Advanced Materials Research Vols. 83-86

Abstract: An attempt has been made to investigate the chatter formation during two machining operations namely, turning and thread cutting on a non-wavy surface of plain carbon steel with a view to observing the formative mechanisms of chatter. Investigations have been conducted on the chips and common types of discreteness in the form of serrated/saw teeth have been identified. Mechanisms of formation of these teeth have been studied and the frequencies of their formation have been determined at different cutting speeds. The different modes of the vibrating components have been extracted by modal analysis and the vibration responses during cutting conditions have also been recorded using an online data acquisition system. All the experiment were conducted on a no wavy surface and thread cutting was conducted on nascent surface, the existence of chatter observed during the experiments prove that the regenerative effect from the waviness of the previous pass, as postulated by the ‘Regenerative Chatter Theory’ is not correct in explaining the primary cause of chatter during metal cutting. The analysis of amplitude of chatter during machining indicates that noticeable chatter appears in the system when the chip serration frequency is equal to or integer multiple of the prominent natural frequency of the system components.

Abstract: End milling is considered to be one of the most commonly applied for both roughing and finishing operations to make flat surfaces, slots and pockets in precision molds and dies. Predictive models were developed for cutting force, flank wear and surface roughness in end milling aluminum alloy by regression analysis. The correlation coefficients for cutting force, flank wear and surface roughness equations were 91.6 %, 89.8 % and 79.6 %, respectively. The goal of these predictive equations is to become a good assistant to the researcher in understanding the machining process. Through the analysis of variance (ANOVA), however, it can be found that the cutter diameter, the helix angle and the feed rate are the important milling process parameters to obtain the machined quality on cutting force, flank wear and surface roughness. The investigations show that cutting force, flank wear and surface roughness can be improved in end milling aluminum alloy by using the lower cutter diameter, medium helix angle and lower feed rate.

Abstract: In grinding process, a stiff air layer is generated around the wheel periphery due to high surface speed of the porous grinding wheel. This stiff air layer restricts grinding fluid to reach deep inside the grinding zone. The formation of stiff air layer has been studied experimentally by obtaining the variation of air pressure around the wheel periphery under different wheel speeds. With the help of the experimental data, a mathematical model has been developed to predict the pressure of the stiff layer of air under different wheel speed. From the model it is found that at close proximity to the wheel, air pressure obtained is very high, and this establishes quantitatively the formation of stiff air layer around the grinding wheel.

Abstract: In order to simulate manufacturing processes, it is essential to have accurate information about mechanical behaviour of material for different deformation conditions depending on the type of the process. In finite element (FE) analysis based techniques for simulation, a constitutive equation is needed to model the mechanical behaviour of material. In the case of metal cutting, the Johnson and Cook (JC) flow stress model is the most suitable constitutive equation to be used in simulation since it contains the effects of strain, strain-rate and temperature. It is needed to evaluate the parameters and constants of the JC model to make it applicable in FE simulations. There are several ways to evaluate the parameters of the equation: experimental such as high strain-rate compression tests called “Split Hopkinson Pressure Bar” which is relatively complicated and expensive technique requiring special testing apparatus; and analytical approach based on Oxley’s theory. An integral method containing quasi-static compression and machining tests have been used in this paper to evaluate the JC equation parameters by fitting data from both tests for a Ti-alloy (Ti6Al4V). Finally the estimated JC model is validated by some other machining tests.

Abstract: In electrical discharge machining (EDM), material is removed by a series of electrical discharge between the electrode (tool) and the workpiece that develops a temperature of about 8,0000C to 12,0000C. Due to high temperature of the sparks, work material is melted and vapourized, at the same time the electrode material is also eroded by melting and vapourization. Electrodes wear (EW) process is quite similar to the material removal mechanism as the electrode and the workpiece are considered as a set of electrode in EDM. In the present study effort has been made to reduce EW by cooling, using liquid nitrogen during the EDM of titanium alloy. Investigation on the effect of cooling on electrode wear (EW), material removal rate (MRR) and surface roughness (Ra) of the workpiece was carried out. Current (I), pulse on-time (ton), pause off-time (toff) and voltage (v) were considered as the machining parameters. Design of experiment (DOE) was used to design the experimental works. Cooling of electrode by this technique reduced the melting and vapourization of electrode material and enhances electrode life. It was possible to reduce EW up to 27% by applying this technique while MRR and Ra were improved by 18% and 8% respectively.

