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Tools & Rules Online -- Measuring Threads
 |  | | Volume 2, No. 2 | Summer, 2008 |
| | | | Measuring screw thread is a common shop task. The level of information required ranges from a simple thread count to detailed information about every aspect of the thread's geometry. Starrett offers a variety of tools, from general to highly specialized, to help you reliably achieve the level of information that you require.
 The Starrett products for the most complex thread measurements are our family of Thread Gages. These complex products and the technical information relevant to their function is covered in our Thread Gage Catalog (No. 355), available through our website literature ordering page or by request to our marketing department. The same information is also in our full line catalog, beginning on page 273 in our current Cat. 31B.
This article is an overview of less comprehensive thread measurement and the Starrett tools used to make them.
Determining Threads per Inch (T.P.I.)
The most simple and common measurement is determining the number of threads per inch. This can be done with an ordinary steel rule. To do this, line up the one-inch line graduation of a steel rule with the crest (point) of a thread (see right) and count the number of crests (points) over that one-inch thread length. That number is the “threads per inch.”
 If the overall thread length is shorter than an inch, count the crests over only a half-inch, then multiply the number by two. For example, if you counted six threads over a half inch, then 6 x 2 = 12 -- there are twelve threads per inch. If you counted five threads over a quarter inch, then there are twenty threads per inch, and so on.
Screw Pitch
Screw pitch and T.P.I. can be determined readily with a screw pitch gage such as our No. 473 (right). The gages consist of a set of thin steel leaves. The individual leaf edges have “teeth” corresponding to a fixed thread pitch.
The leaves on Starrett screw pitch gages are stamped to show the thread pitch number. For example; 28, 24, 20, etc. To determine the thread pitch and T.P.I., simply match the leaf that fits the threads on your part.
Starrett offers a selection of six gages sets to measure V, Unified, American National 60° threads, including No. 473 (above, right). We also offer two sets for International Metric Standard 60 degree threads, and one for Whitworth Standard 55 degree threads.
 Measuring Groove Depth
Starrett 210 and 760 Series Screw Thread Comparator Micrometers provide quick comparisons of thread accuracy in screw cutting operations, measuring in small grooves or recesses where regular micrometers cannot be used.
The electronic 760 micrometers (above, right) measure grooves from 0-1" with the metric version's range from 0-25mm The mechanical 210 Series measure grooves from 0-7/8" with the metric version's range from 0-22mm.
Note that 210 and 760 micrometers do not measure pitch diameter. For such measurements, Starrett No. 575, 585, or thread gaging systems are needed.
Measuring Crest & Root Diameters
 Screw thread micrometers such as Starrett 575 & 585 Series (above, right) with a pointed spindle and V-shaped anvil are used to measure pitch diameters. The point of the spindle and the V-shaped anvil are designed so that contact is made on the side (flank) of the thread.
Major (crest) and minor (root) diameters of threaded parts can vary depending upon the sharpness or fullness (truncations) of the thread. Measurements are usually made at the pitch line to determine the pitch diameter.
Therefore, pitch diameter is the diameter of a cylinder passing through the thread profile, so as to make the widths of thread ridges and widths of thread grooves equal, along the length of the thread. Pitch diameter is the primary control and reference datum for measuring and manufacturing screw threads.
Reference: Gaging Standards
There are several gaging standards, but the primary ones are:
Unified Inch Screw Threads: Denoted by diameter and number of threads per inch. Example: .250-20 UNC (normally referred to as 1/4-20)
ISO Metric Screw Threads: Denoted by the letter “M” for metric, then the diameter and then the pitch in millimeters. Pitch is the distance from a point on a thread to the corresponding point on the adjacent thread. Example: M6 x 1 has a 6mm diameter and threads, which are 1mm from point to corresponding point. Common practice for quick determination is to measure from crest to adjoining crest. |
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