Tap Drill Size
Tap Drill Sizes - Form Taps - Inch
Suggested Percentage of Full Thread in Tapped Holes
To minimize tapping problems and lengthen tool life, use the largest drill possible to produce a minor diameter that will result in the lowest percentage of full thread consistent with adequate strength. A minor diameter that provides a 55% to 65% thread is sufficient for most requirements, but in some cases a higher percentage of thread may be necessary to conform with the minor diameter limits of the thread class specified. * Generally, deeper than 1 1/2 times the hole diameter.
*Deep Hole Tapping
Average Commercial Work
Thin Sheet Stock or Stamping
Material
Aluminum, Brass, Bronze, Cast Iron, Copper, Mild Steel, Tool Steel Cast Steel, Drop Forging, Monel Metal, Nickel Steel, Stainless Steel
Free Cutting
60% - 70% 65% - 70%
75% - 85%
Hard or Tough Cutting
55% - 65% 60% - 70%
Threads Per Inch
Minor Diameter
Tap Drill Diameter - Form Taps
75% Thread 70% Thread 65% Thread 60% Thread 55% Thread (in) (in) (in) (in) (in)
Tap Size
UNC UNF
8-Pitch Min. 2B Max. 2B
0 1
–
80
– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –
0.0465 0.0561 0.0580 0.0667 0.0691 0.0764 0.0797 0.0849 0.0894 0.0979 0.1004 0.1040 0.1110 0.1300 0.1340 0.1450 0.1560 0.1710 0.1770 0.1960 0.2110 0.2520 0.2670 0.3070 0.3300 0.3600 0.3830 0.4170 0.4460 0.4720 0.5020 0.5270 0.5650 0.6420 0.6820 0.7550 0.7980 0.8650 0.9100
0.0514 0.0623 0.0635 0.0737 0.0752 0.0845 0.0865 0.0939 0.0968 0.1062 0.1079 0.1140 0.1190 0.1390 0.1420 0.1560 0.1640 0.1810 0.1860 0.2070 0.2200 0.2650 0.2770 0.3210 0.3400 0.3760 0.3950 0.4340 0.4570 0.4900 0.5150 0.5460 0.5780 0.6630 0.6960 0.7780 0.8140 0.8900 0.9280
0.0536 0.0650 0.0659 0.0769 0.0780 0.0884 0.0899 0.0993 0.1014 0.1123 0.1134 0.1221 0.1253 0.1481 0.1498 0.1688 0.1741 0.1948 0.1978 0.2245 0.2318 0.2842 0.2912 0.3431 0.3537 0.4011 0.4120 0.4608 0.4745 0.5200 0.5342 0.5787 0.5967 0.6990 0.7181 0.8183 0.8386 0.9363 0.9575
0.0540 0.0655 0.0663 0.0774 0.0785 0.0890 0.0904 0.1000 0.1020 0.1130 0.1141 0.1230 0.1260 0.1490 0.1507 0.1700 0.1750 0.1960 0.1990 0.2260 0.2329 0.2861 0.2927 0.3452 0.3552 0.4035 0.4137 0.4634 0.4762 0.5229 0.5361 0.5817 0.5986 0.7024 0.7202 0.8221 0.8410 0.9405 0.9603
0.0545 0.0661 0.0669 0.0781 0.0791 0.0898 0.0911 0.1010 0.1028 0.1140 0.1150 0.1243 0.1270 0.1503 0.1518 0.1716 0.1762 0.1976 0.2002 0.2279 0.2342 0.2879 0.2941 0.3474 0.3566 0.4059 0.4154 0.4660 0.4779 0.5257 0.5379 0.5848 0.6004 0.7058 0.7224 0.8259 0.8434 0.9448 0.9632
0.0549 0.0666 0.0673 0.0787 0.0796 0.0905 0.0917 0.1018 0.1035 0.1148 0.1157 0.1252 0.1278 0.1512 0.1526 0.1729 0.1772 0.1989 0.2014 0.2295 0.2354 0.2898 0.2955 0.3495 0.3580 0.4084 0.4171 0.4686 0.4796 0.5285 0.5398 0.5879 0.6023 0.7092 0.7245 0.8297 0.8459 0.9490 0.9660
0.0554 0.0672 0.0679 0.0794 0.0802 0.0913 0.0924 0.1028 0.1043 0.1158 0.1166 0.1264 0.1288 0.1524 0.1537 0.1746 0.1784 0.2006 0.2028 0.2315 0.2389 0.2917 0.2969 0.3516 0.3594 0.4108 0.4188 0.4712 0.4813 0.5313 0.5417 0.5910 0.6042 0.7126 0.7266 0.8334 0.8483 0.9533 0.9866
64
–
–
72
56
–
2
–
64
48
–
3
–
56
40
–
4
–
48
40
–
5
–
44
32
–
6
–
40
32
–
8
–
36
24
–
10
–
32
24
–
12
–
28
20
–
1/4
–
28
18
–
5/16
–
24
16
–
3/8
–
24
14
–
7/16
–
20
13
–
1/2
–
20
12
–
9/16
–
18
11
–
5/8
–
18
– – – –
10
–
3/4
– 9 –
16
–
7/8
14
– – –
8 –
–
1
12
FORMULA: TAP DRILL SIZE
FORMULA: PERCENTAGE OF FULL THREAD
Tap Major Dia - Drill Dia 0.0068
0.0068 x % of Full Thread # of Threads Per Inch
% of Full Thread = Threads Per Inch x
Drill Size = Tap Major Dia -
% of Thread = 20 x (0.2500 - 0.2262) 0.0068 Example: to determine the % of thread for 1/4”-20UNC Tap using 1.9603” drill
Example: to determine drill size for 1/4”-20UNC tap, 70% thread Tap Major Ø = 0.2500” % of Full Thread = 70% # of Threads per Inch = 20 Drill Size = 0.2500” -
(0.0068 x 70%) 20
# Threads per Inch = 20 Tap Major Ø = 0.2500” Drill Ø = 0.2045”
Drill Size = 0.2500” - 0.0238” = 0.2262”
% of Thread = 20 x 3.5 = 70%
Suggested Pipe Tap Drill Sizes
Tap Size
1/16 1/8 1/4 3/8 1/2 3/4
1 1-1/4 1-1/2 2 2-1/2 3 3-1/2 4
Taper Pipe Tap* Straight Pipe Tap†
C Q 7/16 9/16 45/64 29/32 1-9/64 1-31/64 1-23/32 2-3/16 2-5/8 3-1/4 3-3/4 4-1/4 1/4 11/32 7/16 37/64 23/32 59/64 1-5/32 1-1/2 1-3/4 2-7/32 2-21/32 – – –
Drill Sizes
*Sizes given permit direct tapping without reaming the hole, but only give a full thread for the first two or three threads. †For Dryseal Straight Pipe Threads suggested drill sizes are as shown, except; 1/4” pipe, use .444 drill size.
Threading Technical
783
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