What Straight Threads Do I Need?

Monday, August 14, 2017

Thread Designation Elements

Threads have a few elements that should be considered when trying to identify the correct designation. Here are a few of the most common:

Major Diameter – Diameter across the crests of a male thread.

Minor Diameter – Inside Diameter across the internal crests of a female thread.

Pitch – The description of distance between thread crest

Inch Threads – Designation = Threads per Inch (TPI)
¾-16 = 16 threads per inch

Metric Threads – Designation = millimeters per thread
M22 X 1.5 = 1.5 mm between thread crests

Pitch Diameter - Theoretical diameter measured ¼ length between the crest and root. (Used in manufacturing and inspection only.)

Flank Angle – Angle between the threads.

Using these descriptions will help you better understand the following thread designations for Inch and Metric Screw threads.

Measurement Processes Explained

Let’s say we are measuring male screw thread...

Measure with calipers the diameter across the thread crests. 

The diameter will be equal to or slightly less than an inch or metric designation i.e. ¾ inch, 22mm, etc.

            Example; the diameter measures .747 inch (18.97 mm), this is most likely a ¾ UN inch series thread. 

Now measure the distance between threads.

For inch thread, set calipers to 1 inch, and count the threads.  If the threads are not one inch in length, measure between thread crests instead.

To calculate pitch using this method, divide 1 by the result. In other words, if the measurement between thread crest = .0625 in., then 1/.0625 = 16.  Note, this method only works with inch measurements.

Based on the above example, we determined we have a ¾ inch thread with a pitch of 16 threads per inch, or ¾-16.

For metric threads, the process is the same, except the measurement between thread crests is the pitch.  

Example: if the major diameter is equal to or slightly less the 22 mm, it is most likely a 22 mm diameter thread. Measure between two thread crests - if it measures 1.5 mm, it is a 1.5 pitch. In this example, the thread is an M22 X 1.5

Pitch Identification

Pitch identification can be simplified with a simple Pitch gage, which is readily available. These are simple, visual gages that contain individual cut-out plates for many of the common pitch threads.

Female Threads

Female threads can be a little trickier since the minor diameter does not correlate to a specific thread size; it will be smaller. Minor diameter tables are readily available through a simple search on the internet.

Flank Angle

All screw threads have a flank angle of 60 degrees, with the exception of British Standard Parallel Pipe (BSPP). The flank angle for BSPP is 55 degrees. The most efficient way to identify this is with an optical comparator.

Our team can help determine which class will be required based on whether the threads are plated or not (plated threads will be covered in another article).

Threads are complex, and these simple designation techniques are just the tip of the iceberg. Keep in mind, these procedures should only be used to identify a thread, not for thread inspection. However, following these simple guidelines will get you on the right track to identifying the threads you already have or need.

If you need some professional assistance, don’t hesitate to contact us at any time – we are here to help!