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Full text of "Modern Mechanical Engineering Vol-I"

5^                               PATTERN-MAKING
The Elements
Pattern-work includes two very broad aspects, that of the method of
moulding to be adopted, and that of the actual construction. It is necessary
to determine the first before the second can be proceeded with.
Castings may be made from (a) complete patterns, (b) incomplete or
skeleton patterns, (r) loam patterns, (d) moulds swept directly in loam.
(a) Complete patterns are those whose shapes, except for cored portions,
are identical with their castings. With the employment of these, many
side-issues are involved: the directions of their jointing; the amount of
shrinkage allowance and taper; the adoption of middle parts, loose pieces,
and drawbacks or false cores; and the formation of internal portions by
self-delivery or with independent cores.
(ft) Incomplete patterns are made of strips or frames that have the out-
and-out dimensions and the main contours the same as for their castings,
but which leave interior spaces to be completed with sand cores, or with
strickles. The object here is to economize timber, and incidentally to
lessen weight.
(c) and (d) Loam patterns, and moulds swept out in loam are only used
for circular bodies, so that flanges, bosses, and brackets must be prepared
in wood as complete pattern elements and attached to the main pattern
or set in the loam.
On the pattern-maker falls the responsibility of deciding by which of
methods the pattern-work and the moulds are to be made.   In many
the most suitable method is self-evident to a man with experience.
Full patterns are always made for work of small and medium dimensions.
Skeleton patterns, those of loam and loam moulds, have preference for very
articles, but subject in a measure to the number of castings required.
A            casting, though of medium dimensions, would seldom have a full
pattern, provided  its  shape were  suitable for skeleton  construction  or
sweeping; a large one, if repeated in considerable numbers, would.   The
problem always is just one of the relative costs in the pattern- and moulding-
A large pattern is expensive, but so is a large loam mould, for'which
numerous attachments may have to be prepared.   It will often happen
that a quantity of castings of large dimensions can be more cheaply
from a skeleton pattern, perhaps from a complete one, than from loam
or loam, patterns.   In such a case the moulder has a grievance if the
him with unnecessary expense in order to lessen the
of its own. department.