Threads, Taps, and Tapping - 1: Thread Designations
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.
Before discussing tapping an understanding of thread systems must be gained. The graphic on the right explains how thread systems are defined showing the diameter, number of threads per inch, series, class, direction, length, and tap depth.

There are five basic thread series in American National and Unified Screw Thread system. They are:

  1. Coarse (UNC)
  2. Fine (UNF)
  3. Extra Fine (UNEF)
  4. Special (UNS) National
  5. Pipe Thread (NPT)

ISO Metric threads have a somewhat different coding system. For example a metric thread would be noted as M 10 x 1.5-6H.

  • M indicates a Metric thread.
  • 10 indicates a basic size of 10 millimeters.
  • 1.5 is the pitch in millimeters. Note that the M 10 taps are also available as M 10 x 1.00 - 6H and 10 x 1.25 -6H.
  • 6 identifies the grade of tolerance. Six (6) is commonly used and comparable with class 2 Unified American Standard. The grade of tolerance ranging from 3 to 5 fine, 6 medium tolerances 7-9 are coarse.
  • H means that it is an internal thread with no allowance. Internal threads are defined with capitals, for example G designates a tight allowance.
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