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Thread Root Definition

The root is a flat (or rounded) surface connecting between the flanks at the bottom of the thread grooves, whether internal or external.
The root width depends on the amount of tip truncation, which is defined by the specific thread standard.

Int root and crest

Internal Thread

  • Root Width of 60° standard threads: Frn=0.125 X P
  • The major diameter is measured on the root.
Extr root and crest

External Thread

  • Root Width of 60° standard threads: Frs=0.25 X P
  • The minor diameter is measured on the root.
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