Screw Plates and Taps

Hoping you wondrous Orchidians can help me again! I have recently
purchased a screwplate and tap set. The screwplate has two sets of
holes, threaded, which are in line with the engraved Gauge size (at
either side) The holes run from 20 down to 7. One row of threaded
holes in the screwplate has a small slit accompanied by a second
hole, then the second row of threaded holes do not have the

So questions!

  1. Why the slit - it would seem logical to assume that the slit is
    some sort of expansion provision, or perhaps it is to allow the metal
    being trimmed away to clear? Help!?

  2. Why two sets of holes? Is the second hole perhaps a 'finishing’

  3. What is the general rule (if there is one) as far as tolerances
    go? i.e. what size wire is appropriate for what size hole? and also
    what size chenier for what size tap I need to be able to make a thread
    in chenier and then have a screw made of wire to suit.

Do I allow 0.5mm for the thread, 1.0mm? I hope that makes sense!

  1. In the past, I had a jeweller friend make some threaded
    components for me and we had problems with the thread stripping. I
    wondered if this was possibly due to incorrect tolerances having been
    allowed (an engineering friend explained hat sometimes if a thread
    was cut too deep the outer edge was fragile and therefore could strip
    more easily). The other possible problem was perhaps metals that
    were in differing states of hardness. My jeweller friend was using
    manufactured chenier and annealed wire so I did postulate that
    perhaps one surface being hard and the other surface being soft
    might not be a good combination…

I’m sure there will be Orchidians with reams of experience so before
I go and stuff anything up…here’s hoping for some good advice
and thanking you in advance!

:slight_smile: Kimmyg
Queensland - Australia

Hi Kim,

The two rows of holes on your screwplate are cutting and finishing -
the slit on the cutting holes is to clear the swarf. To select the
correct hole for and size wire, push the wire into the holes until
you find one that the wire will only just go through. The correct
tapping hole is then two sizes smaller. You need to be a bit careful
in using these screwplates as they don’t work in the same way as an
engineers die nut. They use a combination of cutting and forming to
produce the thread and, indeed, the finishing holes without the slit
use pressure only to form the metal. So, it is important that you
lubricate the wire, particularly in the plain holes, or it will stick
and break off - then the only way to clear the hole is to drill out
the stub of wire - not an easy task!.

Best Wishes
Ian W. Wright
Sheffield UK