[Fig. 1715] represents a piece of work that can be readily drilled with a twist drill but not with a flat one, such work being very advantageous in cutting out keyways. All that is necessary is to drill the three holes b first, and if the drill runs true and the work is properly held and the drill fed slowly while run at a quick speed the operation may be readily performed.

The speeds and feeds for twist drills are given in connection with the use of the drill in the lathe, but it may be remarked here that more duty may be obtained by hand than by automatically feeding a drill, because in hand feeding the resistance of the feed motion indicates the amount of pressure on the drill, and the feed may be increased when the conditions (such as soft metal) permits, and reduced for hard spots or places, thus preserving the drill. Furthermore, the dulling of the drill edges becomes more plainly perceptible under hand feeding.

The commercial sizes of both taper and straight shank twist drills are as follows:—

Diameter.Length.Diameter.Length.Diameter.Length.Diameter.Length.
1⁄461⁄8 25⁄3297⁄815⁄16141⁄4127⁄32163⁄8
9⁄3261⁄4 13⁄1610 111⁄32143⁄817⁄8161⁄2
5⁄1663⁄8 27⁄32101⁄413⁄8141⁄2129⁄32161⁄2
11⁄3261⁄2 7⁄8101⁄2113⁄32145⁄8115⁄32161⁄2
3⁄863⁄4 29⁄32105⁄817⁄16143⁄4131⁄32161⁄2
13⁄327 15⁄16103⁄4115⁄32147⁄82 161⁄2
7⁄1671⁄4 31⁄32107⁄811⁄215 21⁄32161⁄2
15⁄3271⁄21 11 117⁄32151⁄821⁄1617
1⁄273⁄411⁄32111⁄819⁄16151⁄421⁄817
17⁄328 11⁄16111⁄4119⁄32153⁄823⁄1617
9⁄1681⁄413⁄32111⁄215⁄8151⁄221⁄4171⁄2
19⁄3281⁄211⁄8113⁄4121⁄32155⁄825⁄16171⁄2
5⁄883⁄415⁄32117⁄8111⁄16153⁄423⁄818
21⁄329 13⁄1612 123⁄32157⁄827⁄16181⁄2
11⁄1691⁄417⁄32121⁄813⁄416 21⁄219
23⁄3291⁄211⁄4121⁄2125⁄32161⁄8
3⁄493⁄419⁄32141⁄8113⁄16161⁄4

Twist drills are also made to the Stubs wire gauge as follows:—

Numbers by gauge.Length.Numbers by gauge.Length.
1 to 54 31 to 3525⁄8
6„10311⁄1636„4027⁄16
11„1531⁄241„4521⁄4
16„2031⁄446„5021⁄16
21„2531⁄1651„6013⁄4
26„30213⁄1661„7011⁄2

Fig. 1716.

[Fig. 1716] represents the flat drill, which has three cutting edges, a, b, and c. The only advantages possessed by the flat drill are that it will stand rougher usage than the twist drill, and may be fed faster, while it can be more easily made. Furthermore, when the work is unusually hard the flat drill can be conveniently shaped and tempered to suit the conditions.