Figure 4-10.-Transferring an angle.equal in size to one that is already drawn.This procedure, called transferring an- angle, isshown in figure 4-10. Here, the draftsman desiredto lay off from O´ a line that would make an anglewith B´O´ equal to angle BOA. To do this, drawan arc through OB and OA, with O as a center,as shown in figure 4-10, view A. Then, draw anarc of the same radius from B´O´, with O´ as acenter, as shown in figure 4-10, view B. Next,measure the length of the chord of the arcbetween OB and OA and lay off the same lengthon the arc from B´O´, as shown in figure 4-10,view C. A line drawn from O´ through A´ makesan angle with B´O´ equal to angle BOA, as shownin figure 4-10, view D.BISECTION OF AN ANGLETo bisect an angle means to divide it in half.If you know the size of the angle, you can bisectit by simply dividing the size by 2 and laying offthe result with a protractor.Geometric construction for bisecting an angleis shown in figure 4-11, To bisect the angle AOB,first lay off equal intervals from O on OA andOB. With the ends of these intervals as centers,strike intersecting arcs of equal radius at P. Drawa line from O through the point of intersectionof the arcs, P. The line OP bisects angle AOB.PLANE FIGURESThis section explains how to construct certainplane figures, such as the triangle, rectangle,square, and regular polygon. You must under-stand the geometrical construction of plane figuresbecause they appear in engineering drawings.Figure 4-11.-Bisecting an angle.Figure 4-12.-Constructing a triangle with three sides given.TRIANGLE: THREE SIDES GIVENTo draw a triangle with three sides given, firstdraw a straight line AB, equal in length to oneof the given sides (fig. 4-12). With A as a center,strike an arc with a radius equal to the given lengthof the second side. With B as a center, strike anintersecting arc with a radius equal to the lengthof the third side. Draw lines from A and B to thepoint of intersection of the arcs.RIGHT TRIANGLE: HYPOTENUSEAND ONE SIDE GIVENFigure 4-13 shows a method of drawing a righttriangle when the hypotenuse and one side aregiven. The line H is the given hypotenuse; the lineS is the given side. Draw AB equal to H. Locatethe center of AB (by bisection), and, with themidpoint as a center and a radius equal to one-half of AB, draw the semicircle from A to B asshown. Set a compass or dividers to the lengthof S, and, with A as a center, strike an arcintersecting the semicircle at C. Draw ACand BC.4-4
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