Contrails ! US machine screw dimensions

UNC and UNF machine screw dimensions

Screw			Thread/inch				Clearance hole
Size	dia mm	dia inches	Coarse TPI	Pitch mm	Fine TPI	Picth mm	Bit	mm	inches
0	1.500	0.0600	---	---	80	0.317	52	1.613	0.0635
1	1.854	0.0730	64	0.397	72	0.353	47	1.994	0.0785
2	2.184	0.0860	56	0.453	64	0.397	42	2.375	0.0935
3	2.515	0.0990	48	0.529	56	0.453	37	2.642	0.1040
4	2.845	0.1120	40	0.635	48	0.529	31	3.048	0.1200
5	3.175	0.1250	40	0.635	44	0.577	29	3.454	0.1360
6	3.505	0.1380	32	0.794	40	0.635	27	3.658	0.1440
8	4.166	0.1640	32	0.794	36	0.705	18	4.305	0.1695
10	4.826	0.1900	24	1.058	32	0.794	9	4.978	0.1960
12	5.486	0.2160	24	1.058	28	0.907	2	5.613	0.2210
1/4	6.350	0.2500	20	1.270	28	0.907	F	6.528	0.2570
5/16	7.938	0.3125	18	1.411	24	1.058	P	8.204	0.3230
3/8	9.525	0.3750	16	1.587	24	1.058	W	9.804	0.3860

This table offers UNC and UNF machine screw diameters in millimeters and inches up to 3/8 in.

Clearance holes as per ASME B.18.2.8.

Dimensions are rounded off to the nearest thousandth of a mm and ten thousandth of an inch.

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US machine screw standard

The Unified Thread Standard

Puplic Domain

The American machine screw UTS standard was adopted in 1949. It includes a fine thread UNF and a coarse thread UNC, as well as some special threads.

The informations in this article are in accordance with standard ANSI B1.1.

Designation

US machine screws are described 0-80, 2-56, 3-48, 4-40, 5-40, 6-32, 8-32, 10-32, 10-24, etc. up to 12, and by fractional inch beyond.
The first number expresses a diameter, the second is the number of threads per inch (TPI).

For each size there is a coarse thread and a fine thread.
The fine thread is preferred in thin materials and is slightly stronger. The coarse thread is frequently used in Lycoming engines.

Converting screw number to dimension

The machine screw number can be converted to its equivalent inch or mm diameter as follows :

D_inch = (N × 0,013) + 0,060), where N is the machine screw number.
D_mm = ((N × 0,013) + 0,060)× 25,400)

For instance, a #10 screw has a diameter of [(10 × 0.013) + 0.060] inch, i.e. 0.190 inch or 4,826 mm.

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Last modified
23-03-2021 @ 11:39:15