My advice would be to cut the thread gauge with a chaser or a single point tool that
produces a precise thread form, Photograph 2, as using a tool that does not produce
the radius on the outside of the thread will complicate the issue in terms of its
diameter,for this I would recommend +0.02 to + 0.05mm. However, using a basic single
point tool would be acceptable when eventually cutting the internal thread. I will
assume the viewer is conversant with cutting threads and will not elaborate on the
subject. If guidance is though required then seek out further reading on the subject,
such as "Screw cutting in the Lathe" Workshop Practice book number 3
When making the parallel gauge do make a short length, say the first 3mm, 0.05mm
under size as this will give an indication that the bore is close to the required
size and at which point considerable care has then to be taken when proceeding.
Attempting a tolerance of only + 0.002mm, which is easy to write down but far from
easy using normal turning techniques to achieve, how then is this accomplished? The
lathe's cross feed in likely to be calibrated in 0.02mm or even 0.025mm divisions
in which case we are attempting to reliably work to a depth of cut of less than 1/20
of one interval on the dial, virtually impossible using the cross slide!
Achieving precise diameters.
Fortunately, the solution is relatively easy to adopt and is to set the top slide
to 0.6° as this gives a ratio of 100:1 between the axial and radial movements of
the cutting edge. My method of achieving the 0.6° angle is briefly as follows and
can be done quite quickly if the required accessories are available. First, set the
top slide precisely parallel with the lathe's axis using a DTI and testing along
the length of a "between centres test bar" Sk. 1-1. Next, place a piece of steel
under the top slide clamp and, with its edge firmly against the test bar, clamp in
place, Sk. 1-2. Finally, loosen the top slide to enable it to rotate and at a point
100mm from the left hand end of the piece of steel place a 1mm diameter drill shank
between the piece of steel and the test bar with the other end against the bar. With
that done clamp the top slide in this position Sk.1-3, see also Photograph 3.
For less demanding situations an angle of 6° gives a ratio of 10:1 between axial
and radial movement with the lathe's calibration for setting the angle of the top
slide sufficiently accurate for this requirement.