For use on steel, wrought iron, and cast iron the lip drill shown in [Fig. 1724] is a very efficient tool. It is similar to the flat drill but has its cutting edge bent forward. It possesses the keenness of the twist drill and the strength of the flat drill, but as in the case of all drills whose diameters are restored by forging and hand grinding, it is suitable for the rougher classes of work only, and requires great care in order to have it run true and keep both cutting edges in action. It is sometimes attempted to give a greater cutting angle to a flat drill by grinding a recess in the front face, as at a in [Fig. 1725], but this is a poor expedient.
Fig. 1726.
[Fig. 1726] represents what is known as the tit drill. It is employed to flatten the bottoms of holes, and has a tit t which serves to steady it. The edges a, b of this drill may be turned true and left without clearance, which will also serve to steady the drill. The tit t should be tapered towards the point, as shown, which will enable it to feed more easily and cut more freely. The speed of the drill must be as slow again as for the ordinary flat drill, and not more than one-third as fast as the twist drill.
To enable a drill to start a hole in the intended location the centre-punch recess in the centre of that location should be large enough in diameter at the top to admit the point of the drill, that is to say, the recess should not be less in diameter at the top than the thickness of the drill point.
Fig. 1727.
If the drill does not enter true the alteration is effected as shown in [Fig. 1727], in which a represents the work, b a circle of the size of the hole to be drilled, and c the recess cut by the drill, while d is a recess cut with a round-nosed chisel, which recess will cause the drill to run over in that direction.