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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.

22/12/2018: I've been having some fun with oil again.
If you've read my articles on oiling the action you'll know that I have a keen interest in this rather geeky field, and that I maintain a healthy scepticism when it comes to some of the claims made for esoteric lubricants.
I'm also of the opinion that it's hard enough to get players to oil the action as it is, without the added hassle of having to track down a very specific oil that isn't widely available. As such I'm a big fan of keeping it simple, and looking for practical solutions that provide credible results for little or no effort at all.
My oil of choice remains synthetic gear oil, but I also make use of high tack (HT) silicone grease in certain situations. It's good stuff - it's reliable, easy to find, cheap...and tends to stay put. However, it also tends to be quite stiff - which means it's not so good for general use because it can induce a noticeable amount of drag in the action. It's no big deal on the keys that are typically sprung closed (palms, sides etc.) and works well on the larger keys (low C/Eb etc.) as well as being a good lubricant for point screws. But if you use it on the main stack keys it can sometimes knock a bit of response off the keywork, more so if the action is nice and tight.

Gear oil and silicone grease mixThis is annoying, because (as the Oiling the action - for geeks article shows) grease has some significant advantages over oil in terms of hanging on in the key barrels. The solution, at least partially, is to use a thinner grease - but you still run into the grey area where neither oil or grease seems to do a thorough job.
And so it was that I was wondering whether it was possible to combine the two types of lubricant in some fashion - to make it 'multigrade'. I'd been experimenting with applying a drop of oil on a rod screw treated with silicone grease. The initial experiments were more of an accident; I'd lubed up a rod with grease, promptly forgotten I'd done so and then later added a drop of oil. I found this affected the viscosity (the stiffness) of the grease considerably - but had no idea as to the long-term implications, nor indeed how the two lubricants were working with each other (or not, as the case might have been).

Well, there was only one way to find out and that was to run some tests.
I was particularly interested in whether the oil and grease mixed or formed some sort of emulsion. If they mixed, the resultant lubricant would simply be a thinner grease...or a heavier oil, depending on your perspective - and this wouldn't really have any benefits over and above what was already available.
If, however, they formed an emulsion, it could have some interesting qualities.
The easiest way to do this was to simply mix the two things together (I'm clever like that) and see what happened - and this (on the right) is what happened.
When the two lubricants are mixed they appear to form a new grease, but once left to settle it becomes clear that the mix starts to separate. This blob has been sitting for a few hours, and bubbles have formed in the mix. Some of them, particularly the larger ones, are air bubbles - but most of them are pockets of oil.
The longer the blob is left, the smaller the bubbles become...until it ends up looking like rather more like a watery milk.

Grease and oil mix in tubeDo the two lubricants eventually settle out of each other? And do they mix of their own accord (i.e. without being agitated)?
To test this I popped some silicone grease into a clear plastic tube, then dropped a layer of oil on top - and left it for a couple of weeks.
Absolutely nothing happened. The oil simply sat on top of the grease. This wasn't very entertaining, but it at least told me that mechanical intervention is required to initiate the mix.
So I mixed the grease and oil together around the boundary point, just to see if the mixture would more readily integrate into the pure grease. Again, no go - it just sat on the top. But the really interesting thing is that it showed no signs of separating beyond the point where the bubbles broke down to such an extent that they formed a milky cloud within the grease.

So what does it all mean?
Well, I know that oil in a key barrel is displaced when the key is operated (due the the pumping action of the key barrel against the pivot) and flows back into the void when the key is released. I know that grease is also displaced, but is much less inclined to flow back - and tends to stay put in areas that aren't subject to quite so much pressure. As such, both methods of lubrication have their pros and cons - but what would be brilliant is a lube that covered both bases; to have some sticking power and yet also have the ability to flow...and to do so with the least amount of drag.
This oil/silicone emulsion appears to have those properties. The grease acts like a filler and deals with any inaccuracies between the pivot and the barrel, and the oil flows in and out of any gaps that appear as a result of the key flexing.

It all sounds rather too good to be true, and yet early experiments with a transparent key barrel seem to indicate that the oil will readily recombine under pressure...and equally readily emulsify within the grease by mechanical action (stirring, essentially). Better still, the stiffness of the grease acts like a plug in the ends of the key barrels and helps limit the amount of oil that gets pushed out of the barrel under pressure - and the amount of grit/moisture that may get sucked back in. The whole effect is like lubricating the action with thousands of microscopically-sized ballbearings.
It's still very early days mind you, and there's a need for long-term and stress tests - as well as seeing how the emulsion reacts over a range of temperature/humidity, and how the mechanism wears - but so far it's looking very interesting indeed.

If you fancy trying it out, it's very simple. All you do is duplicate my original mistake.
Select a suitable key - one of the palm or side keys perhaps, or even the side F# or low C/Eb if they're mounted on rod screws. Remove the key, degrease the rod screw and key barrel, apply a blob of grease to the rod screw and work it into the key barrel. Remove the rod, give it a quick wipe with a tissue (don't degrease it), then pop a drop of oil on the rod and insert it in the barrel again. Work it back and forth, then remove the rod and refit the key. Pop another drop of oil on the rod, if you like. And if you don't fancy taking a key off, you can use the method on one of the rollers.

Let me know how it goes. In the meantime I'll carry on testing the mix until I can prove that there's something fundamentally wrong with it - and you can find more details in the Oiling The Action - For Geeks article.

 

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