The fundamentals for self-clinching success

Paul Boughton

Self-clinching fastener technology enabled the development of ever-thinner and lighter end-product designs. Leon M Attarian reports.

 In the universe of fastening and joining technologies available to designers for attaching thin metal assemblies, self-clinching fasteners display a wide array of advantages.  Their overarching function is to provide permanent and reusable load-bearing threads to accept mating hardware in metal sheets too thin to be tapped or where extruded or stamped threads would be impractical.

Upon installation (usually during the fabrication process), self-clinching fasteners become integral parts of an assembly, will not loosen or fall out (even when the mating thread is removed), never have to be restrained from rotation with a tool, and never have to be handled again. They allow for component removal and re-attachment for access or service and can dramatically reduce the required amount of hardware in an assembly by eliminating the use of loose washers, lock washers, and/or nuts. Fewer parts enable lighter designs, quicker assembly, lower production costs, and optimised levels of end-product reliability.

Dozens of types and thousands of variations of self-clinching fasteners (steel, stainless steel, or aluminium) have been engineered over the years. Notable product families include threaded and through hole spacers and standoffs, captive screw assemblies, cable tie mounts and hooks, and face-to-face panel mounting hardware, among others. 

The introduction of ‘micro’ self-clinching fasteners has expanded application possibilities, especially in the consumer electronics marketplace, by realising smaller thread sizes and thinner sheet capabilities than legacy product.  Innovative fastener designs continue to be introduced to meet new and emerging application needs.

Each attachment application will present particular requirements or challenges for designers when considering and evaluating self-clinching fastener options.  Fundamental facts ultimately can help guide designers toward successful outcomes.

Among the basic tips:

* The fastener must always be harder than the sheet in which it will be installed.  Regardless of type, self-clinching fasteners install permanently in thin ductile metal sheets by pressing them into place in a properly sized hole and then applying sufficient squeezing force. 

The fastener’s serrated clinching ring, knurl, ribs, or hex head is consequently into the panel surface, displacing sheet material into a specially designed annular recess in the shank or pilot of the fastener, known as an undercut. The metal displaced into the undercut secures the fastener against axial movement, while a non-round displacer secures the fastener against rotation to result in the fastener’s permanent installation.

Due to the requirements of this process, metal sheets into which the fastener will be installed must exhibit adequate ductility to allow the displaced sheet material to cold flow into the undercut without fracturing.

In addition, the host metal sheet must always be sufficiently softer (typically 20 points on the HRB or HRC scale) than the fastener to prevent fastener deformation during installation and to promote reliability in service.

* Host sheets must meet the minimum thickness recommended for the fastener. If the host sheet is too thin to accommodate the fastener, failure can follow. Most legacy self-clinching fastener product families can be installed reliably into sheets as thin as 0.76mm, with several traditional families going down to 0.51mm. Trends toward thinner sheets have prompted new product families to satisfy those applications with sheets as thin as 0.3mm (and, in some cases, even thinner). Typically, there will be no limitation on the maximum thickness of a sheet.

* Specify the proper stainless alloy for fastener installation into stainless steel sheets. A prevalent misconception is that all stainless self-clinching fasteners will perform as intended in all stainless steel sheets. This is not true. Standard stainless self-clinching fasteners made from 300 Series cannot be expected to perform reliably in 300 Series stainless sheets, because of the relative hardness issue involving fastener and sheet. Appropriate stainless fasteners for use in 300 Series stainless sheets will include types manufactured from 400 Series stainless or parts made from special alloy or precipitation hardened stainless. 

* Pay close attention to centreline-to-edge fastener placement requirements. When a self-clinching fastener is installed closer to the edge of a sheet than recommended by the manufacturer, the sheet may bulge or blow out. Once bulging occurs, some reduction in performance of the fastener can be expected due to some of the material not flowing into the undercut of the fastener. Performance in service will be difficult to predict and will be influenced by the installation and the application.

These same challenges can be anticipated in multi-sided close-to-edge applications. In either case, supporting the edge with special anvils can be used for reinforcement during fastener installation, but this technique generally should be applied with caution and with minimal expectations.

