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. M m , I''