Overcoming the problem
From the above I have attempted to say that the lathe headstock, and the cutting
tool, are both capable of machining to fine limits but that this is prevented by
the inability to feed the tool reliably by the small increments necessary. How then
can this be overcome.
The simple answer is to set the top slide to an angle of 6 degrees, precision is
not required, the lathe's top slide calibration should suffice. In this case the
ratio between the traverse of the top slide and the radial movement of the tool is
very close to 10 : 1. Therefore, it can be taken, that feeding the top slide by 0.01mm
will cause the tool to infeed by 0.001mm. This overcomes the problem of placing on
very small increments using the cross slide.
However, if one is working to a face, then traversing the top slide will increase
the length of the parallel portion made. In this case, therefore, the parallel portion
should initially be made slightly on the short side and finished to length once the
required diameter has been achieved.
If wishing to work to even closer tolerances, typically if making a hole gauge, then
an angle of 0.6 degrees gives a ratio of 100 : 1. Unfortunately, using the lathe's
top slide calibration will not be adequate for this, but can easily be set up providing
a between centres test bar is available, see Sk. 1.
The above assumes that the workpiece being machined is of a diameter and length that
prevents it from being deflected, other than minutely, by the cutting forces, if
not, then turning precision diameters becomes much more demanding. All I can add
regarding this is firstly to try to avoid the requirement. If this is not possible,
then a sharp tool is absolutely essential, and when nearing the required diameter
very shallow depths of cut should be attempted. Also, make multiple passes at the
same cross/top slide settings in an attempt to machining out the effect of the workpiece
deflecting (spring cut).
A very slow feed rate should also be used which probably means that it will have
to be hand fed using the saddle leadscrew as the power feed may be too fast. Of course,
try the power feed first and check the result.
Even at larger diameters, making a second, or even a third, pass at the same cross/top
slide settings is always worthwhile before measuring and adding more depth to the