Milling ceramics with the ductile cutting mode becomes more efficient the smaller the parts and the smaller the drills and grooving are.
Since 2021 Kern Microtechnik GmbH’s job shop manufactures different ceramics parts which are being used, for example in analytical applications. The ductile cutting mode, developed by Kern, plays a vital role. Engineers and technicians succeeded together with the customer in generating a reliable process which provides highest quality and zero rejects.
Many different industries, such as analytics, chemical and waveguides have been waiting for this just like manufacturers of highly precise parts such as for measurement devices, jewellry and watch making and satellite technology and many others. It’s all about reliable manufacturing of technical ceramics like silicon carbide, aluminum oxide etc. The extreme hardness, stiffness, heat and chemical resistance of these materials and their excellent thermal connectivity make ceramics a very interesting material for many different applications.
However, the many advantages pose a challenge at the same time: Manufacturing is very demanding. Also, because ceramics products need to be no less than 100% perfect. Cracks, breakoffs, or damaged material result in a failure of the part. Unlike metal, there is no protection to limit the spread of a crack. Furthermore, the parts need to be manufactured in highest dimensional accuracy.
Lapping, honing, and grinding have been established as a traditional manufacturing process. However, this strongly limits the manufacturable shapes. Also, reliability becomes an issue as soon as contours and outlines need to be cut. This is where the new ductile cutting mode, developed by Kern engineers, comes into place. Special milling tools, with a geometrically defined cutting edge remove material from brittle ceramics with flying chips.
Manufacturing ceramics with highest quality and a reliable process
Alexander Stauder, head of applications, explains: „We gain many advantages with this process. Most important are the excellent quality in terms of accuracy and surface quality and the reliability of the process.” Depending on the quality of the ceramics, roughnesses Ra/Sa in the range of a few 10 nm and accuracies in the lower micrometer range can be achieved.
Often it is possible to cut machining times considerably, which results in more efficiency. However, there is a rule of thumb: the larger the parts and respectively the drills and grooves are, the more it makes sense to do the first removal of material with grinding.
Finishing is then the only thing remaining for milling, where tools are sometimes more costly. The smaller the parts and respectively the drills and grooves are, the more productive it becomes to directly work with the ductile cutting mode.
For either process, a reliable machine with highest accuracies is essential. A high stiffness and running smoothness, a spindle with a high RPM and persistence against the abrasive ceramic removals are prerequisites for working with either ductile cutting mode or grinding.
Many requirements for the machining center
The Kern Micro HD is very well suited for these requirements. With the smart design of the axes the mechanics of the high-end five-axis machining center are well protected against the dust resulting from removing ceramics. The design of the machine as well as the unique micro-gap hydrostatics provide an extreme stiffness. Together with the 15 kW high-speed spindle, RPM up to 42.000 min-1 and an HSK 40 interface, the machining center ensures highest running smoothness and efficient removal of material.
The Kern Micro HD is a master of jig grinding as well as of milling with the ductile cutting mode. It is possible to first do the grinding and then switch to milling without re-clamping. Less time and more accuracy are only two advantages.
Of course, there are more factors relevant for the success. Besides the reliable five-axis machining center, communication is the key: „Nobody knows the part to be made of ceramics better than our customer, who is very often also the developer of the part. On the other side nobody knows the machine better than we do in our applications department and in our job shop. This knowledge needs to be combined.“
Just in this manner the first projects were already successfully realized. The percentage of grinding and ductile cutting vary. For example, since parts in the watch and jewellry industry are always very small and demanding usually milling is the way to go.
Reality: Zero Rejects
Kern recently machined different parts being used in analytical applications. These parts used to be grinded in another job shop. Stauder says: „The customer told us that small inaccuracies and tiny chamfers often led to the failure of an entire assembly group. We were able to eliminate these failures when changing the process in our job shop in Murnau.”
The entire process is done in eight steps – from powder to the final part – in house. The final steps are on a Micro HD. The first removal of the material takes place with jig grinding with mounted points. After this – and this is crucial for the success of the process – the grand finish comes with the ductile cutting mode. Together with the customer, Kern’s application team developed extensive tests to find the best medium chip thickness which is necessary to have the clean cut. “It’s relevant to meet the tight process“, explains Stauder, „otherwise the quality cannot be kept up and as a result there’s a high wear and tear rate of expensive tools.
Once the parameters for working with this unique ceramic material were defined it was all about finding a 100% reliable machining process. One task that the Kern Micro HD in Murnau, Bavaria, Germany, fulfills perfectly well today. The project has been running for over a year now. This means: The planned oxide ceramics parts are being milled fully automated in serial production 24/7 – without any rejects and an accuracy of < 2 µm.
Ductile Cutting Mode needs corresponding tools
While researching the ductile cutting mode Kern worked closely together with 6C Tools. Many tests lead to today‘s results. 6C Tools offers many tools for milling in the ductile cutting mode. Meanwhile there are more suppliers offering tools for working with ceramics.
www.kern-microtechnik.com