Fig. 1058.
In [Fig. 1058] is shown a twist drill in which one cutting edge is ground longer than the other, and the two cutting edges are not at the same angle to the axis a a of the drill.
Here we find that the axis of drill rotation will be on the line b from the point of the drill as before, but both cutting edges will perform some duty. Thus edge e will drill a hole which the outer end of f will enlarge as shown. Thus the diameter of hole drilled will be determined by the radius of corner c, from the axis of drill revolution, and will still be larger than the drill. A drill thus ground would drill a more true and round hole than one ground as in [Fig. 1057], because as both cutting edges perform duty the drill would be steadied.
The rate of feed, however, would require to be governed by that length of cutting edge on f that acts to enlarge the hole made by e, and therefore would be but one-half what would be practicable if the drill were ground true. Furthermore, the corner c would rapidly dull because of its performing an undue amount of duty, or in other words, because it performs double duty, since it is not assisted by the other corner as it should be. In both these examples the drill if rigidly held would be sprung or bent to the amount denoted by the distance between the line a a, representing the true axis of the drill, and line b b, representing the line on which the drill point being ground and one-sided compels the drill to revolve; hence one side of the drill would continuously rub against the walls of the hole the drill produced, acting, as before observed, to grind away the clearance that was shown in [figure] and also to dull corner c.
Fig. 1059.
[Fig. 1059] shows a case in which the point of the drill is central to the drill axis d d, but the two cutting edges are not at the same angle. As a result all the duty falls on one cutting edge, and the hole drilled will still be larger in diameter than the drill is, because there is a tendency for the cutting edge e to push or crowd the drill over to the opposite side of the hole.