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Application Report

More than 14 x D -
and still process reliable

Reliable and dimensionally accurate with more than 14 x D overhang length. Direct milling brings a significant reduction in process costs compared to die-sinking EDM.

Siebenwurst has redefined the limits of rib milling -
with the support of Moldino
Siebenwurst was faced with a decision: Should the complex rib structure, up to 56 mm deep, be die-sunk or milled into the mold plate, which measures around four square meters? The high-tech mold maker from Dietfurt in Altmühltal opted for the latter, saving more than half the time and a lot of money, thanks in part to the mold and process technology support from Moldino.

in cooperation with

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Maximilian Siebenwurst (left) and Xaver Ferstl from Siebenwurst together with Christian Gräml from MOLDINO.

The production of negative ribs is part of the daily plastic mold-making business. However, these are still usually die-sink molded in a tried and tested manner, even though the costs are higher than milling. This is because many shy away from the risk. At Christian Karl Siebenwurst GmbH & Co. KG in Dietfurt an der Altmühl, north of Ingolstadt, took the plunge into rib milling some time ago and has not regretted it. The high-tech company from the Upper Palatinate is not only in the premier league regarding toolmaking but also a development partner, lightweight construction specialist, and full-service provider. "Although we are still operating on two tracks, rib milling is becoming increasingly important to us. Numerous rib fields distributed over a 2,200 mm x 1,900 mm molding plate - that was a bit of a challenge," says Xaver Ferstl.

"In addition, we had to mill groove depths of 56 mm for the most part, which meant a overhang length of over 14 x D." Xaver Ferstl is Head of Tool Management at Siebenwurst. He is referring to the large-format mold plate of the injection mold for producing a battery tray for electrically powered commercial vehicles. A battery case is being developed as part of the "HyFiVe" research campus project led by the Open Hybrid LabFactory e. V. in Wolfsburg (funded by the German Federal Ministry of Education and Research BMBF).

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Milled to perfection: Xaver Ferstl (left) and Maximilian Siebenwurst next to the mold plate clamped in the machine with the finished milled, two-layer ribbed structure.

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With the 5 mm free grinding, the needed overhang length of over 14 x D was achieved. The two-layer ribbed structure was thoroughly milled in just 353 hours.

Siebenwurst is one of 15 project partners from industry and academia who play a vital role in implementing HyFiVe's tooling. The project aims to develop a modular, scalable battery carrier housing in a plastic-metal hybrid design. This lightweight design is intended for use in electric vehicles. It also includes developing all processes for large-scale production with a wide range of variants.

And the geometry of this battery tray is something special. "The frame structure is a combination of aluminum profiles with a back-injected rib structure," explains Maximilian Siebenwurst, a member of the extended management team. The son of Christian Siebenwurst, who, along with his brother Roland, is the fourth generation to run the family business, Maximilian has been involved in the HyFiVe tool project as head of project management and design. These U-profiles provide mechanical stability and prevent buckling. In the event of an accident, they also protect the battery from point impact forces along its entire length. The resulting leakage of electrolytes is a major cause of fire. For this reason, two rib structures were combined on top of each other in what Siebenwurst calls a locally redundant structural design: "The ribbed structure is designed to absorb a large portion of the impact energy."

Rib milling workshop triggered the idea

For more than 15 years, cutting tools from the Japanese manufacturer Moldino Tool Engineering (with European headquarters in Hilden near Duesseldorf) have been used in many places in Dietfurt: mainly carbide cutters, but also CBN, drilling tools, and indexable tools. The high level of satisfaction with the tools, which were previously sold under the name MMC Hitachi Tool, goes hand in hand with the technical support provided by Christian Gräml, a process optimizer, as Moldino's application engineers call themselves, who has been looking after Siebenwurst for several years. In addition to intensive technical process support, he regularly organizes on-site workshops. Christian Gräml always works according to the Production50 method (P50) developed by MOLDINO. Some time ago, rib milling was also the subject of a several-hour workshop. Since then, everyone involved has known how to successfully implement this regarding CAM, machine, tool selection, and cutting data. The profitability calculation always accompanying P50 was a significant factor in moving away from die-sinking ribs at Siebenwurst to milling instead.

