Parallel shanked twist drills are driven by chucks, while taper, shanked ones, are driven by sockets, such as in [Fig. 1042], from c to d, fitting into the lathe centre hole, while the bore at the other end is the Morse standard taper, to receive the drills e e, which have a projection such as shown at a, which by fitting into a slot that meets the end of the taper holes in the socket, lock the drill and prevent its revolving in the socket, while affording a means of forcing the drill out by inserting a key k, as shown in the figure.[14]
[14] See also [Shanks and Sockets for Drills used in the Drilling Machine].
Each socket takes a certain number of different sized drills, the shanks of the smaller drills being in some cases longer than the drill body.
| Number | 1 | socket receives | drills from | 1⁄8 | to | 19⁄32 | inch | inclusive. | ||
| „ | 2 | „ | „ | 5⁄8 | „ | 29⁄32 | „ | „ | ||
| „ | 3 | „ | „ | 15⁄16 | „ | 1 | 1⁄4 | „ | „ | |
| „ | 4 | „ | „ | 1 | 9⁄32 | „ | 2 | „ | „ | |
| „ | 5 | „ | „ | 2 | 1⁄32 | „ | 2 | 1⁄2 | „ | „ |
These sockets are manufactured ready to receive the drills, but are left unturned at the shank end so that they may be fitted to the particular lathe or machine in which they are to be used, no standard size or degree of taper having as yet been adopted.
Fig. 1043.
A twist drill possesses three cutting edges marked a, b, c respectively in [Fig. 1043], and of these c is the least effective, because it cannot be made as keen as is desirable for rapid and clean cutting, and therefore necessitates that the drill be given an unusually fine rate of feed as compared with other cutting tools.