Too late, as usual, I've come in from the workshop where Im working my way through my latest project. I had to reduce the diameter of some 3mm screws to fit the design I'm trying to breathing life into. Although I have an ER11 collet chuck and set of collets, I have also produced a number of fixtures for the lathe when I needed to reduce the length of some crews for another (different) project, in various sizes. It also happens to double up for holding a screw t'other way round, so that the head is exposed and and thus can be reduced in diameter. This is the fixture in bits:
![screw fixture screw fixture](data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==)
Quite simply, it's a piece of BMS turned and threaded at both ends. One end is the thread size for the screw to be machined and the other end is threaded to accept the socket head bolt seen on the left. Insert the victim screw in the large end until the required cut-off length is protruding. Then screw in the shouldered bolt until it nips up on the screw. It acts as a sort of back-stop. Reduce length to the right-hand face of the fixture. Similarly, either insert the subject screw into the small end until it contacts the socket head screw, or slip a nut onto the screw to stop it turning in the fixture. Then reduce the head diameter.
![reducing screw head diameter reducing screw head diameter](data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==)
That's the latter method. For scale, the screw is a 3mm pan head.
It's taken much longer to explain how to do it than actually machine all four screws.
John