Metric Threads
- Format is M major diameter (mm) X pitch (mm)
- Example: M12 X 1.75 is a thread having a nominal major diameter of 12 mm and a pitch of 1.75 mm.
- The letter M is the clue to the metric designation.
- Similar classification are given for other threads
Unified Threads
- Dimensions specified in inches
- Coarse and fine series exist and are termed UNC and UNF respectively.
- Format is major diameter – threads per inch series
- Example: 5/8” – 18 UNF is a thread having nominal major diameter of 5/8”, 18 threads per inch, and belongs to the fine thread series.
- Screw sizes with diameter under ¼” are numbered (i.e., small screw diameters are designated by gauge sizes, and not by fractions of an inch).
- Unified threads have thread angle of 60.
Fine Threads vs. Coarse Threads
Reading Data Sheets
- Static Load Rating (always stay below) – maximum stationary load before total permanent deformation of the fastener.
- Operating Load Rating (always stay below) - maximum permissible live load before total permanent deformation of the fastener.
