Process to cut hour glass screws

Process to groove an hourglass screw consisting of the steps of setting a screw blank on a first shaft, having a second shaft transverse to the first shaft, setting a tool inside the second shaft, setting expansion means inside said second shaft to locate the position of said tool, rotating the two shafts in a synchronized way, starting the grooving with the tool retracted in said second shaft, locking the tool to the second shaft at least while the tool is engaged inside the blank, actuating the expansion means to feed the tool out when the tool is out of engagement with the blank and unlocking at least partly the tool, at least when actuating the expansion means, and expanding the tool by a succession of sequences of locking, unlocking and expansion until full tool penetration in the screw is achieved.