Author Topic: Brake Drum Skimming  (Read 5058 times)

Offline Duncan R

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Brake Drum Skimming
« on: 11.09. 2010 18:31 »
Hi ,

In order to try and sort out my woeful front brake I think I need to have it skimmed and oversize shoes fitted. Can it be done with out dismantaling the wheel?

Thanks
Anglo - Indian A7SS (Actually is a 650)
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Offline anita

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Re: Brake Drum Skimming
« Reply #1 on: 11.09. 2010 18:36 »
I think this depends on if the lathe used has a big enough clearance to take the whole wheel in it.
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Offline MG

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Re: Brake Drum Skimming
« Reply #2 on: 11.09. 2010 18:43 »
The brake drum gets out of round by irregular spoke tension, so skimming it when mounted in the wheel with all spokes tighened is the only thing that makes sense.
1955 A7 Shooting Star
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1961 Matchless G12 CSR

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Offline Riderusty

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Re: Brake Drum Skimming
« Reply #3 on: 11.09. 2010 21:14 »
MG's comment is logical--if you have access to a wheel jig for lacing, set up the wheel and check the inside of the drum with a dial indicator. See if adjusting the spoke tension makes a difference in the indicator reading.

As far as turning the drum with wheel intact, at one time lathes with special headstock risers were made specifically for the purpose and gave sufficient swing to turn car and truck drums with wheels assembled. I dont know how many of these exist any more.

Tom B. 
1953 BSA A7 Plunger-under resto.
1962 BSA B40 -awaiting resto.
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1959 BMW R50-restored

Offline Duncan R

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Re: Brake Drum Skimming
« Reply #4 on: 11.09. 2010 21:28 »
Just had a look on the SRM site , they do it but I should imagine the postal charges would be quite high
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Re: Brake Drum Skimming
« Reply #5 on: 11.09. 2010 21:45 »
I have that problem on the cafe after re-spoking. Originally the rim was close to true (about 1mm) and the brakes were great. Now I have the rim with 0.25mm side ways and 0.3mm vertically the front brake is s**t. The thick Akront rims take a lot of spoke tension to true.
I was thinking of making a jig to skim the drum with the wheel in the forks. Might be a while before I get around to it, but will post if successful.
Cheers
'51 A7 plunger, '57 A7SS now A10CR, '76 XT500, '77 AG175 '83 CB1100F, '81 CB900F project.
Australia
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Offline anita

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Re: Brake Drum Skimming
« Reply #6 on: 11.09. 2010 22:06 »
Just checked with Bob, you need to find someone with a gap bed lathe, as this will take the whole wheel.  Do you know any jobbing engineers.  Bob has an old lathe in the garage that will do this and he can't be the only one.  He's not an engineer by trade but I know some of the engineers at work have set up's at home.  There must be other similar engineering companies about where you could enquire.
1951 BSA A10 plunger
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Online bsa-bill

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Re: Brake Drum Skimming
« Reply #7 on: 12.09. 2010 09:55 »
if you know any lorry drivers or owners ask them, had my wheel skimmed local and they apparently skim lorry brake drums
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
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Offline Duncan R

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Re: Brake Drum Skimming
« Reply #8 on: 12.09. 2010 10:18 »
Thanks for your help everyone.

The mechanic at the shop where I had my MOT done is getting a hairline crack in my brake plate welded for me, I will check with him as he owns a Triumph to see if he knows anyone in the local area. I might drop SRM a mail and get an idea of postal costs as well.
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Online BSA_54A10

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Re: Brake Drum Skimming
« Reply #9 on: 12.09. 2010 10:57 »

First of all and extremely important.
BSA used an alloy called "nickasil" for their drums
It is very, very, very hard and even harder to machine.
It needs to be ground, not machined with any thing that vaguely resembles a lathe tool.
here is the trick.
knock out the wheel bearings and mount the wheel on a spindle.
the spindle needs to go all the way through the chuck and you can use a live center on the other end to add extra support.
Use a centerless grinder mounted on a pedistal behind the lathe

