Insights into a rapidly developing technology

Extremely high demands are placed on tools for punching, stamping and forming sheet metals. In some cases, accuracies of between 1 µm and 2 µm are required during manufacturing. The level of challenge increases drastically the larger the tool and the thinner the sheet. The stamping plates for the sheet-metal parts in fuel cell bipolar plates are a prime example. Bipolar plates are thin structures made from welded sheet-metal half shells that enclose the filigree flow fields. They are built up one after another in many layers, with the membrane electrode assemblies sandwiched in between, to produce the final stack.

Bipolar plates for fuel cells that will be used in automotive applications commonly consist of stamped, punched sheet-metal half shells that are welded together to produce hollow pieces. The manufacture of suitable stamping and punching tools is a constraining factor given the current technology available. Thinner sheets would indeed reduce the weight of fuel cells. However, as the material becomes thinner, the die clearance becomes narrower and the geometry must be more accurate. The accuracies asked of stamping and punching tools and presses are therefore extremely demanding.

Interest is focused on the development of a suitable process chain for manufacturing stamping and punching tools for the production of sheet-metal parts. Key points are the demands on the steel for the tools, the computer aided design/manufacturing software (CAD/CAM software), the necessary micro-milling tools, the properties of the machine tool, the lubrication and cooling of the milling cutters as well as the metrological testing and documentation of quality.

Companies working in this area include, for example, Hufschmied, MHT, Röders, Open Mind, Voestalpine and Zeiss. Together they outlined the current state of development as part of a seminar with more than 50 attendees. The results presented at the event are not only relevant for those involved with bipolar plates, but also for other sectors such as micro-production, precision engineering, medical technology or aerospace.

Ultra-hard steel: Böhler K888 Matrix
The stamping tool must have an extremely high dimensional accuracy, excellent wear resistance and low adhesion tendency in order to economically produce the extremely fine structures present in bipolar plates. Another prerequisite is excellent machinability. This presumes a low proportion of primary carbides in a hard matrix structure (matrix steel). Furthermore, the carbides should only be very small and distributed evenly across the whole cross section since coarse examples can break up during cutting and may cause surface imperfections. This is why steel produced from powder metal is used.

The Böhler K888 Matrix was chosen which is a material with a maximum carbide proportion of less than 2 percent. This is supplied in an annealed condition with a Brinell hardness of under 280 HB and achieves a Rockwell hardness of 63 +1 HRC after hardening at temperatures between 1,070 °C and 1,120 °C. This material thus demonstrates excellent wear resistance even in comparison with high-carbide materials.

Machining trials by Hufschmied have shown that the material is still extremely workable and can achieve extremely high surface qualities. It also responds well to coating which in turn leads to an increased service life.

CAD/CAM software
A suitable numerical control (NC) program is essential for optimal component quality. To create these NC programs, Open Mind offers a CAD/CAM system called hyperMILL which meets all requirements. The software calculates the tool paths with utmost accuracy and thus provides NC data with the appropriate exactness. Nevertheless, several factors need to be borne in mind: To fully take into account the topology of the component in order to calculate the tool paths, geometric features such as sharp edges, recesses and the condition of the surface transitions must be analyzed and identified. This information is then fed into the calculations and roughly controls the point distribution in the tool path.

In addition, further optimization can be carried out, for instance the adaptation of the feed. This allows the milling tool to machine the component at a constant feed rate. The “soft overlap” option prevents visible transitions by using various milling tools or strategies and reduces the time spent on manual finishing to virtually zero.

It is also important to link geometrically identical structures within a component that are either automatically or manually identified or defined. The appropriate tool paths that were initially created for an individual area can then be transposed to the previously identified or manually defined positions and connected fully automatically using the transformation function. This removes the need for unnecessary movements. This process allows calculation times in the CAM system to be significantly reduced.

Milling machine requirements
The machining of dies for bipolar plates is characterized by high material hardness, small tools with diameters of well under 1 millimeter as well as stringent demands on surface quality and accuracies down to the 1-µm range. What is more, the small contours require long running times which presuppose a very high long-term thermal stability of the machine tool.

