The basic tapping screws are classified as thread forming and thread cutting.

Sellers called his talk, "On a Uniform Standard of Screw Threads". He complained that American nuts, bolts and screws were all individually made by hand, and that they still had to be hand-matched ...

From here, multiply the pitch by 0.9 to get the pitch of the 1BA screw, and do that again to get the 2BA screw pitch. A website called sizes.com is kindly hosting the original discussion papers that ...