Fig. 1860.
Fig. 1861.
A common method of avoiding this is to cut away all the teeth save for a width of die equal to the diameter of the bolt, as shown in [Fig. 1859]. An equally effective and much simpler plan is to form the dies as in [Fig. 1860], the diameter at the back b being slightly larger than that at the mouth a, so that the back teeth are relieved of cutting duty. This enables the dies to undergo more grindings and still retain sufficient teeth. For example, the chamfer at a may be ground farther towards b, and still leave in action sufficient teeth to equal in width of chaser the diameter of the bolt. To enable the threading of dies in this manner the hobs or master taps employed to thread them are formed as in [Fig. 1861], the proportions of the master taps for the different sizes of bolts being as given in the following table:—
| Diameter of bolt. | —— | —— | Length at A. | Length at B. | Length at C. | Length at D. | Length at E. | ||||||||||
| 1⁄4 | Dia. from | G to H | 15⁄64 | At J | 7⁄32 | 1⁄2 | 1 | 1 | 1 | 1⁄2 | 1⁄2 | ||||||
| 5⁄16 | „ | „ | 19⁄64 | „ | 9⁄32 | 1⁄2 | 1 | 1 | 1 | 1⁄2 | 1⁄2 | ||||||
| 3⁄8 | „ | „ | 23⁄64 | „ | 11⁄32 | 1⁄2 | 1 | 1 | 1 | 1⁄2 | 1⁄2 | ||||||
| 7⁄16 | „ | „ | 27⁄64 | „ | 13⁄32 | 1⁄2 | 1 | 1 | 1 | 1⁄2 | 1⁄2 | ||||||
| 1⁄2 | „ | „ | 31⁄64 | „ | 15⁄32 | 1⁄2 | 1 | 1⁄2 | 1 | 1⁄2 | 1 | 1⁄2 | 3⁄4 | ||||
| 5⁄8 | „ | „ | 39⁄64 | „ | 19⁄32 | 1⁄2 | 1 | 1⁄2 | 1 | 1⁄2 | 1 | 1⁄2 | 3⁄4 | ||||
| 3⁄4 | „ | „ | 47⁄64 | „ | 23⁄32 | 1⁄2 | 1 | 1⁄2 | 2 | 1 | 1⁄2 | 3⁄4 | |||||
| 7⁄8 | „ | „ | 55⁄64 | „ | 27⁄32 | 1⁄2 | 1 | 1⁄2 | 2 | 1 | 1⁄2 | 3⁄4 | |||||
| 1 | Dia. at G | 31⁄32 | At J 1⁄100 less | 1⁄2 | 4 | 4 | 1 | 1⁄2 | 1 | ||||||||
| 1 | 1⁄8 | „ | 1 | 3⁄32 | —— | 1 | 4 | 4 | 1 | 1⁄2 | 1 | ||||||
| 1 | 1⁄4 | „ | 1 | 7⁄32 | —— | 1 | 4 | 4 | 1 | 1⁄2 | 1 | ||||||
| 1 | 3⁄8 | „ | 1 | 11⁄32 | —— | 1 | 4 | 4 | 1 | 1⁄2 | 1 | ||||||
| 1 | 1⁄2 | „ | 1 | 15⁄32 | —— | 1 | 4 | 4 | 1 | 1⁄2 | 1 | 1⁄4 | |||||
| 1 | 5⁄8 | „ | 1 | 19⁄32 | —— | 1 | 5 | 5 | 2 | 1 | 1⁄2 | ||||||
| 1 | 3⁄4 | „ | 1 | 23⁄32 | —— | 1 | 5 | 5 | 2 | 1 | 1⁄2 | ||||||
| 1 | 7⁄8 | „ | 1 | 27⁄32 | —— | 1 | 6 | 6 | 2 | 1 | 3⁄4 | ||||||
| 2 | „ | 1 | 31⁄32 | —— | 1 | 6 | 6 | 2 | 1 | 3⁄4 | |||||||
| All over 2 in. same length as the 2 in. Shanks J turned to bottom of last thread. | |||||||||||||||||
The cutting speeds for the dies and taps are as given in the following table, in which it will be seen that the speeds for bolt factories are greater than for machine shops. This occurs on account of the greater experience of the operators and the greater care taken in lubricating the dies and keeping them sharp:—
| Diameter of bolt. | Revolutions of dies for machine shops. | Revolutions of dies for bolt factories. | Diameter of bolt. | Revolutions of dies for machine shops. | Revolutions of dies for bolt factories. | ||
| inch. | inch. | ||||||
| 1⁄8 | 450 | 600 | 1 | 5⁄8 | 33 | 48 | |
| 1⁄4 | 230 | 300 | 1 | 3⁄4 | 30 | 45 | |
| 3⁄8 | 150 | 200 | 1 | 7⁄8 | 28 | 40 | |
| 1⁄2 | 100 | 150 | 2 | 25 | 38 | ||
| 5⁄8 | 75 | 125 | 2 | 1⁄8 | 23 | 36 | |
| 3⁄4 | 65 | 100 | 2 | 1⁄4 | 22 | 34 | |
| 7⁄8 | 55 | 85 | 2 | 3⁄8 | 21 | 32 | |
| 1 | 45 | 75 | 2 | 1⁄2 | 20 | 30 | |
| 1 | 1⁄8 | 42 | 65 | 2 | 5⁄8 | 18 | 25 |
| 1 | 1⁄4 | 40 | 60 | 2 | 3⁄4 | 15 | 20 |
| 1 | 3⁄8 | 38 | 55 | 2 | 7⁄8 | 12 | 18 |
| 1 | 1⁄2 | 35 | 50 | 3 | 10 | 15 | |
| Taps same speed as dies. | |||||||
