The screw is the part that is in control of opening and closing the jaws of the vice.
The body of the screw is threaded and its end is outfitted with the vice handle which controls the screw’s movement.
A machine vice commonly has a small crank handle which allows the user to apply maximum clamping force when working, but without having to turn the handle excessively. This is because the crank handle provides extra leverage when rotating, meaning the screw can exert greater pressure onto the jaws but without any extra effort from the user.
Other models may not have a handle at all, but a simple rubber coating on the screw end which is turned to open and close the vice jaws.
Machine vices are designed with these handles so as to avoid obstruction by the drill press or milling bed once it is bolted down.
Machine vices are often manufactured with a flat bottom base design which fits firmly against the machine’s table. This allows the vice to fit on the table in horizontal alignment with the drill bit.
The base is also designed with holes through the bottom, in order to allow the drill to pass through during applications.