If a pipe is required to have a twisted seam as shown in Fig. 311, the rake of the pattern strip can be quite easily determined by the method of construction as seen in the figure. The pipe girth is laid out first and the line A B drawn perpendicular and made equal in length to the height of half a twist. Line B C is then marked off equal to half the pipe circumference, and the pattern completed as shown.
If A B had been made equal to the pitch of the spiral seam - that is, the vertical height of one complete twistthen B C would have to be made equal to the complete pipe girth.