In response to these challenges and recognising that designers may have little or no choice when working with especially restrictive design envelopes, specific self-clinching fastener types have been developed to address the issues. Such fasteners feature smaller diameters, lower heights, and permit shorter ‘edge of sheet’ to ‘centre of fastener’ distances. An example is a family of 300 Series stainless steel self-clinching nuts ideally suited for installation into ultra-thin steel or aluminium sheets. Their minimal ‘edge of sheet’ to ‘centre of fastener’ distance (3.7mm/.150”) enables close-to-edge mounting and their small diameter (5.6mm/.220”) and low height (1.4mm/.065”) combine to provide overall low-profile fastener solutions.

* Keep multiple fasteners sufficiently spaced apart. Multiple self-clinching fasteners must be spaced far enough apart to avoid their interfering with each other’s mounting holes. Otherwise, sheet distortion and/or ‘oil canning’ can lead to failures. The proper distance between two or more fasteners can be calculated by the formula C/L to edge +½ the diameter of the second mounting hole (where C/L represents ‘centreline’).

* Evaluate performance-testing values to determine a fastener’s expected reliability in service. Three tests can be applied to ascertain how well a fastener will remain intact and serve as intended. The first test is torque-out, which is the fastener’s ability to resist rotation within a sheet.  This test often is made at the head of the fastener (with values usually exceeding the ultimate torsional strength of a mating screw or nut).

The second reliability measure is pushout, which indicates the axial resistance of a fastener to remove it from the sheet opposite to the direction from which it was installed. (This should be roughly 5% to 10% of the force used to install the fastener). A final test is pull-through, or the resistance of a fastener to pulling through the metal sheet when a clamping torque is applied. 

Typically, a fastener manufacturer will conduct fastener tests on its own and publish the resulting values as guidelines and for evaluation.

* Ensure integrity of fastener design and manufacture. The production of quality self-clinching fasteners begins with good engineering research, design, development, and testing. Precision is necessary in all facets of fastener production. Dimensional accuracy and consistency are critical and, if these are lacking, the outcome may be rejected assemblies upon fastener installation. Even minute size variations among parts can cause automated fastener-installation equipment to jam, increasing unscheduled downtime and diminishing production time and efficiencies. So-called ‘equivalents’ rarely, if ever, rise to the occasion or to the demands.

Significant variations among fastener materials, manufacturing processes, quality controls, and component dimensions can potentially make the ‘same’ product from different manufacturers much different from one another.

Most fasteners are formed on cold headers or cold formers, while others are formed on automatic bar machines. After primary operations, many will require secondary operations (such as slotting, tapping, or lock forming) during manufacture.

* Identify potential secondary benefits of a fastener type. Some self-clinching fasteners will demonstrate unique performance capabilities – often more than one – to maximise their effectiveness and contribute meaningfully to end-product assembly.

For example, a self-clinching fastener designed to mate two panels at a right angle delivers added value by enhancing EMI/RFI shielding (since any need for cutouts in the middle of panels is eliminated).

* Keep pace with material trends. These include the integration of unconventional materials offering newfound capabilities beyond those provided by standard steel, stainless, or aluminium self-clinching fastener types.

An example: hybrid fasteners incorporating a combination of metal and injection-moulded plastic elements. Depending on type, these may be less expensive, lighter, and easier to manipulate and install compared with standard mechanical fasteners. Their plastic caps introduce an opportunity for colour-coded parts serving as visual identifiers to ‘flag’ components, designate restricted or limited access areas, or correlate with equipment instructions. They can also be specified to match the colour of a panel or other component or to simply enhance overall cosmetics. In this way, fasteners effectively become multi-functional devices.

Self-clinching fastener technology has advanced significantly in more than 70 years since first introduced and has enabled the development of ever-thinner and lighter end-product designs. Designers can reinforce proper decision-making when specifying a fastener for an application by enlisting a supplier’s support and in-house technical and engineering resources in the early stages of the design process.  Even after a fastener is specified, new assembly challenges or changes in end-product design may occur and such a partnership cultivated at the outset can help keep the door open to invaluable experience and assistance regardless of application challenge.

Leon M Attarian is Director of Global Marketing for PennEngineering, Danboro, PA, USA.

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