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Full text of "Modern Mechanical Engineering Vol-I"

I

I

92                                   PATTERN-MAKING

Methods of Union.—The union of corners and the formation of
curved ends are shown in figs. 68 to 72. In fig. 68 sides and ends abut at
right angles, and a square block is glued in, which when set is worked to
interior and outside curves. In fig. 69 a similar method is employed for
interior and exterior radii, all being connected with a plated covering, after
the inner as well as the outer curves have been cut, the interior being for
self-delivery, as is the previous figure.

The next group represents portions of patterns, the interiors of which
have to be cored. This permits of making stiffer constructions. The
thicknesses of stuff are greater, and screws can be used if thought desirable
to assist the glue. Figs. 70 and 71 are alternative methods for the outside
curves. Both are strong, and are reinforced with the screws that secure the
plated portions, halved at the corners, to the verticals. Fig. 72 is a semi-
circular end, made in the strongest way. Risk of shrinkage is reduced by
making the segmental blocks short, and they are reinforced with the strips
glued in the angles. The inside curve is made of two pieces having the
grain running perpendicularly, in order to avoid short grain at the ends.
The plate is made with strips having half-lap joints, and is screwed to the
sides. This method is suitable for the semicircular ends of beds and of
other patterns of that type.

6. SCREWS

This work includes the production of helices in pile screws, conveyors
for elevators, worms, and propeller blades, cut in wood or swept up. The

patterns for these contain one or more than
one revolution, or a fractional portion of a
revolution. Although made in different
ways, the principle involved in all is the
same, viz. the development of a helix is an
inclined plane, or conversely a helix may
be imagined to be an inclined plane wound
round a cylinder. This is translated into
actual practice in many small patterns by
cutting an inclined plane in paper and
wrapping it round a cylinder as a guide for
working by. One templet of this kind may
be used for the base of the screw, the other
for its tip. The pitch is alike in each, but the
lengths of the envelopes and the angles of the
helices differ. The pitch is the distance be-
tween the centres of a helix or blade when
FIR. 73.—Marking the Tip of a Pile Screw it has made one revolution. The diameter

is measured across the tips of the blade.

Pattern Construction.—In pile and conveyor screws (figs. 73-75),
and in worms, which are members of the same family, the blades or