Technology suitable for small quantity production

Micro ultrasonic machining (MUSM) is a method derived from conventional ultrasonic machining, in which a tool and free abrasives are used. The tool that is vibrated at ultrasonic frequency drives the abrasive to create a brittle breakage on the workpiece surface. The shape and the dimensions of the workpiece depend on those of the tool. Since the material removal is based on brittle breakage, this method is suitable for machining brittle materials such as a glass, ceramics, silicon and graphite [9]. The chip size required for micromachining can be realized when submicron particles are available for the use as an abrasive. Microtools can be supplied in the same way as that in micro-EDM, because the same types of tools are used to specify the corresponding shapes of products, although the microscopic removal phenomenon is completely different. The major problems are the accuracy of the setup and the dynamics of the equipment. Ultrasonic vibration of the machining head makes accurate tool holding difficult. The implementation of a transducer and rotation mechanism is too difficult to maintain high equipment precision. However, recent development has overcome most of these difficulties. In the earliest works, the vibrations were applied to the tool, resulting in problems in tool holding and in machining accuracy. In order to overcome tool holding problems, the on-the ­machine tool preparation was introduced. In this approach the tool was soldered to the machine head prior to its preparation then machined by wire electro discharge grinding (WEDG) to the desired dimensions and the subsequent machining of workpiece took place on the same machine tool. However this method prevents measurement of the size and the shape of the fabricated tool. By means of the on-the-machine fabrication of the tool, holes in silicon with 20 µm in diameter have been produced [10].

The main problem in tool holding accuracy arises from the soldering process that has been applied to solve the problem of loosening caused by ultrasonic vibration. If the tool material is soldered before it is fabricated into a microtool, the completed tool setting is free from the low accuracy of soldering.

A further development is the introduction of vibrations applied to the workpiece instead of to the tool [10]. This enables a better tool holding and the use of a high-precision spindle mechanism. A high- precision rotation/feed mechanism is essential for most machine tools. The introduction of a vibration mechanism such as that in USM is, therefore, an idea contrary to the concept of the micromachining. One of the solutions to this problem is to attach the vibration mechanism to the workpiece side [10]. This will cause little problem in terms of accuracy because a transducer can simply be inserted between the worktable and the workpiece. Such a setup enables the use of a universal, high­precision machine head and its influence on workpiece holding accuracy is negligible.

With MUSM, micro holes of 5 µm in diameter were machined in quartz glass and silicon, using a tool with diameter of 4 µm and an abrasive with average grain size of 0.2 µm. Furthermore a microhole with diameter 9 µm and an aspect ratio of 4 was realized in quartz glass. A major problem in MUSM is the high tool wear ratio. Tungsten carbide is used for tool fabrication because tools of approximately Ø 5 µm can be machined and it is tough enough to withstand machining load. However a wear ratio always higher than 0.5 leads to limitations of the machining efficiency with the impossibility of machining deep holes or multiple holes with the same tool. The introduction of sintered diamond tools has enabled to overcome the problem, giving a wear ratio of 0.01 when machining soda glass. However, tools in sintered diamond are limited to a minimum diameter of 15 µm since they are fragile and tend to break during 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 removal rates than those obtained by either diamond machining or USM alone. As main tool is made of diamond, steel is not machinable, and its main advantage is obtained machining fragile materials (ceramics, glass), so its application to metals is quite marginal. Expected accuracy is in the order of few microns and roughness of 0.2 microns Ra.

In pure ultrasonic machining (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. Abrasive slurry composed of water and small abrasive particles is supplied between the tool tip and the workpiece. Material removal occurs when the abrasive particles impact the workpiece due to the downstroke of the vibrating tool.

In rotary ultrasonic machining (RUM), a rotating core drill with metal bonded diamond abrasives is ultrasonically vibrated in the axial direction while the spindle is fed toward the workpiece at a constant pressure. Coolant pumped through the core of the drill washes away the swarf, prevents jamming of the drill and keeps it cool. By using abrasives bonded directly on the tools and combining simultaneous rotation and vibration, RUM provides a fast, high-quality machining method for a variety of materials.

In [A_4], micro USM is widely described. Material removal in micro USM is by the mechanical action of abrasives as well as by the cavitation erosion due to rapid pressure changes caused by the ultrasonic vibration of fluid in working zone [A_5] [A_6]. This non-thermal, nonchemical and non-electrical process is especially suitable for the micro machining of hard brittle and inert insulators such as glass, ceramics, composites, quartz, precious stones and for the machining of fragile and porous materials such as graphite. Irregular shaped hard abrasive particles are dispersed in a liquid medium (called abrasive slurry) and fed into the gap between tool and workpiece. The tool is vibrating with an ultrasonic frequency (usually 20~40 kHz) with an amplitude of several to tens micrometers. When static load is applied between tool and workpiece, abrasive particles impact and chip away material from both workpiece and to a lesser extent from the tool [A_7].

