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issue: May 2003 APPLIANCE European Edition

Assembly & Fastening
The Advantages of Clinching


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by Lisa Bonnema, Managing Editor

One fastening technique that is rapidly gaining acceptance among appliance producers is clinching, the process of creating a joint via punching and squeezing, without the need for additional fasteners such as screws, rivets, or pins.

According to ATTEXOR, Inc., clinching offers several advantages over other fastening methods, specifically spot welding. “While it is difficult to tell whether a spot weld is a good one or whether a screw thread has been stripped, the quality of a clinched joint can be controlled at any point in time without destroying or disturbing the assembled structure,” explains Dr. Hans Bergkvist, president of ATTEXOR Inc. (Springfield, MA, U.S.) and ATTEXOR Tools (Ecublens/Lausanne, Switzerland). “This has been one of the most important arguments in favor of clinching, as well as the fact that clinching works perfectly on pre-coated, enameled, and galvanized material found in the appliance industry without destroying the surface finish.”

One of the main advantages of clinching over spot welding is that clinching can easily join material couples that are difficult to weld, Dr. Bergkvist explains. “It is a well-known problem that the punches tend to stick in aluminum, particularly in larger thicknesses, which can be a major difficulty when low-weight aluminum profiles are to be joined with sheet metal.

Various ways of solving this drawback have been tried, such as using a 50/50 alcohol and water spray on the punch side. This mixture has the advantage of evaporating without leaving any traces that could have a negative influence on subsequent painting operations. However, installing and maintaining the spray equipment means additional complications and costs.”

As an alternative, ATTEXOR has developed patented punch geometries that are said to significantly reduce the sticking problems. These punch geometries use double-stroke clinching, the company’s proprietary clinching technology.

Dr. Bergkvist also believes that clinching equipment such as his POT CLINCH® Tiger line and the SPOT CLINCH® Grip line will replace stitchfolding machines in applications like vending machines, commercial refrigerators, and washing machines. One of the main reasons for this, he says, is operator safety.

“ In the stitchfolding process, tabs are cut out of the overlapping material sheets, and they are then folded back to give a staple-like joint where the staples are made from the base material itself. While this process is very fast, it presents additional drawbacks over the risk for hand injuries,” Dr. Bergkvist explains. “Operating quickly means that the tool often gets moved laterally before the punches have been fully retracted and as a result, they break. Even if replacing them is rapid, the cost for punches can run up to a considerable amount. Additionally, a stitchfold joint is not foam tight, requiring tape masking of the joined material lips before foam is injected between the outer shell of an appliance and the inner lining.”

Steve Sawdon, president of BTM Corp. (Marysville, MI, U.S.) agrees that recent advancements in clinching have made it the ideal fastening method in most cases. Specifically, new tooling materials have allowed his company to further improve its Tog-L-LOC® clinching system.

One example of the system’s expanded capabilities was a microwave application performed by BTM’s German distributor. “The microwave was joined together with the Tog-L-LOC clinching process even though it had some stainless steel components in its wave guide and body,” he says. “That is fairly new because stainless steel has traditionally been difficult to join. However, with the advent of some new tool steels, we’ve been able to accomplish sufficient joint strength, and the tooling is durable enough that it can join this harder stainless steel together and can give the customer acceptable tool life, even in the range of 30,000-50,000 cycles per tool.”

According to Mr. Sawdon, because of the tougher tooling, BTM can now join most metals, including ferritic stainless steel and austenitic stainless steel, as well as more ductile metals such as aluminum, copper, brass, and coated steels.

Also from the May 2003 APPLIANCE Magazine European Edition:
Assembly & Fastening: Putting It Together

 

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