The thread standard most commonly used in North America is the Unified Thread Standard (UTS). UTS screws are specified by their nominal diameter and thread count. For example, a ¼-20 is a screw with a diameter of ¼ of an inch and a thread count of 20 threads/in. Screws with diameters less than ¼ inch are specified by an integer number, 0-16. Screws with diameters less than 0 are described by the numbers 00, 000, and 0000. As an example, the screws used in eyeglasses are typically 00-72.

UTS supports multiple thread counts for each screw diameter. The lower or coarser thread counts are specified as Unified Coarse Thread (UNC). The screws with a finer pitch are described as Unified Fine Thread (UNF) (Table 1).

Table 1: The dimensions of screws under the UTS screw standard. (Table source: Art Pini)

Referring to Table 1, note that for each screw diameter there are two possible thread counts, so a #10 screw is available with thread counts of 24 or 32 threads/in.

The absolute diameters of the screws using gauge number integers can be calculated by multiplying the gauge number by 0.013 and adding the product to 0.060 inches. Thus, a #6 screw has a diameter of (6 x 0.013) + 0.060 = 0.138 in.

There is also a metric screw standard. These are designated by the letter M, followed by the screw’s major diameter in mm, then the thread pitch also in mm (Table 2).

Table 2: Metric screw diameters and thread pitches for both the coarse and fine thread series. (Table source: Art Pini)

The fine pitch threads offer more choices of screw diameters than the coarse series, but only the overlapping diameters are shown in Table 2. As in the case of the UTS screws, metric screws are available in two thread pitches for each diameter. Remember that the screw pitch is the reciprocal of the thread count per mm.

Table 2 presents the diameters and thread pitches of metric screws for both coarse and fine thread series (source: Art Pini). The fine pitch threads provide a broader range of screw diameters compared to the coarse series, with overlapping diameters shown in Table 2. Similar to UTS screws, metric screws come in two thread pitches for each diameter, where the pitch is the reciprocal of the thread count per millimeter.

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