A helicoid is the surface generated by a curve which is simultane rotated about a fixed axis and translated in the direction of the axis w velocity proportional to the angular velocity of rotation.[1] An Archime Screw [2] is strictly a finite helicoid in which the generating curve is c (and normally circular) ; but screws, helical in shape, for which the gene curve is a simple finite arc can be—and are—used in practice as Archime Screws, and are considerably easier to manufacture than tubular ones.