UNC (Unified National Coarse) Chrome Alloy Hand Taps - 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 UNC 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). This thread series is commonly used in the mass production of bolts, screws and nuts. - It is used for threading into lower tensile strength materials (bronze, brass, aluminium and plastics) to obtain the best resistance to stripping of the internal thread. It is also used on quick assembly or disassembly or if corrosion or slight damage is possible. - 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.