Abstract: A new cutting force model of micro-milling was developed based on calculating the instantaneously projected area of cutting in this study. According to the rotation trajectory of cutting edge, the instantaneous cutting area projected to xy-plane was first divided into several portions and determined with rectangular integral. After the cutting coefficients were experimentally investigated, the cutting force of micro-milling with different cutting conditions can be determined. Because the developed cutting force was function of cutting parameters, such as spindle speed, feed rate, depth of cut, with taking the cutter deflection into account it was further used to the determine of optimal cutting parameters for obtaining better cutting efficiency and machining accuracy. To verify the proposed cutting force model and the way to determine the optimal cutting parameters, micro-milling experiments were conducted. The results have shown the feasibility of the proposed model and method.

Abstract: Micro Electric Discharge Machining is mostly used micromachining technique for manufacturing of micro holes and mini cavities. It is a thermal material removal process achieved by application of recursive electrical sparks between two conductive materials immersed in dielectric liquid. There are many electrical and technological parameters, which are effective in the machining characteristics and machined material surface integrity. In this study, effects of pulse energy on characteristics of surface damage in micro electric discharge machining of micro holes are investigated. To observe the influences of investigated pulse forms, a series of micro-holes were machined and cut from cross sections for metallurgical examinations. Unconventional etching agents were applied to examine the metallurgical transformations using optical and scanning electron microscopy. It was observed that heat-damaged segment is composed of three distinctive layers, have relatively high thicknesses (0.2 to 10 μm) when compared with the size of the micro holes and varies noticeably with respect to drilling dept. Different morphological structures and deformations in the subsurface of micro-hole walls at several sections were illustrated. Finally, analyzed machining parameters and variation of hat damaged layer thicknesses were discussed in terms of drilling depth.

Abstract: The scroll profile is complex with involute curve and high precision is required. Owing to the interpolation method has significant effects on the dynamic behaviour and to obtain the flexibility of the machining, this paper focuses on the development of a computer-aided manufacturing methodology for precision scroll by the combination of curve fitting and CNC interpolation. In this paper, the method for design and manufacture of the scroll profile is established based on the differential geometry and the enveloping theory. The cutter location using the cylindrical end mill is derived and the cutting path sequences are planned. Three types curve fitting methods: linear fitting, circular biarcs curve fitting, and NURBS curve fitting are adopted to approximate the toolpath of the scroll profile. The error control is used as the basis for generating appropriate toolpath to ensure the machining accuracy. The circular biarcs fitting and NURBS curve fitting can obtain continuous smooth toolpath. In the identical chordal deviation, the circular biarcs curve fitting provides the best performance. The toolpath generated by the proposed method is verified through the solid cutting simulation and the trial-cut on a three-axis machine tool.

Abstract: In this paper, the effect of machining parameters including cutting velocity, feed rate, and tool material on machining power of EN-AC 48000 aluminium alloy has been studied. A L27 Taguchi's standard orthogonal array has been applied as experimental design to investigate the effect of the factors and their interaction. Twenty seven machining tests have been accomplished with two random repetitions, resulting in fifty four experiments. EN-AC 48000 is an important alloy in automotive and aerospace industries. Machining of this alloy is of vital importance due to build-up edge and tool wear. Machining power is an essential parameter affecting the tool life, dimensional accuracy, and cutting efficiency. Three types of cutting tools including coated carbide (CD 1810), uncoated carbide (H10), and polycrystalline diamond (CD10) have been used in this study. Statistical analysis has been employed to study the effect of factors and their interactions using ANOVA analysis. Moreover, optimal factor levels have been presented using signal to noise ratio (S/N) analysis. Also, regression model have been provided to predict the machining power. Finally, the results of confirmation tests have been presented to verify and compare the adequacy of the predictive models.

Abstract: This paper presents a study of the thrust forces in the aluminium alloy, UNS A97050-T7. The measurement of these forces during the dry drilling in plates is evaluated under cutting different conditions and in through and blind holes. The experiments have been performed in a CNC milling and HSS Co drills have been utilised with unlike diameters (6.3 mm and 4 mm). The thrust forces have been measured by means of a piezoelectric dynamometer. In addition, the results have been compared with theoretical forces, obtained by the Kronenberg equation. The ANOVA study has allowed confirming the similitude between the forces obtained, independently of number of holes drilled, with the exception of through holes of 4 mm, in whose case the Kruskal-Wallis test has corroborated the dissimilitude. Moreover, the comparison with Kronenberg forces is coherent for diameters 6.3 mm, but it is very different for diameter 4 mm. Thus, the results respect to through and blind holes are dependent of diameter of drill, and also the differences found in the theoretical forces. The high variability of the forces for minor drills, in through and blind holes, points to an inadvisable use in this material.