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Posted by Gordon Maney [208.126.138.206] on Tuesday, December 28, 2010 at 10:23:46 :
In Reply to: Re: Brake Drum Question posted by TGP (IL) [64.12.116.76] on Monday, December 27, 2010 at 18:23:22 :
....for saving me a lot of time!
:-)
Then I got to thinking….
Regrettably, there is a lot of conventional wisdom we have to toss out the window when considering these hub/drum assemblies. Eli Whitney said all mass produced parts were the same and could be interchanged. In theory, one machined hub can be substituted for another, and all the assemblies will still be the same. Eli obviously never worked with Power Wagon hub/drum assemblies.
Factors affecting the operational concentricity of a hub drum assembly include variations in the hub, drums, and bearing-set parts. Removing and replacing bearing races is never to be regarded as a thoughtless process. There is too much at stake there. Did the new race go home to the same place occupied by the old one? Did you raise any sort of burr in that counterbore during your work? Did a piece of something [even shop rag fabric] get behind the race in the counterbore?
As an automotive instructor I had a brand new Ammco brake lathe [a beautiful and highly accurate lathe] and I had all the correct truck adapters, including the extended shaft for their drum micrometer. I had the right adapters to turn the drums by engaging with the bearing races, rather than a hubless setup.
When you set one of those drums up you need to spend a lot of time verifying your setup. If your setup is wrong you will put more runout into the drum; you will destroy that thin drum.
Stepping back from these drums to brake service in general, it has always been good form to skim cut a drum or rotor at time of lining replacement. Why? Because the typical brake service job is replacing linings after a normal service period. The meaning of that can vary, depending on vehicle, its application, whether it is drum or disc, and the good or bad habits of the driver. To offer a figure, a normal service period for the brakes on an axle could be anywhere from 40,000 to 120,000 miles.
During that time the drum or disc surface becomes impregnated with lining material. The drum wears and the disc wears. A light machining removes the lining impregnation and also makes the drum round and straight; it makes the rotor have no runout and causes it to be flat.
Setup is key. When the setup is good, and the change in service is small, you can refresh the surface with a very small cut. I have, in passenger care sized components cleaned a surface by removing less than .010 in. The traditional service guideline is that a drum diameter can go to .060 over standard unless otherwise marked. Rotors should always be marked with minimum thickness.
People have stepped back from turning at every brake service for, unfortunately, all the wrong reasons. Really, there are no right reasons to not refresh a friction surface. Bad brakes kill.
What are the reasons? In our case, the combination of increasingly rare drums with unskilled, apathetic, and profit motivated brake lathe operators. As I type this I have no access to a brake lathe, and I realize that if I had some drums turned, the only way that would occur is if I was standing at the operator's side, moderating the process. All I need is some pimply faced, part time, high school aged employee ruining my drums in a few minutes.
In the case of contemporary vehicles, reasons include the difficulty of removing rotors on many applications, and the use of nickel alloy rotors that make them poorly machinable. Chattering is extremely common. The advent of the on-car lathe helped with the removal problems, but not necessarily the chatter and finish problems.
Another dynamic is [at least purported to be] the lower prices of service rotors. Chinese, mostly, and we have our feelings about that, but the sentiment is that you just buy new ones and toss the old ones.
I have personally seen rotors on late model, used vehicles in dealer lots — on vehicles they supposedly have inspected and approved — that were clearly junk. Late model vehicles are affected by the craze the make the vehicle lighter. The rotors are thinner. Any significant scoring and surface wear renders them unacceptable.
Maybe what we really need is ccsammy to start making drums as soon as he gets done with sheet metal parts.
The disc brake solution is great in many ways. However, authentic restoration demands drums. I can tell you from experience, from having properly machined drums on Power Wagons, that a newly serviced drum system, fitted with straight, round, machined drums, with all shoes adjusted correctly, is an excellent brake. A vacuum booster makes it even better. It may be that none of you have every experienced that. Most people encounter these trucks with junk drums and very poorly adjusted brakes.
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