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Full text of "Modern Mechanical Engineering Vol-I"

54

PATTERN-MAKING

Fig. 3.—Rib and Boss dowelled to come with Top Box

the mould joints are made along the centres of the convex edges, but the
pattern joints are elsewhere.

Taper.—The deeper the mould, the more difficult is the pattern to with-
draw and the greater the care that must be exercised to avoid disturbing the
sand. The first inch or two is the stage at which fracture of the sand is

most likely to occur. After the
pattern has been loosened by
rapping, and drawn slightly out
of the sand, the principal care
necessary for the remainder of
the lift is to keep the pattern
level, the difficulty of which in-
creases directly with area. Slight
rapping is continued until the
pattern, by reason of its taper,
has cleared the encircling walls
of sand. Taper or draught there-
fore assumes much importance.
Its amount varies widely. Some
patterns have a large amount, in

localities where it does not interfere with the fitting of parts, as on the
outsides of deep bedplates, of sewer boxes, of machine frames, or of stiffening-
brackets. No rule can be stated to meet all cases, but common practice is
to give J in. to J in. per foot of depth.

Loose pieces.—These are a particular provision, made to ensure free
delivery of portions on pattern sides, without making down-joints. In some

cases they afford alternatives to
coring and to drawbacks or false
cores. In others, as in copes, pieces
are often left loosely on the main
pattern (figs. 3 and 4) in order to
permit of their being taken out of
the mould after turning the cope
over in preference to lifting the
mould off them. In the latter case,
the alternative is to impart as much
taper as is permissible to the por-
tions that come in the top, and to
avoid keen edges and angles there.
In a sheave wheel the convexity of

the internal rim (fig. i) provides for a free delivery. In a trolley wheel,
internal taper and a well-rounded edge are necessary. Top bosses are
almost invariably left loose, unless they are very shallow. Thin facing
pieces and core prints are fast, but these have well-tapered edges.

Generally, when loose pieces are visualized, the most frequent case of
those attached to the vertical sides of patterns occurs to the mind. In the

Fig. 4.—Rib dowelled to come with Top Box