Sheet metal screws for moving grain along a trough, as shown fitted to a shaft in Fig. 312, are usually made up out of rings which are shaped by hammering and riveted together. Being a twisted surface it is not strictly developable, but a very good approximation can be obtained by the diagram shown in Fig. 312. A C is set out equal to the shaft circumference, and A B made the same height as the pitch; the line B C will then give the length of the inner circumference of the pattern ring; the corresponding diameter being determined by dividing B C by 3 1/7.
By calculation it can be shown that the outside of the ring is a little longer than that required for the outside of the spiral, but when the pitch is small and the ring narrow the difference is very slight. To give the twist without buckle the rings will require to be carefully hammered, the blows falling heaviest on the inner part of the ring.