If however, your lathe does not have an adjustable tailstock and you are unable to
achieve the accuracy required, then there is little point in continuing with making
the between centres test bar as it will not met the requirements of the future use
for it. Whilst this is unfortunate, it is not a disaster, as the error will not be
a problem in the vast majority of tasks undertaken on the lathe.
If though all is well, then continue to make adjustments and take further cuts of
0.05mm to 0.1mm deep until the part becomes parallel. Do now take further light cuts
endeavouring to get to 24mm diameter say +/- 0.005mm. Whilst not crucial, having
a simple dimension as its diameter may prove beneficial when the test piece is put
to use in the future.
Do note that the fine edge on the finishing tool will quite quickly deteriorate,
if therefore you make many cuts to get to this point the edge may well require honing
Remember also, that when machining between centres the part will warm up and expand,
possibly resulting in the need for the tailstock to be reset. In this case, the cuts
taken are only light and the temperature rise will be low but tailstock adjustment
should nevertheless be checked. Also, remember that a break in proceedings can cause
the part to cool and contract, similarly making adjustment necessary.
During this machining operation do not move the tailstock along the lathe's bed but
make all adjustments by advancing and retarding the barrel. The reason for this
is that the barrel may not be exactly parallel with the lathe's axis. Remember that
all components have an allowable machining tolerance, even super precision items,
so that this does not necessarily indicate a fault in the lathe's manufacture.
The effect of this would be that the centre would not be on the same axis if the
extension of the barrel was different to that on the previous pass of the cutter.
An important point to take note of is therefore that the lathe may not turn parallel
in subsequent applications if the barrel extension is different and small adjustments
may need to be carried out each time a very critical item is turned.
The process just carried out will though have familiarised you with the theory. In
any case, my concern for the accuracy of the alignment of the barrel is probably
unfounded in most cases, as the lathe being used will be sufficiently accurate for
it not to be a problem. You do not need to wait though for experience to be gained
over a number of applications to see how the lathe performs in this direction, it
can be determined with the test piece just made.
With the test piece in place and the barrel in the same position as it was when the
test piece was machined, move a dial test indicator along its length with its plunger
on the front edge and deflection of the indicator should be minimal. Now carry out
the same test with the barrel fully extended and with it fully home, if in both cases
the indicator pointer remains essentially static along the length the tailstock can
be considered sufficiently accurate.
Without going into a lot of detail regarding its uses, one application for the test
piece can be to use it for setting the top slide accurately parallel with the lathe's
axis using a similar approach. This is better than turning a test piece using a turn,
measure, adjust, turn, measure, adjust approach.
Whilst there is very little to see resulting from these pages, just a piece of parallel
steel,(two if you also made the Cylindrical Square on other pages) the experience
and knowledge of the lathe's capabilities should be invaluable.