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Guided tooling KO's stainless steel stampings: to wring the maximum quality production from its new turret punch presses, a fabricator switches to fully guided tools to cope with the challenges of 304 stainless steel.

Custom Air-Plenums (Bean Station, TN), a world-leader in the manufacture of air handling systems for schools, trucks, and commercial buildings, fabricates stainless steel panels and requires high speed punching of different hole shapes and sizes for positioning air flow openings, wiring, and rivets.

With the recent addition of several new products, all requiring punched stainless steel cabinetry, the company set up a new punching department using a new Murata Wiedemann Motorum 2044 precision punch press.

Setting up new punching operations requires a thorough analysis of the materials to be punched, the punch presses' production capabilities, and the best tooling to achieve quality and maximum productivity. The company's first tooling option was standard 114 tooling. However, when punching stainless steel, standard 114 tooling doesn't always measure up performance-wise in today's high performance punch presses.

"We really tried to make the 114 tooling perform in our new machine, but it was just too much for the parts we had to punch," reports Eddie Williams, plant manager for Custom Air-Plenums. "We weren't getting very good edge-quality on our stainless steel parts using the 114 tooling and had to stop the machine during the part run for tool-sharpening.

"On one stainless steel part we tested, we punched two rows of 0.250" x 3.000" rectangles in 18 ga. 304 stainless steel. The part had a 0.375" web between holes and 1.000" between rows of rectangular holes. In the web area of the part, the punch often pulled the material upward and twisted it so the part had to be scrapped. And we never got more than 8,000 hits before the tool had to be sharpened. We were losing press time and parts so we acted quickly to find a better way."


Side-By-Side Testing

The company called on Mate Precision Tooling (Anoka, MN) to evaluate tool performance and make recommendations. A trial of Mate's Marathon tooling, which is a fully guided system with a good track record for high quality punching of difficult materials such as stainless steel, was recommended.


"The problems we were having with the 114 tooling went away when we substituted Mate Marathon tooling," reports Williams.

"We tried both tools side-by-side in the machine, both punching 0.250" x 3.000" rectangles in the 18 ga, stainless steel. The difference was unbelievable. The 114-punched holes had significant burrs and the web portion was twisted upward. The Marathon-punched holes were perfect. Edges were clean and smooth and the web was flat."

Tool sharpening and machine downtime were also major concerns for the company. With the introduction of the Marathon Tooling, no interruption of part runs for sharpening occurred with the tools operating well over 100,000 hits, compared to fewer than 8,000 hits for the 114 tooling.

Williams also reports that overall punching results using the Marathon tooling were equally effective. Custom Air-Plenums' parts require 0.145" holes for riveting and a variety of 3/16" and 3/32" square and round holes. Other airflow holes range in size from 3/4" x 1/4", 2" x 1/4", and 1" x 1/4". Parts are fabricated from 48" x 96" stainless steel sheets using the Marathon tooling for the parting operations.


"The Marathon tools handled all of these operations without problems," notes Williams. "Our press operators just loaded the tools in the turret, started the machine and waited until the part run was complete. After seeing the results, making the decision to convert all of our tooling to Marathon was easy."

Today's high-performance, high-speed punch presses operate well with many tooling brands and styles. However, punching difficult materials can challenge any press and pose special problems. Since most of Custom Air-Plenums' parts are punched out of stainless steel, and its Murata Wiedemann Motorum press operates at up to 500 strokes a minute, tooling must not only keep pace with these high hit rates on the very hard stainless steel material, it must last longer, provide flatness of the workpiece, and do so with dependability. Also, at these high press speeds, there is less opportunity to individually inspect each part and there is a greater chance to produce scrap if the punching process is not predictable.

Side-by-side tooling tests like the one Custom Air-Plenums set up are a good way to see the value differences in action of one style of tooling versus another.

The Marathon punching system, for example, is fully guided, uses high-speed steel punches, and spring-loaded steel strippers. The standard 114 tooling lacks these. The Marathon tooling system overcomes web pulling problems because during the punch stroke, the entire guided assembly is driven down until the stripper contacts the material. Then the punch slides down through the sleeve, with the internal springs holding the stripper firmly against the material and punches the holes. The guiding prevents any side loading or twisting forces so the punch is delivered squarely and evenly to the material surface. On the return stroke, the spring action continues to exert downward force until the punch clears the hole, at which point the entire assembly retracts to the top of the stroke. For Custom Air-Plenums, holding the material in position while it is being punched is critical to part quality.

Rigid Support

The Marathon tooling also provided rigid support of the return stroke. Most hardened tool steels are very tough in compression. However, this is not the case under tension. Micro chipping is the dominant factor in punch wear, and that occurs most often as the punch is exiting the material. With the hard stainless steel Custom Air-Plenums punched, the fully guided Marathon tooling reduced this micro chipping with an immediate increase of tool life by a factor of more than ten.

But the real improvement from the fully guided tooling feature was to the quality of the part web. That was due to the tool's fitted metal stripper and spring stripping action. Since the stripper is metal, it is very rigid and exerts a firm and consistent pressure over a relatively wide surface area around the punched hole. There is less material flow and lateral deformation, which means the material is less likely to close in around the punch after it has penetrated or create friction as it exits the hole. The pressure of the stripper exerted during the punching cycle flattens the sheet, which results in better flatness in the workpiece. This is especially important with the stainless steel Custom Air-Plenums works with because it has a tendency to adhere to the punch, causing material webs to pull out of their desired shape. These problems are eliminated with the Mate Marathon system.

The side-by-side tooling performance comparison also reveals how the Marathon guiding features delivers the tool at a perfectly square angle to the material. The springs allow the punch to withdraw at the same angle, reducing tool deflection and wear. The punch holder virtually eliminates turret press wear because the holder transmits less pressure to the turret bore than non-guided tools. All side-loading and rubbing are contained within the punch holder, rather than in the turret bore.

Quick Set-Up

Another benefit of Marathon tooling for Custom Air-Plenums is tool set-up. A single unit containing all tool components (holder, punch, die, and stripper) is easily loaded in the machine turret. After sharpening, punch length is easily reset with a built-in adjustment feature. Sharpening may take a little longer because of the harder tool steel, but this is more than compensated for by fewer sharpenings. Mate Precision Tooling or Circle 205 for more information
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Publication:Modern Applications News
Date:May 1, 2004
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