Parametric studies on modulation assisted machining and its effect on chip characteristics

Abstract

Sustainability in manufacturing can be realized by developing a machining system, that reduces consumption of resources for converting raw materials into useful product. Moreover, it must produce waste that can directly be used by another production system as an input material. Intentionally inducing vibration to make machining system sustainable in this regard started from the work of Kumabe [2]. Based on the direction of modulation with respect to workpiece motion, three distinct effective conditions develop; Vibration Assisted Machining (VAM) having low amplitude and high frequency vibration in the cutting velocity direction, Modulation Assisted Machining (MAM) having high amplitude and low frequency vibration in tool feed direction and Elliptical Vibration Assisted Machining (EVAM) possesing low amplitude and high frequency vibration both in tool feed as well as cutting velocity directions. Vibration in machining helps to increase the tool life and reduce overall cost of production. Moreover, it also helps in improving tool chip contact conditions so as to improve surface finish. For superimposing vibration to tool motion, the use of piezoelectric transducer device based tool holder attachments has been reported and investigated. Imposition of oscillation in machining reduces the cutting forces and burr suppression, which further decrease the energy consumption and hence make the system energy efficient. Moreover, MAM has been used to produce chips of required shape and size distribution, which can be used by another production system as an input material without any post processing. The austenitic stainless steel AISI 316 (316 SS) is considered as one of the difficultto-machine materials due to its low thermal conductivity and high work-hardening rate. Despite the fact that these steels are of great significance, still there exist some technical gaps about their behavior during machining. During drilling, severity of contact conditions at chip-tool interface plays a significant role in tool life and quality of the hole produced. Different techniques such as different types of coatings, lubrication systems and drilling strategies have been used for improving the conditions of tool-chip interface. Glass fibre reinforced plastics (GFRP) can offer several promising mechanical properties, which include high specific strength, stiffness and damping, along with low thermal expansion coefficient. These properties of laminates have made them to replace metallic materials as structural elements in several defence and aerospace applications. It has been reported that during the conventional drilling of GFRP, rejection rates are on higher side due to the damage around the holes. Several measures to control such damages have been investigated, which include proper tool material selection, tool point design, use of optimized feed rates and cutting speeds, development of special manufacturing strategies and modeling of cutting forces. The scope of the present investigation has been chosen to evaluate the effect of tool-chip contact disruption during modulation assisted drilling (MAD) of 316 SS and GFRP on quality of the holes produced, cutting forces and tool wear. Moreover, in MAM disruption of tool-chip contact during machining helps in formation of discontinuous chips from the alloys, which otherwise produce continuous chips in conventional machining. This capability of MAM has been investigated to produce controlled size and shape chip particulates of brass. Furthermore, characteristic studies of these particulates have been done to explore the effect of MAM on their properties. A patented tool holder (TriboMAM) was retrofitted on the CNC lathe for centerline drilling during drilling experiments. Piezoelectric sensor based dynamometer was used for force and torque measurements. A roughness tester (Talysurf) was used for surface roughness measurements. An optical microscope (Leica) was used to take images of the surface produced using modulation and conventional drilling. Maximum flank wear, (V Bmax), was measured by a tool maker’s microscope. The used drills were inspected under SEM to establish the possible wear mechanism. It was observed from the drilling force data that the effect of feed rate was remarkable on the thrust force in conventional drilling (CD) as well as modulation assisted drilling (MAD). With increase in feed rate, thrust force increased drastically. The effect of rotational speed on the thrust force was found to be insignificant and no noticeable change was observed in thrust force with increase in rotational speed during CD and MAD for the investigated range of parameters. Moreover, mode of drilling has been observed to be significant with regard to its influence on thrust force. As the mode of drilling changed from CD to MAD, significant reduction in thrust force was observed. It was observed from the results that the highest surface finish could be obtained in the case of modulation assisted drilling. Whereas,the worst surface finish was produced in the case of conventional drilling. Modulation assisted drilling outperformed conventional drilling in surface finish at almost every investigated feed and speed combinations. A comparative analysis of the wear performance of tungsten carbide drills used in CD and MAD indicated that the drills used in MAD were better in resisting tool wear than those used in CD. The recorded percentage reduction in the tool wear of drill used in MAD over the drill used in CD is about 44.4% for the cutting speed of 32.9 m/min and feed of 0.015 mm/rev. However, at a higher feed of 0.03 mm/rev during MAD, a catastrophic failure of drill occurred due to fracture of cutting edge. The impact-type cyclic loading on cutting edge is believed to increase with the increase in value of the feed. The direction of force acting on tool edge and the enhanced magnitude of cyclic loading may be the reason for tool edge fracturing in MAD at higher feeds. An in-depth analysis of SEM and optical micrographs of flank wear on the drills used in MAD and CD revealed that the MAD effectively enhanced the cutting life of drills by resisting adhesion wear and plastic deformation. In the case of drilling experiments on GFRP, tool maker’s microscope was used to measure delamination around the holes produced during CD and MAD. Moreover, hole size was measured using