Modern Mechanical Engineering Vol-I

EXAMPLES OF WORK

93

threads would suffer from very short grain if they were cut in one solid
with the cylindrical body. This therefore is prepared first, being jointed
along the centre and dowelled. The blade is fitted in short segmental
divisions and glued permanently into shallow grooves cut around the body,
but if the blades are deep they must be screwed temporarily, to be with-
drawn from the mould after the delivery of the cylindrical body.

It is generally convenient to mark the width of the grooves on the templet

Fig. 74-—A Pile Screw

sheet of paper, through which they are cut on the turned body. The length
of the pitch and of the circumference at radius r form a rectangle, whose
diagonal is the development of the helix at radius r. Another line is drawn
parallel with this to represent the width of the groove to be cut in the
cylinder. If there is more than one turn of the helix, the construction is
repeated. The paper when glued round is an accurate guide to the work-
man who cuts the spiral groove with saw, chisel, and router. The segmental
blades are fitted into this, prevented from overlapping by the insertion of a
flat dowell next the outer edges, and secured with screws put in from the
joint faces, for which the pattern has to be removed from the lathe, and
taken apart.

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