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Guide to exploiting self-clinching fasteners.

An integral fastener is a device, usually threaded, that's installed in a chassis, bracket, or panel to become a permanent part of the unit. Permanently mounted fasteners can be mechanically clinched, swaged, riveted or welded in place, or may be of a self-clinching design.

Integral fasteners are available in hundreds of sizes and configurations. One family--the self-cliching types--is particularly attractive in terms of ease of installation, dependability, and low installed cost. Because self-clinching fasteners are precision parts, it's important to install them in carefully designed products.

Fastener facts

Each fastener type has its own characteristics, and some designs present problems in installation and use. For example, clinch nuts are hex or round nuts with a knurled shank or pilot that projects from one end. The shank is inserted in a mounting hole in the metal sheet, then spread, crimped, or rolled over the other side.

While resistance to "push-out" (i.e., being pushed through the sheet) is good, the fastener can be spun readily on its axis. In additon, special tooling is needed to spread the shank, and the cost of countersinking is added if flush installation is specified.

Anchor or plate-type nuts are large, relatively expensive, integral fasteners that generally require three holes in the panel, plus two rivets, to secure them. Again, flush mounting requires counter-sinking, and the costs of tooling, installation, and the fasteners are high.

Other types of integral fasteners include weld nuts and studs. While projection weld nuts and studs are fabricated mostly in steel, stainless units also are available. Projection weld fasteners assure good resistance to both push-out and torque-out when they are welded into place properly. Welding fabrication generally must be completd prior to metal finishing, and fasteners can't be welded satisfactorily in aluminum.

Self-clinching fasteners provide assembly and service economies as well as cosmetic benefits. The fastener is squeezed into a hole in the metal sheet, using any parallel-acting press.

The fastener must always be harber than the sheet metal that cold-flows under force into an annular groove around the shank and into a knurled step of the self-clinching fastener, Figure 1. This provides a strong, permanent fit in metal as thin as 0.020". Self-cliching fasteners provide substantial installed-cost savings because of the many installation economies they allow.

Self-clinching fasteners offer great potential for use in aerospace, appliance, automotive, and electronic industries, wherever the metals to be joined are too thin to provide effective thread length or strength by tapping. Even if the sheet is thick enough to sustain tapping, it may actually be more economical to use self-clinching fasteners. They can be installed during fabrication or during final assembly to eliminate loose hardware.

Self-clinching fasteners allow using thinner sheet metal and permit a real reduction in installed cost vis-a-vis other fastener designs.

The self-clincher

Before assuming that a custom-designed integral fastener will be required, manufacturing engineers should become familiar with the variety and sizes of self-clinching fasteners available off the shelf. These include several types of nuts with free-running or self-locking threads; floating insert nuts, self-locking or nonself-locking; flush-head, heavyhead, and concealed-head studs; trough-hole, blind-hole, and concealed-head standoffs; panel fasteners with captive screws; spring-loaded plungers or pins; grounding solder terminals, flush-head pins; and many other devices in thread sizes from #0-80 to 3/4"-10.

Self-clinching fasteners are made from free-machining, cold-drawn, low-carbon bar stock, from medium-carbon steel, 303 free-machining hardened stainless, aluminum 7075-T6 and 2024-T4, and CDA 510 phosphor bronze.

Wire stock also is used for basic blanks formed on cold headers or cold formers; piercing, drilling, tapping, thread rolling, and slotting may be required.

Tolerances of 0.002" to 0.003" are mandatory for quality fasteners. With respect to the height of a knurled ring or hex head, 0.003" can represent a 20- to 30-percent variance in the volume of metal cold flowing into the undercut.

Dimensions and thread fits must be referenced accurately to commercial and military specs. This manufacturing precision must be applied to all metalforming operations, heat treament, electroplating, and hydrogen-embrittlement relief. A mechanical or metallurgical shortfall on the fastener manufacturing line could become a failure in the field.

Selection criteria

There are four basic criteria to consider when specifying self-clinching fasteners.

* Function of the fastener: Where will it be used? How can the fastener expedite assembly and field service disassembly? Can it assure the lowest installed cost?

* Reliability in service: Reliability of a self-clinching fastener depends on many factors, beginning with a properly sized hole, thickness and hardness of the metal sheet, proper installation, and design of the fastener and the unit into which the fastener is inserted.

There are three tests for determining reliability. 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, and the value is given as a maximum, often exceeding the ultimate torsional strength of the mating screw.

A second test is push-out. These values indicate resistance of a fastener to being removed from the sheet opposite the direction from which it was installed, and are roughly 5 to 10 percent of the force used to install the fastener.

The final test is torque-through, which represents the resistance a fastener gives to pulling through the metal sheet when normal torque is applied (e.g., inserting a screw in the final assembly operation). The screw thread generally will fail before the self-clinching nut rotates in the sheet or exhibits thread failure.

* Tooling and automatic installation: Since all self-clinching fasteners must be squeezed into place, any press or vise that provides the necessary parallel force--pneumatic, arbor, mechanical or hydraulic--may be used.

An automatic air/hydraulic press should be considered for high-volume assembly, though. For example, our Pemserter Series 1000 uses an "intelligent" main cylinder containing an optical encoder that transmits absolute ram position to a microprocessor-based logic control system, Figure 2. An end-of-ram sensor provides an operating window to allow the machine to perform at its maximum rate while prviding for operator and tooling safety.

Operations is simple. A vibratory bowl is filled with fasteners. The bowl, in conjunction with a feed track, properly orients each fastener and presents it to the work area. The operator places the fastener mounting hole of the panel the an anvil and activates a guarded foot switch. If the end-of-ram sensor is uninterrupted, the ram proceeds to the work.

At the bottom of the stroke, high force is applied, securely installing the fastener. Then the ram retracts. Standard installation rates are 1200 fasteners/hr.

The unit uses an air-over-oil system, which saves power and permits faster system cycling. It's also quiet and eliminates heat build-up. Only periodic maintenance of air filters is required. The tooling is a gravity-track top-feed system. The press features modular design and easy access to make maintenance and service simple.

* Installed cost--the bottom line: If integral fasteners are being welded, cost of materials, labor, and overhead must be factored into the final cost per installed fastener to obtain an accurate comparison with the final calculated cost per self-clinching fastener. Rejects and spoilage because of failure of poorly designed or manufactured fasteners will contribute to high production costs.

For more informaton about selfclinching fasteners, circle E27. For more regarding automatic insertion presses, circle E28.
COPYRIGHT 1985 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1985 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Ernest, Richard B.
Publication:Tooling & Production
Date:Sep 1, 1985
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