This is the C key from the upper joint of a Laubin english horn, in for an overhaul. Since tight key fit is essential to stable padding when working with cork pads, I spend a lot of time in my overhauls fitting keys to remove any "slop" in the mechanism. A lot of times that involves sweding the tube on the key to shrink it a little bit, and make it fit more snugly on the hinge screw. However, this is one of several keys on an oboe that can't be swedged, because of the long arm that's attached along its length. There's not a good way to get a tool in there to shrink the tube if you don't have access to the entire circumference of the tube. If you look at the end of the tube on this key, you can see that it's been "wallered out" somehow, so the diameter is way too large on that end. That allows the key to wobble on the hinge. The normal way of dealing with this would be to make an oversized (greater diameter) hinge screw and ream the inside of the tube to fit it, thereby enlarging the tube enough to cut out the "wallered" section. However, I'd already made an oversized hinge screw for the entire left hand section on this instrument. There were other keys in that stack had also been loose, and they all fit beautifully after reaming and lapping to the new screw. However, this one was still loose. Not wanting to make an even larger screw and ream all the other keys again, I decided to make an insert for this tube that would allow it to fit snugly.
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| The rough surface on the inside of the tube is where it was still loose on the oversized screw. The reamer that I ran through didn't even touch that area because it's so blown-out. |
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| This thick-walled brass tube has a very small center hole, much smaller than the diameter of the hinge screw. After the insert is installed, that hole will help guide the reamers through as I gradually enlarge it to the correct size. It's being turned down to a diameter that will fit in the tube on the key. |
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| Here's that same piece after being cut off and cleaned up in the bench motor. |
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| The piece was "tinned" to make it easier to solder in place. The whole part was heated and solder flowed over the surface, then the solder was wiped off while still liquid, leaving a thin layer on the surface. When soldered in to the key, solder will flow much more easily over this surface than it would the raw brass underneath. |
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| After soldering the insert. The end of it sticks out beyond the end of the key tube, and will be trimmed off later. The finished insert was intentionally made larger (greater diameter) than the hole in the key, and the key was reamed up to match it. That created a nice clean surface inside the key for soldering, and the inner diamter of the key tube was made uniform to match the outer diameter of the insert. |
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| Reaming the insert up to size. Progressively larger reamers are used, each one removing a few thousandths of an inch of material. |
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| The last reamer sticking out, having passed all the way through the insert. The chips of material it removed are visible on the end. |
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| The interior surface of the insert is much smoother and more uniform than the original key tube. |
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| After a little bit of lapping to remove to make the fit smooth, the hinge screw fits nice and snug in the key now, with no wobbling on either end. |
