What I can remember after decades of working with BMW motorcycles
Turning a drum
The reason for wanting to turn a drum is because the drum is out of round. When braking, it has a pulsing feel. I wrote this up in 2014 because it wasn’t on my website.
We found a local brake shop that would go to the trouble to turn a drum with the rim in place. You really do want the entire wheel assembled and complete when you turn the drum because if you spoke it up later, you might pull the drum out of round with the spokes and you would have a pulsing brake. If the wheel has a newly spoked rim, then I would prefer to ride it a bit and give the wheel a chance to settle in. Then, when you check for spoke tension you might find a couple that had loosened up slightly. You really want to know that you have a stable spoke job before you turn the drum.
If this is a front wheel with the double leading brakes, I highly suggest that you remove the two shafts that operate the cams and clean out the rusty bushings and add a bit of lube. Getting those two front springs on and off is a chore. If this is a rear wheel, then you can ignore cleaning the shaft with the single cam to operate the brake shoes.
The shaft on the brake turning machine was too large to go through the cone of the wheel bearings. We found that we were able to leave the cups (outer race) in the hub and the brake shop had a variety of cones to slip on the shaft and center the hub. The seal holder must be removed in order to get it on the shaft.
There is no way to tighten the set up enough that the wheel would be held in place on the shaft when the tool is cutting the drum. They had a tool called a “dog” that went on the shaft and an arm that stuck out and grabbed a spoke. That dog provided the force and did the turning quite well.
By this time you have already had your shoes relined. The /5 and later are bonded onto the shoes. The /2 and earlier were riveted in place. The brake industry has standard brake linings that are available in a few oversizes. That way they can turn the drum out by .010” (some set amount) and then the new diameter of the drum is .020” larger. Then the mechanic just gets a box marked .020” over and installs them. BMW didn’t do it that way. I suggest that you cut the very minimum out to just clean up the grooves. The machine that we used had two speeds, one for fast cutting and the much slower one for a finish cut. We really want the finish cut, as it gives the drum a nice surface that won’t grind off much of the friction material while seating in the new brakes.
I highly suggest that you have the brake shoes go through a procedure called “arcing the shoes” as that will give the friction surface the correct radius to fit the newly cut drum. It will if your technician knows what he is doing.
For the bedding in process, I suggest that you test the “arcing” process for accuracy. Ride it around enough to give it some wear, maybe a couple of hundred miles, maybe less. You just want to see the wear pattern on the shoes. The ideal wear pattern would show equal wear along the entire length of the shoe. If it is more worn on the heel, that means that the arcing didn’t take enough off. It the wear is mostly on the toe of the shoe, then too much was taken off. You may have to take the shoes back to get them ground off a bit more, or do some hand work to make it even.
This is new and from Scottie Sharp.
I made a bushing for my vintage hub lathe to turn /2 and /3 wheels. It works pretty good.
I made a jig that simulates the geometry of the backing plate with the brakes slightly shimmed that I can spin in my lathe.
I’ve been having folks mail me their wheels and brake backing plates and I can turn their hubs, and machine new brake linings made from material I purchase from the company “Vintage Brake” (very high performance). I cut it, counter drill, rivet and bake it (it cures when baked). Then I machine it to fit the diameter of the newly machined hub.
My opinion is that while I haven’t had Scottie cut a drum for me, I wouldn’t hesitate to try him, as he does very good work.