Z-Axis feed screw locking nut found on the HiTorque bench mills and SIEG SX3.5 bench mills. This mechanism fits into the Z-Axis feed screw base and is threaded onto the bottom of the feed screw. It is secured into place with a M6x8 set screw.
See Used On tab for reference to machines using this part and the corresponding drawing number.
In your instruction manual, this part appears on the parts list as: