TechArchive

Original article date: April 1998

A square self-clinching fastener said to cut assembly time of thin aluminium sheets by up to 80% has been unveiled.

Faced with the need to attach thin metal sheets too thin to tap, how best to provide permanent load-bearing threads, while reducing the amount of required hardware and time required for installation?

Broadly defined, 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 specifically designed annular recess in the shank or pilot of the fastener.

The shank or pilot of the fastener is inserted into a previously punched, drilled, or cast hole. Then force is applied until the head of the nut contacts the sheet. Some types of fasteners will be fully installed when the head is flush within the sheet.

When the mating piece is installed from the side opposite the head of the fastener, the assembly is complete. Installation results in a flush surface on one side of the panel.

A serrated clinching ring, knurl, ribs, or hex head prevents the fastener from rotating in the host material once it has been properly inserted.

Upon installation, self-clinching fasteners will not loosen or fall out, and they can dramatically reduce the number of parts to be handled or kept in inventory. They are manufactured from low- and medium-carbon steels, martensitic and austenitic stainless steels, aluminum, and phosphor bronze.

For years, automakers, tier-one suppliers and aftermarket manufaturers have been applying self-clinching fasteners to assembly problems, including airbag and fuel rail assemblies, water pump covers, sun roofs, engine mounts, door strips, window controls, dashboards, button carriers and ignition coils.

As a rule, a self-clinching fastener can be specified whenever a component must be readily replaced and where “loose” nuts and hardware would not be accessible. If the attaching “nuts” and “screws” cannot be reached after a chassis or cabinet is assembled, self-clinching fasteners can be installed during metal fabrication.

The R’ANGLE clinch fastener was first introduced in the US late last year as a method to provide a strong right-angle attachment point in aluminum sheets down to 1 mm without the need for brackets, metal bending or loose fastening hardware. It is intended as an alternative for bent tabs at edges of sheets, bent tabs in the middle of sheets, bent flanges, right-angle brackets and tack welds. Tab cut-outs are unnecessary, giving reductions in materials and assembly costs, loose hardware problems and improved EMI/RFI shielding.

The fastener is permanently installed into a properly sized rectangular hole using the self-clinching principle. After installation, a sheet or adjacent part is attached perpendicular with either M3 or M5 thread forming screws.

The self-piloting screw forms its own thread as it is installed, providing a very secure tight thread fit, which is resistant to vibration and has excellent strip-out characteristics. The screw can be removed and re-installed without loss of performance.

An early application is at Fillon Pichon is for a two-piece enclosure for digital electronic weighing scales. The enclosure is an extruded metal chassis which reportedly previously required tabs to be punched and bent to create right-angle anchor points. Using six such fasteners has reduced assembly time from 90 to 15 minutes, according to engineers at Fillon Pichon. The fasteners are installed prior to assembly and coated with a crackle finish.

Fastener FAQ

Does the shape of the self-clinching fastener change during installation?

No, the fastener does not deform in any way. There is no flaring, crimping, swaging, peening or riveting.

What holds the fastener in the sheet?

The squeezing force on the fastener cause the sheet material beneath the ahead to cold flow into the back-tapered shank or undercut of the fastener.

Is special equipment needed to install self-clinching fasteners?

No. Self-clinching fasteners are installed using any type of parallel acting press which will squeeze them in place.

Won’t these fasteners fall out, twist out or spin in their mounting holes if I tighten down too hard?

No. Typical torque-out values are generally quite high compared to the rotational force that will be put on them. In fact, for most quality self-clinching nuts, the screw will fail before the nut rotates in the panel.

Can a self-clinching fastener be installed with a hammer blow?

No, a self-clinching fastener must be installed using a squeezing action. An fast impact installation will not allow sufficient time for the sheet material to cold flow.

Is there a self-clinching fastener that will go into a stainless steel panel?

Yes. Self-clinching fasteners for stainless steel applications are available. These fasteners are generally made from a specially hardened stainless steel and will install into sheets up to HRB 88 in hardness.

When using self-clinching fasteners what are the general panel requirements?

Generally, there are two basic requirements:

• The panel must be a ductile material softer than the fastener which is going into it. This is why it’s important to know the MAXIMUM panel hardness for each fastener type.
• The panel must meet the minimum sheet thickness required by the fastener. Some self-clinching fasteners can be installed into sheets as thin as 0.51mm, but generally 0.76mm or 1mm is the minimum thickness necessary.

Is there a maximum sheet thickness for self-clinching fasteners?

Generally, there is no specified maximum sheet thickness limitation. However, because of their special design and function a few fastener types do specify a thickness range which includes a maximum.

Some of the fasteners have a hexagonal shaped head. is it necessary to punch a hexagonal mounting hole to install these?

No. All self-clinching fasteners are installed into a round punched or drilled hole. A hexagonal head will cause the sheet material to cold flow around the head to provide high torque-out resistance. The hexagonal head will be flush in the sheet when installed.

Can you install these blind, from one side, without access to both sides of the sheet?

Generally, you must have access to both sides of the sheet to properly install self-clinching fasteners. However, there are some larger nuts which can be drawn in from one side using an impact-torque wrench.

• PEM International
• 01302 886961

April 1998