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Full text of "Modern Mechanical Engineering Vol-I"

THE  ELEMENTS                                61
all sections are made in this way, parallel (fig. 12), bevelled, and semi-spherical
(fig. 13). The pieces are glued singly, with carefully planed joints, checked
with chalk. In general it is not necessary to reinforce the joints, but as a
precaution wire nails are frequently driven in. When the section of a
pattern is that of a cone, as in the rim of a bevel wheel, or has any curved
outline, wooden pegs are preferable, because, if they should happen to come
to the exterior, they will not damage the turning-tools.
Methods of Union.—The union of elements is in some respects
peculiar to pattern-work, being due to the necessity of making alterations
from time to time. The tenon and mortise joint is seldom used. Other
forms of joints are frequently not glued, but screwed only. Unions of a
more or less temporary character made with battens, the impressions of
which are filled up in the sand, and do not appear in the casting, occupy a
useful place in alterations. Very many alterations are made only partly in
the pattern, being completed in the mould by the method of " stopping-off ".
One of the commonest joints is the half-lap, used for uniting flat strips. It
is either a plain half-lap fastened with screws only, when there is a proba-
bility of future alterations being called for (fig. 6), or, if permanent, the
dovetail form is cut (fig. 7), and the joint is both glued and screwed.
Screws occupy a larger place in pattern-work than in the more permanent
methods of the joiner. They take the place of tenons and mortises and
dovetails in the attachment of parts.
Dovetails.—These are employed chiefly for the corners of deep open
frames that deliver their interiors, of which sewer boxes are typical, and for
loose pieces, as an alternative to the skewers. Their use is generally restricted
to standardized work. They are safer than the skewers, since these afford
but a doubtful indication, by their small holes, of the position of a loose
piece if mislaid.
Dowels.—These play a large part in pattern-work. They include the
tightly fitting dowels used in open joints, and those loosely fitting in one
piece, in the joints of patterns that are divided for delivery between bottom
and top parts (figs. 3 and 4). These are of wood or, for permanent work,
of brass or malleable-cast iron.
Angles, Fillets, or Hollows.—These are peculiar to pattern-work,
being employed to fill up re-entrant angles that would, without them, invite
fracture in the castings. They should never be omitted. They are made
of wood, leather, or soft metal, to be bent round curved portions. Illustra-
tions of all the elements here noted will occur in the subsequent sections.
3. CORE PRINTS AND CORE BOXES
Core Prints.—The function of a core print is to locate, by the im-
pression which it leaves in a mould, the exact place for the insertion of a
core. There are exceptions to this general statement, since some large cores
are set without print impressions, as when moulds are made from sectional
and skeleton, patterns. Also, when portions of metal are cored over, in