As the part has long term uses, it is desirable that the centred ends are perfectly
in line. Sketch 2 shows, much exaggerated, the error that could have occurred as
a result of them being drilled on the drilling machine and which we are trying to
avoid. To eliminate this possibility the centres will now be trued on the lathe.
With the part still in the chuck , remove the centre and fit the fixed steady to
locate on the reduced diameter, re-engage the centre to prevent the part moving whilst
the steadies arms are set. Remove the centre and, with a centre drill in the drill
chuck deepen the drilled impression to about 8mm diameter. This action will, sufficiently
for this case, eliminate any error that may have existed in the alignment of the
centres. It will also remove any unmachined end due to facing the end whilst using
a full centre. With the first end now centred accurately, reverse the part and centre
drill the second using the same sequence.
The part must now be located between centres so that its outside diameter can be
turned along its length. Fit the driving dog onto one end of the test piece using
some protection to prevent damage to the already machined end. Whilst turning between
centres, the peg on the drive plate acts against the arm of the driving dog to keep
the workpiece rotating.
If the part is not round, maybe when turning a round portion on a square bar, the
cut will be intermittent and the drive dog will bounce about on the drive peg, this
is not ideal. With a continuous cut the problem is much less, but can still occur
with light cuts. It is therefore good practice in all cases to tie the arm on the
dog to the drive peg on the drive plate by using some strong twine or copper wire.
When using centres it is good practice to very slightly over advance the tailstock
(and I do mean very slightly) to ensure that both centres are fully home, always
having first thoroughly cleaned both centres and their.
sockets. With this done release the pressure and readjust to a running position,
do not forget the oil. The part is now ready to have its outer diameter turned along
its length. For this operation it is highly desirable that a fine feed is set up
making it probable that a better finish will result. It will also make the task less
laborious as the part is going to be turned along its length a number of times.
No matter how careful you were at centring the part it is likely to run slightly
out of true to begin with (the truing of the centres above only made them accurate
with each other and the reduced diameter ends). Because of this first turn the outer
diameter to machine out any inaccuracy, do though remove the minimum necessary, say
take cuts of 0.1mm depth.
With the part now fully machined replace the knife tool with a finishing tool. Typically
a knife tool with a small flat, see Sk. 3. This must be honed to a fine edge and
as at the depths of cut being taken dictate that it must not be above centre, set
just below centre as this is the only way that it can be certain that it is not above.
With this done, take a cut of 0.05mm depth, Photograph 5, and measure along its length.
If already parallel, and we are looking for no more than 0.005mm, ideally 0.002mm,
difference end to end, your tailstock is already set accurately. It is though more
likely that the tailstock will need adjusting, inspection of the tailstock should
make it obvious how this is made, if not, then consult the lathe's manual.