Condition of the abrasive and its grain size affect the machining rate [A_8]. A continuous flow of abrasive slurry flushes away the debris from the working zone. Since actual machining is carried out by abrasive particles, the tool can be softer than the workpiece.

A vibration of the workpiece improves the machining accuracy [A_9]. It not only simplifies the structural design of the tool system, but also stirs the abrasive slurry during the machining to improve renewing particles in the working zone and facilitates debris removal [A_10]. Precise measurement of the vibration amplitude at micron level is a challenging task. An online measurement method proposed in [A_10] drives the tool tip to touch the workpiece surface and captures two vertical positions of the surface with respect to turning on and turning off of the vibration. The difference between two positions is treated as the vibration amplitude. The accuracy of this measurement method is highly affected by the precision and responding time of driving components and force sensor. A force sensor with short responding time and high resolution is required for monitoring and controlling the static load to avoid tool breakage during machining. In some studies, the static loads are recorded under constant tool feed rate [A_11]. The static load under constant tool feed rate usually shows cyclic fluctuate pattern. A closed loop control can be employed for the better evaluation of the effect of static load as a parameter on machining characteristics. In this case, the tool feed rate is adjusted to obtain a stable static load [A_10].

The mechanism and modeling in macro USM are not yet fully understood. Several models have been proposed to predict the material removal rate, and most of them are with rough accuracy in prediction. Material removal mechanism in micro USM is believed to be similar to conventional USM. However theoretical work in micro USM has rarely been reported and there is a lack of knowledge about the process behaviours of micro USM under various conditions. Theoretical models of macro USM may not exactly be applicable to micro USM due to effects such as difficulty of refreshing of abrasive particle and debris removal caused by the downscaling of tool and abrasive particle [A_10]. Existing knowledge is far from sufficient to provide a complete understanding and instructive rules for industrial users [A_12]. A tentative mechanistic modeling of material removal in micro USM was proposed in [A_13]. The basic assumptions in this modeling are similar to those in Rotary Ultrasonic Machining (RUSM).

All materials tending to a brittle fracture behavior can be machined by micro USM. Examples include highperformance ceramics, glass, graphite and a part of the fiber-reinforced plastics [A_50] [A_14]. Geometrical capabilities of micro USM have been testified by drilling, slot machining and 3D machining. Micro holes with a diameter less than 10 μm were successfully drilled on silicon, quartz glass and alumina [A_9] [A_15] [A_16]. Slotting on low melting glass has been reported in [A_11]. The tool wear compensation strategy “Uniform Wear Method” originally developed for micro EDM has been applied in micro USM to successfully generate 3D micro cavities as shown in Figure 17 [A_17].

Micro USM has not yet been commercialized with a functional machine tool similar to micro EDM. However, it is believed that this process could provide solutions to easily and quickly achieve the larger MEMS structures as well as packaging for both prototype and production in silicon, glass and ceramic [A_7].

Proper selection of micro USM process parameters at present is not well understood due to lack of related experimental results [A_17]. Abrasive particle size, vibration amplitude, static load and tool rotation are the main parameters influencing the micro USM machining speed for the given workpiece material [A_10] [A_13] [A_16]. It was found that a slight tool rotation drastically improves the drilling speed. However, there was no significant improvement for speed higher than 50 rpm [159]. The debris accumulation affects the machining speed in micro USM due to the poor fluidic circulation around the machining zone [A_13]. The dependence of dimensional accuracy on tool diameter, vibration amplitude, and abrasive size needs further research [A_12]. By means of calculating the maximum impact force, combined with microcrack models obtained from the research on indentation, it is possible to correlate the depth of microcracks with process parameters. A predictive model for the microcrack depth can be employed to optimally select the process parameters [A_12].

High tool wear is an intrinsic drawback of micro USM. It is difficult to get a constant depth of cut due to longitudinal tool wear. Tool wear is affected by parameters such as vibration amplitude, static load and tends to increase when harder and coarser abrasives are used. As a consequence, harder abrasives, like diamond, cause higher tool wear than softer abrasives such as silicon carbide [A_16]. Therefore, it is necessary to account for and to compensate the tool wear during machining. The feasibility of applying the “Uniform Wear Method” for generating accurate 3D microcavities by micro USM has been tested and found that the tool shape remains unchanged and the tool wear has been compensated [A_17]. Due to inadequate research done on tool wear in micro USM, the selection of tool materials is not well supported by the experimental data and related analysis. Also, the tool wear mechanisms, the wear rate dependence upon tool hardness, toughness, abrasive type and size, abrasive hardness and material toughness need to be studied to reduce and control the tool wear in micro USM [A_12].

