1. Basic Units
- Metric System (ISO):
- Uses millimeters (mm) for diameter, pitch, and length.
- Example: M4 × 12 → 4 mm diameter, 12 mm length.
- Imperial System (UTS – Unified Thread Standard):
- Uses inches (″) for diameter and length.
- Thread pitch is measured in threads per inch (TPI) instead of mm.
- Example: #8-32 × 1/2″ → size #8 screw, 32 threads per inch, 1/2 inch long.
2. Thread Pitch
- Metric: Directly specified as the distance between threads in mm.
- Example: M6 × 1 → 1 mm between each thread.
- Imperial: Specified by the number of threads per inch (TPI).
- Example: 1/4″-20 → 20 threads per inch.
3. Diameter Measurement
- Metric: Uses the actual diameter in mm (M3 = 3 mm).
- Imperial: Uses gauge numbers or fractions of an inch.
- Example: #8 screw ≈ 0.164″ (4.16 mm) major diameter.
4. Length
- Metric: Always in millimeters (M4 × 20 = 20 mm long).
- Imperial: In inches or fractions (1/4″, 3/8″, 1/2″, etc.).
5. Standards
- Metric: ISO, DIN, JIS.
- Imperial: UNC (Unified Coarse), UNF (Unified Fine), UTS (Unified Thread Standard).
Conversion Formula
- Length/Diameter:
- 1 inch = 25.4 mm
- 1 mm ≈ 0.03937 inch
- Threads per Inch (TPI) ↔ Pitch (mm):
Example:
- 1/4″-20 UNC → 20 TPI → Pitch = 25.4 / 20 ≈ 1.27 mm.
- M6 standard pitch = 1.0 mm → TPI ≈ 25.4 / 1.0 = 25.4 TPI (finer than 1/4″-20).