Choosing a Screw by the Thread Size


#1

Thread Size contains two parameters - Major Diameter and Pitch. Major Diameter is the maximum diameter of the thread. Pitch is the distance between threads.
screwscrew1
Generally, there are two international standards used on thread size in the industry, namely UTS (Unified Thread Standard) and ISO Metric (International Organization for Standardization Metric Screw Thread) .

  1. UTS: The Major Diameter and Pitch are represented with a set of codes,
    for example, # 4-40, or ¼ "-20; and the two main parameters are measured in term of inch.

The first part of the code represent the Major Diameter:
In case major diameter greater than or equal to 0.25 inches, it will express in terms as a fraction;
Otherwise, if less than 0.25 inches, it will present with a symbol “#” plus 1 digit expression.

The second part of the code represents the pitch, which using the TPI (Threads Per Inch) value as the code.
For example, Keystone Electronics 9906 model
Thread size is #8-32, that 8 mean 0.1640inch major diameter; 32 means 0.03125inch per pitch.

  1. ISO Metric
    ISO Metric is also using a set of codes to represent the major diameter and pitch. The main difference of UTS and ISO metric is measured in millimeters (mm).
    The code is in term of Maa X bb, where aa is the major diameter; bb is the pitch.
    For example, Hammond Manufacturing 1591MS100 model, its screw size M3x0.5, that mean 3mm diameter X 0.5mm pitch

Product Tips: 1) If there is no custom-made screw, all the matching of major distance and pitch are standardized under the ISO Metric Standard.
In some case, the pitch is standardized in 0.5mm for M3 screw. Some manufacturers will simpliy the expression of the thread size in “M3”.

https://www.digikey.com/products/en/hardware-fasteners-accessories/screws-bolts/572