Edited by Martha K. Raymond
Penn Engineering & Manufacturing Corp.
Most mechanical fasteners, such as nuts and bolts, are just loose hardware. During assembly or adjustments to hard-to-reach components a few fasteners are bound to fall on the floor or roll under equipment, never to be seen again. Self-clinching fasteners, on the other hand, are easier to work with because they stay where they’re put, even on vertical surfaces.
A self-clinching fastener is any device, usually threaded, that when pressed into ductile metal, displaces the host material around the mounting hole, causing it to cold flow into a specially designed annular recess in the fasteners shank or pilot. A serrated clinching ring, knurl, ribs, or hex head prevents the fastener from rotating in the host material once it has been properly inserted. Self-clinching fasteners then become a permanent part of the panel, chassis, or bracket.
Other advantages of self-clinching fasteners let them take up less space and require fewer assembly operations than anchor or caged nuts. Compared to sheet-metal screws, self-clinching fasteners have greater reusability and more holding power. They can be installed during either fabrication, final assembly, or when fasteners are not easily accessible in later assembly. Automated assembly equipment can be used to further reduce costs in high-volume applications.
Self-clinching fasteners are most often used when good pullout and torque loads are required in sheet metal that is too thin to provide secure fastening by other methods. Even if the sheet is thick enough to sustain tapping, it may be more economical to use self-clinching fasteners especially when they can be installed during fabrication. Their compact design and low profile also provide neat appearance.
Knowledge of self-clinching fastener characteristics can give engineers an edge when it’s time to specify a joining system for streamlined assembly and sleek-looking products.
After deciding to use self-clinching fasteners, looking at their composition helps select the best ones for the job. Fastener characteristics include material, manufacturing processes, quality controls, and component dimensions.
Many self-clinching fasteners are made from free-machining, cold-drawn, low-carbon bar stock, a variety of free-machining and cold-formed stainless steels, aluminum, and phosphor bronze. They are formed on automatic bar machines and generally undergo secondary operations such as slotting, tapping, or lock forming.
Wire stock is also used to make self-clinching fasteners. Basic blanks are formed on cold headers or cold formed, and fasteners may require subsequent operations such as piercing, drilling, tapping, thread rolling, and slotting. Finishing operations such as heat treating and plating are equally sophisticated, and quality-control measures must be monitored throughout the manufacturing process.
With so many manufacturing variables to control, there are several quality areas which must be considered, including:
Dimensional tolerances — A self-clinching fastener requires tight tolerances to maximize performance. A variation of only two thousandths of an inch can make a 20% difference in a part that is specified with a dimension 0.010 in. (0.25 mm).
Thread fit — A part may be specified because it meets one or more government regulations for thread tolerances. Equivalents must meet the same specifications.
Heat treatment — Improper tempering can cause fastener brittleness and eventually cracks in the fastener. Inadequate treatment can make fasteners so soft that they are literally crushed during installation.
Plating — These standards set limits for metal preparation, plating thickness, adhesion, rust-corrosion protection, hours of salt spray testing, and other operations. A poorly plated part will diminish the appearance and performance of a final product.
Performance — Fasteners should be tested to meet the manufacturer’s published performance data. In addition to basic performance testing, fasteners must meet vibration resistance, thread locking, heat, and electrical parameters.
Quality control — Ensures fasteners were manufactured under comprehensive quality control systems and methods.
Once chosen, self-clinching-fastener reliability depends on factors such as a properly sized hole, the thickness and hardness of the host panel, correct installation, and proper fastener selection.
Three tests help determine whether a self-clinching fastener will be reliable in service. The first, called torque-out, determines the fastener’s ability to resist rotation within the panel. This test often is made at the head of the fastener, many times with values exceeding the ultimate torsional strength of the mating screw or nut.
A second reliability test is pushout. Its values indicate the axial resistance of a fastener to removal from the sheet opposite to the direction from which it was installed. Pushout loads should be roughly 5 to 10% of the force used to install the fastener.
A final test, torque-through is the resistance of a fastener to pulling through the metal sheet after applying a clamping torque.
After selecting a self-clinching fastener for an application, engineers are faced with a variety of styles. To make the selection process more manageable, a description of the most basics types can help narrow down the choices.
Right-angle styles provide a strong right-angle attachment point in sheet metal as thin as 0.040 in. (1 mm) and offer a cost-effective alternative to forming tabs at edges, bent flanges, right-angle brackets, tack welds, and loose hardware.
Standard nuts have thread strengths greater than mild steel screws and are commonly used wherever strong internal threads are needed for component attachment or fabrication assembly.
Studs are externally threaded fasteners which position attachments before fastening. Flush-head studs are normally specified, but variations can be used for high torque, thin sheet, or electrical applications.
Spacers and Standoffs are used to stack or space components away from the panel. Through-threaded or blind types are generally standard.
Flush fasteners are completely flush within the sheet when installed. These fasteners are often installed into a flat sheet so that the fasteners won’t interfere with subsequent bending and forming operations.
Floating nuts compensate for mating hole misalignment by having a floating threaded element.
Panel-fastener assemblies are generally used on enclosures where the screw must remain with the door or panel.
Locking fasteners provide a prevailing torque-locking feature to restrict screw rotation under vibration.
Nonthreaded fasteners allow quick assembly or component removal without screws or additional fastening hardware.
Concealed head designs are installed into milled, blind holes so that one side of the panel remains unmarred. Studs and standoffs are generally available in this type.
Blind fasteners have closed ends that limit screw penetration and are useful for protecting internal components from damage by inadvertent insertion of extra long screws. Threads are also protected from damage and foreign matter.