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 level of detail that defines the internal and external thread characteristics and it gets horribly complicated, very quickly.
When you specify fasteners (a.k.a. screws to the rest of us) you have a lot of stuff to define:
Inch sizes come in either Unified Coarse (UNC) or Unified Fine (UNF) which define the number of threads per inch (TPI) which are typically standardized based on the Gauge (the fastener's diameter).
So you could specify a small fastener as being 4-40 which from the table below indicates a Unified Coarse (UNC) (its UNF equivalent is 4-48) or 8-36 which is UNF (its UNC equivalent is 8-32). A larger fastener would be 1/4-28 (UNF) or 1/4-20 (UNC).
The diameter is measured at the outside of the thread (some manufacturers refer to a pitch diameter which is measured to the inside of the thread).
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.
Note: British threads use the Whitworth (more correctly BSW - British Standard Whitworth) standard. Whitworth threads use a 55° with rounded ends, whereas UNC/UNF and metric threads use a 60° with flat ends.
Gauge numbers are used for diameters less than 1/4" inch. Above that they are defined by the diameter in fractions of an inch in increments of 1/16 up to 5/8 then in increments of 1/8.
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 count for nuts.
|UNC TPI||Short Form||UNF TPI||Short Form||NUT Size||Millimeters (mm)
|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|
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 0.6mm. 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.
|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|
|M5.5 x 0.5||12-24||12-28|
|M6 x 1||1/4-20||1/4-28|
The following - incomplete - list extends the normal UNC/UNF designations for aerospace/military applications:
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