Fabrication Notes -- Tapping and Clearance Drills for Fractional Machine Screws

Fabrication

Tapping and Clearance Drills for Fractional Machine Screws

Screw Size					Tapping Drills				Clearance Drills
Nominal Diam. (in.)	Threads/Inch	Series	Dec. Equiv.		Drill Size (65-75% thread)	Equivalents			Close Fit		Free Fit
Nominal Diam. (in.)	Threads/Inch	Series	Dec. Equiv.		Drill Size (65-75% thread)	(decimal)	(mm)		Drill Size	Dec. Equiv.	Drill Size	Dec. Equiv.

1/16	64	NS	0.0625		3/64"	0.0469	1.1913		52	0.0635	51	0.0670
1/16	72	NS	0.0625		3/64"	0.0469	1.1913		52	0.0635	51	0.0670
5/64	60	NS	0.0781		1/16"	0.0625	1.5875		46	0.0810	45	0.0820
5/64	72	NS	0.0781		52	0.0635	1.6129		46	0.0810	45	0.0820
3/32	48	NS	0.0938		49	0.0730	1.8542		41	0.0960	40	0.0980
3/32	50	NS	0.0938		49	0.0730	1.8542		41	0.0960	40	0.0980
7/64	48	NS	0.1094		43	0.0890	2.2606		34	0.1110	32	0.1160
1/8	32	NS	0.1250		3/32"	0.0938	2.3825		30	0.1285	29	0.1360
1/8	40	NS	0.1250		38	0.1015	2.5781		30	0.1285	29	0.1360
9/64	40	NS	0.1406		32	0.1160	2.9464		27	0.1440	26	0.1470
5/32	32	NS	0.1563		1/8"	0.1250	3.1750		20	0.1610	19	0.1660
5/32	36	NS	0.1563		30	0.1285	3.2639		20	0.1610	19	0.1660
11/64	32	NS	0.1719		9/64"	0.1406	3.5712		16	0.1770	15	0.1800
3/16	24	NS	0.1875		26	0.1470	3.7338		11	0.1910	9	0.1960
3/16	32	NS	0.1875		22	0.1570	3.9878		11	0.1910	9	0.1960
13/64	24	NS	0.2031		20	0.1610	4.0894		4	0.2090	3	0.2130
7/32	24	NS	0.2188		16	0.1770	4.4958		2	0.2210	1	0.2280
7/32	32	NS	0.2188		12	0.1890	4.8006		2	0.2210	1	0.2280
15/64	24	NS	0.2344		10	0.1935	4.9149		C	0.2420	D	0.2460
1/4	20	NC	0.2500		7	0.2010	5.1054		F	0.2570	H	0.2660
	24	NS			4	0.2090	5.3086
	27	NS			3	0.2130	5.4102
	28	NF			3	0.2130	5.4102
	32	NEF			7/32"	0.2188	5.5575
5/16	18	NC	0.3125		F	0.2570	6.5278		P	0.3230	Q	0.3320
	20	NS			17/64"	0.2656	6.7462
	24	NF			I	0.2720	6.9088
	27	NS			J	0.2770	7.0358
	32	NEF			9/32"	0.2812	7.1425
3/8	16	NC	0.3750		5/16"	0.3125	7.9375		W	0.3860	X	0.3970
	20	NS			21/64"	0.3281	8.3337
	24	NF			Q	0.3320	8.4328
	27	NS			R	0.3390	8.6106
	32	NEF			11/32"	0.3438"	8.7325
7/16	14	NC	0.4375		U	0.3680	9.3472		29/64"	0.4531	15/32"	0.4687
	20	NF			25/64"	0.3906	9.9212
	24	NS			X	0.3970	10.0838
	27	NS			Y	0.4040	10.2616
	28	NEF			15/32"	0.4688	11.9075
1/2	12	NS	0.5000		27/64"	0.4219	10.7163		33/64"	0.5156	17/32"	0.5312
	13	NC			27/64"	0.4219	10.7163
	20	NF			29/64"	0.4531	11.5087
	24	NS			29/64"	0.4531	11.5087
	27	NS			15/32"	0.4688	11.9075
	28	NEF			15/32"	0.4688	11.9075
9/16	12	NC	0.5625		31/64"	0.4844	12.3038		37/64"	0.5781	19/32"	0.5938
	18	NF			33/64"	0.5156	13.0962
	24	NEF			33/64"	0.5156	13.0962
	27	NS			17/32"	0.5313	13.4950
5/8	11	NC	0.6250		17/32"	0.5313	13.4950		41/64"	0.6406	21/32"	0.6562
	12	NS			35/64"	0.5469	13.8913
	18	NF			37/64"	0.5781	14.6837
	24	NEF			37/64"	0.5781	14.6837
	27	NS			19/32"	0.5937	15.0800
11/16	11	NS	0.6875		19/32"	0.5937	15.0800		45/64"	0.7031	23/32"	0.6562
	16	NS			5/8"	0.6250	15.8750
	24	NEF			41/64"	0.6406	16.4084
3/4	10	NC	0.7500		21/32"	0.6562	16.6675		49/64"	0.7656	25/32"	0.7812
	12	NS			43/64"	0.6719	17.0663
	16	NF			11/16"	0.6875	17.4625
	20	NEF			45/64"	0.7031	17.8587
	27	NS		23/32"	0.7187	18.2550
13/16	10	NS	0.8125	23/32"	0.7187	18.2550	53/64"	0.8281	27/32"	0.8438
13/16	20	NEF	0.8125	49/64"	0.7656	19.4462	53/64"	0.8281	27/32"	0.8438
7/8	9	NC	0.8750	49/64"	0.7656	19.4462	57/64"	0.8906	29/32"	0.9062
	12	NS		51/64"	0.7969	20.2413
	14	NF		13/16"	0.8125	20.6375
	18	NS		53/64"	0.8281	21.0337
	20	NEF		53/64"	0.8281	21.0337
	27	NS		27/32"	0.8437	21.4300
15/16	9	NS	0.9375	53/64"	0.8281	21.0337	61/64"	0.9531	31/32"	0.9688
15/16	20	NEF	0.9375	57/64"	0.8906	22.6212	61/64"	0.9531	31/32"	0.9688
1	8	NC	1.0000	7/8"	0.8750	22.2250	1-1/64"	1.0156	1-1/32"	1.0313
	12	NF		59/64"	0.9219	23.4163
	14	NS		15/16"	0.9375	23.8125
	20	NEF		61/64"	0.9531	24.2087
	27	NS		31/32"	0.9687	24.6050
	32	*		31/32"	0.9687	24.6050
1-1/16	18	NEF	1.0625	1"	1.0000	25.4000	1-5/64"	1.0781	1-3/32"	1.0938
1-1/8	7	NC	1.1250	63/64"	0.9844	25.0038	1-9/64"	1.1406	1-5/32"	1.1562
	12	NF		1-3/64"	1.0469	26.5913
	18	NEF		1-1/16"	1.0625	26.9875
1-3/16	18	NEF	1.1875	1-1/8"	1.1250	28.5750	1-13/64"	1.2031	1-7/32"	1.2188
1-1/4	7	NC	1.2500	1-7/64"	1.1094	28.1788	1-17/64"	1.2656	1-9/32"	1.2812
	12	NF		1-11/64"	1.1719	29.7663
	18	NEF		1-3/16"	1.1875	30.1625
1-5/16	18	NEF	1.3125	1-1/4"	1.2500	31.7500	1-21/64"	1.3281	1-11/32"	1.3438
1-3/8	6	NC	1.3750	1-7/32"	1.2188	30.9575	1-25/64"	1.3906	1-13/32"	1.4062
	12	NF		1-19/64"	1.2969	32.9413
	18	NEF		1-5/16"	1.3125	33.3375
1-7/16	18	NEF	1.3693	1-3/8"	1.3750	34.9250	1-29/64"	1.4531	1-15/32"	1.4688
1-1/2	6	NC	1.5000	1-11/32"	1.3438	34.1325	1-33/64"	1.5156	1-17/32"	1.5312
	12	NF		1-27/64"	1.4219	36.1163
	18	NEF		1-7/16"	1.4375	36.5125
1-9/16	18	NEF	1.5625	1-1/2"	1.5000	38.1000	1-37/64"	1.5781	1-19/32"	1.5938
1-5/8	5 1/2	NS	1.6250	1-29/64"	1.4531	36.9087	1-41/64"	1.6406	1-21/32"	1.6562
1-5/8	18	NEF	1.6250	1-9/16"	1.5625	39.6875	1-41/64"	1.6406	1-21/32"	1.6562
1-11/16	18	NEF	1.6875	1-5/8"	1.6250	41.2750	1-45/64"	1.7031	1-23/32"	1.7188
1-3/4	5	NC	1.7500	1-9/16"	1.5625	39.6875	1-49/64"	1.7659	1-25/32"	1.7812
1-7/8	5	NS	1.8750	1-11/16"	1.6875	42.8625
2	4 1/2	NC	2.0000	1-25/32"	1.7813	45.2450

