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Tech Stuff - 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 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:

  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 (a.k.a. gage 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. Defined as Threads-per-Inch (TPI) or frequently simply as the Pitch (though technically this the distance between adjacent threads). UNC, UNF and Metric define standard values. To convert from inches (TPI) to millimeters (pitch) 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, non-ferous such as nylon and other polyamides.

UNC, UNF and Metric

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 and Inch Sizes

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.

Gauge No. Decimal
Inches
(Dia.)
UNC TPI Short Form UNF TPI Short Form NUT Size Millimeters (mm)
Approx.
No.0000 0.021 160 - - - - 0.53
No.000 0.034 120 - - - 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 0.6mm. 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.

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
M5.5 x 0.5 12-24 12-28
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:



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