Clinching,

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Clinching is a form of Mechanical Fastening, used generally for joining overlapping sheet metal elements.

This type of joining has commanded much interest and the number of successful applications is expanding.

Did you ever strap together...

...two or three sheets of paper by folding one corner a few times and then ripping a short tear across the fold?

That is somewhat similar to the joining process described in this page...

Two characteristics stand out which distinguish this method from all others:

  • No additional joining elements or consumables are needed
  • By using the combined mechanical action of a punch-and-die pair, an interlocking joint is formed in the base sheet metals themselves.

Many applications suitable for resistance spot welding, can be considered appropriate candidates for this joining process.

But Clinching can readily join also many incompatible materials combinations, not weldable by resistance processes.

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Clinching Characteristics

Clinching, the high speed mechanical fastening of sheet, is best suited for automated mass production, like that of automotive and consumer appliances.

No specialized skills are needed from the operator, the equipment is economic, with low energy requirements.

No fumes or heat are generated, low noise is emitted, and tools enjoy long life.

Clinching may need to be excluded from certain applications.

That is if the remaining imprint on the surface of the object being processed is objectionable because of aesthetic or practical reasons.

Thickness of sheet metal joined by Clinching typically ranges from 0.5 to 3.0 mm.

Materials include steels of various types, bare, coated or prepainted, stainless steels, aluminum and also dissimilar combinations.

The punch part of the tooling, usually on top, deforms or shears the sheets, while the die, of closed type or of split type, forms the button and completes the self locking joint.

Joint per joint, properties are slightly lower than those of comparable spot welds, but applications can be designed to exhibit sufficient strength as required.

Furthermore unique situations may exist, like for aluminum to steel Clinching.

Here, apart from other mechanical or adhesive fastening, it may be the only practical joining solution.

Different joint designs exist, in part proprietary, to satisfy special requirements.

Equipment

The punching and forming process is described and illustrated in stages.

In practice Clinching is performed at one stroke at a time.

Tooling parts must be perfectly aligned and the workpiece is to be correctly presented to the tooling.

For best results the thicker or stronger sheet is always facing the punch.

Once selected Clinching as the process of choice, different types of equipment exist, suitable for performing the work.

Small equipment can be hand held or suspended.

Pedestal stand alone machines may be used, or complex robotic cells can be assembled, sometimes with multiple working heads.

Design

Design should follow a few simple rules, based on experience and common sense.

The flange width (or distance from the edge) must be sufficient to contain the interlocking button.

The minimum suggested edge distance is of 1.5 times the punch diameter from the joint center.

Spacing between joints should be adequate: a current recommendation establishes a minimum joint spacing of 2 to 3 times the button diameter.

The joints number should be sufficient to satisfy strength requirements. Tolerances and fit-up of sheet elements should be acceptable and consistent.

It is considered that the overall strength, stiffness and rigidity of an assembly depends more on the number and location of all the joints than on the strength of a single clinched joint.

Parameters

The main operating parameters of Clinching machines are stroke length and punch force.

The application of a pre-clamp force may assist in avoiding unwanted gap between sheets and in producing better results.

This is controlled by checking that the joints are fully closed after pre-clamp.

Material should be of constant thickness, clean and lightly lubricated as necessary. Too much lubricant may shorten tooling life.

Quality

Visual inspection should assure consistent shape and dimensions of the resulting buttons.

As each new production lot is started, a few destructive tests on typical sections, with precise measurement of buttons shape, thickness and diameter will alert of impending problems.

Punch and die surfaces should be inspected frequently and carefully, to perform in time required maintenance as needed.

Performance of proposed design is best verified by running and testing limited pre-production lots, by submitting the specimens to loading conditions similar to those of the intended applications.

As many of the commercial systems available depend on proprietary development, there are no reliable data on which to base universal quality standards and requirements.

Therefore the industries establish internal Standards suitable to their quality requirements.

Watch the following Video:

Clinching 18 Gauge Galvanized Paint Booth Assembly

http://www.youtube.com/watch?v=kEESEzQqpfQ

* * *

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