There is a geometric construction for laying off, on another part of the same drawing or on a different drawing, an angle

equal in size to one that is already drawn. This procedure, called transferring an angle, is shown in figure 4-10. Here, the draftsman desired to lay off from O a line that would make an angle with BO equal to angle BOA. To do this, draw an arc through OB and OA, with O as a center, as shown in figure 4-10, view A. Then, draw an arc of the same radius from BO, with O as a center, as shown in figure 4-10, view B. Next, measure the length of the chord of the arc between OB and OA and lay off the same length on the arc from BO, as shown in figure 4-10, view C. A line drawn from O through A makes an angle with BO equal to angle BOA, as shown in figure 4-10, view D.

To bisect an angle means to divide it in half. If you know the size of the angle, you can bisect it by simply dividing the size by 2 and laying off the result with a protractor.

Geometric construction for bisecting an angle is shown in figure 4-11, To bisect the angle AOB, first lay off equal intervals from O on OA and OB. With the ends of these intervals as centers, strike intersecting arcs of equal radius at P. Draw a line from O through the point of intersection of the arcs, P. The line OP bisects angle AOB.

This section explains how to construct certain plane figures, such as the triangle, rectangle, square, and regular polygon. You must under-stand the geometrical construction of plane figures because they appear in engineering drawings.

To draw a triangle with three sides given, first draw a straight line AB, equal in length to one of the given sides (fig. 4-12). With A as a center, strike an arc with a radius equal to the given length of the second side. With B as a center, strike an intersecting arc with a radius equal to the length of the third side. Draw lines from A and B to the point of intersection of the arcs.