Diameter of Tap Drill Machinery's Handbook, 31st Edition
2169
Table 5. Tap Drills and Clearance Drills for Machine Screws with American National Thread Form Size of Screw No. of Tap Drills Clearance Hole Drills
Close Fit Free Fit Drill Size Decimal Equiv. Drill Size Decimal Equiv.
Threads per Inch
No. or Diam.
Decimal Equiv.
Drill Size
Decimal Equiv.
.0469 .0595 .0595 .0700 .0700 .0785 .0820 .0860 .0890 .0935 .1015 1040 .1065 .1130 .1360 .1360 .1495 .1590 .1770 .1820 .1935 .2010 .2010 .2130 .2570 .2720 .3125 .3320 .3680 .3906 .4219 .4531
52 48 43 37
.0635 .0760 .0890 .1040
50 46 41 35
.0700 .0810 .0960 .1100
0 1 2 3
.060 .073 .086 .099
80 64 72 56 64 48 56
3 ∕ 64 53 53 50 50 47 45 44 43 42 38 37 36 33 29 29 25 21 16 14 10 7
36 a 40 48 40 44 32 40 32 36
4
.112
32
.1160
30
.1285
5 6 8
.125 .138 .164 .190 .216 .242 .250
30 27 18
.1285 .1440 .1695 .1960 .2210 .2460 .2570 .3230 .3860
29 25 16
.1360 .1495 .1770 .2010 .2280 .2570 .2660 .3320 .3970
24 32 24 28
10 12 14
9 2
7 1 F
20 a 24 a 20 28 18 24
D
7 3 F I
F P
H Q X
1 ∕ 4
.3125
5 ∕ 16
16 24 14 20 13 20
5 ∕ 16 Q
W
.375
3 ∕ 8
U 25 ∕ 64 27 ∕ 64 29 ∕ 64
.4375
.4531
.4687
7 ∕ 16
29 ∕ 64
15 ∕ 32
1 ∕ 2 .5312 a These screws are not in the American Standard but are from the former A.S.M.E. Standard. .500 33 ∕ 64 .5156 17 ∕ 32 Factors Influencing Minor Diameter Tolerances of Tapped Holes.— As stated in the Uni- fied screw thread standard, the principle practical factors that govern minor diameter tolerances of internal threads are tapping difficulties, particularly tap breakage in the small sizes, availability of standard drill sizes in the medium and large sizes, and depth (radial) of engagement. Depth of engagement is related to the stripping strength of the thread assembly, and thus also to the length of engagement. It also has an influence on the tendency toward disengagement of the threads on one side when assembly is eccentric. The amount of possible eccentricity is one-half of the sum of the pitch diameter allowance and tolerances on both mating threads. For a given pitch, or height of thread, this sum increases with the diameter, and accordingly this factor would require a decrease in minor diameter tolerance with increase in diameter. However, such decrease in tolerance would often require the use of special drill sizes; therefore, to facilitate the use of standard drill sizes, for any given pitch the minor diameter tolerance for Unified thread classes 1B and 2B threads of 1 ∕ 4 inch diameter and larger is constant, in accordance with a formula given in the American Standard for Unified Screw Threads. Effect of Length of Engagement of Minor Diameter Tolerances: There may be applications where the lengths of engagement of mating threads is relatively short or the combina tion of materials used for mating threads is such that the maximum minor diameter tolerance given in the Standard (based on a length of engagement equal to the nominal diameter) may not provide the desired strength of the fastening. Experience has shown that for lengths of engagement less than 2 ∕ 3 D (the minimum thickness of standard nuts)
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