Many lathe's have a means of moving the tailstock bodily across the lathe's axis,
providing the facility of deliberately producing tapered work, though no more than
a degree or two. As this permits the centre to be off centre it must also be used
to get the centre back on the lathe's axis for more normal turning. If this adjustment
is not available then it will be a major fitting exercise to correct any fault and
is well beyond the scope of these pages, especially as the method will vary from
one lathe type to another.
Even if adjustment is not provided the following test piece should still be attempted
so as to be aware of the situation that exists. It may first prove that the tailstock
is sufficiently accurate, the most likely case, and secondly, the test piece produced
can be a useful accessory.
A Between Centres Test Bar
Cut a 150mm length of 25mm diameter mild steel and centre drill the ends. There are
many ways of doing this and the method chosen will depend on the level of accuracy
required, and of course the equipment available. For this application I would suggest
the simplest, as any error present will be machined out due to the part being machined
over its complete length. However, if you have a fixed steady you could try the method
described in my pages on using steadies. This will give a precise result.
Otherwise, set the height of your surface gauge to 12.5mm, as accurately as can be
achieved. Mark both ends of the bar with marking blue and place it on the surface
plate or some other reasonably flat surface. Scribe a line on the end, rotate the
bar about 120 degrees and scribe again and a further 120 degrees and scribe a third
time. If your 12.5mm was spot on then the three lines will cross in the exact centre.
If however there was a small error the centre of the bar will be in the centre of
the small triangle produced.
Unfortunately, you will be working with a sawn end so the marks may not be the clear,
but with care should be adequate. Centre punch the end and centre drill to about
5mm diameter on the drilling machine. Do make sure the bar is accurately upright
whilst being drilled. Repeat the process on the other end.
With the piece mounted in the three jaw chuck and the outer end supported by the
tailstock centre, face the end as close to the centre as the tool being used will
permit. If you have a "half centre" then use this, even if only a drill chuck mounted
version made in the workshop, Photograph 4 shows an example being used on another
project. Follow this by machining the reduced diameter.
If you have a tool that presents an angled leading edge, then use this, it will give
a less abrupt transition between the reduced and full diameters than would a 90°
knife tool, see test bar drawing, Sk. 1. Reverse the part and produce the reduced
diameter on the other end, as this time it is being held on the now machined portion
do protect it from damage.