Manufacturing Horn Bearing Plates

This sad old Gretsch horn (as in the guitar maker!) came in to the shop from a local charity that we do work for. Two of the bearing plates were just missing and while it was well beyond the value of the instrument to make new ones, I was interested in doing it for pro-bono practice. At worst, I'd mess it up and they'd have a horn that was no less functional than when it came in. At best I'd get it right and they'd have a horn that was marginally more functional than when it came in. Marginalliy because, you know, it's a Gretsch horn.
A bearing plate is a pretty simple shape to machine, the hardest part being to bore out the spindle hole so that the spindle will be tight but not binding. A hole that's a little tight can be lapped in to make a smooth fit, but a hole that's too loose is useless. I threw out a few practice plates before getting two that worked.

1" brass stock would have been ideal to work with since the plates measured just under an inch at their widest, but I was out and only had a day to work on this, so a couple thick brass slugs, about 2.5" diameter, had to be turned down quite a bit. After turning a couple duds, I figured out that it would be best to turn the spindle hole first. If it turned out too large, I could cut off and start over, without having invested any time in turning the other parts of the plate. Here the slug is being center-drilled to start that hole.

After center drilling and through-drilling the hole with a drill bit that was slightly undersized to the final target diameter, it's bored out to pretty close to that diameter.

Checking the fit of the spindle in the hole. This one was a little snug, which is great because it can be lapped in later.

Turning the under-side of the plate. The raised area around the hole is the bearing surface that will make contact with the bearing surface on the rotor. The rotor only contacts the plate on that narrow surface and inside the spindle hole.

The underside of the plate, on its way to being finished. The thick shoulder running around the outside edge had to be thinned out to allow the plate to sit at the right depth in the rotor casing. I did that by repeatedly checking the fit on the instrument and removing small amounts of material until achieving the right fit.

The top of the 2nd plate, with a big ugly 2 stamped in.

All set to go. The 1st and 2nd casings are new, the 3rd is the original I used as a template. I didn't turn in that decorative step on the new ones just to save a couple minutes. Without witness marks on the new rotors, they had to be ported by looking down the slide tubes and sighting the alignment, then trimming the bumpers as needed. That's how I port all horn rotors anyway, though, since the witness marks aren't always perfectly aligned, especially on mass-produced student level horns.