14 http://www.model-engineer.co.uk Model Engineers’ Workshop
threaded 5/16” BSF for a distance of around
3/8”. The blanks can be screwed onto
this mandrel and the recently parted face
cleaned up to give an overall thickness of
3/8”. Once all have been cleaned-up the
mandrel can be transferred to an indexing
head and the 6 holes around the periphery
added. These holes are 2.4mm in diameter
for a depth of 1/8” to avoid breaking into
the threaded bore.
3.3 Bronze bush
The bronze bush is a straightforward
turning exercise. Aim to make the reduced
section of the outside diameter (1”) an
easy fi t in the turret body so that it can be
secured with a Loctite retainer.
In fact, now would be a good time to
fi t the bronze bush in preparation for
machining the indexing hub (3.3) below.
Simply apply the Loctite retainer and place
the bush in the turret body with the fl ange
facing to the rear as seen in photo 4. and
the general arrangement fi g. 1 - ‘Section
through the turret’.
3.4 Locking collar - indexing hub
The chances are that you will have to
screw cut the 26tpi thread on the indexing
hub, so it would be sensible to make the
locking collar next so that it can be used as
a gauge. It is probably easiest to cut a disc
slightly longer than required and facing
both ‘ends’ to machine it back to 3/8” thick.
Also, while in the chuck drill a hole through
the disc (say 1/2” diameter) so that it can be
mounted on a simple mandrel (don’t use
your best mandrel for this as you will almost
certainly drill a hole into it).
Fit the disc onto the mandrel and turn
the diameter down to 1 3/16” and then
break the edges with a small chamfer. It
is easiest to produce the holes around
the periphery before fi nal boring to make
thread, so transfer the mandrel to the
indexing head and drill and tap the 2BA
hole for the locking screw - this is where
you will probably drill into the mandrel.
Now index 120 degrees and drill 3.2mm x
1/4” deep for a 1/8” tommy bar, then index
another 120 degrees and repeat.
The disc can now be removed for the
mandrel and refi tted into the 3-jaw chuck,
when the central hole can be opened up
and tapped 5/8” x 26tpi (see photo 9).
While doing this take a very light skim over
the face to remove any ‘wobble’ and then
the bore will be exactly square to the face- mark this face as the one which will be
closest to the bronze bush. In this way there
will be complete contact between the two
components.
Finally, the means of locking the collar
onto the indexing hub should be provided
as this collar controls backlash in the turret
assembly. Here, I would recommend that a
small slug of copper should be placed under
the grub screw to avoid damaging the
threads on the indexing hub.
3.5 Indexing hub
This item is made from mild steel, and
initially it is a nice easy turning job. It is
best to ensure that the 1.248” and 0.750”
diameters are a good running fi t in theturret body and bronze bush respectively.
Then turn the indexing hub and cut the
thread, using the locking collar as a gauge.
At this stage the 0.500” dimension
marked * should initially be made slightly
longer than shown on the drawing so that
it protrudes from the turret body. This will
allow you to determine where the indexing
holes in the hub should be drilled. There
will almost certainly be some deviation in
the dimensions shown on the drawing - the
bush may protrude into the main body
of the turret more or less than the 1/16”
allowed for, or the turret body may be
slightly thinner or thicker than intended -
we will need to compensate for this when
drilling for the detents. We need indexing
to be smooth but positive, so it is important
that the holes in the turret body and the
indexing hub align accurately.
Anyway, we can complete the turning of
the indexing hub and screw-cut the thread
for the locking collar. Now fi t the indexing
hub into the turret body and measure the10 11
12
Measuring the turret body and indexing boss as an assembly Spotting the index positions in the indexing hub (posed)
Drilling tapping size for the peg keys (posed)