PERFORMANCE ANALYSIS OF ROTARY TOOL MICRO-USM PROCESS

Abstract

Miniaturization of components is the need of hour because it saves raw material, processing time and energy. Therefore, micro components in the form of microfluidic devices are gaining huge importance in the field of electronics, optics and biotechnology etc. Hard and brittle non-conductive materials such as glass and silicon are the most commonly used materials for the fabrication of microfluidics devices. In recent past, micro-ultrasonic machining (micro-USM) process has emerged as a promising technique for machining of micro features such as microholes and microchannels on hard and brittle materials irrespective of their electrical conductivity. Despite several advantages, micro-USM process has some limitations such as low material removal rate (MRR), high tool wear rate (TWR) and poor form accuracy. The accumulation of abrasive inside the machining zone is the major factor responsible for the abovementioned problems. In past, several attempts have been made to overcome these limitations by augmenting micro-USM process. However, machining of dimensionally accurate, high aspect ratio microfeatures using micro- USM process is still a challenge. Thus, in order to resolve the aforesaid issues, rotary tool micro-USM, a process variant of micro-USM process has been experimentally investigated in the present research endeavor. In rotary tool micro-USM, the tool vibrates and rotates simultaneously and abrasive slurry is pumped between the tool and work material. The present research work was aimed to enhance the performance (i.e. productivity and accuracy) of micro-USM process by providing modification in the conventional micro-USM system. The modification was provided in the form of rotary motion of tool without making any manufacturing complexity in the micro-USM facility. An extensive experimentation was performed to investigate the effect of tool rotation and other process parameters of micro-USM process on its performance characteristics. Also, multi-criteria optimization was carried out to obtain maximum productivity and accuracy. During experimentation, microholes and microchannels were machined and subsequently characterized using different types of characterization techniques such as optical microscope, stereo zoom microscope, field emission scanning electron microscope (FE-SEM) etc. v The major objectives of the present research work are outlined as follows:  To develop a rotary tool micro-USM process setup for machining of microfeatures in hard and brittle materials.  To investigate and analyze the effect of tool materials and their properties on performance of rotary tool micro-USM process.  To evaluate the performance of rotary tool micro-USM process during machining of microholes and microchannels on borosilicate glass.  To investigate and analyze the influence of various process parameters of rotary tool micro-USM process during drilling of microholes on hard and brittle materials.  To investigate the tool wear phenomenon and its effect on form accuracy of microchannels during rotary tool micro-USM process.  To develop a predictive model of material removal rate for rotary tool micro- USM process. In the present research work, initially, the experimental setup of rotary tool micro- USM process was designed and fabricated using the in-house facilities. After that pilot experimentation was performed on rotary tool micro-USM process. The objective of pilot experimentation was to select the working range of tool rotation speed and to select the suitable tool material for rotary tool micro-USM process. The one-factor-at-time (OFAT) approach was used in pilot experimentation. The experimental results revealed that tool rotation speed for microhole drilling can be varied from 100 rpm to 500 rpm, whereas for microchannels fabrication, it can be varied from 100 rpm to 600 rpm. Tungsten carbide tool material was found to be suitable candidate tool material for rotary tool micro-USM process due to its high abrasion resistance and better acoustic properties. After selecting the range of tool rotation speed and tool material, the performance of rotary tool micro-USM process was experimentally evaluated. During performance evaluation, the rotary tool micro-USM process was compared with stationary tool micro-USM process while drilling of microholes and fabrication of microchannels. Experiments were conducted by varying the four micro-USM process parameters. The responses measured in this experimentation were MRR, hole overcut (HOC) for microholes and depth of channel (DOC) and form accuracy for microchannels. The vi results revealed that the rotary tool micro-USM process performed better than stationary tool micro-USM process. Thus, rotary tool micro-USM was selected for further experimentation. A qualitative analysis of tool wear and form accuracy during stationary tool and rotary tool micro-USM processes was also carried out with the help of field emission scanning electron microscope (FE-SEM) micrographs. The results revealed that rotary tool micro-USM process exhibits lesser tool wear and better form accuracy as compared to stationary tool micro-USM process. Subsequently, the rotary tool micro-USM process was employed for drilling of microholes in hard and brittle materials such as glass, silicon and zirconia. The aim of this investigation was to examine the drilling capability of rotary tool micro-USM process for different hard and brittle materials. In order to fulfil this aim, an extensive experimentation was performed on rotary tool micro-ultrasonic drilling (USD) process. Three types of work materials i.e. glass, silicon and zirconia were selected. The experimentation was performed using OFAT approach. The machined surface were analysed qualitatively to investigate the mode of material removal during rotary tool micro-USM of hard and brittle materials. Eventually, desirability approach was used to optimize the responses of rotary tool micro-USM for microhole drilling. The experimental results revealed that rotary tool micro-USM can be employed for drilling of microholes in all type of hard and brittle materials. The machining rate and HOC were found to be higher during drilling of silicon followed by glass and zirconia. Maximum tool wear was observed during machining of zirconia, whereas minimum tool wear was observed when silicon was machined. In all the work materials, pure brittle fracture was observed as the mode of material removal. At optimal parametric settings, microhole of depth 4355 μm was successfully machined in glass using rotary tool micro-USD process. Tool wear greatly affect the performance of micro-USM process as the shape of the tool governs the shape of machined microfeatures. In order to control the tool wear, an investigation was performed on tool wear and its effect on form accuracy of microchannels during rotary tool micro-USM process. In this investigation, initially, mechanism of tool wear and types of tool wear were identified considering tool, abrasive and workpiece interaction. Thereafter, a geometrical model of tool wear was developed to calculate volumetric wear of tool quantitatively during rotary tool microvii USM process. Later, the effect of tool wear and other rotary tool micro-USM process parameters were investigated on dimensional and form accuracy of the machined microchannels. The results revealed that rotary tool micro-USM has two types of tool wear i.e. longitudinal wear and edge wear. The form accuracy of microchannels was found to be more affected by edge wear as compared to longitudinal wear. Whereas, the DOC was found to be more affected by longitudinal wear as compared to edge wear. The desired DOC at the best possible form accuracy (i.e. at lowest edge rounding wear) of the microchannel can be obtained by providing longitudinal wear compensation to the tool. The optimal parametric combination of rotary tool micro- USM process provided maximum MRR 2.89 mg/min, DOC 517.48 μm and form accuracy of 87% and minimum width of microchannel 663 μm and TVW 0.017 mm3. Further, the rotary tool micro-USM process was utilized for machining of complex shaped microchannels to check its machining feasibility for microfluidic applications. Additionally, an attempt was made on development of material removal rate model for rotary tool micro-USM process considering brittle fracture theory. The model was developed by selecting tetrahedron geometry of abrasive particle. The pure brittle fracture was considered as mode of material removal during development of the model. The developed model was experimentally verified and statistically analysed. The predicted results were in good agreement with the experimental results within the selected range of input parameters. Statistical analysis also conformed the prediction accuracy of the developed model.