Page:Sheet Metal Drafting.djvu/223

Rh With point 2 as a center and a radius equal to the upper base of section 2 to B, an arc is drawn bearing away from point A. This is intersected by an arc drawn from point A with a radius equal to distance AB of the true section, Fig. 296, thereby establishing point B. In this manner all of the points of the pattern are fixed and the curved and straight lines of the pattern drawn in. Care should be observed with regard to these items:

(a) The spacing between the points of the pattern must be taken from corresponding spaces in the true sections.

(b) Distances 1-2 and 7-6 are greater than any of the other spaces and must be connected by straight lines.

(c) A chisel point must be used in the compass to assure fine lines without which the necessary accuracy cannot be attained. The whole pattern may be produced by copying on the other side of line A-1 the half which has already been drawn.

The pattern may be checked for accuracy by ascertaining whether or not the angle E-7-6 is a right angle. If this angle is of more or less than 90°, the pattern is incorrect. In this connection it should be observed that a slight error in any measurement will throw the whole pattern "out of true."