text: structure and function of screw? any structure and function of screw  pic. or words also needed. thx~ screw  (skr)    1. a cylindrical rod incised with one or more helical or advancing spiral threads, as a lead screw or worm screw.          2. the tapped collar or socket that receives this rod.    2. a metal pin with incised threads and a broad slotted head that can be driven as a fastener by turning with a screwdriver, especially:          1. a tapered and pointed wood screw.          2. a cylindrical and flat-tipped machine screw.  http://dictionary.reference.com/search?q=screw  a screw is a shaft with a helical groove or thread formed on its surface. its main uses are as a threaded fastener used to hold objects together, and as a simple machine used to translate torque into linear force. it can also be defined as an inclined plane wrapped around a shaft.  a screw used as a threaded fastener consists of a shaft, which may be cylindrical or conical, and a head. the shaft has a helical ridge or thread formed on it. the thread is essentially an inclined plane wrapped around a shaft. the thread mates with a complementary helix in the material. the material may be manufactured with the mating helix (tapped), or the screw may create it when first driven in (a self-tapping screw). the head is specially shaped to allow a screwdriver or wrench to grip the screw when driving it in. it also stops the screw from passing right through the material being fastened and provides compression.