Fig. 1064.
Drills ground by hand may be tested for angle by a protractor, as in [Fig. 1061], and for equal length of cutting edge by resting them upon a flat surface, as b in [Fig. 1062], and applying a scale as at s in the figure. In the case of very small drills, it is difficult to apply either the protractor or the scale, as well as to determine the amount of clearance on the end face. This latter, however, may be known from the appearance of the cutting edge at the point a in [Fig. 1063], for if the line a is at a right angle to e, there is no clearance, and as clearance is given this line inclines as shown at b in the figure, the inclination increasing with increased clearance, as is shown at c. When this part of the edge inclines in the opposite direction, as at d in the figure, the curved edges e f stand the highest, and the drill cannot cut. The circumferential surface of a drill should never be ground, nor should the front face or straight side of the flute be ground unless under unusual conditions, such as when it is essential, as in drilling very thin sheet metal, to somewhat flatten the corner (c in [Fig. 1062]), in order to reduce its tendency to run forward, in which case care must be taken not to grind the front face sufficiently to reduce the full diameter. In [Fig. 1064], for example, that part of the circumference lying between a and b being left of full circle, the faces of the flutes might be ground away as denoted by the dotted lines c d without affecting the drill diameter.
Fig. 1065.
[Fig. 1065] represents the Farmer lathe drill, in which the flutes are straight and not spiral, by which means the tendency to run forward when emerging through the work is obviated.
Fig. 1066.