Blowing the picture up to that size shows a degree of roughness that I wouldn't have let a drum leave my lathe with. Bad thing is, the more you machine one, the more metal you're removing, even if it's just .002" cut. That's still 4 or 5 thousandths of metal that's not there to be used by normal braking action.
Agree. At least, assuming that the photo represents the actual surface roughness and isn't an optical illusion.
measure the piece in multiple spots, to try to see if it was out of round or warped. Bear in mind, they often are. So what I would do, is take a fast cut, but not deep. Just a couple thousandths. See where it's at true wise. Then take another fast cut, small as possible. Keep mic'ing it between cuts. A guy's coming to get his drums or rotors machined, it's likely because he can't afford new ones. So I will do what I can to save as much metal as I can. But this takes time and patience, and most machine shops or parts stores aren't going to have that
I did similar, but tried to estimate remaining machinable thickness of drum vs. deepest wear. I'd make at least one rough cut at my "best guess" depth...with the intention of a) not overloading the brake lathe by cutting deeper than it was capable of, b) staying within the machining limit of the drum, c) deliberately leaving some minimal scarring after this rough-cutting. As long as there was some non-machined area of the braking surface, I knew I hadn't accidentally machined excessively.
The light scarring would hopefully vanish during the final, fine cut. This fine cut was never in excess of the machining diameter of the drum. If there was a minor spot of scarring after the final cut...we didn't get too worried. Didn't seem to affect braking, and we saved some iron that would have otherwise gone beyond the machining limit.
And if it came out under/oversized, I had that in writing on the receipt, and we both signed off on it. Wrote the finished size on it with paint marker and on the receipt. CYA, and that way everyone can see you're doing your due diligence.
We never had to worry about marking oversizes on the receipt, or customer sign-off on oversized drums/thin rotors because we never cut them that far. We'd cut to the machining limit, but no farther.
TYPICAL machining limits for drums was .060. The typical "discard" limit cast or stamped into drums by Federal law was .090 over the original size. That meant that a remachined drum had .030 diameter that could wear away during the life of the new brake shoes.
TYPICAL machining limit for rotors was .030. Discard limit cast or stamped into the rotor would be another .015--.020 thinner. So again, having machined the rotor, the pads could wear the rotor some before hitting the discard limit. But rotors are so soft that in many cases they've worn beyond the machining limit, maybe even worn beyond the discard limit.
The discard thickness/diameter
cast or stamped into the iron was the absolute authority, superseding all other conflicting information including the service manual. Our machining limits would adjust to suit the discard diameter.
I looked in the '97 Service Manual. They're saying they want a rotor surface finish no rougher than 60 RA. (But who has the kind of equipment to measure RA?) They're suggesting a non-directional surface finish using either 120-grit aluminum oxide sandpaper, or 150-grit aluminum oxide sandpaper on a sanding block, and to sand for a minimum of 60 seconds. I recall using finer sandpaper--hand-held 180-grit emery cloth comes to mind. The service manual says nothing about surface finish or sandpaper on drums. I'm surprised. All they say is that the crossfeed speed for the rough and the final cuts is the same as the rough (.006--.010) and final cut (.002) crossfeed speed for rotors. And we know that they think that after a final-cut and sandpaper, you'd achieve 60RA or better.
