ansi 9standards0 code issues - un tpi for large holes

huangyhg

ansi un tpi for large holes
hello,
i'm trying to find out:
1. when to use un and when to use unc in a drawing.
2. how to determine the un threads per inch designation for holes greater than 6 inches as in the machinery's handbook 27th ed.  specifically, i have a hole that is 7.5" and want to know what tpi would make it un/unc/unf etc.
thanks.
sharafe