فرسایش ابزار در ساخت میکرو کانال ها در میکرو ماشین‌کاری التراسونیک (USM)

ABSTRACT Tool wear studies in fabrication of microchannels in ultrasonic micromachining

Tool wear can be considered as one of the most important parameters which decides the form accuracy of a machined component. Form accuracy, on the other hand manifests as how close the machining process replicated the dimensions of cutting tool on the workpiece surface [1]. The efficacy of ultrasonic micromachining can also be measured by form accuracy of the machined component. As the tool gets worn out with time, a noticeable change is observed in form accuracy forcing the component towards rejection in the quality check. However, form accuracy can be achieved significantly by controlling and maintaining the tool shape and size. Tool material properties are important parameters effecting the tool wear. Maintaining form accuracy becomes even more challenging while machining at micro levels. Flexibility and feasibility of machining micro components using ultrasonic machining (USM) have already been demonstrated [2–4]. Hard and brittle materials like glass, silicon wafers and ceramics could easily be machined by USM in the micro domain too [5]. Moreover, while compared to chemical methods of etching and lithography, USM is preferred as a cleaner and faster process.

Extensive work have been reported in macro drilling of various hard and brittle materials [6]. Studies were carried out on tool wear at macro levels. In one of the studies, the tool wear pattern was divided into two types – lateral wear and longitudinal wear [7]. Lateral wear at the tool edges occurred due to the abrasive rubbing phenomenon between the tool and workpiece walls. Lateral wear was also found responsible for reduction in tool diameter. On the other hand, longitudinal wear was responsible in reduction of tool length which occurred due to microchipping and cavitation phenomenon [8]. The complete tool wear during USM of macro holes was thus, a combination of both the wear patterns. Some researchers reported that with an increase in machining time, microhardness of the tool was found to be more at the edges in comparison to the middle part. Plastic flow and work hardening were found to be the major reasons for increased hardness [9].

In macro machining, if the tool gets slightly worn out, the machined product can still fall in the desired range. However, in case of machining in the micro regime, for example, fabrication of components like microchannels and microholes, the form accuracy gets affected drastically with minimal tool wear. Consequently the parts are liable to be rejected. One of the ways for achieving desired form accuracy is to minimize tool wear to the extreme possible.

Tool wear in USM mainly depends upon a number of factors like workpiece material, tool material, amplitude, applied frequency of vibration, abrasive type, abrasive size, static load and concentration of abrasive particles. Proper combination of the above mentioned parameters is essential to minimize the eventuality wear. But the tool material is the most crucial parameter deciding the wear. Nimonic alloys, thoriated tungsten, tungsten carbide (WC), high speed steel, silver steel, titanium, maraging steel and mild steels have been the most commonly used tool materials for ultrasonic macro machining [10]. While macro drilling, it was reported that stainless steel (SS) tool is far better than WC tool [7–9,11]. This implied that SS could be an excellent candidate for ultrasonic micromachining. It is easily available and is a cost effective solution for machining. The ultrasonic micromachining can be differentiated from the macromachining by the size of machined features developed, which is generally less than 1 mm. However, conflicting opinions were revealed for SS tool in USM at micro levels [2,3,12–14]. Some studies in ultrasonic micromachining include development of models for predicting tool wear [10].

These contradictions encouraged the present study on tool wear of SS and WC tool while fabricating microchannels on borosilicate glass. The tool wear effect on the form accuracy of developed microchannels was investigated in this study. Further, this study deals with identification of tool wear mechanism. Subsequently this article defines the type of tool material for microchannel fabrication. A one factor at a time approach was used to study the effect of step feed and abrasive size on the form accuracy of developed microchannels using SS and WC tool.