Twist drills are hardened by special processes which, generally speaking, are not understood outside the shop where the drills are made. Very good results, however, may be obtained if the drills are heated somewhat and dipped into a solution of the following:

Pulverized charred leather...................

1 pound

Fine family flour.................................

1 1/2 pounds

Fine table salt......................................

2 pounds

The charred leather should be ground or pounded until fine enough to pass through a No. 45 sieve. The three ingredients are thoroughly mixed while in the dry state, and water is then added, slowly, to prevent lumps, until the mixture formed has the consistency of ordinary varnish.

After the drill has been dipped in the mixture it should be laid in a warm place to dry; when thoroughly dried it should be heated in a tube, or preferably in a crucible of red-hot lead, until it is a low red, and then plunged into a bath of lukewarm water or brine; small drills may be dipped in a bath of oil. The drill must not be put in red-hot lead until the coating is thoroughly dried, as the moisture may cause minute particles of lead to fly in all directions, endangering the eyes of the operator. After hardening, the temper should be drawn to a full straw color. If several drills are hardened at one time, the temper may be drawn by placing them in a kettle of oil over a fire, gaging the amount of heat by a thermometer, as explained in the section on the tempering of tool steel.

A bath that insures excellent results when drills and similar articles are hardened, is shown in Fig. 79. This bath has perforated pipes extending up the sides, as shown. The water from the perforations is projected against the drill and to the bottoms of the flutes, so that uniform results are assured.


Although most shops are provided with a special machine for grinding twist drills, yet at times it is necessary to grind such tools by hand, and every workman should practice until he is able to do this properly without the use of a special machine. The cutting edges must make a proper and uniform angle with the longitudinal axis of the drill; they must be equal in length, and the lips of the drill sufficiently backed off for clearance; otherwise they will not cut easily, or if they do cut, they will make a hole larger than the size of the drill. Drills properly made have their cutting edges straight when ground to a proper angle, which is 59 degrees, Fig. 40. Grinding to an angle less than 59 degrees leaves the lip hooking, which is likely to produce a crooked and irregular hole. A very satisfactory form of an angle-gage for this work is shown in Fig. 41. The graduations on the upper part of the gage show when the lips are ground to an equal length, which is essential if the drill is to cut the proper size. As the operator becomes experienced, he can gage the angle and length of lips very accurately by the eye, but until he has had the necessary experience, it is advisable to use some form of gage.