This section is from the book "Practical Sheet And Plate Metal Work", by Evan A. Atkins. Also available from Amazon: Practical Sheet And Plate Metal Work.

To find the radius of the pattern disc, graphically, the line A N (fig. 239) is made equal to the length of the arc A D E, and N P drawn square to it and equal to the radius O E. Line E R is then drawn parallel to N P, and the point R cut off by joining P to A. O G is then made equal to E R by joining R to O and drawing E G parallel to R O. The line K L is cut off equal to K G and L M drawn parallel to G A. Now, using A as centre, and A N as radius, the point S is marked off. A semicircle is next described on S M intersecting the line A N in V. The line C V is the length required for the radius of the pattern disc; and this, on measurement, will be found to be 10 in., as calculated before.

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