The micro tools used in micro USM can be prepared by a WEDG unit [A_9]. A micro tool with multi tips was made using batch mode micro EDM. Tool material with enough abrasive wear resistance and small deflection under mechanical load is preferable in micro USM. A PCD tool is helpful in reducing tool wear [A_9].

Some of the important micro USM issues requiring systematical research include the study of material removal mechanism, innovative tooling, tool wear mechanism and reduction, on-line sensing, subsurface damage control, and surface roughness improvement. In addition, in-process monitoring and model-based selftuning strategies are needed for improving the process stability and performance.

[9] Masuzawa, T., 2000, State of the art of micromachining, Annals of the CIRP, 49/2: 473-488

[10] Egashira K., Masuzawa T., 1999, Microultrasonic machining by the application workpiece vibration, Annals of the CIRP 48/1: 131-134.

[A_4] Micro and Nano Machining by Electro-Physical and Chemical Processes, K.P. Rajurkar, G. Levy , A. Malshe, M.M. Sundaram, J. McGeough, X. Hu, R. Resnick, A. De Silva, Annals of the CIRP Vol. 55/2/2006, 643-666.

[A_10] Hu, X., Yu, Z., Rajurkar , K. P., 2005, Experimental Study of Micro Ultrasonic Vibration Machining. Fourteenth International Symposium on Processing and Fabrication of Advanced Materials, Pittsburgh, Pennsylvania, USA, 197-210.

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[A_5] Kremer, D., Saleh, S. M., Ghabrial, S. R., Moisan, A., 1981, State of the Art of Ultrasonic Machining., Annals of the CIRP, 30 /1: 107-110.

[A_6] McGeough, J. A., 2002, Micromachining of Engineering Materials, Marcel Dekker Inc., New York.

[A_12] Zhang, C., Rentsch, R., Brinksmeier, E., 2005, Advances in Micro Ultrasonic Assisted Lapping of Microstructures in Hard-Brittle Materials: A Brief Review and Outlook, International Journal of Machine Tools and Manufacture, 45 /7-8: 881-890.

[A_7] Medis, P. S., Henderson, H. T., 2005, Micromachining Using Ultrasonic Impact Grinding, Journal of Micromechanics and Microengineering, 15/8: 1556-1559.

[A_8] Adithan, M., Venkatesh, V. C., 1978, Appraisal of Wear Mechanisms in Ultrasonic Drilling, Annals of the CIRP, 27 /1: 119-121.

[A_9] Egashira, K., Masuzawa, T., 1999, Microultrasonic Machining by the Application of Workpiece Vibration, Annals of the CIRP, 48 /1: 131-134.

[A_11] Moronuki, M., Saito, Y., Kaneko, A., Miura, A., Aikawa, C., 2004, Vibration Micromachining of Low-Melting-Temperature Glass. 7th International Symposium on Advances in Abrasive Technology, Bursa, Turkey, 489-494.

[A_13] Yu, Z., Hu, X., Rajurkar , K. P., 2005, Study of Micro Ultrasonic Machining of Silicon. Proceedings of 2005 ASME International Mechanical Engineering Congress and Exposition, Orlando, Florida USA, 1-8.

[A_14] Ghahramani, B., Wang, Z. Y., 2001, Precision Ultrasonic Machining Process: A Case Study of Stress Analysis of Ceramic (Al2O3), International Journal of Machine Tools and Manufacture, 41 /8:1189-1208.

[A_15] Egashira, K., Masuzawa, T., Fujino, M., Sun, X. Q., 1997, Application of USM to Micromachining by On-the-Machine Tool Fabrication, International Journal of Electrical Machining, 2: 31-36.

[A_16] Choi, H.-Z., Lee, S.-W., Lee, B.-G., 2003, Micro-Hole Machining Using Ultrasonic Vibration, Key Engineering Materials, 238-239: 29-34.

[A_17] Yu, Z. Y., Rajurkar, K. P., Tandon, A., 2004, Study of 3D Micro-Ultrasonic Machining, Journal of Manufacturing Science and Engineering, 126 /4: 727- 732.

[A_50] IWF, IPT, 2002, Investigation of the International State of the Art of Micro Production Technology MickroPRO.