EDM was also an option

Milling the two-layer rib structure accurately was challenging despite the company's previous experience. As a result, the company initially considered EDM and outsourcing the work. "Because the size of the workpiece would not have allowed eroding even on the largest EDM machine, the OPS Ingersoll Gantry 1400, that we have here in our company," says Maximilian Siebenwurst. As a result, quotes for eroding were obtained. They were in the mid-five-figure range, including electrode production. "At that point, we said we would mill the ribs despite the project's challenges. Especially since Siebenwurst's 5-axis Parpas XS is a gantry milling machine on which the mold plate can be easily clamped thanks to its several meters of vertical movement. Despite its impressive size, equivalent to that of a small terraced house, the Parpas XS with its HSK 63 interface mills so precisely that the +/- 0.01 mm tolerance required for the ribs could be achieved, Xaver Ferstl was confident in milling the ribs. At this point, the names MOLDINO and Christian Gräml come into play.

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Rib milling with more than 14 x D overhang length: The preset 4 mm Moldino ETMP-4040-40-10 torus cutting tool in the HSK-63 tool holder. The additional 5 mm long free grinding can be seen above the convexity area.

The ETMP torus cutter should handle everything

Milling specialist Xaver Ferstl called Christian Gräml, who immediately offered to help. Together, they determined the basic procedure, i.e., where to start milling the ribs on the panel and which steps to follow. The appropriate cutting tools were quickly selected. Strictly speaking, only one cutter was available, the ETMP-4040, with a diameter of 4 mm, but in two different lengths of 32 mm and 40 mm. The ETMP is a 4-flute toroidal tool with 1 degree of convexity. Thanks to the TH45+ nano-coating applied by the PVD process, this tool can machine hardened material up to 60 HRC combined with a very long tool life.

Even though the material was non-prehardened tempered steel 1.2738, the long tool life and the resulting process reliability were essential. "Another advantage is that the cutting edge of the ETMP has an oval cut," explains Christian Gräml. "Two cutting edges are on the diameter, and two are below the diameter, so the tool is calm during milling, and the vibrations are low, especially with the long overhang lengths. The tool is much more relaxed in the corner area.". This is particularly important with high projection lengths. "Here, we had to mill with an overhang of a good 14 x D. In other words, grooves up to 56 mm deep with a 4 mm diameter cutter, like walking a tightrope.".

High feed rates

The NC programming was done with Tebis CAM, which Siebenwurst also uses to generate the milling programs (in addition to Work NC). The CAM software has special roughing and finishing functions for grooves and opposing ribs, which can also be automated. That is a ready-made strategy in Tebis CAM (R-Plan). "Here, the final contour is milled to zero immediately, without measuring," says Xaver Ferstl, describing the process. The strategy also smooths the surface in the intersection area, where larger surfaces occur. "Among other things, this optimizes the dynamics of the machine to achieve the necessary path feeds. The cutter makes flat, Z-constant movements along the contour."

Process optimizer Christian Gräml then specified a precise lateral infeed ae of 1.6 mm. For the less deep ribs, the slightly shorter ETMP was used. Moldino suggested the standard cutting conditions.

However, the 56 mm deep ribs required some fine-tuning regarding speed, feed rate, and depth infeed. "Our goal was to ensure that each of these segments is milled in five hours," explains Xaver Ferstl. That was the previously determined tool life at which no wear was visible on the cutting edge of the long ETMP. The goal was to replace a sister tool after each rib segment had been milled to the final dimension. Therefore, a series of tests were done on a test workpiece in a segment to determine this machining time. With a feed rate of 2.3 m/min at just under 10,000 rpm and a depth infeed ap of 0.1 mm, these five hours per cavity were achieved precisely.

5-mm-free-grinding was the solution

But how would that work? At 40 mm, the longest available ETMP with a convexity range of 34 mm plus 6 mm cutting edge length was too short for this groove depth. Xaver Ferstl: "The groove has a convexity of 1 degree and a width of 4.2 mm at the bottom. We would have been zero to zero with the taper of the cutter."

