DoN. Nichols wrote:
On 2008-10-14, Christopher Tidy wrote:

Tim Wescott wrote:


[ ... ]


It doesn't have to be tidy, it just has to _work_.

No, I mean the actual length of the blade is 3/16".


The length from handle to tip, or the length of the parallel
surfaces to go into the slot in the screw head? 3/16" is awfully short

Sorry, it looks like I've created some confusion. I should probably have
said 3/16" wide, but I wanted to avoid confusion between width and
thickness. I am talking about the tip only.

I have a set of "precision screwdrivers" from Radio Shack (US), the
largest one is 9/64" x 0.025 or so. You ought to be able to grind this
freehand on a wheel -- just hold some 3/16" rod straight down to get a
nice radiused cut on each end. I'd be tempted to grind it to a knife
edge, then go straight in until I'd achieved my desired thickness.


Except that this will leave the blade tapered, so it will be
particularly thin at the bottom of the slot, and likely to twist and cam
out of the slot.

Good point. Unless you can grind away a curved piece of material, so
that the blade faces are parallel at the tip. I think this is what
you're suggesting below.

I just don't want something which is a bad fit in the slot, and which
may damage the screw heads. That's a good idea to grind it to a knife
edge first. It might help to get a neat shape.


Grind on the top of the wheel (wheel coming towards you),
allowing the wheel surface to produce nearly parallel surfaces at the
tip. Put something soft and large on it (soft wood like pine will do) so
if it digs in from a slight misalignment, it won't have a small end to
be driven into your body.

This sounds a bit risky. Something that, going by my gut reaction, I
would prefer not to do. Also, the wheels on the grinder I have access to
are covered at the top. The are only exposed at the front.

If you want a non-slip tip, make the final grinding at right
angles to the shank so the grinding marks will act as a gripping
surface.


I guess I could try making one from a piece of drill rod, but it might
need quenching, I'm not sure. Or I could just start with a screwdriver.


Don't get it that hot and quench. This will make it harder,
more brittle, and more likely to break off in the slot. Just grind
gently -- keep it fairly cool, and the default temper of drill rod should
be very good for your purpose.

Surely drill rod in its raw form isn't as hard wearing as a good
screwdriver, is it?

Note that it is standard practice for gunsmiths to make
individual screwdrivers to precisely fit the screw slots in individual
guns, so it is not difficult to do. And an old clock mechanism sounds
like something which should get similar care -- especially since screw
slots and screwdriver blades were not standardized during the times when
some of these were made.

I was referred to the gun tools supplier Brownells yesterday
(http://www.brownells.com). They have a set of especially thin
screwdriver bits which look suitable. But I e-mailed them and found out
that it's $35 to have them shipped to England. That put me off a bit,
but I might still go for them as I think they'd be useful. Anyone know
how good the Brownells tools are?

Many thanks,

Chris