August 2019 15
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Turret Stop
protrusion.
So, we now need to measure the
protrusion (if any) of the indexing hub
from the body as shown in photo 10. (We
can either face the indexing hub back to
be fl ush with the turret body or take the
deviation into account). As you will see from
the photograph my indexing hub is slightly
proud of the turret body.
It is now time to drill for the detent
positions and for the peg keys. Measure the
protrusion (if any) and add to the 0.250”
dimension specifi ed. Set on a mandrel and
dimple in 6 places (at 60 degree increments)
with a 3mm drill on the centre line of the
indexing hub. This size will allow for positive
engagement of the balls while not locking-
up as the ball is not at risk of disappearing
into the hole. Photograph 11 shows the
general idea.
Once these six holes have been spotted,
don’t remove the mandrel from the
indexing head or deviate from centre line,
but reposition to drill and tap the holes for
the two peg keys. It is best to tap these
holes using the milling machine spindle to
ensure that they are truly in line with the
centre line of the hub as the peg keys will
fi t into the keyway to be milled on the stop
rod. Photograph 12 shows the tapping
drill being used prior to tapping.
3.6 Peg key
These look quite tricky to make, but they
are easy if you use a special holder. The
holder is drawn in fi g. 9 and shown in
photo 13. It’s also shown in use holding
the grub screws for machining the 0.092”
diameter tip which engages with the stop
rod.(8.1) - see photo 14. I made these items
from 2BA x 3/8” long grub screws.
The fi rst operation is to reduce the depth
of the key socket by around 0.08”, which
is no problem in a light duty item such
as this. The screw is held with the socket
outermost. Unless this is done the socket
is eff ectively too deep and when trying to
machine the tip you will encounter the inner
end of the socket and the newly machined
tip will fall off.
Now measure the length of the remaining
screw and calculate how much the screw
needs to be reduced in length to achieve
the fi nished overall length of 0.230”. This
fi gure will be around .065”. That done, fi t
the grub screw with the plain end outwards
by passing an Allan key through the central
and ‘do-up’ until tight. Face to length by
removing the calculated fi gure, and thenturn to achieve the reduced diameter of
0.092” for a length of 0.80”. Job done.
Photograph 15 shows the completed
indexing hub, together with the two peg
keys awaiting assembly. During the fi nal
assembly process the depth of the peg keys
will need to be set in relation to the keyway
in the stop rod, so that will have to wait
until the stop rod is available. To assemble,
fi t the peg keys and then back-off slightly- the keys can be secured with thread-lock
if desired. It may also be necessary to dress
the tops of the keys slightly if they fi nish
proud of the surface. Photograph 16
shows the keys fi tted into the indexing hub,
complete with the stop rod.
Assembling the turret
The good news is that the turret can fi nally
be assembled as shown in photo 17. Adjust
so that turret turns freely without any
backlash by adjusting the locking collar,
which should itself then be locked with
the grub screw. Add the detent balls and
springs and test for positive ‘indexing’. I
would suggest that you start off by using
three positions, adjusting, adding or
subtracting to suit the ‘feel’ which suits you.
All that needs adding now are the length
screws with locking collars and the turret
assembly itself is complete.13 14
15
16
The peg keys with the indexing hubThe peg keys have now been fi tted to the indexing hub - shown with the stop rodThe holder for machining the peg keys and the special centre punch Machining a peg key in the special holder