Technology Research Manager
PEM Fastening Systems
Edited by Victoria Reitz
Design engineers have lots of options when it comes to joining thin sheet-metal panels. But some are better than others. Tack welding can be awkward and inconsistent, mounting brackets involve a lot of hardware, and adhesives can fail with cyclic temperatures. Self-clinching fasteners, on the other hand, join panels with little preparation, assembly, or equipment and provide strong threads in metal sheets too thin to be tapped.
In broad terms, self-clinching fasteners are usually threaded and when pressed into ductile metal, displace the host material around the mounting hole. Pressing causes the panel material to cold flow into a specially designed annular recess in the shank or pilot of the fastener. A serrated clinching ring, knurl, rib, or hex head prevents the fastener from rotating in the host material. Thus, self-clinching fasteners become a permanent part of the panel, chassis, or bracket into which they are installed.
Generally, self-clinching fasteners take less space and require fewer assembly operations than caged or anchor nuts. They are also reusable and have more holding power than sheet-metal screws. They are used chiefly where good pullout and torque loads are required in sheet metal that is too thin to securely fasten by any other method. Even when sheets are thick enough to tap, selfclinching fasteners with gaugeable threads may be more economical. In fact, because self-clinching fasteners eliminate the need for tapping, they often permit the use of thinner sheet metal, giving a real reduction in installed cost over other fasteners. A compact design and low profile provide for a neat appearance, too.
As a rule, self-clinching fasteners should be specified whenever a component must be readily replaced and where loose nuts and hardware wouldn't be accessible. If nuts and screws can't be reached after a chassis or cabinet is assembled, self-clinching fasteners can be installed during metal fabrication.
The electronics industry commonly captivates screws that will be removed during service work. Captive screws cannot fall into a chassis or cause damage when removed. For these applications self-clinching panel fasteners retain screws using less hardware than other methods.
Like other self-clinching fasteners, panel fasteners press into drilled or punched holes. Most any type of parallel-acting press will squeeze the fastener in place, no special equipment is needed. However, panels must be a ductile material softer than the fastener, and must meet minimum sheet thickness required by the particular fastener. Some can be installed into sheets as thin as 0.51mm.
Panel fasteners with plastic knobs eliminate the traditional panel-fastener spring. The spring is built into the knob and shape of the retainer. Spring-action plastic "fingers" hold the knob in the retracted position. The plastic also lets designers color code screws. For example, green knobs in products such as copiers and printers signify that users can handle them. The knob material is ABS, with a temperature limit of 200°F. The screwhead is still exposed for assembly.
For applications plagued by mating hole misalignment between panels, experts recommend floating panel fasteners. Though they are not self-clinching and not permanent, they compensate for up to 0.060-in. offset in the mating fastener. To install, the fastener is placed in a recessed anvil, the panel and a washer are placed over the fastener shank and a flaring punch swages the fastener into place.
Another type of nonclinching fastener, snap-in fasteners, can be installed with finger pressure into any thin material. These are a less costly, nonpermanent option typically used to mount computer chassis and telecommunications devices. They have thumbscrews that won't fall out when unscrewed.
Clinch fasteners traditionally clinch and fasten along the same axis of an assembly. Right-angle fasteners, however, clinch into one sheet and allow fastening of a second sheet at a right angle. They can replace bent-edge tabs, flanges, angle brackets, and tack welds. The compact fasteners also save material and space. Some designers have reduced assemblies from four or five components to two by switching to right-angle fasteners, also improving design for manufacturing/assembly.
Right-angle fasteners clinch in a rectangular hole in one sheet, then screw to a perpendicular sheet. Repeated removal and reinstallation of the screw does not affect the fastener's holding power. The clinched panel remains flush from the outside, so it does not mar the surface and can be painted.
The fasteners are available with either threaded steel or aluminum for threadforming screws. Threadforming screws do not generate metal residue, so they are widely used in electrical and electronic assemblies. Threadforming screws also resist vibrationinduced loosening.
An interesting outgrowth of self-clinching fasteners is the cable tie-mount — typically found inside panels to organize wires and cables. Cable tie-mounts work like other selfclinching fasteners. A rectangular hole is cut and the mount is clinched into the panel. A cable tie can then slip through the "eye" to secure bundled cables. It does not fasten panels together.
This is an alternative to plastic tie holders with nuts, washers, and screws, or to adhesives that can fail with temperature fluctuations. The panel exterior also remains flush and closed with tie mounts, minimizing EMI/RFI.
Following is a review of some of the commonly used terms relating to the self-clinching fastening process.
Anvil. An insert, either solid or hollow, used on the underside of a panel to resist installation force.
Blind hole. A hole, usually threaded, which is open from only one end.
Cold flow. The movement of a ductile material under pressure.
Floating. The ability of a fastener to move in a direction parallel to the mounting panel and allow for mating hole misalignment.
Interference fit. The insertion of a member into a smaller-diameter part.
Minimum distance.The minimum distance from the center of a fastener mounting hole to the nearest edge of a panel which prevents the edge from deforming. Suitable fixturing or increasing panel thickness may reduce this distance.
Pull through. A fastener's resistance to force applied in the installation direction.
Pushout. The force required to remove a fastener from a panel opposite the installation direction.
Shank. The portion of a fastener slightly smaller than the mounting hole that locates the fastener in the hole. A shank also incorporates an annular groove that fills with panel material as the fastener is installed. The retention of this material provides pushout resistance.
Torque-out. The torsional holding power of a self-clinching fastener in a sheet. Test torqueout by applying pure torsion (no axial load) to the fastener.
Undercut. The reduced diameter of a fastener which receives the sheet material when a fastener is installed. Depending on the type of fastener, the undercut may be rectangular or back-tapered.