This is (barely involves or doesn't involve a lathe) how I deal with school-owned rotary-valves tubas whereby a rotor stem (not lost, but available) is broken off: (ie. the "good enough" fix)
> Drill down into the dead center of the rotor body (from the broken-off point).
> Tap (usually) an M3 x .5 thread into that hole.
> "Chase" the (same threads) stem threads with a tap (to clean up those threads).
> Screw a short piece of stainless steel "all-thread" (again: same thread) into the piston body coated with acid flux, and solder it in place.
> Very lightly "tin" the exposed threads with solder, and then "chase" the tinned threads with a die (so as to not prevent them from threading).
> Screw the broken-off stem (on to the "tinned" stainless steel all-thread (paying attention to "clocking" it back to its original position), and solder it back in place. Again, the stainless steel can't be too long, or else it will interfere with the rotor's center screw.
> One last time, chase the threads (from the top of the reattached stem) with a flat-bottomed M3 tap, to make certain that the center screw will screw down easily.
> Hope that this fix is (actually) "good enough", and charge 'em money.
With one of those crappy "axial flow" trombone valve (aluminum) stems, I've done the same thing - except with 24-hour epoxy. The epoxy held for years. I did this on a "professionally-owned" trombone, because it's owner told me that they were going to replace the valve...yet they never got around to replacing it.
![Smilie6 :smilie6:](./images/smilies/e21576.gif)
bloke "Ridicule this, but - after I've done it - stuff works."