COARSE THREADS VERSUS FINE THREADS
Five reasons for selecting coarse threads:
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Parts with coarse threads assemble faster than fine threads. For example, a 3/8-24 screw takes 24 revolutions to advance it one inch, while it takes a 3/8-16 only 16 revolutions to advance the screw one inch.
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Coarse threads have a greater resistance to stripping out than fine threads. Coarse threads have a greater amount of material than a fine thread to resist stripping.
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Coarse threads are less affected by nicking. Fine threads are shallower than coarse threads and more prone to nicking. Thread gaging and thread interference are frequent during assembly due to nick threads.
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Coarse threads are less affected by plating build-up. A given plating thickness consumes a greater percentage of the plating allowance on a fine thread than on coarse threads. Fine threads have more gaging and assembly problems than do coarse threads due to plating build-up.
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In stainless threaded assemblies, coarse threads are much less likely to gall than fine threads. Surface friction between mating stainless steel threads causes thread galling. The closer the fit between mating threads and the more rotations the threads must experience, the greater the chances become for thread galling. Since fine threaded components have less pitch diameter clearance, they have to rotate more times than coarse threaded components. Therefore, the more likely to experience galling.
ONE REASON FOR CONSIDERING FINE THREADS:
The one point that usually motivates users to select fine-thread parts over coarse-thread parts is that with a fine-thread having a slightly larger tensile stress area, therefore, should have a higher tensile strength than do corresponding coarse-thread parts. This is a valid point only when both parts have the same hardness.