How to understanding the screw thread and Types of threads

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The screw thread is understanding by the dimensions and representing the dimension lines.

screw thread

Minor Diameter

Minor Diameter is the smallest diameter of an Internal (or) External  thread screw. The minor diameter is also known as root diameter (or) core.  The minor diameter is denoted by the dc. The thread which bounds  the root of an external thread.

Major Diameter

Major Diameter is the largest diameter of a screw thread . The major diameter is also known as nominal diameter (or) outside diameter. The threads which bounds the crest of an internal thread. The major diameter is denoted by the ‘Dc’.


Pitch it is the measured  parallel to the axis from a point of one thread to the another point of next thread. The thread is denoted by ‘P’.


Lead is the distance for screw thread advances axially in one complete rotation. Lead is equal to one pitch in case of screw leads single start threads.

Thread angle

Thread angle is the angle between the flanks of the threads measured in an axial plane Thread angle is varies for different type of screw threads. The thread angle is 60 for ISO metric thread.

Types of threads

  1. Acme Thread
  2. Buttress Thread
  3. British Standard White worth (BSW) Thread
  4. Knuckle Thread
  5. Square Thread
  6. Worm Thread