Thread Cutting Jig
From DT Online
A carbide tipped suitably shaped cutter (i.e. normally ‘V’ shaped) can be fitted into a Router or Laminate Trimmer and Jig used to cut an external screwthread. In operation, it is similar to the action of a Thread Box but with a rotating cutter instead of the single blade.
Jigs are available which comprise a piece of suitable material (e.g. plastics or aluminium) into which the thread required has been cut (e.g. using a large Tap or by cutting with a screw-cutting Centre Lathe). The threaded hole is Counterbored to, say, half the width of the Jig and provision made for the Router cutter to be lowered radially into the centre and adjusted such that exactly the right amount of cutter showing on the inside of the bore can be determined by trial and error.
Dowel stock is simply fed into the Counterbored end of the Jig and slowly turned clockwise (if a right-handed screw thread). The cutter starts the thread which is then picked up by the pre-cut thread beyond it in the bore, and once engaged, the pre-cut thread continues to pull the dowel stock through the Jig as turning continues at a steady rate (e.g. approximately one revolution per second normally).
Taps are used to cut the internal thread.