Skeleton-free punching It was while jogging that Stefan Büttner and Wolfgang Laib developed a concept for skeleton-free punching, a concept that actually delivers what it promises.

When the scrap skeleton is eliminated, then profitable parts fill up the sheet right to the edge. That increases the number of parts per sheet by at least ten percent.

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Sheet skeletons eliminated after 400 kilometers

It wasn’t at the drawing board, but rather while jogging, that Stefan Büttner and Wolfgang Laib came up with a bright idea for punching that is truly free of sheet skeletons.

Running isn’t just healthy and good for the physique. It is known to stimulate the intellect, as well. That’s something that Stefan Büttner, product manager for punching and combination machines, and Wolfgang Laib, who develops punching modules at TRUMPF, can readily confirm. Away from the office, during their weekly rounds of jogging, these two developed — over an estimated four hundred kilometers and literally step by step — a concept for punching without a sheet skeleton, and one that really lives up to the name.

In the past, the punching process made it necessary to leave an L-shaped sheet metal edge, in spite of perfect part nesting. This truly bothered these two engineers and recreational joggers who have been punching specialists at TRUMPF for twenty years. “Efficient material use and process reliability were both in the foreground in our thoughts,” explained Büttner. “Losing time when the sheet skeleton gets caught was annoying to our customers — as was the amount of operator time needed to extract this unwieldy waste,” adds Laib.

Many steps toward the objective

“Even when we’re off work, we just can’t seem to get away from shop talk,” says Büttner with a grin. And so, the two worked on skeleton-free punching lap by lap during their runs in the forest. The result was an overall concept that brought together many small ideas. The sheet skeleton can now be entirely eliminated when using the TruPunch 3000 and the TruPunch 5000. “Nesting parts so as to make individual cuts do double duty is nothing new,” says Büttner, “but now we are filling the entire sheet, right up to the edge, with good parts, and that raises average yields by at least ten percent.” One customer involved in testing found that the share could be raised by more than fifty percent while, at the same time, reducing waste by more than twenty percent.

The wheat from the chaff…

“Essentially we work a sheet of metal just like you eat a milk chocolate bar — one row at a time. This means that we punch good parts and, in parallel to that, we separate out the waste areas lying between them — instead of leaving them there until the entire bar has been nibbled away,” Laib explains. The blanks are ejected immediately, already sorted into good parts and waste. This is possible because the machine can work with the parts chute open. And a divider below that chute separates the good parts from the waste.

“To make sure that ejection is successful — without any parts getting caught because their center of gravity keeps them from spontaneously tipping onto the parts chute — we came up with a separate stripper with about one millimeter milled away on one side,” Büttner explains. After the punching cycle, the parts — both good ones and waste — can be put into the correct tipping position with the rotating single punch head in the TruPunch. The raised surface of the stripper grasps the part to be ejected and the milled area makes it possible to move the metal sheet away. The part can be rotated on the chute and thus is certain to fall. It works even for small pieces at the edge of the sheet.

Energy efficient

“Workpiece clamps that can be opened individually, in conjunction with the rotation of parts, also make it possible to eject the residual strip so that ultimately, there’s nothing left of the sheet of metal,” summarizes Laib. From the overall ecological viewpoint, too, the concept devised by these two “out-of-the-box” thinkers is an absolute knock-out. “If we remember that the energy needed to manufacture a sheet of metal is some thirty to eighty times more than the energy expended in an hour of operation, then the materials savings take on an entirely different meaning. The process cannot be implemented on combination units — at least not yet. But that might well be in the cards since Stefan Büttner and Wolfgang Laib are always running across great new ideas.


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This article was first published in spring 2011.

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Skeleton-free punching is possible when using the TruPunch 3000 and the TruPunch 5000.

Once punching has been carried out, the rotating single punching head in the TruPunch puts the good parts and the waste in the right positions for tipping and removal.

Each part is reliably discharged through the parts flap – even the very last bit of the gripping edge.

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