DoN. Nichols wrote:

But I'm really glad I had the foresight to get my feeler gauge set out
and measure a few screw heads in the electrical equipment and clocks I
intend to work on. Some slots turned out to be 20/1000", perhaps half of
the total. The rest were 25/1000".


More commonly written 0.020" and 0.025"

From the point of view of consistency, I feel a bit bad describing a
screwdriver tip as 3/8" x 0.025". I was using fractions for the larger
dimension earlier, so I decided to be consistent. Using fractions and
decimals together seems weird.

The feeler gauges fitted the slots
really closely, so I'm pretty sure they're intended to be these sizes.


What this really means is that the slots were cut by milling
cutters of those thicknesses. I've set up to make screws for English
system concertinas (I've settled on M3.5x0.45 since the original thread
matches no current standard), and among other operations, I made a block
which will hold 20 screws in four rows. I have four slotting cutters on
the horizontal mill's arbor so I can slot all of them in a single pass.
IIRC, the cutter thickness for these is 0.0156" (1/64th") -- selected to
match the slots of older concertina endbox screws. Once you are set up
with a single slitting saw size, you really don't want to change
frequently, so they used what they had for all the screws, instead of
making each to a dimension appropriate to the job.

Strange size. I thought the standard M3.5 pitches were 0.35 mm (fine)
and 0.6 mm (coarse).

Many modern screws are made by "heading" the stock which carried
the thread -- bashing it into a die which causes it to take on the
appropriate shape -- and often also drives the slot into it at the same
time, instead of cutting it. Given that, such modern screws are likely
to have slots which are a better fit to a taper-ground screwdriver than
to a parallel ground one. :-)

Good point. I imagine that if the slot is formed in a die, it will end
up being slightly tapered.

The gauge fitted better than a taper ground screwdriver bit. You could
be absolutely sure when you had the right size. I guess this is one of
the disadvantages of a taper ground bit.


It depends on how the screws were made. Clock screws will have
the slot cut by slitting saws in a milling machine (or a clockmaker's
lathe acting as a milling machine).


So I need a set with bits which are 20/1000" thick. The Brownells set is
increasingly looking like the best option. But I'm going to have to
steal myself to make the purchase,


You mean that you will have to illegally take other's property?
Or did you really mean "steel yourself"? :-)

You're right. I did mean "steel myself". Thanks for correcting me!

as the delivery and likely import tax
will be high.

I notice that Brownells offer two handles. A magnetic handle and a
"clip-tip" handle. Initially I was inclined to go for the magnetic
handle so that I could use it to hold steel screws. But now I'm not sure
as I think the "clip-tip" handle may hold the bit more tightly. Some
magnetic handles do not hold the bit tightly enough. Anyone have an
opinion on which handle is better?


I would avoid the magnetic style when working with clocks and
tape recorders, as both can be damaged by magnetic fields. In a clock
(or a watch) the magnetic field can cause alternate layers of the spiral
spring on the balance wheel to stick together. (Not a problem with
pendulum clocks, of course), and with tape recorders, the magnetic field
can magnetize the tape heads resulting in a serious increase in hiss.

I hadn't thought of this. Thanks for pointing it out. I'll probably go
with the "clip-tip" style of handle now.

But the selection really is normally a function of the bits
being purchased. The ones to be held magnetically are very short bits,
while those intended to be held by the "clip-tip" are longer, and have a
groove turned near the end into which a ball detent drops to keep the
tip from falling out. I tend to use the latter design most of the time,
but you can get magnetic holders which will fit into the "clip-tip"
style body when you need to handle less common bits. For example, I
have a set of bits which contains 100 pieces, of which all but 4 are
different sizes of bits. There are duplicates of the most frequently
used sizes and style -- 5 copies of then #2 Phllips, while only one each
of #0, #1 and #3. Same for the Pozidrive bits (Phillips with a extra
set of points in the inner folds of the socket).

It seems that the Brownells bits will fit either type of handle.

Many thanks for the advice.

Best wishes,

Chris