Machining Ceramics with Rotary Ultrasonic Machining
Rotary ultrasonic machining (RUM) is a hybrid machining process that combines the material removal mechanisms of diamond grinding with ultrasonic machining (USM), resulting in higher material removal rates (MRR) than those obtained by either diamond grinding or USM alone. Experiments with calcium aluminum silicate and magnesia-stabilized zirconia have shown that the MRR obtained with RUM is six to 10 times higher than that of a conventional grinding process under similar conditions, and it is about 10 times faster than USM. It is also easier to drill deep holes with RUM than with USM, and the hole accuracy is improved. Other advantages of this process include a superior surface finish and low tool pressure.
History of RUMAlthough the principle of ultrasonic machining was recognized in 1927, the first useful description of the USM technique wasn’t given in industry literature until about 1940. Since then, ultrasonic machining has attracted a great deal of attention and has found its way into industry on a relatively wide scale. By 1953-1954, the first ultrasonic machine tools, mostly based on drilling and milling machines, had been built. By about 1960, ultrasonic machine tools of various types and sizes for a variety of purposes had been seen, and some models had begun to come into regular production.
In ceramic applications, USM provides a number of advantages compared to conventional machining techniques. Both conductive and nonconductive materials can be machined, and complex three-dimensional contours can be machined as quickly as simple shapes. Additionally, the process does not produce a heat-affected zone or cause any chemical/electrical alterations on the workpiece surface, and a shallow, compressive residual stress generated on the workpiece surface can increase the high-cycle fatigue strength of the machined part.
However, in USM, the slurry has to be fed to and removed from the gap between the tool and the workpiece. As a result, the material removal rate slows considerably and even stops as the penetration depth increases. The slurry can also wear the wall of the machined hole as it passes back toward the surface, which limits accuracy, particularly for small holes. Additionally, the abrasive slurry “machines” the tool itself, which causes considerable tool wear and, in turn, makes it very difficult to hold close tolerances.
Rotary ultrasonic machining was invented by P. Legge in 1964. In the first rotary ultrasonic machining device, the slurry was abandoned, and a vibrating diamond-impregnated tool was used against a rotating workpiece. However, because the workpieces were held in a rotating four-jaw chuck, only circular holes could be machined, and only comparatively small workpieces could be drilled with this device.
Improvements led to the development of a machine comprising a rotating ultrasonic transducer. The rotating transducer head made it possible to precisely machine stationary workpieces to close tolerances. With different shaped tools, the range of operations could be extended to end milling, tee slotting, dovetail cutting, screw threading, and internal and external grinding.
RUM EquipmentRUM devices contain a uniquely designed spindle that is coupled to an ultrasonic transducer. The ultrasonic power supply converts conventional line voltage into 20 kHz of electrical energy. This output is fed to the piezoelectric transducer located in the spindle, and the transducer converts electrical input into mechanical vibrations. By changing the setting of the output control of the power supply, the amplitude of the ultrasonic vibration can be adjusted. The spindle speed (measured in revolutions per minute [rpm]) is programmable using the CNC controller for speeds up to 8000 rpm.
A variety of tool shapes are used for rotary ultrasonic machining, and ceramic and technical glass machining applications typically use either a diamond-impregnated or electroplated tool. Diamond-impregnated tools are more durable, but electroplated tools are less expensive, so the selection depends on the particular application.
One of the major differences between USM and RUM equipment is that USM uses a soft tool, such as stainless steel, brass or mild steel, and a slurry loaded with hard abrasive particles, while in RUM the hard abrasive particles are diamond and are bonded on the tools. Another major difference is that the RUM tool rotates and vibrates simultaneously, while the USM tool only vibrates. These differences enable RUM to provide both speed and accuracy advantages in ceramic and glass machining operations.
Current ApplicationsIt is difficult to discuss actual applications due to the proprietary nature of the work being performed. In many instances, the rotary ultrasonic machining method yields a competitive edge, and application information is not disclosed to maintain the proprietary nature of this work. However, following are some generic examples that indicate the type of work being performed.
Cost-Effective Research. Today’s research personnel often have a difficult time procuring aluminum oxide tubes to their exact specifications cost-effectively and in a timely fashion. When using conventional machining methods, researchers must obtain small material quantities, often at a high expense, and go through time-consuming pressing and firing processes to produce the material to their specifications. Using the rotary ultrasonic process, the researcher can obtain already fired alumina stock in block form in the proper density from alumina vendors and have it easily machined to the required dimensions and tolerance in a reasonable cost and timeframe. (Conventional machining methods are not easily used for fired material, and USM is extremely slow for this application and is limited to practical machining depths of 3⁄4-in. or less.) Thin or thick wall ceramic tubes up to 16 in. (406 mm) in length, as well as thin discs from .005 in. (0.127 mm), can be easily machined using the RUM method.
Semiconductor Solutions. The ability of RUM to drill hundreds of .022-in. (0.55 mm) diameter holes to depths of .400 in. (10.06 mm) or more in materials such as silicon, quartz, sapphire and alumina can provide the semiconductor market with unique solutions to an ever-changing, fast-paced technology industry.
Laser Rods and Fiber Optic Preforms. The rotary ultrasonic process is able to machine 10-in. (254 mm) long (and often longer) rods of quartz, glass, sapphire, ruby, etc. that are extremely round to within .001 in. (0.025 mm). The technique can also produce long holes in the same materials or in borosilicate glass, aluminum nitride, alumina, silicon carbide or other ceramic materials where the need for a high tolerance hole is required or where parallelism from hole to hole is required.
Future AdvancementOne area of interest for future use of RUM technology is in the rotary ultrasonic face milling of ceramics. RUM is currently limited to machining only circular holes due to the rotary motion of the tool. However, attempts have been made to extend rotary ultrasonic machining to machining flat surfaces or milling slots, and research in this area is ongoing.
A new approach to extend rotary ultrasonic machining to face milling was proposed by Dr. Z.J. Pei in 1999. In this approach, the cutting surface is a conic surface, rather than the cylindrical or bottom surface. The advantages of this approach are that material removal mechanisms are kept the same as in rotary ultrasonic machining, and that flat surfaces on large workpieces can be machined. This technology has been transferred to industry and is under further research and development.
With this and other advances in RUM, it is likely that this machining method will continue to gain popularity for glass and ceramic applications in the near future.
For more information:For more information about rotary ultrasonic machining, contact:
• Professor Z.J. Pei, Department of Industrial and Manufacturing Systems Engineering, Kansas State University, 237 Durland Hall, Manhattan, KS 66506; (785) 532-3436; fax (785) 532-3738; e-mail email@example.com; or visit http://www.ultrasonicmachining.net.