blog of everyday life on and around the workbench
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.
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.
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
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.
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.
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.
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.
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.
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.