Tek Motion BT40 CNC Tool Holder Tightening Fixture This BT40 Universal CNC fixture tightening tool has multiple uses and includes NMTB (ISO), BT and DIN tapers. It is designed to provide maximum protection for the tool holder that has extraordinary gripping power for cutting tools in horizontal and vertical positions. The tightening fixture is lightweight and compact making it easy to mount on a work table or tool cart. When using ER collets, you must snap the collet into the chuck nut first and then screw the nut/collet on to the holder together. Dropping the collet into the holder and screwing the nut on is incorrect and will damage the tools.