We went back and forth on how to solve the problem. "We then decided to extend the 1-degree taper of the cutter by 5 mm on a grinding machine here at the company," he says, describing the pragmatic approach that ultimately led to success. So, the 6mm shank of the cutter was reduced in length by 5mm to match the convexity of the diameter. "That way, we could get into the rib and still have about 1/10 mm of clearance. We needed that to be able to swing safely along the shoulder. When milling ribs in general, and for this project in particular, the machining process must be as effective as possible", emphasizes Christian Gräml: "We, therefore, opted for a rough finish and milled from solid material, which is no problem with the ETMP cutting tool." Further optimization involved the rib geometries. In cooperation with the in-house design department, the contours were modified in some places to make them easier to mill.

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The NC programming was done with Tebis CAM, which has special roughing and finishing functions for grooves and opposing ribs. This project involved milling directly from the solid to the finished dimensions using a combined roughing and finishing strategy.

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This rib-segment test part was tested until reaching five hours of machining time. When the segment is milled to its final dimensions, the cutting tool is changed.

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Almost like new: The Moldino ETMP-4040-40-10 4 mm torus cutting tool on the tool presetting and measuring device has been running for five hours. The image shows that the oval grinding of the cutting edge has yet to show any wear.

"Regarding process reliability, it was now guaranteed that each segment could be milled with a new cutting tool without reworking. All ribs were milled dry with air cooling only."

Project success thanks to new milling cutters

Christian Gräml used the abovementioned Production50 method for this project. The basic idea of P50 is to analyze existing processes with the customer to gain a new perspective on the production process. That includes profitability calculations that document the added value of a changeover.

"With the HyFiVe tool, the advantages of rib milling were particularly great," says Maximilian Siebenwurst, head of project management and product design. "Choosing rib milling instead of die-sinking EDM resulted in potential cost savings of up to 30% in this area. In addition, there would have been the cost of the much higher polishing effort."

All in all, the rib milling project was a complete success, concludes Xaver Ferstl. This also applies to the high quality of the milled side surfaces, which is extremely important for the de-molding process. He is still impressed by the low-vibration operation of the 4 mm ETMP cutting tool despite the enormous overhang length of over 14 x D. "The fact that it worked so smoothly is a highlight in itself," he points out. "And that with infeed and speed. Let's remember we only needed five hours per ribbed panel."

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The ETMP-TH is available with diameters ranging from
2 - 6 mm and with usable lengths of 6, 8 & 10xDC.
For this application, a 5 mm long clearance grinding was
added above the convexity area (not visible in this picture).

Cooperation as the key to success

Christian Karl Siebenwurst

The main focus of the family-owned company, which has headquarters in Dietfurt in Altmühltal, is designing and manufacturing complex injection molds. This also includes complete solutions for unique processes for producing large and complex workpieces, focusing on lightweight solutions. The specialists at Siebenwurst plan and manufacture injection molding and press tools for various methods and technologies, such as Organosheets, hybrid processes, hydroforming, RIM RTM, and many more. They also build stamping and forming tools. The focus is primarily on the automotive sector, although customers are also increasingly coming from other sectors, such as the environment or energy. The entire process chain is covered, from the design model to try-out and small series production, to which the in-house injection molding shop contributes. In 2020, Siebenwurst expanded its portfolio to include unique machine construction. Around 150 tools are manufactured annually at the Dietfurt site, which employs 300 people. In lightweight construction, the company collaborates with numerous universities and other research institutions, such as in the HyFiVe project. Last but not least, Siebenwurst's commitment to process optimization and innovative technologies has earned it the 'Toolmaker of the Year' award three times (2009, 2012, 2018). This is awarded annually in four categories by the Machine Tool Laboratory WZL at RWTH Aachen University and the Fraunhofer Institute for Production Technology IPT.

www.siebenwurst.com

Contact us and take advantage of our strengths, we look forward to your challenge.

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