Method 2
Use a mill with a powered rotary feed.
Very few mills will spin quick enough to use a grinding wheel but if yours dose not then use a strait cutter.
It is very important to cut the full face of the drum as point cutting tools will flex & skip giving you a durm that resembles a corrigated iron tank.
Bike Beesa
Trevor

Offline MG

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Re: Brake Drum Skimming
« Reply #10 on: 12.09. 2010 11:18 »
Trevor,

Please don't take offense, but unless Nikasil is something completely different in English language use than in German, this can't be right.
"Nikasil" is a trademark owned by Mahle and was first used in the late 60s (afaik). It is a silicone-carbide coating (nickel matrix, hence the name) deposited in thin layers, mainly used as coating for alloy cylinders (with direct contact to the piston, thus cast iron liners can be omitted). So Nikasil (again, in German!) is a coating material, but definitely no casting alloy.
Maybe you meant Dural or Silumin?

The Ariel hubs I have simply have a steel ring cast in.

Cheers, Markus
1955 A7 Shooting Star
1956 A10 Golden Flash
1961 Matchless G12 CSR

www.histo-tech.at - Restoration, Repairs, Racing

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Offline Duncan R

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Re: Brake Drum Skimming
« Reply #11 on: 12.09. 2010 11:22 »
Thanks Trevor

I think its cast iron ,would it still have the Nikasil coating? My BMW has Nikasil coated bores and there is hardly any wear on them after 90,000 miles so it must be good stuff.But would it be useful in a situation where you need friction?
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Re: Brake Drum Skimming
« Reply #12 on: 12.09. 2010 11:56 »
I know it's very amateur and Heath Robinson and you can all chuckle, but faced with the same problem, not having a lathe big enough nor a deep enough table on my modest miller, and not wanting to be sending wheels away for ages, I made the attached to rectify the drum on the front of one BSA.
All it is is a fly-cutter bit held in a bit of round mild steel bar with a milled flat one side for locking it by means of the smaller tommy bar, mounted on a piece of thick steel bar in which the tool can slide (and be locked for and aft and side to side by the screws and the ugly 6mm rod with nuts on). The big piece of flat bar is drilled to go exactly over the wheel spindle and be done up fast with the nut that holds the backplate.
With the wheel spindle mounted in clamps in a big vice, tool locked on and held rock steady (with an extra arm attached to stop it budging under any circs), I then rotated the wheel slowly by hand while introducing the tool very gently by means of the long screw, having set the depth of cut on the sliding block to take a thou or two at a time at the narrowest points of the drum. It got rid of 35 thou of ovality in many stages and didn't leave me with a bit of corrugated iron either. I just kept running the tool down (which is spring-loaded in the bit of blue tube to get it to come back up again) at the same settings time after time. Finish not per factory obviously, but smooth to the touch, and more importantly, a round drum, concentric with the spindle, which works a million times better.
With a slight mod (collars and spacers made up to suit), it finish-cut a home-made drum liner for an AMC front end after shrink-fitting it, with another OK (to me) result. Not something to do on a perfect and beautiful machine perhaps, but on my old clunkers it's worked fine and no-one's said to me 'wot, no brakes!'.
Bill

Offline NickSR

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Re: Brake Drum Skimming
« Reply #13 on: 12.09. 2010 12:13 »
Hi Duncan
Most wheel building firms carry out his service, last time I was quoted £75 / £85 for a skim and new lining, but ended up new rim etc.

Its important that the new oversize lining are machined to the new drum size.

Regards
Nick  
1962 Super Rocket
1955 BSA C11G
1998 BMW R850R

Offline MG

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Re: Brake Drum Skimming
« Reply #14 on: 12.09. 2010 12:23 »
groily,

that's a brilliant tool, no need to hide your light under a bushel!
It might take a bit longer, but I can imagine you will get as good a result than with large machinery, the basic principle being the same.
1955 A7 Shooting Star
1956 A10 Golden Flash
1961 Matchless G12 CSR

www.histo-tech.at - Restoration, Repairs, Racing

Austria