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A rolling blog of everyday life on and around the workbench

 

03/12/2018: I had a King Super 20 tenor in for a shakedown this week - a late model, though still a very nice horn. Among the things that needed sorting out was a bit of play in the octave mech. The client felt it was a little imprecise and sloppy - and indeed it was. It's a swivel mech, so my first suspicion was that the tips of the swivel arms had worn down, along with the sockets they sit in - but having dismantled the horn and taken a closer look at the mech, it didn't seem like the small amount of play in the swivel tips could account the the amount of play in the mech. The key barrels were reasonably snug and there was little axial (end-to-end) play to speak of - which left the likely culprit being the swivel pin itself.
Lapping the swivel pinWith the mech dismantled it was pretty easy to see that the pin was indeed somewhat the worse for wear, and as a result the mech was losing a fair bit of its motion between the thumb key being pressed down and one or other of the octave key pads rising. And if the pin's worn you can be sure the swivel is too.

This is a fairly common problem on swivelling mechs, which means it's also a fairly common repair. However, the pin on the King is rather long - at least twice as long as on, say, a Selmer or a Yamaha - and this has a bearing (spot the pun) when it comes to choosing the best method to restore its accuracy.
A typical repair would be to ream out the pin socket and fit a Teflon tube to the pin. This is a cheap and effective repair and comes with the benefits of making for a quieter action and a simple fix when the mech wears again (you just replace the tube). It works fine on shorter pins, but because the King's pin is so long it tends to accentuate any inaccuracies in the fit - so I decided to restore the mech to its original state...and perhaps even make it a tad more accurate.

The standard way to achieve this is to drill the swivel pin hole oversize, fit a bush and then ream it to size - but there's no point in doing this if the pin is out of spec. They typically wear into a slight oval, as well as a taper that narrows from the base of the pin (neither of which a Teflon sleeve would address). So the first order of business is to bring the pin back to being perfectly cylindrical, thus making it a reference to which all other dimensions will be matched.

Drilling the swivelThis was done with a series of lapping tubes, each fractionally smaller than the last. Very fine lapping compound was applied to the pin, which was then lapped into each tube in turn. This takes out the ovalness of the pin while also dealing with the taper - and the end result is a perfectly round, perfectly straight pin.

With the pin more or less sorted it's time to make and fit the bush for the swivel.
It starts with drilling and reaming out the swivel. There's a lot of 'meat' on the swivel, so I have the luxury of being able to fit a relatively large bush - which'll prove to be an advantage a little later in the process.

Testing the fit of the bushHere's the bush after having being turned to size and drilled out fractionally smaller than the diameter of the pin.
It's a friction fit in the swivel, and given that it isn't going to be subjected to a great deal of torque it would probably remain in place forever if it was merely pressed into the swivel.
However, I like a belt and braces approach so I'm going to soft solder the bush in place....just to be certain. As it's a tight fit I can't be sure that the solder will wick into the joint, so I'll file a small, shallow flat halfway along the length on one side of the bush to make sure that some solder flows into the joint.

With the bush secured in place and trimmed to size it's time to match it to the pin.
I left the bore of the bush slightly undersized - and at this point the usual technique would be to ream it out to fit. This gives a good fit, if done right, but I'm going to push the boat out on this job and lap the bush to fit. Now, I could lap the bush straight to the pin - but this would cut more metal from the pin and may even introduce a very slight taper. It'll also be rather difficult because the design of the keys won't allow for the swivel to be fully rotated on the pin (there are a couple of key arms in the way).
So what I'm going to do is repeat the same process I used to bring the pin back to spec...but in reverse.

Lapping the bushI've made up a series of lapping pins, each very slightly larger than the last. Because I made the bush quite large its thick walls have kept it quite dimensionally stable during the fitting process and it hasn't compressed as much as a thinner one would, so there really isn't very much work to do to bring it to a good fit. By gently lapping the bush out in stages it ensures the bore remains parallel and round - and I'll continue to lap the bore until I get a slightly stiff sliding fit against the pin. At this point I can finally fit the bush to the pin with an extremely fine polishing fluid so that it's almost an airtight fit.

The finished mechHere's the finished job. Unfortunately there's not really a lot to see - which I suppose is the general idea. You'd need a particularly sharp eye to spot that the swivel has been bushed, even if you knew it had been. What's really needed here is 'feel-o-vision', because that swivel glides on its pin like an eel in a bucket of butter.
I'm pretty sure, too, that it's better than its ever been. The action on these horns is remarkably sturdy, and tends to stand up well to general wear and tear - but the wear on the swivel pin seemed disproportionate compared to the rest of the action. As such I think it more than likely that it left the production line with a fair amount of play already built in.

It all looks like a lot of fuss and bother to go to in order to bring the mech back to spec, but in truth it probably took longer to write about it than it did to do the actual job.
Could it have been done any quicker? For sure, but while the results would still have been good, I don't feel they would have taken advantage of the mech's design - and they wouldn't have been quite so good as those achieved by this technique. When you have as much bearing surface to play with as this design affords, it seems a crime not to make good use of it.
With a few tweaks to the wear in the key barrels and sorting out the minor play in the swivel tips, I can happily sign off the job in the knowledge that it'll be many decades before this mech needs any significant attention.

 

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