American standard nuts and bolts (UNC - Unified Coarse Threads) have less threads per inch than machine nuts and bolts (UNF Unified Fine threads). For example, given a 3/8" bolt, the UNC version has 16 threads per inch (TPI). Where a machine style thread for 3/8 bolt has 24 threads per inch - (TPI).
The picture below shows how bolts can look the same and yet are not cross compatible because of subtle differences in pitch and TPI threads per inch. Note in the picture below, you will see two threaded shafts.
|LEFT SHAFT||RIGHT SHAFT|
|Size||M8 (Metric) ISO Metric Coarse||5/16 UNC Unified Coarse thread or American Standard|
|Pitch||1.25mm = 0.049213 inches||0.056 "|
|TPI Threads per inch||20.32||18|
The one on the left is a M8 metric size which has a TPI of 20 threads per inch. The shaft on the right is a 5/16 inch size which has a TPI of 18 threads per inch. Note that within the the span of 10 threads on the left there are 9 threads on the right.
The ratio is 10 to 9. That is for a half inch distance. If you double both those numbers, you get 20 and 18 which jives with the TPI values in the table below.
As you can see, the differences are subtle and can cause much frustration for the avid garage shop tinkerer who thinks he /she has the perfect stash of nuts to bolts but can't get them to work together!
American standard or "unified coarse- UNC" thread is defined by the thread having a 60° angle, and the Flat being 1/8th the size the pitch or
Flat = Pitch / 8
The third defining characteristic of the American standard coarse thread is that the depth is 0.64951 of the Pitch or
Depth = Pitch x 0.64951
A 3/8' coarse threaded nut or bolt (also known as American Standard thread or UNC), has 16 threads per inch or "TPI" . So it has a pitch of
Pitch = 1" / 16 = 0.0625"
UNC and UNF American Standard / Machine screw threads per inch table