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
- For metric threads, pitch is the distance between threads.
Example shown to the right
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Diameter vs. Pitch |
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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.
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Unified Sizing |
Fine Threads vs. Coarse Threads
Different design applications call for different threads!
Coarse Threads(UNC)
- Rugged applications
- Typical on larger diameter fasteners
- Used on softer materials because it has more bite and resistance to stripping
- Faster to remove and replace
- Less precise
- Not as easily cross-threaded
Fine Threads (UNF)
- Marginally stronger bolts (due to larger minor diameter
- Typically used for smaller diameter fasteners
- More precise
- More threads engaged per material thickness
- Easier to tap on tougher or harder material (ex. stainless steel)
- Fine threads will not wear with vibration as easily as coarse threads
- Fine Threads vs. Coarse Threads
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Fine 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.
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Fastener Data Sheet with Multiple Data Points |
Standards (Page link)
- ASTM's fastener standards specify, test, and evaluate the material, dimensional, mechanical, and metallurgical properties of various fasteners. These fastener standards allow manufacturers and users to examine fasteners to ensure their strength and quality for safe utilization1.
Materials and Fastener Grades (Page Link)
- A page discussing some common materials and grades of fasteners that you will come across during work terms.