– Screw Thread Metrology Basics

Ensures dimensional compliance and functionality of threaded parts.

• Thread element gaging: quantify specific dimensions critical to thread fit and function.
  • Pitch diameter: theoretical cylinder where thread width equals groove width.
  • Effective diameter: diameter at which mating threads would engage properly, accounting for variations.
  • Major diameter: largest diameter of external or internal thread.
  • Minor diameter: smallest diameter at thread root.
  • Thread pitch: axial distance between two adjacent thread crests.
  • Drunken thread (variation in pitch): threading defect where pitch changes inconsistently along the length.
• Thread profiling: capture and evaluates the full thread shape.
  • Stylus profiler or CMM: scan the thread profile to acquire detailed shape data across the thread’s geometry.
  • Optical projector: project thread image for dimensional comparison against overlays.