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Full text of "Modern Mechanical Engineering Vol-I"

64

PATTERN-MAKING

The fitting of drop prints is often associated with the presence of boss
facings which have to be left loose (fig. 17). These are cut to fit round the
print, or the print is notched to fit over them. The portion of the boss that
is covered by the print has to be made good in the mould during the stopping-
over (fig. 18), or, for permanent work, it is put in the core box.

Fig. 18.—Stopping-over a Core with Boss Facing in Mould

Fig. 19.—Iron
Box for Round
Cores

Core Boxes.—When determining the forms of these, similar methods
and precautions have to be observed as in the construction of patterns, with
regard to freedom of delivery, taper, loose pieces, and so on. In addition
there is the excess length necessary for the location of the cores in the print
impressions. Often a main core will contain prints, the im-
pressions of which will serve for the location of other cores.
Box portions must generally be taken apart to permit of
the removal of the core, so that they are only held tem-
porarily with dowels, clamps, or screws. The subject of
core-box work is therefore nearly as extensive as that of
pattern construction.

Standard  boxes of iron have  their  halves fitted  with
tongues and grooves (fig. 19);  those of wood are very similar (fig. 20).
The  ends of rectangular boxes may be retained  in place with blocks
screwed against the sides, and the sides may be screwed to the ends
(fig. 21).    This entails loss of time in removing the screws as often as
the sides  have to  be taken away from the core.
Clamps of wood (fig. 23) or of iron are to be pre-
ferred.  The sides of long boxes will become rammed
outwards, with consequent enlargement of the core,
unless they are retained about the centre with a bolt
(fig. 22) or with a clamp.   The fitting of ends into
shallow grooves (fig. 22) is to be preferred to their
abutment against end blocks.  Frequently the interior
of a rectangular  frame  is occupied  with contour
fittings.    In one example shown (fig. 23) for making one of the numerous
cores for a turbine ring the blocking of yellow pine is lined with mahogany
to favour durability in service.   Here the actual width of the turbine ring
* that of the curved strip which represents the metal separating three tiers

Fig. 20.—Wooden Box for
Round Cores