a coordinate measuring machine (CMM). It has been observed that damage due to delamination in GFRP laminates depended on the mode of drilling, feed and rotational speed. Among them, the mode of drilling has been observed to be the most significant factor for the drilling induced damage. Moreover, the damage around hole was found to increase with speed and feed in CD. Holes with less damage were produced in MAD at higher feed and speed. A maximum value of delamination factor was found in CD at a speed of 2400 rpm and feed of 0.105 mm/rev. At the same drilling conditions with MAD, approximately 49% reduction in delamination factor value was observed, which indicates the benefits of MAD over CD. A reduction in the damage zone around holes may be attributed to the intermittent contact breakage between tool and chip during MAD. It is believed that due to the intermittent contact in MAD, instantaneous force becomes zero during each modulation cycle, thus reducing average thrust force. Cutting force analysis showed that the effect of feed rate is remarkable on the thrust force in CD as well as MAD and with increase in feed rate, thrust force increased drastically. It is perceptible that an increase in feed rate increased the section of sheared chip, so the GFRP resisted the rupture to a greater extent and it required larger efforts for the chip removal. The effect of rotational speed on the thrust force during drilling of GFRP has been found to be insignificant and no noticeable change was observed in thrust force with increase in rotational speed during CD and MAD. Moreover, the mode of drilling has been found to be a significant parameter for its influence on thrust force. As the mode of drilling changed from CD to MAD, a significant reduction in thrust force was observed. It is perceptible that the contact breakage in intermittent cutting during MAD helped in zeroing the instantaneous thrust force. Zeroing of instantaneous thrust force further reduced the mean thrust force value. ANOVA analysis for means value of output response (hole size) showed that only feed was the significant parameter. Other two parameters (mode of drilling and rotational speed) have been found to be insignificant. Oversize of the hole was found to reduce with the increase in feed value. Holes with less deviation could be produced in conventional drilling at higher feed and lower speed values. Increase in feed has been found to limit the skidding motion of drill, which may further have reduced the hole oversize. Empirical relation between delamination factor and investigated parameters was established using nonlinear regression analysis. For particulate production experiments a specially made tool of HSS was used. SEM and Tool maker’s microscope were used to analyze the size and morphology of the particulates. In MAM, modulation frequency (fm) and workpiece rotational frequency (fw) determine the contact time of the tool with the workpiece and affect the resultant particulate length. This is evident that, an increase in workpiece rotational frequency requires increase in modulation frequency to create particulates of equivalent length. Feed rate (h0) and modulation amplitude (A) describe the axial position of the tool at any point of time, defining the chip thickness and cross-sectional shape of the resultant particulates. Similarly, increased feed rates require increased modulation amplitudes to ensure interrupted cutting and deformation of particulates. Any determination of particulate morphology must account for each of these variables. In MAM, particulate formation will occur as long as the modulation amplitude is sufficiently large for undeformed chip thickness (s0) to reach zero during each cycle of the modulation and frequency of modulation (fm) is not an integer multiple of frequency of workpiece rotation (fw). For deformation analysis of the bulk brass and chip particulates, X-ray diffraction (XRD) data was used. After obtaining X-ray patterns, line broadening analysis was used to measure the level of deformation. In order to obtain internal strain, Williamson-Hall method and for calculating crystallite size, Scherrer method were used. Metallurgical studies of the bulk brass and chip particulates were done using optical microscopy, SEM and EBSD, whereas for mechanical characterization, micro-hardness and nano-hardness testers were used. The particulates of different shapes and sizes ranging from 100 µm to 5 mm with aspect ratio of ∼ 10 were produced using different modulation and machining conditions by MAM. Average shear strain for the chip length ratio of 0.511 was found to be 2.32 for the particulates. Shear strain analysis showed that the variation in deformation level in chip particulates produced at different fm/fw ratio is less than 5%. It has also been found that the modulation parameter did not affect the level of deformation in chip particulates. It has been observed that with a decrease in chip particulate size, the crystallite size also decreased, while the internal strain increased. With an increase in the value of fm/fw ratio, the chip lengths became increasingly smaller; however, there was no noticeable difference in microstrain and crystallite size of these chips. In comparison to the bulk brass, the chips produced at different fm/fw ratios have been found to have smaller crystallite size, however with increased microstrain. This change in crystallite size and microstrain may be attributed to the severe plastic deformation (SPD) taking place during MAM. This SPD may have increased the defects in chip particulates, thereby changing the microstrain and the crystallite size of the particulates. It has been observed that with decrease in particulate size, internal strain increased and crystallite size decreased. The results of microstructural analysis showed that there existed an ultrafine grained microstructure at the edge of chip particulates, while the central area of the chip particulates had elongated and equiaxed grained microstructure. EBSD analysis show that chip particulates have a refined grain structure in comparison to the bulk brass. Texture analysis from pole figure and misorientation plots showed the evolution of different texture in bulk brass and chip particulates. The results of the nano-indentation showed that hardness and Young’s modulus of chip particulates were higher, which may be attributed to the bimodal ultrafine microstructure observed in the chip particulates.