...there is maybe a hairline crack and a welding seam in the front brake drum

[Jordan]

Nice milling machine.
I hope you got the hub centred well!

[Wolf]

Jordan, of course I took utmost care to center the hub right before milling. The crack inspection says: no crack. So, the process was not really necessary but now I have a perfect brake drums on both sides, before milling they were not really round. But now I need new slightly (+0.5mm) oversized brake shoes.
Cheers
Wolf

[blethermaskite]

That's been a nice bit of work and has proved there was nothing amiss .........has the skim made sense of the bit that looked like it had been welded or did it just disappear once cleaned up? maybe you could show us a photo of that strange area now its cleaned up?
Cheers,
George

[Jordan]

Jordan, of course I took utmost care to center the hub right before milling.
(...)
But now I need new slightly (+0.5mm) oversized brake shoes.

Hi Wolf,
I'm sure you took great care to centre and squarely locate the hub, but it is an unconventional method you chose to do this job.
Normally, the drum would be machined in a turning type machine (that is, a lathe), rather than a mill.
Set up on a good lathe, with the wheel bearings mounted between centres, you have guaranteed true concentricity and squareness of the drum to the axle, in quick time.
A big enough lathe is needed, especially if the tyre remains on the rim.
May I suggest a possible way to use your existing shoes? Make shims for the cam pads.

[Wolf]

Hi Jordan,
yes - I agree - the lathe might have been the better option. But I'm afraid that the largest lathe in my vicinity is not able to be used with a 19" wheel, even without the tire. I've prepared soft chuck jaws (I love this expression, sounds very nice to me!) on the lathe and transformed them to the mill. How do you make shims for the cam pads? This sounds interesting!

@ George - the suspect looking area vanished completly - I'll try to make a photo.

Cheers
Wolf

[Jordan]

Thanks for the photo, Wolf.
It's good to see the setups to solve machining problems.

The brake shoe shims are simple.
They are like the steel "cam follower" pads that are set into the shoes, added to them so the linings are closer to the drums.
Cut sheet steel into rectangles, width equal to the pads, and longer so the ends can be bent over the pads.
Once installed they are captive and will be secure.

[Wolf]

Jordan, thank you for the explanation how to shim the brake shoes - I think I can do this. I have understood that the brake shoes when machined to fit the brake drum should have a preload of 0.5 mm per shoe. This is depicted in the picture with the vernier caliper where I inserted two feeler gauges with 1mm, so the standard brake shoe should work. The stainless-steel sheet metal has a thickness of 0.5 mm and is easy to handle.
Cheers
Wolf

[DBDBrian]

Hi Wolf
When I machine oversize linings to size, rather than use shims, I always set the shoes in what I would describe as the brake applied position. Especially with a SLS arrangement, as the cam does not open both shoes equally, due to the different radius of the cam contact points on the shoe pad. This does not apply to a TLS as both shoes move the same amount. But in either event, I always set the shoes open using the set up pictured.
I accept the difference in movement is minimal, but setting the shoes by opening the cam, eliminates what ever discrepancy there my be in the system.
I also always measure across the centre line of the shoes in relation to the spindle, when machining to size.

P1080333.JPG

[blethermaskite]

So when I set up a s.l.s. drum brake with new shoes I always spec the fitted shoes + linings to be overall about 1.5 mm bigger than the diameter of the drum, shoes are fitted to the brake plate, brake plate is mounted on the lathe centred on a dummy axle and before a sizing skim I fit two temporary shims made from small strips of tin can between the cam and the shoes, I then skim the shoes to the exact diameter of the drum.
If this is all done carefully you should have the best shoe to drum fit while still allowing a free spinning drum with no bind, I was instructed on this process by respected British drum brake guru Dick Hunt (a man who knows what he's doing) one point I would make about the double sided brake you are working on Wolf.............the cam spindle's fit in the brake plates are very sloppy with no bush's fitted, the decent setting up (as above) and operation of this particular brake will be much improved if you fit a couple of bronze bush's to the cam spindle boss's this will ensure the shoes will make parallel contact with the drum in operation and will not have the tendency to tilt to one side or lift with a degree of unequal force. All little things but they all help.
Cheers,
George

[Jordan]

I have understood that the brake shoes when machined to fit the brake drum should have a preload of 0.5 mm per shoe.

Yes, when machining the linings the shoes should be held slightly "on" in the brake plate, so there is some clearance from the drums when in the "off" position.
This should be done by turning the cam shaft and holding it there. The lever can be rigged to do so.
If you place temporary spacing shims of the same thickness at the pads, with a single leading (per drum) system, the result will be that only one shoe will touch the drum when using the brake, because there is a difference in how far the cam pushes each shoe with this design.
It's too complicated and prone to mistake, to try to work out exactly the correct spacer thickness for each shoe, so the lever method is better.

Somebody has provided a photo of the method.

ScrFrontBrake1.jpg

The axle should be accurately central, of course - not guaranteed with a 3 jaw chuck.