PARALLEL AND PERPENDICULAR LINES

Engineering Aid 3 - Beginning Structural engineering guide book
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be measured. Extend line AO on to C; extend line BO on to D. When you set the center of the protractor at O, make sure that both points c and a are on line AC. Take your reading at point d as well as at point b when you measure the angle. If you are laying off the angle BOA, protract and mark point d as well as point b; this gives you three points (d, O, and b) for establishing line DB. If you are using a semicircular protractor, you can’t, of course, locate point d; but your accuracy will be improved by lining up c, O, and a before you measure or lay off the single angle BOA. PARALLEL AND PERPENDICULAR LINES To draw a line parallel to a given line (fig. 3-6, view A), adjust the hypotenuse of a triangle in combination with a straightedge (T square or triangle) to the given line; then, holding the straightedge firmly in position, slip the triangle to the desired position and draw the parallel line along the hypotenuse. To construct a line perpendicular to an existing line, use the triangle and straightedge in combination, with the hypotenuse of the triangle resting against the upper edge of the straightedge (fig. 3-6, view B). Adjust one leg of the triangle to a given line. Then slide the triangle along the supporting straightedge to the desired position and draw the line along the leg, perpendicular to the Figure 3-6.-Drawing parallel and perpendicular lines. leg that was adjusted to the given line. In the same manner, angles with multiples of 15 degrees may be drawn, using the triangle combinations shown in figure 3-4. CURVED LINES Many drawings that you will prepare require the construct ion of various curved lines. Basically there are two types of curved lines: circles and segments of circles, called arcs, which are drawn with a compass; and noncircular curves, which are usually drawn with french curves. In this chapter we will discuss only techniques for using the compass and the french curve. Application of compass techniques in geometric construction will be covered in chapter 4. Use of the Compass When you are drawing circles and arcs, it is important that the lines produced with the compass are the same weight as corresponding pencil lines. Since you cannot exert as much pressure on the compass as you can with pencils, you should use a compass lead that is Figure 3-7.-Sharpening the compass lead and adjusting the point. 3-6

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