A SCREW-THREADED FASTENER

摘要

A screw 10 for soft material 20 has an asymmetrical thread form 16, 18 which includes an upper flank 28 comprised of a plurality of continuous helical surfaces 34, 36 or 34, 35, 36. The upper flank surfaces are shaped so that a helical protrusion 36 is formed on the upper flank 28 which increases friction between the screw 10 and the material 20 only after the screw's head 12 has seated on a workpiece 24 and the soft material 20 is deformed.