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Thread A helical projection of uniform section on the internal or external surface of cylinder or cone. Also, the operation of cutting a screw thread.
- Left Hand Thread
- A screw thread that is screwed in by rotating counterclockwise.
- Right Hand Thread
- A screw thread that is screwed in by rotating clockwise. Most threads are right handed.
- Symmetrical Thread
- Both flanks of the thread profile are inclined at the same angle.
- Thread Angle
 The angle formed by the two sides of the thread (or their projections) with each other.
- Thread Axis
 A line running lengthwise through the centre of the screw.
- Thread Crest
- The top surface joining the two sides of a thread.
- Thread Depth
 The distance between the crest and the root of a thread.
- Thread Flank
- The thread flanks join the thread roots to the crest.
- Thread Height
- This is the radial distance between the minor and major diameters of the thread.
- Thread Length
- The length of the portion of the fastener with threads.
- Thread Micrometer
- A micrometer in which the spindle is ground to a point having a conical angle of 60 degrees. The anvil, instead of being flat. has a 60 degree V-Shaped groove which fits the thread.
- Thread Pitch
 The distance from a point on one screw thread to a corresponding point on the next thread.
- Thread Pitch Diameter
- The diameter of a screw thread measured from the thread pitch line on one side to the thread pitch line on the opposite side.
- Thread Root
- The bottom surface joining the sides of two adjacent threads. On external threads the roots are usually rounded so that fatigue performance is improved.
- Thread Runout
- The portion at the end of a threaded shank which is not cut or rolled to full depth, but which provides a transition between full depth threads and the fastener shank or head.
Historical Notes - 500 BC The philosopher Archytas first invented the screw. They were made out of wood and used for wine and olive oil presses.
- 1400′s Metal screws first appeared. Since they were handmade, they were very rare and considered a specialty tool.
- 1744 The screwdriver was developed.
- 1789 Patent application for a machine to mass-produce screws.
See also: Acme Thread, Bolt, British Association Threads, British Standard Whitworth, Chasing Threads, External Thread, Metric Threads, Multiple Thread Screw, Nut, Rolled Thread, Screw, Screw Thread, UNC, UNF.
  
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