Standard UN and UNJ tap drill sizes

Standard UNJ and UN Tap Drill Sizes

It's my understanding that you can use the same tap drill hole size (the actual tap is what varies) for both UNJ and UN thread forms, however; if you want to maximize the percentage of thread (side wall) depth, the hole may have to be varied as well. This table is based on a percentage of thread depth that is around 60 to 80%. Included in this calulation is the probable oversize (for each thread size) in accordance with FED-STD-H28/2. Note: In order to comply with AS8879 and FED-STD-H28/2 the selected drill sizes must fall within the min/max ranges listed for each thread form (UNJ and UN). Having said that, your drawing might need to specify SAE AS8879 or FED-STD-H28/2 in order to insure the quality of thread form, for example; "THREADS SHALL BE IAW SAE AS-8879 (UNJ PROFILE)" or "THREADS SHALL BE IAW FED-STD-H28/2 (UN PROFILE)". Here's another chart , and another, if you don't like the one below..

 

	            ------------UNJ, CLASS 3B------------ -------------UN, CLASS 2B-------------
      THREAD      |SERIES      NOM       1)MIN 1)MAX   % |SERIES      NOM      2)MIN 2)MAX   % |
  --------------  |------ -------------   ----  ----| THD|------ -------------- ---- -----| THD|
  | #0(.060)x80   | UNJF | 1.25mm(.0492) .0479 .0511| 57 | UNF | 1.25mm(.0472) .0465 .0514| 70 |
  | #1(.073)x64   | UNJC | 1.5mm (.0591) .0578 .0619| 61 | UNC | 1.45mm(.0571) .0561 .0623| 71 |
  | #1(.073)x72   | UNJF | #53   (.0595) .0595 .0631| 66 | UNF | #53   (.0595) .0580 .0635| 66 |
 *| #2(.086)x56   | UNJC | 1.75mm(.0689) .0686 .0732| 66 | UNC | 1.7mm (.0669) .0667 .0737| 75 |
  | #2(.086)x64   | UNJF | 1.8mm (.0709) .0708 .0749| 66 | UNF | 1.8mm (.0709) .0691 .0753| 66 |
  | #3(.099)x48   | UNJC | 2.05mm(.0807) .0787 .0841| 61 | UNC | 1.95mm(.0768) .0764 .0845| 75 |
  | #3(.099)x56   | UNJF | #45   (.0820) .0816 .0862| 65 | UNF | 2.05mm(.0807) .0797 .0865| 71 |
 *| #4(.112)x40   | UNJC | 2.25mm(.0886) .0877 .0942| 66 | UNC | #44   (.0860) .0849 .0939| 74 |
  | #4(.112)x48   | UNJF | 5.35mm(.0925) .0917 .0971| 65 | UNF | 2.3mm (.0906) .0894 .0968| 72 |
  | #5(.125)x40   | UNJC | #38   (.1015) .1007 .1072| 65 | UNC | #40   (.0980) .0979 .1062| 76 |
  | #5(.125)x44   | UNJF | #37   (.1040) .1029 .1088| 63 | UNF | #38   (.1015) .1004 .1079| 72 |
 *| #6(.138)x32   | UNJC | 2.75mm(.1083) .1076 .1157| 67 | UNC | 2.7mm (.1063) .1040 .1140| 72 |
  | #6(.138)x40   | UNJF | 2.9mm (.1142) .1137 .1202| 65 | UNF | #33   (.1130) .1110 .1190| 69 |
 *| #8(.164)x32   | UNJC | 3.4mm (.1339) .1336 .1417| 67 | UNC | 3.4mm (.1339) .1300 .1390| 67 |
  | #8(.164)x36   | UNJF | 3.5mm (.1378) .1370 .1442| 65 | UNF | #29   (.1360) .1340 .1420| 70 |
  | #10(.190)x24  | UNJC | #25   (.1495) .1494 .1600| 69 | UNC | 3.7mm (.1457) .1450 .1560| 76 |
 *| #10(.190)x32  | UNJF | #20   (.1610) .1596 .1675| 64 | UNF | 5/32  (.1562) .1560 .1640| 75 |
  | #12(.216)x24  | UNJC | #16   (.1770) .1754 .1852| 66 | UNC | 11/64 (.1719) .1710 .1810| 75 |
  | #12(.216)x28  | UNJF | #14   (.1820) .1812 .1896| 66 | UNF | #15   (.1800) .1770 .1860| 76 |
 *| 1/4(.250)x20  | UNJC | 13/64 (.2031) .2013 .2121| 66 | UNC | 5mm   (.1968) .1960 .2070| 76 |
  | 1/4(.250)x28  | UNJF | 7/32  (.2165) .2152 .2229| 64 | UNF | 5.4mm (.2126) .2110 .2200| 72 |
 *| 5/16(.3125)x18| UNJC | 5.5mm (.2598) .2584 .2690| 68 | UNC | 6.5mm (.2559) .2520 .2650| 73 |
  | 5/16(.3125)x24| UNJF | #I    (.2720) .2719 .2799| 67 | UNF | 6.8mm (.2677) .2670 .2770| 75 |
 *| 3/8(.375)x16  | UNJC | 8mm   (.3150) .3141 .3250| 68 | UNC | 7.9mm (.3110) .3070 .3210| 73 |
  | 3/8(.375)x24  | UNJF | 8.5mm (.3346) .3344 .3417| 65 | UNF | 8.4mm (.3307) .3300 .3400| 74 |
  | 7/16(.438)x14 | UNJC | 9.4mm (.3701) .3680 .3795| 68 | UNC | 9.25mm(.3642) .3600 .3760| 74 |
  | 7/16(.438)x20 | UNJF | 9.9mm (.3898) .3888 .3970| 66 | UNF | 9.75mm(.3839) .3830 .3950| 75 |
 *| 1/2(.500)x13  | UNJC | 11mm  (.4331) .4251 .4368| 62 | UNC | 27/64 (.4219) .4170 .4340| 73 |
  | 1/2(.500)x20  | UNJF | 11.5mm(.4528) .4513 .4591| 65 | UNF | 11.5mm(.4528) .4460 .4570| 65 |
  | 9/16(.562)x12 | UNJC | 31/64 (.4844) .4814 .4914| 68 | UNC | 12mm  (.4724) .4720 .4900| 79 |
  | 5/8(.625)x11  | UNJC | 35/64 (.5469) .5365 .5474| 62 | UNC | 17/32 (.5312) .5270 .5460| 75 |
* PREFERRED USAGE

