In addition to the chuck's mounting, another task that requires accuracy is the
internal angle of the bore that matches the collet. This needs the top slide to be
set accurately to 8 degrees, drawing 2 showing a possible method using a between
centres test bar and a dial indicator with Photograph 6 showing the task being undertaken.
For an angle of 8° the indicator should deflect 4.00mm for 28.74mm traverse of the
top slide. If imperial, then 0.150" for 1.078" traverse.
Between centres test bar
If you have a between centres test bar fit this up between the mandrel and tailstock
centres. Do clean the bores into which these fit and the centres themselves. Ideally,
you should check that all is well using a dial indicator whilst traversing it using
the lathe's saddle. If you do not have a test bar perhaps now is the time to make
one, alternatively, place a piece of steel in the chuck and machine a parallel portion
just long enough to carry out the following operation.
With that proven, set up the dial indicator onto the top slide and set the top slide
to 8° using the lathe's calibration. Now traverse the top slide and check that the
indicator's deflection corresponds to that required, making any necessary adjustments
to arrive at precisely 8°. Do bear in mind that the indicator's plunger must approach
the test bar at 90°. It must also be pointing through the bars centre and be horizontal.
A degree or two error may not be a problem but any more and the setting will not
Now remove the parts used for setting up the 8° angle and thoroughly clean the lathe's
mandrel nose and thread, also the internal diameter and thread of the collet body
and fit it in place. Do ensure that the body is firmly screwed in place, even border
on over tightening it a little, as it would be a disaster if the body were to move
part way through the machining operations.
First, finish the outside diameters but do not be tempted to leave the knurling till
this stage, as the result will be to substantially tighten the body onto the lathe's
mandrel, may making it difficult to remove after completion.
If you are also making the closing ring I feel it would be an advantage to reduce
the 18mm dimension to 14mm as this will give a flange against which the closing ring
could be tightened for final machining the 30° internal angle. You will though need
to reduce the 13mm length of the thread so as to give a wider groove for the threading
tool to run into.