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FEATURE FASTENINGS & ADHESIVES MAKING THE RIGHT DECISION


Not only will every fastening and joining decision impact the early stages of appliance design, but it will affect the integrity and performance of the equipment. So, when evaluating the technologies available to attach thin metal sections and/or components in an application, there will be a number of considerations, writes Leon Attarian from Penn Engineering


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any questions need to be asked before the right fastening or


Many questions need to be asked before the right fastening or joining solution can be selected for an application


joining solution can be selected for an application. Will, for example, disassembly be likely to allow for service or repairs, or will the application require joining dissimilar metals? If so, welding can be ruled out as this not only forms a permanent bond but it is virtually impossible to join dissimilar metals such as steel to aluminium. In addition, if the welded surface is intended to be the finished surface, then additional finishing steps to compensate for burn marks and scale will be needed. Some sheets will also be too thin to weld properly. Adhesives or tapes will provide clean and flush attachments but they are also ‘one time only’ joining solutions. These may not provide the required strength and/or longevity, and may be adversely affected by extreme temperatures, vibration, etc. Permanent epoxies, meanwhile, can be difficult to work with and to apply. Riveting can be used to attach dissimilar thin materials, but rivets cannot install flush without countersinking or some other secondary operation, leaving metal bulging and often limiting their use to utilitarian assembly inside enclosures. Attention must therefore be paid to the clearance necessary for the rivet bulb as it can take up valuable ‘real estate’, especially in appliances where space is at a premium. Sheet-metal screws, meanwhile, can


develop issues related to diminishing holding power over time, reduced thread integrity and may even loosen and fall out. So can introducing threads provide an answer? Sometimes these can be tapped,


but some metal sheets will be too thin for effective tapping. Extruded or stamped threads can prove to be impractical. Fasteners, however, are engineered to provide strong and reliable threads in thin metal assemblies – such as self-clinching threaded fasteners and ‘blind’ threaded inserts. These install permanently into thin sheets, become an integral part of an assembly, reduce the amount of loose parts (such as washers, lock washers and nuts), require only a single mating piece to complete final attachment, and enable component or panel removal and re-attachment (without threatening thread reliability) if needed.


SELF-CLINCHING FASTENERS The benefits of self-clinching fasteners essentially arise from their design, which incorporates an annular recess for permanently locking the fastener in place in thin ductile metal sheets, and an element to prevent fastener rotation in service. These can be installed permanently in


ductile metal sheets as thin as .012”/ 0.3mm for engineered micro clinch versions developed for handheld consumer electronics devices. Fasteners are pressed into place in a properly sized hole and, when sufficient squeezing force is applied, the fastener’s serrated clinching ring, knurl, ribs or hex head is forced into the panel surface. This displaces the sheet material into an annular recess in the shank or pilot of the fastener, known as an undercut. The metal forced into this secures the fastener against axial movement, while a non-round displacer secures the fastener against rotation. Dozens of types and thousands of


variations of self-clinching fasteners (including steel, stainless steel or aluminium) have been engineered over the years, with more being developed.


SUITABLE PRODUCTS For appliances, there are a number of suitable product families. These include nuts, with standard types designed with load bearing thread strengths greater than mild steel screws. Variations focus on nut size, locking-thread properties, and special alloy materials. All clinching during installation occurs on the


12 MAY 2015 | DESIGN SOLUTIONS


fastener side of the sheet – the reverse side remains flush and smooth – and a mating screw finishes the job. Studs are externally threaded self-clinching fasteners which are generally selected for applications where a component must be positioned in advance of final attachment. Flush-head studs are standard, but variations can satisfy high torque, thin sheet or electrical applications. Studs without threads can double as permanently mounted guide pins or pivots. Spacers and standoffs are designed


primarily to stack or space components. The most common types include thru-threaded and blind-threaded versions. All install with their heads flush within the host sheet. Using blind-threaded types, outer panel surfaces of an assembly are smooth and closed. Another type is access hardware.


Self-clinching panel fastener assemblies incorporate captive screws to keep loose parts to a minimum and eliminate risks associated with hardware that can loosen, fall out and damage internal components. These will ideally be specified to attach metal panels or other thin material components in applications where subsequent access will be necessary. Among recent innovations is a


self-clinching fastener type that enables two sheets – even if they are made from dissimilar materials – to be joined permanently in a flush-attachment connection without protrusions on either side. These practical alternatives to riveting or spot welding are a solution for attaching two metal sheets which are too thin to weld, fasten sheets of unequal thicknesses, join dissimilar metals, attach ultra-thin metal sections, or join metal sheets to PCB or plastic panels. Their rotational capability offers additional application advantages by allowing the fastener to act as a hardened pivot point. These install smooth with the top sheet


and flush or sub-flush with the bottom sheet. Unlike rivets that ‘bulb’ during installation, this hardware allows for unobtrusive flush attachment requiring minimal space. A smooth surface is retained for finishing and the fasteners subsequently can be concealed easily with paints or powder coatings.


/ DESIGNSOLUTIONS


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