Wednesday, January 24, 2018

Resetting a Buffet Clarinet Post

This Buffet clarinet came to me twice in the same year because the hinge screw on the Side F# key had snapped off at the threads. The first time I simply manufactured and installed a new screw, but I did notice at the time that it felt a little tight going in. This time I investigated further and discovered that the underlying problem was a misalignment between the holes in the threaded post and the unthreaded post, which hold the screw. The hole in the unthreaded post was angled too far downward, so that when the screw was fully inserted, that post pushed down on it while the threaded post pulled up on it. The screw broke at the end of the threads because that's where the diameter is thinnest and weakest. The fix would be to change the angle of that hole.

The way I saw it, there were three potential ways to address the issue:
1) Raise the unthreaded post so that the angle of the hole was in line with the threaded post. 
2) Drill out the hole in the unthreaded post, then solder in a bushing at the correct angle, using the screw as a guide. 
3) Completely fill in the unthreaded post with a soldered plug, then re-drill it at the correct angle using a post-drilling jig.
At the time, I didn't have a post-drilling jig (I do now!) so option 3 was out, but I did have a drilling jig for the body, as detailed in a previous post, so I chose option 1 to give that tool a workout, though option 2 would have been just as good.

The first step was to remove the post and drill out the post-hole in the body to remove the threads. Because the post's height and radial alignment are both dictated by how far it's threaded in to the post-hole, and because both of those variables have to line up at exactly the same point, new threads would have to be cut into a bushing, then aligned on the body.

The small hole at the center of the picture was the post-hole. This is after it was drilled out.
Below is the bushing installed on the post. It's just a piece of ABS plastic, turned on the lathe and threaded.


The post was threaded into the bushing before installation.
The bushing and unthreaded post were glued in to the post-hole in the body together, using the screw to properly align things with the threaded post. Once the glue set, the pad had to be replaced to account for the slightly different angle at which the key now meets its tone hole, and the problem of the broken screws should now be permanently resolved.


The repositioned post is at the center of the photo. Looking at the base, you can see that it's now raised slightly off of the body. It's not cosmetically perfect, but probably not the kind of thing you'd notice unless you were looking for it.