Here’s an interesting situation that came up the other day. A Kramer guitar came into Robot Monster with a truss rod nut with the hex wrench hole stripped out. It is easy to find replacement nuts for Gibson or Fender guitars but it is often difficult to find them for imports with metric threads. In addition to the threads, the diameter needs to match closely. This was the case and the repair needed to be done quickly so I made a replacement.
Polygonal holes can be made on a lathe through a process called rotary broaching. It requires a cuter the shape of the hole with cutting clearances and holder that is free to spin with the workpiece offset angularly by 1°. I don’t have a rotary broach but I was able to improvise one using a small lathe head clamped to the cross slide.
First I had to make the cutter which gave me an opportunity to use my “new to me” surface grinder. I milled the flats on the cutter at an angle using an indexing fixture and a sine plate to get the 2° cutter clearance.
After making the truss rod nut with the required m5 x .8 thread the cutter was plunged into the rotating nut. With the existing hole the cutter is only required to cut out the corners of the hexagonal shape.
I made a few extras in case this situation arrises again. You can see the threads just peeking through the hex hole but it is a nice snug fit.