Leadscrew C13 Cut a length of 10 mm steel a little over 200 mm and place in the three
jaw chuck, face and centre drill the end. Move out by about 160 mm, supporting the
end with the tailstock centre. Cut the thread at your chosen pitch. The easiest way
of making a suitable cutter is to use a piece of round HSS in a suitable holder and
grind it to the required width, nominally 1 mm if 2 mm pitch or 0.050" if 10 TPI.
Both cutters for the internal and external threads will of course need to be very
marginally on the wide side, say plus 0.05 mm/ 0.002", else if undersize the threads
will not mate as the web will be wider than the space.
Having then ground your cutter rotate it in its holder to suit the helix angle of
the thread being cut. Working to the angle at the core diameter (8 mm), which is
the steepest, the angle will be 4.6° for a 2 mm pitch and 5.8° for 10 TPI. The 10
TPI figure still being based on a core diameter of 8 mm. The angles are not critical
as the side clearance on the cutters will of course allow for some variation. If
you limit the depth of cut at each pass then you should get away without having to
use a travelling steady, though if you possess one I would definitely recommend its
use, Photograph 6.
If you feel hesitant at cutting a thread on the lathe then a copy of “Screw Cutting
in the Lathe, Workshop Practice Series, number 3”, would be worth reading. If though
you are practised at cutting threads on the lathe but have never cut a square thread
there is one important difference. With a conventional V form thread, if having made
one thread then the other can be gradually increased in depth until a fit is achieved.
This though is not the case with a square thread. For example, if cutting the internal
thread, increasing the depth of the cut will increase its outside diameter but not
its core diameter. The initial bore is therefor important.
When making the leadscrew, reduce the diameter at the outer end to 8 mm (the core
diameter) for about 6 mm. As you increase the depth, the tool will eventually contact
the 8 mm diameter, at this point the core diameter will have been reached. This is
easier and more reliable than working to the dial readings of the cross slide.
For those who have made either of my grinding rests, or are thinking about it, Photograph
7 shows a cutter being ground using one. Grinding such a small tool free hand is
not impossible but will need to be done with considerable dexterity and care.