Tap Drill Size
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. Tap Drill Sizes - Cut Taps - Inch
Suggested Percentage of Full Thread in Tapped Holes
*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 Hard or Tough Cutting
60% - 70% 65% - 70% 75% - 85%
55% - 65% 60% - 70%
Threads Per Inch
Minor Diameter
Tap Drill Diameter - Cut Taps
Tap Size
80% Thread 75% Thread 70% Thread 65% Thread 60% Thread (in) (in) (in) (in) (in)
UNC
UNF 8-Pitch Min. 2B Max. 2B
0 1
–
80
– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – 8 – – 8 – – 8 – – 8 8 – 8 8 – 8 – 8 – 8
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 0.9700 1.0350 0.9900 1.0950 1.1600 1.1150 1.1950 1.2850 1.2400 1.3200 1.4100 1.3650 1.4900 1.5330 1.6150 1.7400 1.7590 1.8650 2.0090 2.1150 2.2290 2.3650
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 0.9980 1.0530 1.0150 1.1230 1.1780 1.1400 1.2250 1.3030 1.2650 1.3500 1.4280 1.3900 1.5150 1.5670 1.6400 1.7650 1.7950 1.8900 2.0450 2.1400 2.2670 2.3900
0.0470 0.0568 0.0586 0.0674 0.0698 0.0774 0.0804 0.0860 0.0904 0.0990 0.1014 0.1055 0.1120 0.1315 0.1351 0.1467 0.1575 0.1727 0.1789 0.1980 0.2129 0.2548 0.2692 0.3101 0.3317 0.3633 0.3855 0.4201 0.4480 0.4759 0.5048 0.5305 0.5673 0.6461 0.6851 0.7595 0.8008 0.8701 0.9134 0.9765 1.0384 0.9951 1.1015 1.1634 1.1201 1.2018 1.2884 1.2451 1.3268 1.4134 1.3701 1.4951 1.5422 1.6201 1.7451 1.7691 1.8701 2.0191 2.1201 2.2402 2.3701
0.0478 0.0578 0.0595 0.0686 0.0708 0.0787 0.0816 0.0876 0.0917 0.1006 0.1029 0.1076 0.1136 0.1336 0.1369 0.1494 0.1596 0.1754 0.1812 0.2013 0.2152 0.2584 0.2719 0.3141 0.3344 0.3679 0.3888 0.4251 0.4513 0.4813 0.5084 0.5364 0.5709 0.6526 0.6891 0.7668 0.8054 0.8782 0.9188 0.9858 1.0438 1.0032 1.1108 1.1688 1.1282 1.2126 1.2938 1.2532 1.3376 1.4188 1.3782 1.5032 1.5551 1.6282 1.7532 1.7835 1.8782 2.0335 2.1282 2.2564 2.3782
0.0486 0.0588 0.0604 0.0698 0.0718 0.0801 0.0828 0.0893 0.0931 0.1023 0.1043 0.1096 0.1153 0.1356 0.1387 0.1521 0.1616 0.1781 0.1835 0.2045 0.2175 0.2620 0.2746 0.3182 0.3371 0.3726 0.3920 0.4301 0.4545 0.4867 0.5120 0.5423 0.5745 0.6591 0.6932 0.7740 0.8101 0.8863 0.9242 0.9951 1.0492 1.0113 1.1201 1.1742 1.1363 1.2235 1.2992 1.2613 1.3485 1.4242 1.3863 1.5113 1.5681 1.6363 1.7613 1.7979 1.8863 2.0479 2.1363 2.2727 2.3863
0.0494 0.0598 0.0613 0.0709 0.0728 0.0814 0.0839 0.0909 0.0944 0.1039 0.1058 0.1116 0.1169 0.1376 0.1405 0.1548 0.1636 0.1808 0.1858 0.2078 0.2198 0.2656 0.2773 0.3222 0.3398 0.3772 0.3953 0.4351 0.4578 0.4921 0.5156 0.5482 0.5781 0.6656 0.6972 0.7812 0.8147 0.8945 0.9296 1.0044 1.0546 1.0195 1.1294 1.1796 1.1445 1.2343 1.3046 1.2695 1.3593 1.4296 1.3945 1.5195 1.5811 1.6445 1.7695 1.8124 1.8945 2.0624 2.1445 2.2889 2.3945
0.0503 0.0608 0.0622 0.0721 0.0738 0.0828 0.0851 0.0925 0.0958 0.1055 0.1073 0.1136 0.1185 0.1396 0.1424 0.1575 0.1656 0.1835 0.1882 0.2110 0.2222 0.2692 0.2800 0.3263 0.3425 0.3818 0.3985 0.4400 0.4610 0.4976 0.5192 0.5541 0.5817 0.6721 0.7013 0.7884 0.8193 0.9026 0.9351 1.0137 1.0601 1.0276 1.1387 1.1851 1.1526 1.2451 1.3101 1.2776 1.3701 1.4351 1.4026 1.5276 1.5941 1.6526 1.7776 1.8268 1.9026 2.0768 2.1526 2.3051 2.4026
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 – 8 – 7 – – 7 – – 6 – – 6 – – – 5 – –
16
–
7/8
14
–
1
12
–
1-1/8
12
– – – – – – – – – – – – – – – – –
1-1/4
12
1-3/8
12
1-1/2
12
1-5/8 1-3/4 1-7/8
4-1/2
2
–
4-1/2
2-1/4
– 4 –
2-1/2
FORMULA: TAP DRILL SIZE
FORMULA: PERCENTAGE OF FULL THREAD
0.01299 x % of Full Thread # of Threads Per Inch
Tap Major Dia - Drill Dia 0.01299
Drill Size = Tap Major Dia -
% of Full Thread = Threads Per Inch x
Example: to determine the % of thread for 1/4”-20 UNC using 0.2045” drill
Example: to determine drill size for 1/4”-20 UNC tap, 70% thread Tap Major Ø = 0.2500” % of Full Thread = 70% # of Threads per Inch = 20
Drill Size = 0.2500” - (0.01299 x 70%) 20 Drill Size = 0.2500” - 0.0455” = 0.2045”
# Threads per Inch = 20 Tap Major Ø = 0.2500” Drill Ø = 0.2045”
% of Thread = 20 x (0.2500 - 0.8245) 0.01299
% of Thread = 20 x 3.50 = 70%
Threading Technical
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