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.
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.
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.
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.
• 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 firstname.lastname@example.org; or visit http://www.ultrasonicmachining.net.