Buffet English Horn Day 7 - 2/19/14

Two days ago I wrote about doing some work on the right hand Eb lever after its tube split during swedging. That lever fits between the two parts of the C# lever on an assembly, and it turns out my work with that assembly was not done. Today was focused on making a replacement for a stripped shaft lock screw on the lower C# lever. In this picture, the red circle indicates the hole where that screw should go.

The shaft lock screws hold the lower C# lever to the steel hinge rod that runs through this whole assembly. On the other end, the C# touchpiece is permanently attached to the rod with a spline. The Eb lever between them, however, floats freely on the rod and isn't attached to anything. That enables the player to press on the touchpiece and cause the lower C# lever to move, while the Eb lever can remain stationary. This is all necessitated by the fingering system for the english horn and the location on the instrument of the Eb key (which would be directly below the assembly in this picture) and the C# key (which would be immediately to the left of the lower C# lever). The reason the lower C# lever has screws holding it on (instead of being permanently attached like the C# touchpiece) is so that the lower C# lever and Eb lever can be removed for cleaning, oiling, or other servicing.

A similar mechanism exists on flutes to enable the instrument to play Bb and F#. However on most flutes the keys are secured to the rod with tapered pins that can be knocked out. Most oboe and english horn manufacturers use tiny screws instead. The upside to this is that they're a lot easier to remove without marring the key. The downside is that a tiny screw is a lot harder to replace than a tapered knock pin. That being said, here's the process I used:

The first thing was to clean up the threads on the inside of the hole with a tap. That's a watchmaker's tap going through the key. You can't see its threads in this picture because they're so small. Sorry about that.

With that being done, the first step in making the screw was to turn a piece of drill rod down to the correct diameter, in this case .045 inches, which is measured with a micrometer.

Once the rod was turned down the correct diameter, it was time to thread it. The plate in this picture has dies for making several different sizes of screws, and you can see the newly threaded rod coming through the fifth slot. This is a delicate step, because the rod is so narrow and fragile. As the die cuts metal away from the rod to make threads, it can snag and break the rod. That happened to me twice, and fortunately both times I was able to extract the broken part of the rod from the die plate.

Once the rod was fully threaded I cut off the excess and smoothed out the head of the screw, then cut a slot with a jeweler's saw. This was most easily done with the screw threaded into the correct sized hole on the die plate.

  
Here's what the finished screw looked like. I didn't make the penny. That's just to show how absurdly tiny the screw is.

After that, it was just a matter of installing the screw in the hole, which was a little harder than expected because I used a pretty cheap tap and die set and the hole didn't exactly match. But by threading the screw in nice and slow like, it all got trued up and now everything fits together quite nicely.

And that's how I made a shaft lock screw. It was terrifying!