Screw Thread Metrology

   
 A Screw Thread Metrology deals with the measurements of the threads. To understand what are the measurements we can take from screw threads, we should know about the terms or elements of the threads. The Screw Thread Terminology gives the list of the terms and their definition those terms.


   Screw Thread Terminology:
The following are the terms of the screw threads:

1.  Screw thread
2.  External thread
3.  Internal thread
4.  Mulitple start thread
5.  Axis of thread
6.  Left hand or Right hand thread
7.  Form of thread
8.  Crest of thread]
9.  Root of thread
10.  Flanks of thread
11.  Angle of thread
12.  Flank angle
13. Pitch
14. Lead
15. Thread per inch
16. Lead angle
17. Helix angle
18. Depth of the thread
19. Axial thickness
20. Fundamental triangle
21. Truncation
22. Addendum
23. Dedendum
24. Major diameter
25. Minor diameter
26. Effective diameter

     Screw Thread: It is defined as a helical ridge which is formed by a continuous helical groove of uniform cross section on the external or internal surface of the cylinder or cone.
     The threads formed on cylinder are known as straight threads and the threads formed on a cone or a fulcrum of a cone are known as tapered threads.

     External Thread: Threads formed on the outside of the workpiece body are known as external threads.
Ex: Bolts and Studs etc.

     Internal Thread: The threads formed on inside of the workpiece body are known as Internal Threads.
Ex: Nuts

     Multiple start screw thread: It is a screw which is made by forming two or more than two helical grooves or teeth with equally spaced and similarly formed in an axial section on a cylinder.

     Axis of thread: It is an imaginary line which takes place longitudinally through the center of the screw.

     Right hand or left hand thread:  The Thread is placed in such a way that its logitudinal axis is normal to the observer and the thread is rotated in clockwise direction, if it is moving away from the observer, then it is a right hand thread; if it is moving towards the observer, then it is left hand thread.

     Form of thread: It is an edge shape of one complete thread as seen in axial section.

     Crest of  thread: It is the top most point of the groove forming threads.

     Root of thread: It is the bottom point of the groove forming threads.

     Flanks of threads: These are the straight edge surfaces which joins the crest to root.

     Angle of thread: It is an angle between two opposite flanks or slopes of a thread measured in an axial plane.

     Flank angle: It is an angle between individual flanks and a plane perpendicular to the axis of the thread. This angle is similar to the half of the thread angle.

     Pitch:  It is the distance between the two successive crest points or root points measured parallel to the axis of the thread.

     Lead: It is the distance moved by the screw for one complete revolution with respect to its mating part. Some times the lead is equal to the pitch but not always.

     Thread per inch: It is the number of threads taking place per one inch. It is reciprocal of the pitch.

     Lead angle: It is an angle made by helical curve of the thread at pitch line with a plane perpendicular to the axis of the thread.

     Helix angle: It is an angle made by the helical curve of the thread with the axis of the thread.

     Depth of thread: It is the distance between crest point and root point which is measured along a plane perpendicular to the axis of thread.

     Axial thickness: It is the distance between two opposite faces of the same thread which is measured along the pitch line.
   Pitch line is a line which divides a thread profile into two equal parts.

     Fundamental triangle: This is made by extending the flanks and joing the points B and C. Thus the triangle ABC is referred to as fundamental triangle. The point A is called as Apex.

     Truncation: The truncation at the crest s the radial distance from the crest to the nearest apex of the fundamental triangle. Truncation is given on both sides of a thread profile that is at crest and root.

     Addendum: It is the radial distance between crest point and pitch  line.

     Dedendum: It is the radial distance between root point and pitch line.

     Major diameter: It is defined as the diameter of an imaginary cylinder which passes through the crest points of the thread.

    Minor diameter: It is defined as the diameter of an imaginary cylinder which passes through the root points of the thread.

Mean or Effective or Pitch diameter: It is defined as the diameter of an imaginary cylinder which passes through the pitch line of the screw threads.