EXAMPLES OF WORK 95 their upper edges guide the movements of a sweeping board which pro- the shape of the loam beds on which the strips, that correspond with ^hanging sectional shapes of the blades, are laid. This work is repeated ^any times as there are blades, the templets being set equidistant round circle. Screw Drums.—These, grooved spirally to receive the wire ropes "the chains used on large cranes, are seldom cut in wood, because the is too great. They are sometimes cored, but a cheaper and more Fig. 76.—The Use of a Templet Screw to control the Striking Board for the Loam Pattern of a Spiral Crane Drum ourate method is to sweep them in loam. Drums up to about 3 ft. 6 in. diameter are swept as loam patterns to be moulded. Those over that ce are made as loam moulds with the axis of rotation set vertically. In .cti case the pitch of the screw-thread is reproduced from a templet which urtrols the longitudinal (fig. 76) or the vertical movement of the sweeping >a.rd. The templet is cut by the guidance afforded by inclined planes ajrked on paper and glued within and without. The grooved sections are it on the edge of the sweeping board. Since this is moved through a dis- rtce of one pitch during one revolution, the result is a true screw in loam. space equal to the thickness of the board has to be filled up and made >od by hand, because the board has to be moved back to its starting position times before a smooth loam surface can be completed.