Hi Chris,
It's called a tapping head. It has 3 or 4 sets of "jaws" that have the thread size machined in them. When it reaches the correct depth of thread required, it releases. They cost anything from £200 through to £2000 plus the jaws.
Chris
chris.h
Member Since 07 Nov 2005Offline Last Active Nov 07 2010 08:49 PM