Ceramic Machining Machine Sintered Ceramics with High Precision in Five Axes

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Ceramics, due to their high hardness, could previously almost only be machined using grinding techniques. A ceramics specialist is now milling even sintered aluminum oxide ceramics with a hardness of 1800 HV to micrometer precision.

"We develop high-precision special ceramic components for use in a variety of demanding applications in industry, research, or medical technology," says Dr.-Ing. Torsten Weiß, Managing Director of BCE Special Ceramics GmbH in Mannheim (Germany). The company, founded in 1986, provides customized solutions made from ceramic materials, primarily oxide ceramics such as aluminum oxide (Al₂O₃) and zirconium oxide (ZrO₂), or mixed oxides like ATZ (Alumina Toughened Zirconia) and ZTA (Zirconia Toughened Alumina). Occasionally, nitride or carbide materials as well as special materials are also used.

In doing so, the developers rely on their extensive material expertise as well as a wide range of processes and equipment for the production and processing of ceramics. The focus is on providing consultation in the collaborative development of customized solutions. This also includes the production of individual pieces and prototypes in a relatively short time. Small to medium-sized series are also produced, while large-scale production is later handed over to specialized manufacturers.

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In the first step, the material question is clarified.

Dr.-Ing Torsten Prescher, Application Developer and Sales Manager, BCE Special Ceramics

In addition to the fundamental material properties, other factors need to be clarified, such as potential material pairings with additional ceramic components or dissimilar materials like metal or plastic. For example, in certain components exposed to ionizing radiation, conductivity is increased just enough through suitable additives to prevent static charges from building up. In other cases, the focus is simply on identifying suitable material pairings for use by the customer.

Once these initial questions are clarified, production begins: At BCE, a compact is produced in all cases, on which further processing steps are carried out. The ceramic powder used for this is mixed with a few percent of an organic binder, so the ceramic particles have sufficient strength after pressing.

To achieve the strength required for initial processing, the compact is cold isostatically pressed at a pressure of around 2,000 bar in a liquid-tight enclosure.

Dr.-Ing. Torsten Weiß, Managing Director of BCE Special Ceramics GmbH

The resulting "green body" already has a certain strength but can still be easily machined, for example, by milling, drilling, grinding, or cutting threads. However, dimensions must be given with a significant oversize of around 20 percent linearly, as the component shrinks by this amount in all three dimensions during the subsequent sintering process. Mastering the shrinkage behavior during sintering requires extensive expertise and experience. If the "as-fired" dimensions of the sintered ceramic are insufficient, additional subsequent mechanical processing through grinding is possible.

This significantly impacts the overall costs, as the component attains the typical hardness of fired ceramics after sintering and is consequently difficult to machine.

Dr.-Ing. Torsten Weiß, Managing Director of BCE Special Ceramics GmbH

During the firing process, the binder burns off, while the ceramic grains fuse into a highly dense structure with only about 0.5 percent porosity. Sintering takes place at temperatures between 1300 and 1400 degrees Celsius (approx. between 2370 and 2550 degrees Fahrenheit), and even up to 1700 to 2000 degrees Celsius (approx. 3090 to 3630 degrees Fahrenheit) for aluminum oxide. The components are then characterized by high strength and hardness as well as temperature resistance but can also withstand extreme cold down to absolute zero (-273.15 degrees Celsius, -523.67 degrees Fahrenheit). They are corrosion-resistant and resistant to most chemicals.

One of their main advantages—high hardness of up to 1800 HV and high wear resistance—has a significant drawback when precise machining after sintering is required. Until now, this was almost exclusively possible through grinding processes. Advanced technologies are available for this. However, milling to produce complex three-dimensional geometries or freeform surfaces was hardly possible due to a lack of suitable machines or tools. Consequently, the design of workpiece geometries was limited. Otherwise, extreme manufacturing costs were to be expected.

While searching for suitable technologies to bridge this gap, we spoke with familiar companies that already had experience machining ceramics with Röders milling machines.

Dr.-Ing Torsten Prescher, Application Developer and Sales Manager, BCE Special Ceramics

For milling green bodies, systems from competitors had been used so far, but they were not suitable for the high-precision five-axis machining of sintered ceramics.

Systems Enable Maximum Contour Accuracy

Initial interactions with Röders immediately made a positive impression. They were particularly impressed by the fact that key components of the technology, including the control system, are developed and manufactured with the highest precision by Röders. The result is systems that enable maximum contour accuracy. For example, they allow drilling with roundness deviations of just 1 micrometer. Furthermore, they are robust enough to withstand the extreme stresses involved in machining ceramics. This is made possible by the use of wear-free direct drives, durable linear guides, and efficient seals that effectively prevent the penetration of highly abrasive ceramic dust or grinding emulsion.

It was important, according to Dr. Prescher, that the machine could be used in a variety of ways and that, depending on the component or operator preference, it could be determined whether the control would be fed with CAD/CAM data or whether quick manual programs could be created. After all, both approaches have their specific advantages.

For this reason, the system was also equipped with a dressing spindle, process monitoring via structure-borne sound, and the highly advanced grinding cycles of the Röders control system. For process cooling and lubrication, internal emulsion supply with up to 80 bar is used, which is essential, especially when working with very small tools. Additionally convincing was the fact that there were references from the use of around 50 systems already employed for various ceramic machining tasks by different users.

Another key point, given the extraordinary hardness of the ceramics, was the procurement of suitable tools.

The operation of the system is so simple and intuitive that we only needed two weeks of training when it was delivered three years ago to be able to work with it and program it directly on the control system.

Fabian Henning,Machine operator, BCE Special Ceramics

Originally, he worked with Siemens controls, but the transition was easy. Since he frequently processes components with simpler 2 to 2 ½ D geometries, he programs a relatively large number of NC sequences directly on the machine. Such tasks can be realized very easily with the Röders control system. This allows him to create NC programs for milling or grinding within a short time. For grinding operations, he uses predefined cycles in the control system. These can be simply applied over the available or directly constructed 2D CAD contours on the control system. The parameters required for this are directly retrieved from the processing database. In cases where more complex NC programs, such as for freeform surfaces, are needed, he uses the CAM system VISI. This enables the quick creation of corresponding three- and five-axis programs.

The reliability of the machine and the high responsiveness of the service are also commendable. For example, callbacks usually occur within 1 to 2 hours. In the three years of using the machine, there has only been one instance of a real failure, caused by a defective sensor. In that case, the technician arrived the very next day with the necessary parts, allowing work to quickly resume.

"The decision for the machine from Röders has proven to be an important and correct step to expand BCE's machining capabilities while significantly reducing production costs," says Dr. Weiß with satisfaction.

*Klaus Vollrath is a freelance author

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