A rolling blog of everyday life on and around the workbench

05/02/2021: There's an old axiom that says "Invent a better mousetrap and the world will beat a path to your door". I've long been looking for such a gadget that applies to saxes - but as yet, alas, I haven't come up with a suitable candidate. But I have come up with something that I reckon will tip the old "Why didn't I think of that?" meter into the red among the great and good repairers of this world.

OK, here's the problem. On a typical modern octave key mechanism there's a bar that 'see-saws' on a central pivot pin. Over a period of time both the pin and the socket that fits over it wear - and when this happens you end up with a clunky octave mech...and a lot of free play (or lost motion) on the thumb key. It's a very common problem - and the solution is to fit some sort of bush over the pin to take up this wear.
However, the wear is really quite small - and in most cases it's impossible to find anything thin enough to serve as a bush. In any event, the wear will be uneven - so there's a need to true things up before you fit a bush. And it's often the case that someone's had a go at sorting the wear already - which often means that they've taken a hammer to the pin and bashed a couple of flats on it.
This works - in the short term - it splays out the pin and increases its diameter...but now you have just two points of contact, and that means more wear over time.

So there's a need to reduce the diameter of the pin so that you can fit a bush or a sleeve over it, and there may also be a need to true the pin up...or even restore its roundness.
If you have an engineering bent you might ask why you wouldn't just bore out the socket on the see-saw bar and have done with it. Well, you could - but there's usually not a lot of spare metal to play with, and making the hole larger isn't going to be good for longevity. So your attention must be directed at the pin. But how to reduce its diameter and true it up?
Bunging it in a lathe would be ideal - and you could certainly do this, but it would take you an appreciable amount of time to set the job up. Like wise, you could mill it...but the same caveat applies. This leaves the old-fashioned method of getting busy with a file, a steady hand and a sharp eye. But this takes time, and is a right royal pain in the nether regions. I figured there had to be be a better way. And there is...

Here's an octave mech that has a lot of free play between the swivel pin and the swivel arm; however, it's not quite enough play to allow you to fit a Teflon sleeve - and in any event the swivel pin has a taper on it. You might spot that there's a slot cut into the pin - and this is a rather clumsy way of allowing you to take up the free play. You insert a screwdriver into the slot, give it a slight twist and splay the pin out. It sort of works, but it does so by increasing the taper on the pin...and that's not great. For the purposes of this article I've soldered the slot up, just to prevent things going south during operations.

What's needed is a way to reduce the diameter of the pin so that a sleeve can be fitted. This can be either a Teflon tube or, if you want to go down the posh route, a brass tube. Either will work.
You might ask why not simply ream out the swivel bar? Well, you could in this instance - because there's a fair bit of 'meat' available - but more often than not you'd end up making the swivel socket dangerously thin. It's fine to ream the socket enough to true it up, but I wouldn't recommend going much further.
Now, you can buy reamers that will cut out a hole accurately - they're very common and relatively cheap. You can also buy reamers that have the cutting faces on the interior of the tool - but they're not common at all and are thus often incredibly expensive. And you'll need a selection of sizes, unless you buy an adjustable reamer...and you don't wanna know how much those things cost.
This isn't a job you have to do very often, so if the tool to do it costs a very significant amount of money it's going to be a long time before you recoup the cost of the tool - so you might as well resort to the file. What I wanted was a cheap solution; something that could be made by adapting a common tool meant for a very different kind of job.

The idea I came up with comes from seeing what happens when a drill bit slips in a chuck. You know the score - you're drilling a hole through something tough and suddenly the drill bit stops dead but the drill keeps spinning. This results in the chuck chewing up the drill bit stub.
If we could control that 'chewing', we'd have a means of reducing the diameter of the swivel pin, right? Unfortunately a drill chuck is a pretty crude bit of kit. When fully tightened onto a shaft it does a pretty good job - but if you slacken it off slightly the jaws tend to flap around in the breeze. In other words there's no way to accurately hold the jaws in position unless they're clamped down hard on the shaft.
And despite the chuck's propensity to chew up drill bit shanks, it wouldn't make for a very neat and accurate job on a brass pin. Yes, you could modify the jaws - but that's quite a lot of work, and you still have the problem of the free play in the chuck. And you don't even know if it would shave the pin down parallel. But the idea in principle was a sound one. Three points of contact on the pin (a very stable and self-centring setup) with three cutting edges and a means of adjusting the 'bite'.

The first method I came up with was using a screwthreading tap wrench. A tap has a square-section head on it, and the wrench has a pair of opposing V-shaped jaws to accommodate it. A pair of opposing V blocks would be self-centring - and depending on the size of the Vs you'd either get four cutting edges or, more commonly, two. Either would work. And it did work...in fact it worked very well.
Trouble is, the jaws of the wrench are slightly recessed and this means that any key arms will foul on the tool before the jaws have cut the full length of the pin. I considered building a tool along the same lines, but with jaws that extend beyond the handles. It could be done, but it seemed like a lot of hassle. There had to be a better way - a cheaper and easier way...though if you're stuck with the job, this method will get you most of the way there.

