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.

Clarinet Tenon Rails

The rails are the thin strips at either end of a tenon. While the cork creates an airtight seal and prevents the joints from slipping apart once assembled, the rails provide structural support and prevent wobbling. This is perhaps best demonstrated on clarinet center tenons, which must bear a lot of stress from the weight of both the upper and lower joints. Ideally a long tenon would distribute that stress, but the center tenon has to be made relatively short because of the placement of the tone holes on either side. It's not surprising, then, that we see a lot of older clarinets with worn down rails on the center tenon, especially wooden instruments. A worn tenon will invariably wobble, causing the bridge key regulation to go in and out. It also just feels weird and wrong. 

This clarinet had an especially bad center tenon. Beyond the natural wear, I suspect someone got overzealous and sloppy with their sandpaper when replacing the tenon cork at some point. The rails were almost gone, so the tenon was pretty much a uniform diameter. Sometimes silk-wrapping a tenon can address worn rails, but you need something to start from in order to do that. The only option here was to turn and install new rails, which is a more stable and accurate repair anyway.

After cutting away what was left of the old rails - and a little more on the lower end to create a nice square edge for the new rail to butt against. That groove on the upper end was for epoxy to get in and adhere, but I don't think I'd do it again as it makes the tenon a little weaker.

The new rails, cut from ABS plastic. The lower one is thicker.

Installed with epoxy and ready to go.

After corking and completing the rest of a repad. No wobble anymore!