1) Selected sizes from MIL-STD-8879, SUPERSEDED by SAE AS8879.
2) Selected sizes from FED-STD-H28/2B.
3) The probable mean oversize dimensions used in the caluclations above, were taken from 
   latest copy of FED-STD-H28/2B, which is a little tighter than the dimensions listed in the
   older "USCTI" chart below.

Selection of Tap Drills

Twist drills have a tendency to cut oversize. The table below list this variation in size, and should be considered when determining minimum and maximum values of tapdrill sizes.
Drill Dia	Average Max	Mean	Average Min	Drill Dia	Average Max	Mean	Average Min
1/16	.002	.0015	.001	1/2	.008	.005	.003
1/8	.0045	.003	.001	3/4	.008	.005	.003
1/4	.0065	.004	.0025	1	.009	.007	.004

Courtesy ot The United States Cutting Tool Institute (USCTI).

Maximum Drill Drift

Twist drills tend to drift, when the depth exceeds five times the diameter. Drawings should allow for this drift with true position tolerancing or a notation which specifies the maximum allowable drift.

Maximum Drill Drift
Drill Size	.250	.500	1.000	1.500	2.000	Drill Size	.250	.500	1.000	1.500	2.000
.0135 (80) - .0280 (70)	.031	----	----	----	----	.0995 (39) - .1285 (30)	----	.006	.015	.031	----
.0292 (69) - .0400 (60)	.015	.025	----	----	----	.1360 (29) - .1610 (20)	----	.005	.012	.022	.035
.0410 (59) - .0520 (55)	----	.015	.025	----	----	.1660 (19) - .1935 (10)	----	.004	.008	.018	.031
.0550 (54) - .0700 (50)	----	.010	.020	----	----	.1960 (9) - .2490	----	.003	.006	.014	.024
.0730 (49) - .0980 (40)	----	.008	.018	.035	----	.2500 - .4990	----	.002	.004	.007	.012