As design engineers strive to create smaller and lighter packages composed of thinner materials, hardware can get in the way. Compact designs inherently shrink the “real estate” available to place and install hardware, while thinner materials can reduce the ability for hardware to gain purchase and provide secure attachment over time. Then there is the matter of choosing the best method for a job.
Traditional “permanent” fastening methods, such as adhesives or welding, will fail to allow for product disassembly, which is so crucial in today’s high-tech product world. Welding results in undesirable fumes and burn-outs and often requires complicated electrodes and pilots that can stall production. Even some types of conventional mechanical fasteners have drawbacks in performance, including sheet-metal screws that can fall short in the areas of reusability and holding power.
These emerging issues have given rise to a whole new generation of self-clinching “miniature” threaded fasteners, which offer performance and service advantages that conventional fastening and joining methods cannot. They install quickly and permanently, minimize the need for additional hardware, and promote access and serviceability.
A variety of miniature types and styles fit especially well in the increasingly restrictive design envelopes encountered in industries ranging from electronics to aerospace. All will provide strong, permanent, and reusable threads in very thin (sometimes “ultra-thin”) metal sheets. Usually, only a mating screw or nut is required to complete the attachment process.
These types of fasteners require fewer assembly operations, deliver more holding power than sheet-metal screws, and can offer benefits especially advantageous for small-component assemblies. Some types, for example, will promote installation closer to edges to maximize use of space, while others will solve problems encountered when working with thin materials.
Now designers have the opportunity to think big when deciding how to assemble small packages. Among the “miniature” threaded self-clinching fastener solutions for those working in the shrinking world of component assembly:
- Self-Clinching Locking and Non-Locking Threaded Nuts. This specialized family of miniature locking and non-locking fasteners is characterized by extremely small thread sizes (as low as #0-80 and M2) and the tiniest of footprints. They can be permanently installed in aluminum or steel sheets as thin as .019″/0.76mm.
The tops of the locking types are elliptically squeezed (instead of round) to add tightening action around the mating screw and meet locking torque requirements of NASM25027.
Both locking and non-locking types feature knurled collars, which embed completely in a metal sheet upon fastener installation to guarantee against rotation. (The spin resistance of the knurl exceeds the torque that can be exerted.) Pushout resistances in values up to 420 lbs./1868N can be achieved, due to the displaced sheet material filling the undercut cavity beneath the collar. These fasteners usually come supplied with a dry-film lubricant to provide smooth, non-galling prevailing torque performance. Torque-out values of up to 110 lbs./12.43 Nom can be realized, depending on thread size and sheet material.
- Self-Clinching Threaded Nuts for Ultra-Thin Sheets. Compared with standard clinch nuts, these types exhibit a lower profile and can be mounted up to 50% closer to the edge of ultra-thin steel sheets. The nuts’ diameters (.220″/5.6mm) and height (.065″/1.4mm) contribute to the low profile and maximized use of space in an assembly.
Once installed (in sheets as thin as .025″/0.64mm), these nuts will provide permanent threads in sizes as small as #2-56 and M2.
- Self-Clinching Threaded Studs with Low-Displacement Heads. These types also can live “close to the edge.” Depending on thread size (as small as #2-56 and M2), they can be installed 25% to 50% closer than standard self-clinching studs without causing that edge of the metal sheet to bulge. (Both the head and overall design are contributing factors.)
When installed in aluminum or steel sheets as thin as .040″/1mm, a flush-head assembly is created and the stud locks securely in place.
- Self-Clinching Threaded Studs for Thinner Sheets. Non-flush stud types (thread sizes as small as #2-56 and M2) can be installed in aluminum or steel sheets as thin as .020″/0.51mm. As with the low-displacement head studs, these fasteners will similarly lock with ample torque-out and pushout resistances. Although their heads will not be flush in the assembly, these fasteners will enable “close to edge” installation in metal sheets half as thick as the thickness of materials required for installation of flush-head studs.
- Self-Clinching Threaded Standoffs for Ultra-Thin Sheets. The primary function of these types of fasteners is to enable components made from the thinnest metal sheets to be stacked or spaced securely. They can be utilized in aluminum or steel sheets as thin as .025″/0.63mm and can be specified in various lengths and in the small thread sizes (#2-56 and M2).
Users specify miniature self-clinching fasteners where good pullout and torque loads are required in sheet metal that is too thin to provide secure fastening by any other method. They perform reliably in sheets too thin to tap and will serve as superior threaded alternatives to extruded/tapped or stamped threads. (In general, miniature self-clinching fasteners should be selected whenever a component must be replaced readily and where “loose” nuts and hardware would be inaccessible.)
The fasteners are installed by pressing them into place in properly sized drilled or punched holes using a parallel acting press adjusted to predetermined forces. The pressing or squeezing process causes displaced panel material to cold-flow into a specially designed annular recess in the shank or pilot of the fastener, locking the fastener in place. Depending on fastener type, a serrated clinching ring, knurl, ribs, or hex head prevents the fastener from rotating in the metal when tightening torque is applied to the mating screw or nut.
The outcome is that miniature self-clinching fasteners become a permanent and integral part of the panel, chassis, bracket, or other component in which they are installed.
For high-volume installations, an automated press (instead of manual models) can be considered. Some automated presses are specifically designed to feed self-clinching fasteners automatically into punched or drilled holes in sheet metal, seating them correctly with a parallel squeezing force. Feeding rates are up to six times faster than manual insertions, and squeezing action is adjustable to compensate for variations in thickness and hardness of the sheet and the height of the fasteners.
Installers also have a newfound option to take advantage of significant advances in in-die fastener-feeding equipment. Systems have been developed to work in tandem with stamping presses (and properly tooled die) to feed and install fasteners, which eliminates secondary operations typically required for fastener insertions. This provides a capability to perform two operations (stamping and fastener installation) simultaneously in the die.
Once a miniature self-clinching fastener has been installed, its reliability in service will depend on many factors, beginning with a properly sized hole, the thickness and hardness of the host sheet, proper installation procedure, and the design and quality of the fastener itself.
There are three tests applicable to a self-clinching fastener to determine its reliability in service. The first is torque out, which determines the fastener’s ability to resist rotation within the sheet. This test often is made at the head of the fastener often with values exceeding the ultimate torsional strength of the mating screw or nut.
A second reliability measure is pushout. Pushout values indicate the axial resistance of a fastener in the opposite direction from which it was installed, and should be roughly 5% to 10% of the force used to install the fastener.
A final test is torque-through, which is the resistance of a fastener to pulling through the metal sheet when a champing torque is applied.
As is generally true in the manufacturing marketplace, differences will exist among fastener suppliers and, in turn, among their products in the areas of reliability, quality, and installed cost.
Production of quality fasteners begins with good engineering research, design, development, and testing. Precision is necessary in all facets of fastener production, especially relating to miniature types. Dimensional accuracy and consistency are crucial and, if these are lacking the result will be rejected panels, chassis, or boards upon fastener installation. Even minute size variations among parts can cause automated equipment to jam, increasing downtime and production time.
A good rule of thumb is that product designers take the time to take advantage of a supplier’s support before a package or component is designed and use the resources of the supplier in the early stages of design to develop the best fastener for the application.