This section is from the book "Machine Shop Work", by Frederick W. Turner, Oscar E. Perrigo, Howard P. Fairfield. Also available from Amazon: Machine shop work.
The tool used for cutting square threads is shown in Figs. 158 and 159. It is of the proper
The Tap Drill Diameters in the Table Provide for a Slight Clearance at the Root of the Thread in Order to Facilitate Tapping and Reduce Tap Breakages. If Full Threads Are Required
Use the Diameters at the Root of the Threads for the Tap Drill Diameters Instead.
U. S. Standard Screw Thread
Pitch | = | 1 | ||
No. of Th'ds. per Inch | ||||
Depth of Th'd | . = | 0.6495 X Pitch | ||
Width of Flat | Pitch | |||
8 |
Diameter | Number of Threads per Inch | Diameter at Root of Thread | DlAMETER of Tap Drill | Area in Square Inches | Tensile Strength At Stress of 6000 Pounds per Square Inch | Working Strength at Stress of 6000 Pounds per Square Inch | Dimensions of Nuts and Bolt Heads | |||||
of Bolt | at Root of Thread | |||||||||||
1/4 | 20 | 0.185 | 13/64 | 0.049 | 0.026 | 160 | ...... | 1/2 | 0.578 | 0.707 | 1/4 | 1/4 |
5/16 | 18 | 0.240 | 1/4 | 0.076 | 0.045 | 270 | ....... | 19/32 | 0.686 | 0.840 | 5/16 | 19/64 |
3/8 | 16 | 0.294 | 5/16 | 0.110 | 0.068 | 410 | ....... | 1 1/16 | 0.794 | 0.972 | 3/8 | 11/32 |
7/16 | 14 | 0.345 | 23/64 | 0.150 | 0.093 | 560 | ........ | 25/32 | 0.902 | 1.105 | 7/16 | 25/64 |
1/2 | 13 | 0.400 | 21/64 | 0.196 | 0.126 | 760 | ......... | 7/8 | 1.011 | 1.237 | 1/2 | 7/16 |
9/16 | 12 | 0.454 | 15/32 | 0.248 | 0.162 | 1000 | .......... | 31/32 | 1.119 | 1.370 | 9/16 | 31/64 |
5/8 | 11 | 0.507 | 1 7/32 | 0.307 | 0.202 | 1210 | 260 | 1 1/16 | 1.227 | 1.502 | 5/8 | 17/32 |
3/4 | 10 | 0.620 | 41/64 | 0.442 | 0.302 | 1810 | 680 | 1 1/4 | 1.444 | 1.768 | 3. | 5/8 |
7/8 | 9 | 0.731 | 3/4 | 0.601 | 0.419 | 2520 | 1210 | 1 7/16 | 1.660 | 2.033 | 7/8 | 23/32 |
1 | 8 | 0.838 | 55/64 | 0.785 | 0.551 | 3300 | 1790 | 1 5/8 | 1.877 | 2.298 | 1 | 13/16 |
1 1/8 | 7 | 0.939 | 31/32 | 0.994 | 0.694 | 4160 | 2470 | 1 13/16 | 2.093 | 2.563 | 1 1/8 | 29/32 |
1 1/4 | 7 | 1.064 | 1 3/32 | 1.227 | 0.893 | 5350 | 3470 | 2 | 2.310 | 2.828 | 1 1/4 | 1 |
I 3/8 | 6 | 1.158 | 1 7/32 | 1.485 | 1.057 | 6340 | 4260 | 2 3/16 | 2.527 | 3.093 | 1 3/8 | 1 3/32 |
1 1/2 | 6 | 1.283 | 1 11/32 | 1.767 | 1.295 | 7770 | 5500 | 2 3/8 | 2.743 | 3.358 | 1 1/2 | 1 3/16 |
1 5/8 | 5 1/2 | 1.389 | 1 27/64 | 2.074 | 1.515 | 9090 | 6630 | 2 9/16 | 2.960 | 3.623 | 1 5/8 | 1 9/32 |
1 3/4 | 5 | 1.490 | 1 17/32 | 2.405 | 1.746 | 10470 | 7830 | 2 3/4 | 3.176 | 3.889 | 1 3/4 | 1 3/8 |
1 7/8 | 5 | 1.615 | 1 21/32 | 2.761 | 2.051 | 12300 | 9470 | 2 15/16 | 3.393 | 4.154 | 1 7/8 | 1 15/32 |
2 | 4 1/2 | 1.711 | 1 49/64 | 3.142 | 2.302 | 13800 | 10800 | 3 1/8 | 3.609 | 4.419 | 2 | 1 9/16 |