Nominal Diam. (in.)	Threads/Inch	Series	Dec. Equiv.	Drill Size (65-75% thread)	(decimal)	(mm)	Drill Size	Dec. Equiv.	Drill Size	Dec. Equiv.
Nominal Diam. (in.)	Threads/Inch	Series	Dec. Equiv.	Drill Size (65-75% thread)	Equivalents		Close Fit		Free Fit
Screw Size				Tapping Drills			Clearance Drills

*The thread size of a C-mount lens.

Notes

Thread Series: National Coarse (NC); National Fine (NF); National Extra Fine (NEF); National Special (NS)

1 inch = 25.4 millimeters: JavaScript Calculator for converting between inches and millimeters.

Tap Drill Size (65-75% thread) = Nominal Diameter - One Pitch Length: example: Tap Drill for 1/4-20 NC screw is #7, closest drill to 0.2000 inches (0.2000 = 0.2500 - 0.0500); JavaScript Calculator for calculating tap drill size.

Pitch (in.) = 1 / (Threads per Inch): example: Pitch of 20 Threads per Inch is 0.0500 inches (0.0500 = 1/20); JavaScript Calculator for calculating pitch.

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This page is for INFORMATIONAL PURPOSES ONLY.

Page author: Dawn Rorvik (rorvikd@evergreen.edu)
Last modified: 01/28/2001