Modern Machine-Shop Practice, Volumes I and II

by Joshua Rose

Diameter of Drills.		Speed for Steel.	Speed for Iron.	Speed for Brass.	Diameter of Drills.		Speed for Steel.	Speed for Iron.	Speed for Brass.
inch.					inch.
	1⁄16	940	1280	1560	1	1⁄16	54	75	95
	1⁄8	460	660	785	1	1⁄8	52	70	90
	3⁄16	310	420	540	1	3⁄16	49	66	85
	1⁄4	230	320	400	1	1⁄4	46	62	80
	5⁄16	190	260	320	1	5⁄16	44	60	75
	3⁄8	150	220	260	1	3⁄8	42	58	72
	7⁄16	130	185	230	1	7⁄16	40	56	69
	1⁄2	115	160	200	1	1⁄2	39	54	66
	9⁄16	100	140	180	1	9⁄16	37	51	63
	5⁄8	95	130	160	1	5⁄8	36	49	60
	11⁄16	85	115	145	1	11⁄16	34	47	58
	3⁄4	75	105	130	1	3⁄4	33	45	56
	13⁄16	70	100	120	1	13⁄16	32	43	54
	7⁄8	65	90	115	1	7⁄8	31	41	52
	15⁄16	62	85	110	1	15⁄16	30	40	51
1		58	80	100	2		29	39	49

To drill one inch in soft cast iron will usually require: For ¹⁄₄ in. drill, 125 revolutions; for ¹⁄₂ in. drill, 120 revolutions; for ³⁄₄ in. drill, 100 revolutions; for 1 in. drill, 95 revolutions.

The rates of feed for twist drills are thus given by the same Company:—

Diameter of drill.				Revolutions per inch depth of hole.
	1⁄16	inch		125
	1⁄4	„		„
	3⁄8	„		120 to		140
	1⁄2	„		„		„
	3⁄4	„		1 inch		feed per		minute
1		„		„		„		„
1	1⁄2	„		„		„		„

Taking an inch drill as an example, we find from this table that the rate of feed is for iron ¹⁄₁₀₀th inch per drill revolution, and as the drill has two cutting edges it is obvious that the rate of feed for each edge is ¹⁄₂₀₀th inch per revolution. But it can be shown that this will only be the case when the drill is ground perfectly true; or, in other words, when the drill is so ground that each edge will take a separate cut, or so that one edge only will cut, and that in either case the rate of feed will be diminished one-half.

Fig. 1057.

In [Fig. 1057], for example, is shown a twist drill in which one cutting edge (e) is ground longer than the other, and the effect this would produce is as follows. First, suppose the drill to be fed automatically, the rate of feed being ¹⁄₁₀₀th inch, and the whole of this feed would fall on cutting edge e, and, being double what it should be, would in the first place cause the corner c to dull very rapidly, and in the second place be liable to cause the drill to break when c became dull.

In the second place the drill would make a hole of larger diameter than itself, because the point of the drill will naturally be forced by the feed to be the axis or centre of cutting edge revolution, which would therefore be on the line b b. This would cause the diameter of hole drilled to be determined by the radius of the cutting edge e rather than by the diameter of the drill. Again, the side of the drill in line with corner c would bind against the side of the hole, tending to grind away the clearance at the corner c, which, it has been shown, it is of the utmost importance to keep sharp. But assuming ¹⁄₂₀₀th inch to be the proper feed for each cutting edge, and the most it can carry without involving excessive grinding, then the duty of the drill can only be one-half what it would be were both cutting edges in action.