Screw Thread Terminology Explained

Posted by Hugh Watson | December 14, 2015 4 Comments

Dating back to oil and juice presses in 400 BC, screw threads are the sloped helices spiraling down the surface of a cylinder.

The thread can be external, such as on a bolt or screw, or internal, such as inside a nut.

The most common thread types used in the manufacturing industry today come in two varieties: spaced, designed to form threads within a pre-cut hole, and machine screw, designed to fit a pre-formed thread in a nut or hole.

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Screw thread-1.jpgWhen researching different types of screw threads, you will encounter some basic terms:

  • Major diameter – In an external thread like the edges of a screw, major diameter refers to the diameter of the overall shaft, including the height of the raised helix. It can be measured on the crests with a calliper rule or slot gauge.
  • Minor diameter – Minor diameter measures the diameter of the “root,” or innermost part of the screw, not including the crests of the helix. For accuracy, this measurement requires specialized equipment.
  • Effective diameter – The effective diameter is halfway between the major and minor diameters. In other words, it measures halfway up the helix crest. For accuracy, this measurement also requires specialized equipment.
  • Pitch – The pitch is the distance between two identical threads.
  • Flank – The flank is the angle at which the helix is raised to form a crest on the thread.
  • Crest – The crest is the height at which an external thread is raised, or the depth at which an internal thread is indented. For common applications, screws and bolts are measured at the crests, while nuts are measured at the roots.

Using the information above, you will be able to read and understand a screw thread callout when shopping for a replacement. Here are the five steps to interpreting thread callout:

  1. Look at the numbers in the callout. A few examples are outlined below:
    1. #4-40 x 0.5
    2. 1/4-20 x 5/8
    3. M3-0.50 x 10
  1. Understand the first number in the callout – This indicates the major diameter. Unified threads (in inches) express diameter as a fixed number #0 through #10, like example A listed above. Anything larger than a #10 is listed in fractional inches, like example B. Metric threads express diameter with M followed by the diameter in millimeters, like example C.
  1. Understand the second number in the callout – This indicates the distance between threads. It can be expressed as the number of threads per unit or as the distance between identical threads (the pitch). Unified threads measure threads per inch. In example A, the screw has 40 threads per inch. Metric threads measure millimeters per thread. In example C, the screw has threads every 0.50 millimeters.
  1. Read the length – This is the number that follows the x. Unified threads measure the length in inches, expressed as a decimal or a fraction interchangeably. In example A, the thread length of #4-40 x 0.5 is 0.5 or 1/2 an inch. Metric threads give the length in millimeters. With this in mind, example C, with a callout of M3-0.50 x 10, is 10 millimeters long.
  1. Be mindful of other nomenclature – You may see additional specifications in a callout. Tolerance classes include numbers 1-3; these refer to how loose or tight a screw fits. The letter A indicates an external thread and B indicates an internal thread. 2A and 2B are the most common classes. The abbreviations UNC (unified coarse) and UNF (unified fine) specify thread series.

Now that you understand screw thread terminology and nomenclature, it’s time for a pop quiz. What would you say is the major diameter of 1/2-20 x 0.75? How about the length of M2-0.25 x 8? The more you read and interpret screw thread callouts, the easier it will soon become.

Assembly Fasteners Inc. (AFI) is an award-winning global producer and distributor of industrial fasteners. With over 30 years of experience, we are dedicated to providing you with the perfect screw, nut, or bolt for your application.

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Check out our other blog posts.

1. Screw Points: A Guide to Their Versatile Functionality 

2. Different Kinds of Bolt and Screw Heads

3. Fastener Material Types