Röders machine tools set themselves apart thanks, among other things, to their frictionless direct drives, highly rigid roller guideways, frictionless weight compensation of the Z-axis, high-speed precision spindles and highly accurate tool measurement. A particular feature is the 32-kHz sampling frequency in all control loops which enables the rapid correction of even the smallest deviations. Another key element is the sophisticated temperature management system which keeps the medium that circulates through all the main machine components at a stable temperature to within ± 0.1 K. This allows tolerances to be reliably maintained in the lower micrometer range.

The Hufschmied tools from the Bumble-Bi series used to machine various sections of the demonstrator (50 mm x 40 mm) on the Röders system along with relevant machining times, Source: Röders/Hufschmied

Bumble-Bi micro-tools from Hufschmied
The task of machining stamping tools for bipolar plates presents milling tools with a particular challenge. This is due to the hardness of the material being cut and the long program running time which in some cases lasts well over 100 hours. What is more, the required accuracies allow only minimal wear. To meet this challenge, Hufschmied developed the specially designed Bumble-Bi series of micro-tools. These include high-feed milling cutters for roughing as well as torus cutters, ball cutters and flat ball cutters. The latter are a hybridized version of a torus cutter and a ball cutter. All tools receive a physical vapor deposition or PVD coating, creating extremely smooth surfaces which enable temperature to be well managed. The milling tools used to make the demonstrator are summarized in a table alongside their operating parameters.

The entire sleeve of the MHT medium distributor encloses the tool holder without touching it or rotating with it. Air and lubricant are fed underneath the spindle via the docking station.

Optimal lubrication with the MHT medium distributor
When it comes to cutting processes, the right combination of cooling, lubrication and chip removal from the working area is crucial. The MHT medium distributor enables efficiency while also saving on energy and cost. The key element is a conical sleeve, which is attached to the tool holder and is exchanged with it during a tool change, yet does not rotate with the milling cutter. The sleeve is docked underneath the spindle and from there supplies it with compressed air and lubricant.

Most of the cooling and cleaning work is performed by the compressed air that is sprayed out of the nozzles arranged in a ring on the lower edge of the sleeve. The powerful air jet immediately removes chips and their heat content from the milling cutter and the workpiece. The lubricant, made from carefully selected hydrocarbons, is fed through in extremely low quantities (2 to 10 milliliters an hour). This is sufficient to ensure optimal lubrication for cutting operations. Heat build-up when hard cutting is reduced by around 50 percent. Significant advantages are much longer lifespans for tools, increased cutting performance of the machine and improved workpiece surfaces.

Measuring equipment and quality control
The manufacturing of bipolar plate stamping tools involves the use of milling cutters with diameters as small as 0.2 mm. For quality control purposes, it is necessary to measure extremely small and narrow contour areas, for example on the sides of the flow channels and on the cut edges. As this means measurements as low as single micrometers, the measurement uncertainty of the measuring system used should be 10 times better than the manufacturing tolerances being examined. This is something that few coordinate-measuring machines are able to achieve.

So that these measuring points can be expertly captured and without excessive effort, the task was given to a Zeiss DotScan optical sensor with a measuring rate of up to 1,000 measuring points per second which was moved with an articulated unit in three different angular positions during scanning.

Measurement of the demonstrator using a Zeiss DotScan optical sensor with a mean percentage error of 1.8 µm + L/350. To facilitate better measurement of the sides, the sensor was moved with an RDS articulated unit and on a Zeiss Contura coordinate-measuring machine during scanning. Image: Zeiss

Results
The presented results (spread ±3 µm) prove the efficiency of the process chain outlined in this article. By selecting the correct components and choosing the right methods, it is possible to achieve a high degree of reliability even when machining high-strength or very hard tool steels. It also allows high quality standards to be met, though this requires all aspects to be considered in detail.

Author: Klaus Vollrath