Machine Design

Self-Piercing Nuts

These nuts are internally threaded, precision, work-hardened steel with external undercuts on two sides. Used in sheet-metal panels and installed by automatic machinery, they can be attached in open or blind locations. Self-piercing nuts have high resistance to torque, vibration, tension, and shear loads, and particularly to a combination of such forces.

Self-piercing nuts can either pierce and be clinched simultaneously, or be clinched into prepierced holes. Nut installation may proceed simultaneously with other forming, blanking, and piercing operations at high speed in both single and progressive die setups. The nut can be installed individually, in strip or coil stock, or as multiples in one panel.

Standard self-piercing nuts may be installed in metal panels up to 0.145 in. thick. The work-hardened steel nut has a pilot-head hardness greater than Rockwell B 85. Proper malleability is retained in threaded areas.

Self-piercing nuts for flush mountings are available for thin metals at high stresses. Application of the high-stress nut is generally limited to metal thicknesses of 0.03 to 0.09 in. A locknut feature can be produced by projections on the driver (nut plunger). Controlled distortion of the last threads in the nut provides a locking action that can be made to torque requirements.

Self-piercing nuts can be installed in unplated, plated, or painted panels. Installation in a painted panel has little or no effect on the painted surfaces.

For proper torque resistance, a self-piercing nut should have an asymmetrical shape. A pilot should protrude a minimum of 0.005 in. through sheet metal to ensure proper slug removal. When piercing two metal thicknesses, the pilot should protrude a minimum of 10% of the total maximum thickness of both metals.

Self-piercing nuts can be located on 1.5-in. centers with standard tools, or closer with special tooling. Rotation of the nut on the panel allows some deviation in the center distances.

When the pilot protrudes through the panel metal, the nut should be placed in an embossment great enough to place the pilot surface of the nut 0.010 to 0.020 in. below the mating-part surface.

TAGS: Fasteners
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