Assembly Drawings 299
dharm
d:\N-Design\Des18-2.pm5
nut from the stud 5. It may further be noted that the work piece is machined at the required
surface before loading in the jig. This is so, because, certain machined surfaces of the work piece
may be used for locating it in the jig.
Exercise
Assemble the parts of the drill jig shown in Fig. 18.24 and draw, (i) sectional view from the front
and (ii) view from above.
18.3.12 Indexing Drill Jig
Figure 18.25 shows the details of an indexing drill jig used to drill six holes in the work piece (13).
The jig consists of the bracket 1, on the top of which is fixed the jig plate 2. The plain drill
bush 6 and the slip bush 7 are located in the jig plate. The plain bush is fixed with interference fit
in the plate, whereas the slip bush is provided with sliding fit in the plate and is removed to
facilitate loading of the job. The job is located in the jig by means of the locater 3 which is fastened
to the bracket 1 by means of the nut 11. Quick loading and unloading of the job is carried out by
means of the quick acting knob 4. Two holes are drilled on the job through the bushes 6 and 7.
The location of the remaining holes, which are at 90° intervals is obtained by the simple indexing
mechanism provided by the ball catch assembly 5. The ball catch assembly consists of a M.S plate
which is fastened to the bracket by means of the socket headed screws 10. The required indexing
is obtained by means of the spring 9, loaded ball 8, which is a part of the ball catch assembly.
After the first operation, when the job is rotated in clockwise direction, the ball catch assembly
facilitates location of the job at intervals of 90° rotation.
Exercise
The details of an indexing drill jig are given in Fig. 18.25. Draw, (i) sectional view from the front,
(ii) view from above and (iii) view from the right.
18.3.13 Self-centring Chuck
The self-centring chuck is a work holding device mounted on the headstock spindle of a lathe. It
automatically centres the workpiece by the three jaws, moving simultaneously to and from the
centre. Regular shaped objects such as rounds and hexagons are quickly held and centred in three
jaw self-centring chucks.
Figure 18.26a shows the two views of a self-centring lathe chuck. The details of the assembly
are given in Fig. 18.26b. It consists of a face plate 1, and the scroll plate 3 is fitted into the circular
recess at the back of the face plate. Three pinions 4 are mounted in position in the face plate such
that, their teeth engage with those on the back of the scroll plate. The back plate 2 is fastened to
the face plate by six screws 7. This assembly is fastened to the flange 5 by three socket headed
screws 8. The three jaws 6 are then engaged with the scroll plate. By rotating any one pinion by
a chuck key (not shown), the three jaws move in the radial direction either to or from the centre.
The threaded hole in the flange facilitates the mounting of the chuck in the threaded headstock
spindle.
Exercise
Figure 18.26b shows the details of a self-centring lathe chuck. Assemble the parts and draw to
1 : 1 scale, the following:
(i) The view from the front, as seen from the side of the jaws, and
(ii) The half sectional view from the right. Consider suitable local section for showing other
details.
18.3.14 Four Jaw Chuck
Four jaw chuck is a device used to hold jobs accurately on a lathe. Unlike three jaw self-centring
chuck, it can hold both regular and irregular shapes of objects.
