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Full text of "Modern Mechanical Engineering Vol-I"

I

02                                     PATTERN-MAKING

order to avoid the employment of loose pieces or of drawbacks. Core
prints fall under two fundamental types, the " round " and the " drop " or
" pocket " forms.

Round Prints.—Tne^e are set either vertically or horizontally. The
first-named have taper or draught, the second are, as a rule, parallel, invariably
so when a pattern and its prints are jointed longitudinally through the centre
to be withdrawn from cope and drag. They are tapered when they are
attached to bosses or pieces that have to be left loose and be drawn hori-
zontally back into the mould.

Vertical Prints.—There is no recognized rule for the length or the
taper of these. Both call for the exercise of judgment. As the diameters of
prints are increased, their length or thickness a (fig. 14, A) is lessened re-

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ii

Fig. 14.—Vertical Core Prints, and Cores
latively, because the larger diameters afford better support to the core at
the bottom than at the sides. A print for example i ft. in diameter need not
be more than J in. thick, while one of i in. diameter will be i in. long. Up
to about 3 in., lengths and diameters are about equal, beyond that the pro-
portionate thickness lessens.
Prints are thinner at the top than at the bottom (fig. 14, A). Usually
they need not be more than half the thickness, since they have not to support
the core, but only to steady it against risk of lateral displacement during
pouring (fig. 14, B).
Bottom prints may have from J in. to J in. taper on the diameter. Top
prints should have more, because the sand in the cope has to be lowered on
the upstanding core, with risk of a crush if the taper is not ample. Often,
to avoid this risk, the portion of the core that enters the print impression
has an excess of taper, with the result that close contact does not occur until
the cope is down to its bedding. (Compare c and D, fig. 14.) It is desirable
in work that is standardized to put the taper in the core boxes. Ordinarily
the moulder rubs the taper on the cores to match the print impressions, and
his is a frequent cause of inaccurate setting.