Machine Design

Standard Tapping Screws

The basic tapping screws are classified as thread forming and thread cutting. Thread-forming tapping screws displace the material adjacent to the pilot hole so it flows around the screw threads. Generally, they are used where large internal stresses are desirable to increase resistance to loosening. The mating parts create a fit with zero clearance, since no material is removed from the engaged section. Lockwashers or other locking devices are usually not needed.

Thread-cutting tapping screws have cutting edges and chip cavities that create a mating thread by removing material from the engaged section. These screws are applied in materials where disruptive internal stresses are undesirable.

Tapping screws are available in coarse and fine-thread series. Coarse threads should be used with weak materials. Fine threads are recommended if two or more full threads of engagement must be above the top of the cutting slot, but the thickness is insufficient to allow two full threads of the coarse-thread series.

High-performance, thread-rolling tapping screws are used with thicker-gauge metals, such as steel, brass, zinc, and aluminum forgings and castings. They are also used where high driving torques are encountered, or where chips resulting from thread-cutting screws are undesirable.

Self-drilling tapping screws have points that drill their own holes. No other drilling or punching is needed, but these screws must be driven by a power screwdriver. Once the self-drilling screw pierces the metal, it forms or cuts threads the same way as standard tapping screws.

TAGS: Fasteners
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