Bordo UNF Carbon Button Die

UNF = Unified National Fine

When used on external threads this series has greater tensile stress area than coarse threads of the same size. Fine series threads will resist stripping out better than coarse threads in areas where the external and mating internal threads are subject to loads of equal to or greater than the capacity of the screw or bolt. 

Fine threads are also used where the length of engagement is limited 1-1/4 or where wall thickness demands a fine pitch.