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Historical Background: From USM to RUM 

Illustration of USM

Left Figure is a schematic illustration of USM. The tool  (shaped conversely to the desired hole or cavity)  oscillates at high frequency (typically 20 kHz) and is fed into the workpiece by a constant force. An abrasive slurry comprising water and small abrasive particles is supplied  between the tool tip and the workpiece. Material removal occurs when the abrasive particles, suspended in the slurry between the tool and workpiece, impact the workpiece due to the downstroke of the vibrating tool. One of the major differences between USM and RUM is that USM uses a soft tool (such as stainless steel, brass, and mild steel) and a slurry loaded with hard abrasive particles while in RUM the hard abrasive particles (diamond) are bonded on the tools. Another major difference lies in that the RUM tool rotates and vibrates simultaneously while the USM tool only vibrates.the first useful description of the technique of ultrasonic machining was given in the 1940's (Balamuth, 1945).Since then, ultrasonic machining has attracted great attention and has found its way into industry on a relatively wide scale.By 1953-1954, the first ultrasonic machine tools (mostly on the basis of drilling and milling machines) had been built (Rozenberg et al., 1964). By 1960's, 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.

The rapid progress in this field can also be seen from the number of published papers. Up to early 1960's, some three to four hundred papers had been published covering the various aspects of ultrasonic machining. Much of this material was brought together by two monographs: Ultrasonic Machining of Intractable Materials by Markov (1966) and Ultrasonic Cutting by Rozenberg et al. (1964), both originally published in Russian in 1962 and translated into English afterward.

Ultrasonic machining of ceramics has the following advantages: (i) both conductive and nonconductive materials can be machined, and complex three-dimensional contours can be machined as quickly as simple ones; (ii) the process does not produce a heat-affected zone or cause any chemical/electrical alterations on workpiece surface; and (iii) a shallow, compressive residual stress generated on the workpiece surface may increase the high-cycle fatigue strength of the machined part (Gilmore, 1989).

However, in USM, the slurry has to be fed to and removed from the gap between the tool and the workpiece. Because of this fact, there are some disadvantages of this method: (i) material removal rate slows down considerably and even stops as penetration depth increases; (ii) the slurry may wear the wall of the machined hole as it passes back towards the surface, which limits the accuracy, particularly for small holes; and (iii) the abrasive slurry also "machines" the tool itself, thus causing considerable tool wear, which in turn makes it very difficult to hold close tolerances.

Rotary ultrasonic machining was invented by Legge (1964). In the first rotary ultrasonic machining device, the slurry was abandoned and a vibrating diamond-impregnated tool was used against a rotating workpiece. Because the workpieces were held in a rotating four-jaw chuck, with this device only circular holes could be machined and only comparatively small workpieces could be drilled.

Further 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 (Anonymous, 1966).

The literature on rotary ultrasonic machining in the 60's and 70's can be classified into two groups: (i) the articles devoted to explaining the principle of rotary ultrasonic machining and describing the equipment and diamond tools (Anonymous, 1964, 1966, 1973; Chechins and Tikhonov, 1968; Cleave, 1976; Dawe Instruments Ltd., 1967; Hards, 1966; Legge, 1964, 1966; Markov et al, 1969; Tyrrell, 1970a, 1970b); and (ii) the papers reporting the experimental investigations on the relations between the process parameters (e.g. vibration amplitude, static pressure, rotational speed and grit size, etc.) and the process performance such as MRR, tool wear and surface finish (Petrukha et al., 1970; Markov and Ustinov, 1972; Markov et al., 1977; Kubota et al., 1977).

For a long time, rotary ultrasonic machining was viewed merely as an improvement of USM. In principle, however, rotary ultrasonic machining is a hybrid process which utilizes the fixed-abrasive tool used in diamond grinding and the ultrasonic vibrations associated with USM(Prabhakar, 1992; Prabhakar et al., 1992; Dam et al., 1993).