Tech Info Mechanical - Threads

The following notes apply to what are called 'machine' screws or fasteners for commericial grade applications. Wood and other screw types are defined using slightly different terminology. Aerospace/Military applications have another of detail that defines the internal and external thread characteristics and it gets very complicated, very quickly.

When you specify fasteners (screws) you have a lot of stuff to define:

1. The head style and type.
2. The length required specified in inches, fractions of inches or millimeters.
3. The diameter - when measuring in inches for diameters less than 1/4" there is a gauge number, above that specify the diameter in fractions of an inch. For metric the diameter is specified in millimeters.
4. The thread pitch - distance between threads referred to as Threads-per-Inch (TPI) or sometimes simply as the Pitch. UNC, UNF and Metric define standard values. To convert from inches to millimeters divide 25.4 by the Thread-per-Inch (TPI value, thus a fastener with a TPI of 32 will give 0.793mm (0.8) and one with a TPI of 56 will give 0.45mm
5. The material - brass, stainless steel, nylon etc.

UNC, UNF and Metric

Inch sizes come in either Unified Coarse (UNC) or Unified Fine (UNF) which define the number of threads per inch which are typically standardized based on the Gauge. So you would typically specify a small screw as being 4-40 which from the table below indicates a Unified Coarse (UNC) or 8-36 which is UNF. A larger screw would be 1/4-28 (UNF).

Metric screws are much simpler to specify being Maa x bb where aa is the diameter in millimeters (mm) and bb is the 'pitch' - the distance in millimeters (mm) between threads. So M3.5 x 0.6 has a 3.5mm diameter and a pitch of 0.6 mm. Standard metric sizes with closest UNC and UNF equivalents.

Gauge and Inch Sizes

Gauge numbers are used for diameters less than 1/4" inch above that the diameter in fractions.

The table below shows the normal standards but there are lots of exceptions and non-standard sizes offered by multiple manufacturers. TPI is threads per inch and is given for UNC (Unified Coarse) and UNF (Unified Fine). Nut sizes appear less codified but are typically given as size in fractions across the flats. Clearly you also need to specify the threads per inch for nuts.

Gauge No.	Decimal Inches	UNC TPI	Short Form	UNF TPI	Short Form	NUT Size	Millimeters (mm) Approx.
No.0000	0.021	160	-	-	-	-	0.53
No.000	0.043	120	-	-	-	3/32"	0.86
No.00	0.047	90	-	-	-	3/32	1.19
No.0	0.060	-	-	80	-	5/32"	1.524
No.1	0.073	64	UNC 1-64	72	UNF 1-72	5/32"	1.854
No.2	0.086	56	UNC 2-56	64	UNF 2-64	3/16"	2.184
No.3	0.099	48	UNC 3-48	56	UNF 3-56	3/16"	2.515
No.4	0.112	40	UNC 4-40	48	UNF 4-48	1/4"	2.845
No.5	0.125	40	UNC 5-40	44	UNF 5-44	1/4"	3.175
No.6	0.138	32	UNC 6-32	40	UNF 6-40	5/16"	3.505
No.8	0.164	32	UNC 8-32	36	UNF 8-36	11/32"	4.166
No.10	0.190	24	UNC 10-24	32	UNF 10-32	3/8"	4.826
No.12	0.216	24	UNC 12-24	28	UNF 12-28	7/16"	5.486
1/4"	0.250	20	-	28	-	7/16"	6.349

Metric Sizes

The metric size gives both the diameter and Pitch as Maa x bb where aa is the diameter in millimeters (mm) and bb is the Pitch - the distance in millimeters (mm) between threads. So M3.5 x 0.6 has a 3.5mm diameter and a pitch of 0.6 mm. To convert from inches to millimeters divide 25.4 by the Thread-per-Inch (TPI value, thus a fastener with a TPI of 32 will give 0.793mm (0.8) and one with a TPI of 56 will give 0.45mm.

For convenience we show the closest UNC and UNF sizes.

Metric	UNC	UNF
M2 x 0.4	1-64	1-72
M2.5 x 0.45	3-48	3-56
M3 x 0.5	4-40	4-48
M3.5 x 0.6	6-32	6-40
M4 x 0.7	8-32	8-36
M5 x 0.8	10-24	10-32
M6 x 1	1/4-20	1/4-28

Aeropsace/Military Designations

The following - incomplete - list extends the normal UNC/UNF designations for aerospace/military applications:

• UNJC = Unified National Coarse thread with external thread controlled root radius
• UNJF = Unified National Fine thread with external thread controlled root radius
• UNEF = Unified National Extra Fine thread
• UNJEF = Unified National Extra Fine thread with external thread controlled root radius

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