Machine Design

Special Tapping Screws

Typical specials include self-captive tapping screws and double-thread combinations. Self-captive screws combine coarse-pitch starting threads with a finer pitch farther along the screw shank. Initially, the coarse threads cut the hole, but then the fine threads retap the hole and change the thread pitch. Attempts to back the screw out cause the coarse thread to interfere with the fine threads; consequently, the screw is held captive.

Double-thread screws combine a left-hand thread near the screw head and a right-hand thread over the rest of the shank. These screws can be driven only to the point where the left-hand threads start.

Another large group of special screws comes with preassembled washers. Some have a serrated face on the underside of the head which cuts into the assembly when tightened. This minimizes stripping caused by overdriving and increases holding power.

Sealing tapping screws, with preassembled washers or O-rings, can control leaks and crazing of enamel.

TAGS: Fasteners
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