Some parts when assembled may be secured using a grub screw. for this a plain parallel
mandrel can be made and the part fitted using the grub screw. A flat on the mandrel
for the screw to tighten onto would be a good idea. If there is some clearance between
workpiece and mandrel the action of the grub screw would be to force the part off
centre and concentricity would suffer.
Adhesive can be used to fix a part to the mandrel, the type that weaken with heat
being ideal as this aids eventual removal. However, the part, with mandrel, would
require to be removed from the lathe to have the heat applied. It is therefore only
suitable for a single part if concentricity is vitally important.
Close fit, end fixing
The methods described thus far, also permit with care, turning the end faces fully
as well as the outer diameter. However, if this is not required, an end fixing screw,
as illustrated in Sk.2, works well. The sketch shows that the workpiece also contacts
the end face of the raised portion of the mandrel and this is obviously a requirement
for using this type of mandrel. Photograph 3 though, shows a slight variation on
that method in that the end of the mandrel mates with the bottom of the bore in the
part being machined. This is a die holder as shown in Photograph 4 and is being machined
in this way to ensure that its shank is both concentric to the dies housing and true
to its base.
The viewer may suggest that this could have been turned with the bore made and the
shank then turned with a left hand knife tool(one that cuts on its right hand side).
This would though leave a stub in the chuck and as a number of holders were being
made much more steel would end up as scrap. Using a fixed steady would avoid the
scrap but I preferred the method I have illustrated. Should the viewer like more
detail on the alternative, see using lathe steadies.
Between centres mandrel
It has been explained that for perfect concentricity the mandrels, so far described,
must be machined and used without being removed from the chuck. Reuse will, even
if returned to the four jaw chuck, not achieve that precise result obtained in the
first instance. However, if a mandrel is known to have use more than once, making
it for use between centres will provide concentricity comparable to the original
situation. This providing that the bore of the lathes spindle and the centre used
in it are accurate enough for the centre to run perfectly true even if removed and