UNF (Unified National Fine)This thread form was introduced in 1948 as an attempt to standardise thread forms in the USA, Canada and the UK. The thread profile of UNF is the same as for metric threads and is described in the JIS B0206 standard. The thread is described by its major diameter and the number of threads per inch of length (TPI). 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 subjected to loads equal to or greater than the capacity of the screw or bolt. Fine threads are also used where the length of engagement is limited or where wall thickness demands a fine pitch. Bottoming Taps have a lead of two threads and are used to ensure minimum thread run-out when tapping to the bottom of a blind hole. Bottoming Taps are typically preceded by the use of a Taper or Intermediate Tap.