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Looking at these two self-tapping screws: photo of two types of self tapping screws

The one on the left is 1/4-20 (coarse) thread, the one on the right appears to be 1/4 but I have no idea what TPI, super coarse?

Trying to understand the different applications and pros/cons of the different pitch?

Found this resource: https://www.thomasnet.com/articles/hardware/about-self-threading-self-tapping-screws/

Is the screw on the right a Type A while the one on the left is a Type AB?

vector image of self tapping screws

From what I understand thin materials and stronger fastening are both reasons fine pitched fasteners are used. For example, going into a metal garage door, the sheet metal is a bit thicker so I'm assuming either fastener will work, but the high TPI will have a higher strength because more threads are engaged? If the material is too soft (like aluminum) would the really tight threads not leave enough material in between the threads and actually weaken the connection?

fred_dot_u
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Wait But Why
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  • Finer threads work on thinner material, but have a shallower cut (difference between major and minor diameter), and over-torqueing can result in shearing threads off easier. Coarser threads distribute load deeper, but the angle also directs more of the pull force toward unscrewing (think wedges and friction). Garage doors can be pretty thin. – Abel Dec 13 '23 at 12:29
  • In terms of categorization, I can't even figure out what the fastener on the right is! Clearly it's not a course 1-4 b/c that's the fastener on the left, so could it be a wood screw?? – Wait But Why Dec 13 '23 at 16:32
  • @Abel I do not think that is correct about fine threads shearing off more easily since fine threads are used when more holding force is required. – DKNguyen Dec 13 '23 at 18:32
  • Yes fine threads have more holding torque. Fine threads shear the part's threads easier from the screwing process. Usually involves someone exceeding a torque spec. – Abel Dec 14 '23 at 00:49

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