Although this method is convenient when a continuous cylinder affords
a good basis for the paper, it is not practicable for the tips of the blades,
since the helix is not cut in solid stuff. Then the method of intersecting
lines is adopted. Here the circumference and the pitch are divided into
the same number of equal parts; the larger the number the more nearly
accurate will be the results. A diagonal drawn through successive inter-
sections will delineate the screw thread (fig. 73). A line drawn parallel with
this is required for the thickness of the blade at the tip. As there is a gap
between the threads, the divisions are marked on a slip of wood. The
Fig. 75.—A Group of Conveyor Screws
divisions are scribed off on the pattern revolved in the lathe. The blades
have to be removed to be worked. This is done with a narrow plane,
slightly convex on the face. Radially, every portion of the surface from
centre to circumference must be straight.
Propeller blades are short sections of multiple screws, two, three, or four
in number. When pattern blades are made for these, in the smaller sizes,
the boss is included, and the blade is glued up with strips that overlap at
the edges to embrace the screw formation, worked through with planes.
The method of intersecting lines is adopted.
Screws produced with Templets.—Large propellers are swept up
in loam by the aid of sheet-iron templets having the upper edge cut to the
inclined plane that corresponds with the slope of the face of the blades. As
many templets are cut as there are blades. These are set round in a circle,