=35. The center punch= (_C_, Fig. 19) should be made of the same quality of material as the cold chisel. It can be made of steel from 1/4 to 5/8 of an inch in diameter; octagon steel is preferable. After it has been roughly drawn out, it is ground to a smooth round point, then it is tempered as hard as it will stand without breaking. It is used for marking centers of holes to be drilled, or for marking metal where it is to be bent, twisted, or forged. When used for marking hot metal, it is frequently made with an eyehole in the body, so that a small handle can be inserted; this will prevent burning the hands.
=36. The rule= (_D_, Fig. 19) should be of good quality. The one best adapted for forge work is the 2-foot rule, which is jointed in the center. It is 3/4 inch wide and is made of either tempered spring steel or hard rolled brass.
_A_, dividers; _B_, calipers; _C_, scriber; _D_, square; _E_, bevel.]
=37. The dividers= (_A_, Fig. 20) are used for measuring distances and for describing circles. The points are clamped in a rigid position with the small thumbscrew, which comes in contact with the segmental arc. Close adjustments can be made with the milled-edge nut on the end of the segmental arc. When metal is to be bent to a circular form, a good method is to rub chalk on the surface plate and describe the desired curve on this chalk. As the markings thus made are not easily removed, this plan is much better than drawing upon a board.
=38. The calipers= (_B_, Fig. 20) are used for measuring diameters, widths, and thicknesses. Those illustrated are the kind generally used in forge work. They are called double calipers and are the most convenient because two dimensions can be determined by them. As the accuracy of the work depends on them, they should be well made. In the illustration here given, each bow is held securely by an individual rivet. Sometimes they are secured with one; if so, the rivet should be square in the straight central part and tightly fitted. The projecting ends of the rivet should be filed round, and the holes in the bowed sides should be made to fit the round ends of the rivet; then the sides should be riveted on tight so that each bow may be moved independently of the other.