Robert Sorby #89HS20 Thread Cutting Set - 20TPI This 3 piece thread cutting sets consist of a pair of thread cutters (one internal cuts and one external), a thread and sizing gauge and the Focus on Specialist Woodturning Tools Part 3 DVD. These thread cutters are perfectly designed for making screw tight lids on boxes and caskets Thread cutters include a range of cutting tools that have been produced from High Speed Steel in 10, 16 and 20 threads per inch (TPI) They have been designed with a unique, enhanced helix angle specially designed for use in wood.