Fig. 1046.
It follows, then, that if the angle of the bottom face of the drill be the same from the centre to the corner of the drill, and we consider the cutting edge simply as a wedge and independent of its angle presentation to the work, we find that it has a varying degree of acuteness at every point in its length. This may be seen from [Fig. 1046], in which the end face is ground at a constant angle from end to end to the centre line of the drill, and it is seen that the angle a represents the wedge at point c and the angle b the wedge at the point f in the length of the cutting edge, and it follows that the wedge becomes less acute as the centre of the drill is approached from the point c. If, then, we give to the end face a degree of clearance best suited for the corner c, it will be an improper one for the cutting edge near the drill point; or if we adopt an angle suitable for the point, it will be an improper one for the corner c.
This corner performs the most cutting duty, because its path of revolution is the longest, or rather of the greatest circumference, and it operates at the highest rate of cutting speed for the same reason, hence it naturally wears and gets dull the quickest.
As this wear proceeds the circumferential surface near this corner grinds against the walls of the hole, causing the drill to heat and finally to cease cutting altogether.
Fig. 1047.
For these reasons it is desirable that the angle of the end face, or the angle of clearance, be made that most suitable to obtain endurance at this corner. It may be pointed out, however, that the angle of one cutting edge to the other, or, what is the same thing, its angle to the centre line of the drill, influences the keenness of this corner. In [Fig. 1045], for example, each edge is at an angle of 60° to the drill axis, this being the angle given to drills by the manufacturers as most suitable for general use. In [Fig. 1047], the angle is 45°, and it will be clearly seen that the corner c is much less acute; an angle of 45° is suitable for brass work or for any work in which the holes have been cored out and the drill is to be used to enlarge them.
