Page 9 - Threading

Thread Parameters

Thread can be defined as a continuous helical ridge on the nut (inner part) or screw (outer part) of a
cylinder. The ridge is called a crest, and the dip or space between each crest is referred to as the root.
The side of the thread surface that connects the crest and the root is called the flank. INTRODUCTION

Screw (Outer Part) Nut (Inner Part)
crest root crest root

flank flank

Thread geometry with value, angles, radiuses, etc. contain tolerances defined in thread standards.
The main parameters of thread geometry and thread functionality are: diameters, pitch, lead,
helix angle, lead angle, and truncation.

Diameters
There are three main thread diameters: major diameter, pitch dimeter and minor diameter. Value and
tolerances for each diameter differ according to each thread standard.

Screw (Outer Part) Nut (Inner Part)

minor diameter minor diameter
pitch diameter pitch diameter

major diameter major diameter
• Major Diameter – the largest diameter of two extreme diameters.
In external threads, the major diameter defines the end limit of a thread profile and in internal
threads, the major diameter defines the start of a thread profile. Therefore, for external
threads the major diameter is an outer thread diameter and for internal threads, the major
diameter is an inner thread diameter. Usually the major diameter of external threads is smaller
than the major diameter of internal threads, if the threads are designed to fit together.

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