As soon as the point of the drill emerges through the work the feed should be lessened, otherwise the drill is apt to force through the weakened metal and become locked, which will very often either break or twist the drill. This may be accomplished when there is any end play to the drilling machine spindle by operating the feed motion in a direction to relieve the feed as soon as the point of the drill has emerged through the bottom of the hole, thus permitting the weight of the spindle to feed the drill. In a drilling machine, however, in which the weight of the spindle is counterbalanced, the feed may be simply reduced while the drill is passing through the bottom of the hole.

Drills for work of ordinary hardness are tempered to an orange purple, but if the metal to be cut is very hard a straw color is preferable, or the drill may be left as hard as it leaves the water; that is to say hardened, but not tempered. In these cases the speed of the drill must be reduced.

To assist a drill in taking hold of hard metal it is an excellent plan to jag the surface of the metal with a chisel which will often start the drill to its cut when all other means have failed. It is obvious from previous remarks that the harder the drill the less the angle of the end facets.

In cases of extreme hardness two drills may with advantage be used intermittently upon the same hole; one of these should have its cutting edges ground at a more acute angle one to the other than is the case with the other drill, thus the cutting edge will be lessened in length while the drill will retain the strength due to its diameter, so that a maximum of pressure may be placed upon it. When one drill has cut deep enough to bring its full length of cutting edge into action, it may be removed and the other drill employed, and so on.

The drill (for hard steel) should be kept dry until it has begun to cut, when a very little oil may be employed, but for chilled cast iron it should be kept dry.

Small work to be drilled while resting upon a horizontal table may generally be held by hand, and need not therefore be secured in a chuck or to the table, because the pressure of the drill forces the work surface to the table, creating sufficient friction to hold the work from rotating with the drill. For large holes, however, the work may be secured in chucks or by bolts and plates as described for lathe and planer work, or held in a vice.

The following table for the sizes of tapping holes is that issued by the Morse Twist Drill and Machine Co. In reply to a communication upon the subject that company states. “If in our estimate the necessary diameter of a tap drill to give a full thread comes nearest to a 1⁄64 inch measurement, we give the size of the drill in 64ths of an inch. If nearest to a 32nd size of drill we give the drill size in 32nds of an inch.”

In the following table are given the sizes of tapping drills, to give full threads, the diameters being practically but not decimally correct:—

Diameter
of tap.		Number
threads
to inch.		Drill for
V‑thread.		Drill for
U.S.S. thread.		Drill for
Whitworth
thread.
1⁄416 18 20 5⁄32 5⁄32 11⁄64—— 3⁄16—— 3⁄16
9⁄3216 18 20 3⁄16 13⁄64 13⁄64——————
5⁄1616 18 — 7⁄32 15⁄64—— 1⁄4—— 15⁄64—
11⁄3216 18 — 1⁄4 17⁄64———————
3⁄814 16 18 1⁄4 9⁄32 9⁄32— 9⁄32—— 9⁄32—
13⁄3214 16 18 19⁄64 21⁄64 21⁄64——————
7⁄1614 16 — 21⁄64 11⁄32— 11⁄32—— 11⁄32——
15⁄3214 16 — 23⁄64 3⁄8———————
1⁄212 13 14 3⁄8 25⁄64 25⁄64— 13⁄32— 3⁄8——
17⁄3212 13 14 13⁄32 27⁄64 27⁄64——————
9⁄1612 14 — 7⁄16 29⁄64— 7⁄16—————
19⁄3212 14 — 15⁄32 31⁄64———————
5⁄810 11 12 15⁄32 1⁄2 1⁄2— 1⁄2—— 1⁄2—
21⁄3210 11 12 1⁄2 17⁄32 17⁄32——————
11⁄1611 12 — 9⁄16 9⁄16———————
23⁄3211 12 — 19⁄32 19⁄32———————
3⁄410 11 12 19⁄32 5⁄8 5⁄8 5⁄8—— 5⁄8——
25⁄3210 11 12 5⁄8 21⁄32 21⁄32——————
13⁄1610 —— 21⁄32————————
27⁄3210 —— 11⁄16————————
7⁄89 10 — 45⁄64 23⁄32— 23⁄32—— 23⁄32——
29⁄329 10 — 47⁄64 3⁄4———————
15⁄169 —— 49⁄64————————
21⁄329 —— 51⁄64————————
1 8 —— 13⁄16—— 27⁄32—— 27⁄32——
11⁄328 —— 53⁄64————————
11⁄168 —— 55⁄64————————
13⁄328 —— 57⁄64————————
11⁄87 8 — 29⁄32 15⁄16— 15⁄16—— 15⁄16——
15⁄327 8 — 15⁄16 31⁄32———————
13⁄167 8 — 31⁄321 ———————
17⁄327 8 —1 11⁄32———————
11⁄47 ——11⁄32——11⁄16——11⁄16——
19⁄327 ——11⁄16————————
15⁄167 ——13⁄32————————
111⁄327 ——11⁄8————————
13⁄86 ——11⁄8——15⁄32——15⁄32——
113⁄326 ——15⁄32————————
17⁄166 ——15⁄32————————
115⁄326 ——13⁄16————————
11⁄26 ——115⁄64——19⁄32——19⁄32——
117⁄326 ——19⁄32————————
19⁄166 ——19⁄32————————
119⁄326 ——15⁄16————————
15⁄85 51⁄2—19⁄3215⁄16——13⁄8—123⁄64——
121⁄325 51⁄2—15⁄16111⁄32———————
111⁄165 51⁄2—111⁄3213⁄8———————
123⁄325 51⁄2—13⁄8113⁄32———————
13⁄45 ——113⁄32——11⁄2——11⁄2——
125⁄325 ——17⁄16————————
113⁄165 ——115⁄32————————
127⁄325 ——11⁄2————————
17⁄841⁄25 —117⁄32117⁄32——15⁄8—137⁄64——
129⁄3241⁄25 —19⁄1619⁄16———————
115⁄1641⁄25 —119⁄32119⁄32———————
131⁄3241⁄25 —15⁄815⁄8———————
2 41⁄2——121⁄32——123⁄32——145⁄64——

To drive all drills by placing them directly in the socket of the drilling machine spindle would necessitate that all the drills should have their shanks to fit the drilling machine socket. This would involve a great deal of extra labor in making the drills, because the socket in the machine spindle must be large enough to fit the size of shank that will be strong enough to drive the largest drill used in the machine, hence the small drills would require to be forged down from steel equal to the full diameter of the shank of the largest drill. To obviate this difficulty the sockets already described with reference to drilling in the lathe are used.