2 1/4 | 4 1/2 | 1.961 | 2 1/64 | 3.976 | 3.023 | 18100 | 14700 | 3 1/2 | 4.043 | 4.949 | 2 1/4 | 1 3/4 |
2 1/2 | 4 | 2.175 | 2 15/64 | 4.909 | 3.719 | 22300 | 18500 | 3 7/8 | 4.476 | 5.479 | 2 1/2 | 1 15/16 |
2 3/4 | 4 | 2.425 | 2 31/64 | 5.940 | 4.620 | 27700 | 23600 | 4 1/4 | 4.909 | 6.010 | 2 3/4 | 2 1/8 |
3 | 3 1/2 | 2.629 | 2 11/16 | 7.069 | 5.428 | 32500 | 28000 | 4 5/8 | 5.342 | 6.540 | 3 | 2 5/16 |
3 1/4 | 3 1/2 | 2.879 | 2 15/16 | 8.296 | 6.510 | 39000 | 34100 | 5 | 5.775 | 7.070 | 3 1/4 | 2 1/2 |
3 1/2 | 3 1/4 | 3.100 | 3 11/64 | 9.621 | 7.548 | 45300 | 40000 | 5 3/8 | 6.208 | 7.600 | 3 1/2 | 2 11/16 |
3 3/4 | 3 | 3.317 | 3 3/8 | 11.045 | 8.641 | 51800 | 45000 | 5 3/4 | 6.641 | 8.131 | 3 3/4 | 2 7/8 |
4 | 3 | 3.567 | 3 5/8 | 12.566 | 9.963 | 59700 | 50100 | 6 1/8 | 7.074 | 8.661 | 4 | 3 1/16 |
4 1/4 | 2 7/8 | 3.798 | 3 27/32 | 14.186 | 11.340 | 68000 | 58000 | 6 1/2 | 7.508 | 9.191 | 4 1/4 | 3 1/4 |
4 1/2 | 2 3/4 | 4.028 | 4 3/32 | 15.904 | 12.750 | 76500 | 66000 | 6 7/8 | 7.941 | 9.721 | 4 1/2 | 3 |
4 3/4 | 2 5/8 | 4.255 | 4 5/16 | 17.721 | 14.215 | 85500 | 74000 | 7 1/ 4 | 8.374 | 10.252 | 4 3/4 | 3 5/8 |
5 | 2 1/2 | 4.480 | 4 9/16 | 19.635 | 15.760 | 94000 | 82500 | 7 5/8 | 8.807 | 10.782 | 5 | 3 13/16 |
5 1/4 | 2 1/2 | 4.730 | 4 13/16 | 21.648 | 17.570 | 105500 | 93000 | 8 | 9.240 | 11.312 | 5 1/4 | 4 |
5 1/2 | 2 3/8 | 4.953 | 5 1/32 | 23.758 | 19.260 | 116000 | 103000 | 8 3/8 | 9.673 | 11.842 | 5 1/2 | 4 3/16 |
5 3/4 | 2 3/8 | 5.203 | 5 9/32 | 25.967 | 21.250 | 127000 | 114000 | 8 3/4 | 10.106 | 12.373 | 5 3/4 | 4 3/8 |
6 | 2 1/4 | 5.423 | 5 1/2 | 28.274 | 23.090 | 138000 | 124000 | 9 1/8 | 10.539 | 12.903 | 6 | 4 9/16 |
* Reprinted from Machinery, thickness at the cutting edge, but is somewhat narrow back of this point. The sides of the tool are inclined to the bodyas shown at AB, Fig. 159; the amount of this inclination varies with the pitch of the thread and the diameter of the piece on which the thread is to be cut. To find the inclination, draw an indefinite straight line AB; and at right angles to it draw CD, Fig. 160. Make the length of CD equal to the circumference of the thread to be cut, measured at the root of the thread. On AB, lay off from C a distance EC equal to the pitch; then draw ED. This line will represent the angle of the side of the thread. The angle of the side of the cutting tool must be a little greater for clearance.
Fig. 160. Diagram of Clearance Angle.
For cutting inside threads, the shape of the cutting edge of the tool should be the same as for cutting an outside thread, and the tool must be made so that the cutting edge alone touches the work. This is accomplished by bending the tool as shown in Fig. 161, and giving it considerable clearance.
 
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