My thoughts returned to the drill chuck and how much work it would be to modify it. But there are other kinds of chuck...most notably the collet chuck. The thing about collet chucks is that they can only handle a specific range of sizes. You can typically bung anything from a 1mm drill bit up to a 13mm jobby into a standard drill chuck - but collets tend to offer you about a millimetre of variability. If a collet chuck will accept, say, anything with a diameter from 4 to 5mm, it won't accept anything outside this range. This property makes it an ideal candidate for a tool that can possibly reduce the diameter of a rod because the limitation on the sizes it can accept will also limit the amount of backlash (or free play) in it.
I did some experiments - but initial tests were disappointing. I had hoped that I could simply bung the swivel pin in the collet, tighten it up a bit and then spin it to cut the metal off. Unfortunately it didn't work...in fact all it really did was burnish (polish) the pin. And if I tightened it up too much, the workpiece would simply rotate with the collet and there was no way to stop it. What I needed to do was introduce some cutting edges to the setup - so I ground out the ends of a couple of the collet jaws in the hope that these would pinch down on the work and allow the collet to shave the pin down. Well it sort of worked, but the results weren't spectacular and it still required quite a lot of pressure from the collet...which made it difficult (and risky) to hold the workpiece.

So I had a sit-down and a cuppa and thought about it a bit more.
It seemed to me that any cutting action the collet might have is mostly confined to its face. Once the workpiece enters the bore of the collet, the best that it can do is (literally) rub along quite nicely. Most of the work has to be done before the workpiece enters the collet. The rounded grinds I'd put on the collet had little chance of doing much of anything at all.
So I ground another collet - and this time I put a very steep and flat grind on one of the jaws. And just to beef things up a bit I took a diamond nail file and ran it through the slots in the collet to sharpen up the edges...and then dressed the face of the collet with a stone. Every edge was nice and sharp.
This did the trick. With the collect just snugged up so that it was set to a fraction below the diameter of the pin, it shaved the brass off as the collet was spun and the workpiece pushed into it. Better yet, the burnishing action deeper inside the collet ensured a decent finish. Not perfect...but if you're going to be fitting a sleeve, a very slightly rough surface makes for better adhesion.
Even better, I quickly discovered that I didn't even need to spin the collet; with the face being sharp it was possible to simply push the workpiece into it and it would shave the pin down. All you had to do was turn the workpiece a few degrees with each pass - just to keep things nice and round. And even more betterer (yeah, I know), the collet can restore a tapered pin back to parallel.

Speaking of roundness, another advantage of the collet is that it can cope with an out-of-round part. A common 'bodge' on worn octave swivel pins is to place the pin down on an anvil and give it a couple of smart taps with a small hammer. This puts a pair of flats on the pin, which in turn splays out the remainder. This makes it a tighter fit in the swivel bar...but with a substantially reduced contact area (which means it wears quicker). I say bodge, but it works quite well and will get you out of trouble for quite a while...but it's not what I'd call the 'gold standard' job.
If you're going to put a sleeve or a bush over the pin, you need to restore the roundness - and the collet chuck does this for free - and just to prove it I bashed a couple of flats on the pin.
Here's just such a pin at the halfway stage of restoration. You can clearly see that the collet is cutting the metal around the flats...gradually working the diameter down. Note the burr on the end of the pin next to the key barrel. This is not wanted, but seems to be a result of how the collet works. Fortunately this part of the pin sees no action, but it still looks rather scruffy.

And here's the finished pin, ready for a bush or a sleeve to be fitted. The flats have gone and the pin has been reduced down to a cylinder again. I cleaned up the burred section by running the pin though a larger collet to leave a neat step. If you really wanted to you could go through a series of collets to eliminate that step.
With a sleeve fitted and the octave mech assembled you can see that there's a lot less free space. It's not perfect because this is just a cheap Chinese mech with a sort of ballrace fitting in the rear of the swivel bar socket - but it's a very great deal better than it was. Were it a client job I'd ream the socket out (plenty of meat in this case) - and, if necessary, fit a brass bush to it...but I think you get the gist of it.

Could the system be improved? Oh, undoubtedly. I might, for example, try grinding an opposing cutting edge...or maybe three of them. I'll be honest, using the tool is a bit of a slow process but I can't think of any other method that can be adapted to a range of swivel pin sizes that offer the benefits of rounding out the pin and removing any taper. Unless you file it by hand (which is a pain).
For now, though, it works - and that's good enough.

A few observations though. I initially started my experiments with an ER16 chuck, but this proved to be a bit too large and would often foul on the key arms. So I stepped down to an ER11 collet with a mini nut...and even then I had to shave a little off the leading edge of the nut.
The process works best under power - but you can do it statically is you have no lathe. You can also by straight-shanked collet chucks with shanks small enough to fit into a cordless drill chuck.
As for feeds and speeds - I found that a higer speed gives the best finish, but comes at the risk of the part being snatched out of your hand. It won't damage the part, but you could end up copping a smart whack across one of your fingers. And, of course, there's the risk of getting sleeves caught up if you haven't bothered to roll them up. Slow and steady is safe enough, and you can always speed up for the final finish.

In terms of costs there's no great need for precision, so a set of cheap collets will do just fine. In fact they're probably a better bet given that they're likely to have sharp edges on the jaws - and an equally cheap collet chuck will do very nicely.
I paid £20 for a chuck and a set of collets (including some half sizes) from Banggood. The only extras you'll need is a diamond nail file to sharpen up the slots, a fine stone to dress the face of the collet and a means of grinding a relief on one of the jaws. I used a diamond disc for this (again, from Banggood). All told I probably spent around £25 on the whole deal...which is pretty good for a single-purpose specialist tool.