Screws

Terminology:

Major Diameter: diameter of screw (at the outermost part of the threads)
- Another way of thinking about it: the smallest diameter of a hole that the screw could fall through without torqueing.

Thread Systems

The Unified Thread Standard is a standard for screws common in Canada and the United States. It is controlled by AMSE/ANSI.
UTS screws are differentiated by their major diameter (D) and their thread pitch (P), they are based on the inch rather than the millimeter like ISO metric screws.
- Pitch is the distance between threads.
UTS screw names look like this: #10-32, #8-36, 1/4”-28.
- The first number represents the major diameter of the screw (which for values less than 1/4” is a number).
- The second value represents the distance between each thread (pitch). This value is measured in threads per inch (TPI), which is the inverse of the pitch in inches.

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ISO metric screws are the most commonly used screws globally.
Metric screws are differentiated by their major diameter and pitch in millimeters.
ISO metric screw names look like this: M3, M10x1.25, M15x2, M8×1.25×30, M8x30.
- The number after “M” is the major diameter in millimeters.
- An M6 screw will therefore fit in a 6mm hole! Easy peasy!
- The value after the “x” is the pitch of the screw threads in millimeters.
  - The pitch of a metric screw can be omitted if the pitch is “coarse”.
- The final value of this is the length of the screw in millimeters. Use context to infer if the final value is the length or the pitch.
ISO 262 lists a selection of preferred sizes/pitches/lengths of metric screws. These would be good screws to use in projects.

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Styles:

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Lengths:

Tip: To ensure a nut stays fastened, put on 2 nuts in a row. This is safe.