First, the cutter must be set just in advance of the steady's arms, say 4mm, using
the top slide and the slide locked in this position. Then, the material for the screw
must be held in the chuck with just a small length available and a short length,
say 10mm, turned to the diameter required, using the cross slide to set the diameter.
The cross slide is then locked and the steady's arms adjusted to the diameter just
turned and secured. This must carried out on the actual part being made, rather than
a piece of scrap, as the material will need to be supported at the start of the final
cut. The material is now moved from the chuck sufficient to make the part fully
and with that done, the short length already turned will enable the material to be
supported before the cut commences. Photograph 10 shows the cut in progress but also
evidence of the transition between the initial short length turned and the final
cut, more regarding this later.
Occasionally, a part will need to be supported by the tailstock as well as the travelling
steady, often when a long thread is being cut. For this, the outer diameter can be
supported as it will not change as the depth of the thread is increased. Photograph
11 shows a square thread being cut.
A problem of the method is that the first cut will throw up quit sharp burs which
will damage the steady's jaws on subsequent traverses. Therefore, after the first
cut has been completed the lathe should be set on a higher speed and the burs removed
with a wide fine file. Being a square thread, in theory, no burs will be made on
subsequent passes but repeating the operation may be worthwhile after a few passes.
If a normal thread is being made then the width of the thread will be machined every
time the depth is increased. As a result, deburing is best carried out after every
pass. In both cases the steady should be in advance of the cutter.
As my three jaw chuck is sufficiently accurate the photograph shows I used this though
using the four jaw would have been preferable. In any case the thread stopped well
short of the chuck so there was still some flexibility .
Fixed and Travelling steadies. Points to consider
Above, I mentioned that the arms will be worn by the roughness of a square thread
being cut, but even with smoother workpieces some wear will occur. In this case,
the arms will need adjusting but the method used will depend on the type of workpiece
being supported. If long and slender(photograph 2) use just the top arm as whilst
this will deflect the part down it will not move it back or forward that may cause
a problem if you are turning precision diameters. With short rigid parts that will
have minimal flexibility(photograph 8) then all three jaws should be reset.