If you have been following our manufacturing series of articles, then you know we have fired ceramic and are about to discuss shaping and machining. If you haven't been following, you should be!
When determining how to machine a ceramic component, it is important to remember that the finished electrical properties of the piece are controlled partly by the geometry. Matching the customer's physical and electrical requirements to the geometry ensures the component will work properly in the finished system. S
ince the matching of the ceramic to the finished device requirements depends on the frequency, capacitance, etc. of the device, the parts are precisely machined in all dimensions. This is accomplished using a variety of machining techniques we will discuss below. Because of PZT's hardness and brittleness, machining the material requires the use of diamond or silicon carbide abrasives when machining. The mechanism for material removal is grinding, not cutting. Coolant must be used to cool the grinding interface, lubricate the tool, and carry away the by-products (swarf).
When removing material abrasively, there are some critical factors to consider including surface finish (Ra) of the PZT, tool wear rate, surface speed of the abrasive, and feed-rate of the abrasive into the piece. Successful machining is accomplished through a compromise of all these factors. Ra<0.7um is a typical surface finish target for machined PZT surfaces. Typical abrasives used to support this compromise when grinding would be diamond in the 320 grit (~44um) range, metal bonded for durability, and operating at surface speeds anywhere from 1200-1800 meters per minute (MPM) with feed-rates ranging from 25-200 mm/min.
Lastly, the precision of the completed piece (flatness, perpendicularity, tolerance level, etc.) is achieved using good work-holding techniques and fixture design, while still considering the inherent compromises of machining listed above.
Machining operations include lapping, center-less OD grinding, ID slicing, CNC grinding/dicing, and multi-axis CNC milling and drilling for complex shapes, and we will touch on each of these techniques below:
Lapping:
Lapping is a process in which material is abrasively removed from the top and bottom surfaces of the ceramic component simultaneously. By sandwiching the components between hard, extremely flat plates, adding lubricant and introducing an abrasive material the process can begin. Motion is induced in a planetary fashion, with the components being retained in carrier plates. Each carrier is rotated about itself while being spun around the lapping plate surface. As this occurs ceramic material is slowly abraded from the surfaces. This produces extremely flat parallel surfaces and can be controlled by pressure and time, resulting in the ability to hold tight tolerances and high levels of precision. This makes lapping a great process to control the thickness of ceramic components (especially when using thickness to control frequency), however lapping of other dimensions is not uncommon.
Center-less Grinding:
For reducing the diameter of components (to set a radial frequency or improve surface finish) center-less grinding is preferred. In center-less grinding, the disk shaped components are held between a high speed grinding wheel and a slow-speed regulating wheel. The regulating wheel rotates the components on edge while the grinding wheel slowly removes material. This allows for the removal of material from the circumference of the disks while keeping them completely round. In applications where high throughput may be required and minimal material removal is necessary, components can be through-fed vs. in-fed. With appropriate machine setup, parts can be loaded into one end of the wheel assembly and fed through the slightly narrowing gap, emerging from the other side "in-tolerance."
ID Slicing:
In applications where very thin (<0.25mm) or complex geometries are required, ID slicing is far more practical than lapping. In the ID slicing process, the stainless steel slicing blade is stretched around a frame and tensioned from the outer diameter. There is a diamond-impregnated hole in the center of the blade which becomes the cutting edge. Material is fed through the hole, then across the blade. This minimizes blade wander through the material that is normally encountered when cutting hard materials with a central mandrel mounted blade and OD abrasive (like a cutoff wheel). By reducing blade wander, extremely tight tolerances for thickness and flatness can be obtained.
CNC Grinding / Dicing:
CNC grinding is utilized when custom tapered, radiused, or other complex but generally flat shapes are required. A quick tool change from custom grinding wheels to custom mandrel to support multiple dicing blades and you have a quick solution for dicing sliced plates into smaller custom square and rectangle shapes. Work holding is critical in grinding and dicing applications, as a majority of the precision in the component is controlled by the quality of the work holding.
CNC Milling / Drilling:
Ring shapes, cylinders, long rods, multiple holes, tight tolerance rings, truncated shapes etc. are all possible with CNC milling and diamond drilling or a combination of the two. By starting with large blocks of fired PZT almost any shape can be manufactured. Custom milling and drilling tools can be designed, and when combined with appropriate techniques and programming the most complex of shapes can be manufactured.
In summary, with the basic machining techniques listed above almost any shape can be broken into multiple simple processes and manufactured by mixing and matching the operations, sometimes performing 10 or more individual steps consisting of 3-4 major operations. After machining to the predetermined specifications is complete, a cleaning operation is performed to remove and remnant swarf, abrasive, oil, water, or additives. This step is critical and varies based on the machining operations performed when manufacturing the component. A clean ceramic surface is absolutely essential for the next step: Applying and Firing Electrodes on PZT. Don't miss next months feature article!
