TRIANGLES

straightedge is also excellent for trimming blueprints and cutting heavy illustration board. Steel straightedges are usually made of stainless steel and are available in lengths of 15 in. to 72 in. The one included in the draftsman kit is 42 in. long. Some have a beveled edge, like the one shown in figure 2-9. TRIANGLES TRIANGLES are used in combination with the T square or straightedge to draw vertical and inclined lines. They are usually made of transparent plastic, which allows you to see your work underneath the triangles. Triangles are referred to by the size of their acute angles. Figure 2-10 shows two basic drafting triangles: the 45° (each acute angle measures 45°, and the 30°/60° (one acute angle measures 30°; the other, 60°). The size of a 45° triangle is designated by the length of the sides that form the right angle (the sides are equal). The size of a 30°/60° triangle is designated by the length of the longest side that forms the right angle. Sizes of both types of triangles range from 4 in. through 18 in. in 2-in. increments. Like all other drafting equipment, triangles must be kept in good condition. If plastic triangles are dropped, their tips may be damaged. Also, triangles may warp so that they do not lie flat on the drawing surface, or the edge may deviate from true straightness. To prevent warping or 29.277 Figure 2-10.-45° and 30°/60° drafting triangles. chipping, you should always lay them flat or hang them up when they are not in use. Since there is seldom enough drawer space available to permit laying triangles flat, it is best to develop the habit of hanging them up. If the tips are bent, use a sharp knife to cut off the damaged part. If the triangle is warped, you may be able to bend it back by hand. If this does not straighten it, leave the triangle lying on a flat surface with weights on it or hold the triangle to the opposite curvature with weights. If the triangle becomes permanently warped so that the drawing edges are curved or the angles are no longer true, throw it away and get another. To test the straightness of a triangle, place it against the T square and draw a vertical line, as shown in figure 2-11. Then reverse the triangle and draw another line along the same edge. If the triangle is straight, the two lines will coincide; if they don’t coincide, the error is half the resulting space. PROTRACTORS PROTRACTORS are used for measuring and laying off angles other than those that may be drawn with the triangle or a combination of triangles. Most of the work you will do involving the use of the protractor will involve plotting information obtained from field surveys. Like the triangle, most protractors are made of transparent plastic. They are available in 6-, 8-, and 10-in. sizes and are either circular or semicircular in shape, as shown in figure 2-12. 45.126 Figure 2-11.-Testing a triangle for straightness. 2-10