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Jul 20, 2012 19:42:22 GMT
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Hi,just make sure to leave a slight chamfer at outer edge I normally 45* cut about 1mm done this several times and never had a problem with balance issues.
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mt2man
Posted a lot
Posts: 1,364
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Jul 20, 2012 19:50:34 GMT
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Yeah its fine! People do it all the time with mini discs when they use Fiesta callipers  |
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redsunbeam
Part of things
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Posts: 136
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Jul 20, 2012 20:08:53 GMT
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Thanks for that. It'll save me £350.
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Jul 20, 2012 20:18:02 GMT
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you don't want to take 10mm off though....
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redsunbeam
Part of things
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Posts: 136
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Jul 20, 2012 20:23:42 GMT
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Take 5mm off giving 10mm less diameter. I get it.  |
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redsunbeam
Part of things
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Posts: 136
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Jul 22, 2012 14:39:47 GMT
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I'm assuming the discs being vented makes no difference?
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tuner
Part of things
Posts: 62
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Jul 22, 2012 15:40:04 GMT
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Hi,vents not a problem but be carefull when in the void ,can catch tool tip when cutting ribs i cut from outside on both faces
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Jul 22, 2012 17:13:16 GMT
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Yes totally fine,I used to manufacture brake discs for a living and its near enough how their made anyway.
One thing though is that it may well then be out of balance as your most likely when machining not to take more than about 3mm depth from a balance cut so of course taking 5mm off you can machine the balanced bit out and it may need balancing again,although I'm not sure how much you'll really notice it but worth remembering anyway.
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bl1300
Posted a lot
Posts: 1,678
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Jul 23, 2012 11:13:56 GMT
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I had the discs on my Triumph 1300 skimmed last year to remove some scoring and rust pitting. Unfortunaltly when they did it they went at it too quickly and without any coolant/lubricant. The result was a set of nicely warped discs, and a lovely treatment for blood clots in the legs every time you touched the brakes. In the end I ended up with a set of new ones on it.
You can do it but don't overheat and warp the discs else you'll end up replacing them anyway. I would try it again as I didn't really lose anything as those discs would have been scrap otherwise anyway.
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Current fleet.
1967 DAF 44
1974 VW Beetle 1303s
1975 Triumph Spitfire MkIV
1988 VW LT45 Beavertail
1998 Volvo V70 2.5
1959 Fordson Dexta
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Jul 23, 2012 11:37:20 GMT
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I had the discs on my Triumph 1300 skimmed last year to remove some scoring and rust pitting. Unfortunaltly when they did it they went at it too quickly and without any coolant/lubricant. The result was a set of nicely warped discs, and a lovely treatment for blood clots in the legs every time you touched the brakes. In the end I ended up with a set of new ones on it. I skimmed some discs for a Renault 5 on a college model engineering club lathe. They were new and had only done a token mileage (to and from MOT) before the car was parked up for a couple of years. The surface rust was enough to make me skim them rather than simply clean with emery cloth and I was happy with the result. On refitting I found, as you say, they had warped and caused the brake pedal to oscillate up and down. On discussing this with the college tutor he said the problem was I needed to use a carbide cutter instead of normal tool steel to get though the work hardened "crust" of the cast iron disc. I suppose that is why automotive machine shops normally grind discs to skim them instead of cutting. BTW new discs for the R5 cost under £15 the pair yet I spent hours skimming then old ones  Paul H |
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Jul 23, 2012 11:58:13 GMT
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Correct about carbide tips,these are readily available but the above mentioned coolant is incorrect,never seen that used in any application of brake disc I've been involved with from £5000 fiesta's to £70000 jag's it's not required.
For what the op wants as well it will make little or no difference,as he's not machining the entire face just the o/d.
Chances are your discs were well warped when you put them in the lathe to machine them and you just made it worse!!!
Chucking and clamping were always the big issues with manufacture plus the measuring equipment was in microns so that's why I'd say they grind as its easier to maintain accuracy.
Lol at the £15 new price,how many times have I spent a whole day botching something with superglue/chemical metal/chock blocks etc to fail then find the new part is less than the fuel I used to go and get the bodging bits!!!
Live and learn eh?
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