<VV> drum roundness/concentricity

[djtcz at comcast.net]

snipped and bottom posted 
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----- Original Message ----- 
From: Stephen Upham < contactsmu at sbcglobal.net > 
Subject: Re: <VV> Brakes grabbing (rubbing ?) 

...................... I'm taking the drums in to have 
them checked for round and thickness. I'll let you know what I find 
as I find it. 

Stephen Upham 
================================================= 

Hi Steve, 

Many of the world's problems can be identified with a 1 inch travel dial indicator and a mag base . 

I'd clean up the hub and drum surfaces, mount the drum inside out, and indicate the friction surface runout right on the car. 
Less than 0.002 inch is good. I do not know how much is "good enough." I have a feeling you have 0.03 inch or more 
Then With the drum mounted on the car the right way I'd indicate the sanded clean OD, and write the amount of runout and the high spot right on the drum with a yellow paint stick. 

I'd be interested, to the point of being squint-eyed suspicious, in the fixture and technique the shop uses to check the drum "roundness". 

That's because I have had some heartburn dealing with (using) devices that must mount a wide variety of parts and thus use fixtures consisting of a flat plate or hubs with fingers, and a cone. 
http://www.ammcoats.com/uploadedImages/Products_and_Solutions/Brake_Lathes/GMDE%204000.jpg 
http://www.made-in-china.com/image/2f0j00bhtQsREKqTmgM/Brake-Drum-Disc-Cutting-Machine.jpg 

The flat plate is meant to establish the face of the wheel hub or crankshaft. If that is not perpendicular to the operating axis of rotation, Lets call that wobble. 
The cone is supposed to "pick up" the bore in the wheel, brake rotor, brake drum, flywheel or whatever and position the part concentric to the axis of rotation. 
But being a cone it really picks up the corner or edge of the bore. There is likely a tiny chamfer there, and maybelike a stamped steel wheel, even a formed lip to create a bore with length greater than the thickness of the drum back plate material. That chamfer is not perfect, and it may be a mile off. Or there may be a burr raised that will not keep the drum fronm sliding on the hub. On some cars, the drums (and wheels!) are not even hub-centric!! The result is during machining (or checking) the drum will spin around an axis that is offset from the axis of my car's wheel hub, and when I install the drum on the car the friction surface will be eccentric, even if it is perfectly round!! 
media.mfg.com/.../uploads/ thumbnail/3007812.jpg 

Here is a link to an actual brake lathe operating manual. 
http://partners.ammcoats.com/docman/viewDocument.asp?sParm=ii-t~c-tbfmmhdmo 
It shows the cone adapters used on a brake drum without a hub (page 13) and, the procedure that includes, before ANY machining is done, checking the concentricity of the setup by making a few scratch cuts before and after rotating the drum on the arbor in a special, controlled way that will reveal if the setup is REALLY picking up a used drum's bore. 
Not every operator knows how to make that check, and some of the ones that know how will skip it if the first scratch looks good, or maybe if things are too busy on a Saturday morning. "oh, we never had that problem before Mr Timberlake, it must be your brake drum" 

Nobody loves your car's brake drums more than you. If when mounted on the brake lathe, the drum does not have the same high spot , with similar runout that I got with my on-the-car-check, the newly machined drum friction surface WILL have runout when back on the car.