A screw, or bolt, is a type of fastener characterized by a helical ridge, known as a male thread (external thread) or just thread, wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as a female thread (internal thread), often in the form of a nut or an object that has the internal thread formed into it. Other screw threads are designed to cut a helical groove in a softer material as the screw is inserted. The most common uses of screws are to hold objects together and to position objects.
Phillips
Created by Henry F. Phillips, the Phillips screw drive was purposely designed to cam out when the screw stalled,to prevent the fastener damaging the work or the head, instead damaging the driver. This was caused by the relative difficulty in building torque limiting into the early drivers.
The American Screw Company of Providence, Rhode Island was responsible for devising a means of manufacturing the screw, and successfully patented and licensed their method; other screw makers of the 1930s dismissed the Phillips concept because it calls for a relatively complex recessed socket shape in the head of the screw — as distinct from the simple milled slot of a slotted type screw.
Phillips drive sizes (different from the screw size) are designated 0000, 000, 00, 0, 1, 2, 3, and 4 (by order of increasing size).
