Triangles, or Set Squares. - In the selection of triangles, the draftsman has the choice in material between pear wood; mahogany, ebony lined; hard rubber; German silver, and steel, silver or nickel plated. In style he has the choice between open work, of the form shown in Fig. 107, and the solid, as in Fig. L08. In shape, the two kinds which are adapted to the pattern cutter's use are shown in Figs. 107 and 108, the latter being described as 30, 60 and 90 degrees, or 30 by 60 degrees, and the former as 45, 45 and 90 degrees, or simply 45 degrees. The special uses of each of these two tools are shown in the chapter on Geometrical Problems (Chap. IV). In size, the pattern cutter requires large rather than small ones. If he can have two sizes of each, the smaller should measure from 4 to 6 inches on the side, and the larger from 10 to 12 inches; but if only a single size is to be had, one having dimensions intermediate to those named will be found the most serviceable.

Fig. 107 - Open Hard Rubber Triangle or Set Square, 40 x 45 x 90 Degrees.

Fig. 108.. - Hard Wood Triangle or Set Square, SO x GO x 00 Degrees.

The value of a triangle, for whatever purpose used, depends on its accuracy. Particularly is this to be said of the right angle, which is used more than either of the others. A method of testing the accuracy of the right angle is shown in Fig. 109. Draw the line A B with an accurate ruler or straight-edge. Place the right angle of a triangle near the center of this line, as shown by D C B, and make one of the edges coincide with the line, and then draw the line D C against the other edge. Turn the triangle into the position indicated by D C A. If it is found that the sides agree with A C and C D, it is proof that the angle is a right angle and that the sides are straight.

Fig. 109. - Testing the Right Angle of a Triangle.

Besides the kinds of triangles described above, a fair article can be made by the mechanic from sheet zinc or of heavy tin. Care must, however, be taken in cutting to obtain the greatest possible accuracy. For many of the purposes for which a large size 45 degree triangle would be used the steel square is available, but as the line of the hypothenuse is lacking, it cannot be considered a substitute.

Fig. 110. - Compasses with Interchangeable Parts.

Fig. 111 - Plain Dividers.

Fig. 112. - Hair-Spring Dividers.

Fig. 113. - Steel Spring Spacers.

Compasses and Dividers. - The term compasses is applied to those tools, of various sizes and descriptions, which hold a pencil and pen in one leg, and are used for drawing circles, while dividers are those tools which, while of the same general form as compasses, have both legs ending in fixed points, and are used for measuring spaces. A special form of dividers - used exclusively for setting off spaces, as in the divisions of a profile line - is called spacers, as illustrated and described below.

A pair of compasses consists of the parts shown in Fig. 110, being the instrument proper with detachable points, and extras comprising a needle point, a pencil point, a pen and a lengthening bar, all as shown to the left. In selection, care should be given to the workmanship; notice whether the parts fit together neatly and without lost motion, and whether the joint works tightly and yet without too great friction. A good German silver instrument, although quite expensive at the outset, will be found the cheapest in the end. A pencil point of the kind shown in our engraving is to be preferred over the old style which clamps a common pencil to the leg. The latter is not nearly so convenient and is far less accurate.

Of dividers there are two general kinds, the plain dividers, as shown in Fig. 111, and the hair-spring dividers, as shown in Fig. 112. The latter differ from the former simply in the fact of having a fine spring and a joint in one leg, the movement being controlled by the screw shown at the right. In this way, after the instrument has been set approximately to the distance desired, the adjustable leg is moved, by means of the screw, either in or out, as may be required, thus making the greatest accuracy of spacing possible. Both instruments are found desirable in an ordinary set of tools. The plain dividers will naturally be used for larger and less particular work, while the hairspring dividers will be used in the finer parts. It frequently happens that two pairs of dividers, set to different spaces, are convenient to have at the same time. A pair of spacers, shown in Fig. 113, is almost indispensable in a pattern cutter's outfit. He will find advantageous use for this tool, even though possessing both pairs of dividers described above. In size they are made less than that, of the dividers. The points should be needle-like in their Oneness, and should be capable of adjustment to within a very small distance of each other. It is sometimes desirable to divide a given profile into spaces of an eighth of an inch. The spacers should be capable of this, as well as adapted to spaces of three-quarters of an inch, without being too loose. As will be seen from the engraving, this instrument is arranged for minute variations in adjustment.