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If the screws are to have hex heads then the feedstock would be of hex bar of the appropriate size, ie, the distance across the flats.
If the heads are to be round then the stock would be round bar of the required diameter.
Capstan position 1
Set bar to length
Capstan position 2
Reduce diameter of bar for the threaded part.
Capstan position 3
Chamfer the end of the part that will be threaded.
This not only makes for a better finish but it makes it easier to fit into the female thread and it also helps the diehead to get started on the thread.
Capstan position 4
Use a die or diehead to produce the thread.
Part off the screw with a shaped cut
If the screws are to have round heads with a slot in them then this cannot be done whilst the lathe is setup like this. It requires a completely different “second” operation.
This is best on the milling machine – see “Slotting screw heads”