Alexander Stauder, head of applications, Kern Microtechnik
Since 2021, Kern Microtechnik’s (Kern’s) job shop in Murnau, Bavaria, Germany, has manufactured ceramic parts for a variety of applications. The ductile cutting mode, developed by Kern engineers and technicians alongside customers for the company’s CNC machining centres, plays a vital role, ensuring exceptionally high quality and zero rejects.
A part made of zirconium oxide (ZrO2), produced on the Kern Micro HD CNC machining centre. It was drilled as well as milled in the ductile cutting mode with full polycrystalline diamond (PCD) tools. This reliable machining process results in a very high standard regarding accuracies and surface-quality.
This capability appeals to a great many manufacturers of precision parts across numerous fields, including analytics, chemical, jewellery and watchmaking, and satellite technology. The ductile mode affords reliable manufacturing of technical ceramics such as silicon carbide, aluminum oxide, etc. The extreme hardness, stiffness and heat and chemical resistance of these materials and their excellent thermal connectivity make ceramics an ideal material for many different applications.
However, manufacturing ceramic parts is very challenging, particularly because they need to be no less than 100 percent perfect. Cracks, breakoffs or otherwise damaged material means the part is a failure. 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 traditional manufacturing processes, but significantly limit the manufacturable shapes. Also, reliability becomes an issue as soon as contours and outlines need to be cut. This is where the ductile cutting mode comes into play. Special milling tools, with a geometrically defined cutting edge, remove material from brittle ceramics with flying chips.
A watch house made of zirconium oxide (ZrO2) being milled on a Kern Micro HD with a PCD tool.
Grinding versus ductile cutting mode
There are many advantages of using the ductile cutting mode. The most important of these are excellent process reliability and part quality in terms of, e.g., accuracy and surface finish. Depending on the quality of the ceramic, Ra/Sa roughnesses in the range of a few 10 nm and accuracies in the lower micrometer range can be achieved. It is often possible to cut machining times considerably, resulting in increased efficiency.
However, there is a rule of thumb. 1) The larger the parts and respectively the drills and grooves are, the more it makes sense to use grinding for the first removal of material. Finishing is then the only task that remains for milling, for which tools are sometimes more costly. 2) The smaller the parts and respectively the drills and grooves are, the more productive it becomes to work directly with ductile cutting mode.
For both ductile cutting mode and grinding, a CNC machine must have high-stiffness axes, high running smoothness and a spindle that has a high RPM and can withstand abrasive ceramic removal.
A part made of reaction-bonded silicon carbide (RBSiC) with microthreads (S1 and S0.6) and highly precise microbores (diameter tolerance <2 µm), produced on the Kern Micro HD.
CNC machine requirements
The Kern Micro HD, a high-precision, 5-axis CNC machining centre, meets the aforementioned prerequisites. The design of the machine as well as the microgap hydrostatics ensure high stiffness of the 3-axis system. The machine’s 15 kW high-speed spindle, affording up to 42,000 min-1 RPM, and HSK 40 interface help ensure high running smoothness and efficient removal of material. Furthermore, the smart design of the axes ensures that the mechanics are well protected against the dust resulting from ceramic material removal.
The Kern Micro HD is ideal for jig grinding as well as milling. It is possible to first do the grinding and then switch to ductile cutting mode to do the milling without re-clamping. Less time and more accuracy are only two advantages.
The ductile cutting mode requires corresponding tools. Kern worked closely with 6C Tools, a manufacturer of polycrystalline diamond (PCD) tools, while researching the ductile cutting mode. Many tests led to today‘s results. 6C Tools offers many tools for milling in the ductile cutting mode, but there are other suppliers that offer tools for working with ceramics.
In addition to selection of the right machine, communication is a key factor for part success. Nobody knows the Kern Micro HD better than the staff in Kern’s applications department and job shop; and nobody knows the part better than the customer, and they are very often also the part’s developer. This knowledge needs to be shared and combined.
The percentage of grinding and milling work varies depending on the job. For example, since parts in the watchmaking and jewelry industry are always very small and demanding, 100 percent milling is usually the go-to method.
A PCD tool from 6C-Tools with 6 mm diameter, corner radius and 52 lasered cutting edges, making it perfect for milling ceramics in the ductile cutting mode.
A zero rejects case study
A customer approached the Kern job shop regarding machining different ceramic oxide parts for use in analytical applications. These parts used to be grinded in another job shop, however the customer informed Kern that small inaccuracies and tiny chamfers often led to the failure of an entire assembly group.
Kern eliminated these failures by changing the process, which is undertaken in house, in eight steps, from ceramic oxide powder to final part. The final steps are performed on a Kern Micro HD. The first removal of the material is achieved via jig grinding with mounted points. After this, and this is crucial for the success of the process, the part is milled using the ductile cutting mode. Together with the customer, Kern’s application team developed and carried out extensive tests to find the best medium chip thickness, which was necessary for a clean cut. This is essential, otherwise quality could not be maintained and there would be a high wear and tear rate on expensive tools.
The project has been running for over a year now. The ceramic oxide parts are being milled to an accuracy of <2 µm, in a 24/7, fully automated, serial production process, without any rejects.
Kern Microtechnik