By Orville R. Hagans and D. L. Thompson
Basic Mechanics - Lesson 4
30. MAKING A SET OF FLAT DRILLS
The drills used by the horologist are usually the so-called flat ones, though twist-drills are sometimes used for the larger work. Like taps, they can be bought in assortments suited for the work, and, being' very small and delicate, they are easily broken.
In selecting a drill for a hole which is to be tapped, its diameter is very important. A comparison of the drill diameters with the screw-thread diameters in the above table for "Waltham scre"ws will show that the drills are gauged definitely for the size of the screw. For instance, the size "of the drill for the No. 1 tap is 1.32 mm, which means that the diameter of the tap at the bottom of the threads is 1.32 mm and that this tap will cut a full, clean thread of 1.50 mm in a hole of this diameter, as shown in the sectional drawing in Fig. 16.
A set of drills, to fit t.he set of taps which the student has made, can be gauged by t.he same holes in the screwplate
To make a flat drill, select a piece of drill-rod of somewhat greater diameter than the hole to be drilled, or of the largest drill to be made, and cut off a length of 2.5 cm. Turn the end down to the exact size of the hole in the screw-plate having the number of the largest tap made, and then taper it slightly from the end back for about 8 times the diameter of the drill, as shown in Fig. 17, at a, which is to relieve the cutting end to allow the chips to clear away from the hole which it makes. The drill blank is then to be placed in a square block and the blade ground flat on opposite sides to the shape shown in Fig. 17, at b, leaving it somewhat thicker than necessary, after which it is to be hardened and then tempered to pale straw in the same manner as for taps.
After hardening and tempering, the blade is to be further ground until the end is of the proper thickness, which· should be only sufficient for strength and varies with the size of the drill. The cutting edges are then ground so that they will be relieved as shown in Fig. 17 , at c, this being done by holding the drill in a pin-vise and up to the emery-wheel in the manner shown in Fig. 18.
Drills smaller than 1 mm in diameter should be sharpened with a soft Arkansas stone, the drill being held in a chuck in the lathe and the stone being applied to the end at such an angle to the blade as to form the proper angle of the cutting edge and to at the same time slightly relieve it, the stone being drawn downward, as shown in Fig. 19. '1'he drill can be held in a pin-vise on the edge of the bench, and sharpened in the same way.
One cutting edge is to be sharpened and then the drill turned a half-turn and the other one sharpened, being careful to get the point central with the axis of the drill, so that it will have a proper guide to drill a straight hole.
The correct and incorrect angles of the cutting edges of a drill are shown in Fig. 20. The correct angle is shown at a, which is an included angle of 120 degrees, and will cut fast and drill a straight or true hole. The shape shown at b is too acute and the drill will break easily. That shown at c is too obtuse and will not drill a true hole as it does not have a proper guide point.
The set of drills should be numbered on the shank in the same manner as the taps, using the corresponding plate hole numbers.
A pivot drill of very small size is to be made in substantially the same manner as the larger ones. The tapered end is to be ground to size and shape with a soft oilstone slip; the drill hardened by wrapping a piece of iron wire around it and holding the shank in a flame until the tapered end is heated to dull red, then quickly cooled, which is best done by holding the water or oil in a test-tube or a wide-mouthed bottle and close up to the flame; and the blade ground flat and sharpened on a soft oilstone-wheel running in the lathe. The smallest sized pivot drills need not be tempered, as they are usually used on tempered steel and are, therefore, required to be especially hard. Small drills are easily sharpened as shown in Fig. 19. The use of these drills will be shown and explained under the subject of pivoting.
The student should make a set of pivot drills from .5 mm to a .05 mm in .05 mm steps.