A study of Elliptical Vibration Cutting in Ultra Precision Machining

Ph.DDOCTOR OF PHILOSOPH

Vibration assisted machining: Modelling, simulation, optimization, control and applications

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University, 30/11/2010.Increasing demand for precision components made of hard and brittle materials such as glasses, steel alloys and advanced ceramics, is such that conventional grinding and polishing techniques can no longer meet the requirements of today's precision manufacturing engineering. Particularly, in order to undertake micro-milling of optical glasses or other hard-machining materials, vibration assisted machining techniques have been adopted. However, it is essential and much needed to undertake such processes based on a scientific approach, i.e. the process to be quantitatively controlled and optimized rather than carried out with a trial-and-error manner. In this research, theoretical modelling and instrumental implementation issues for vibration assisted micro-milling are presented and explored in depth. The modelling is focused on establishing the scientific relationship between the process variables such as vibration frequency, vibration amplitude, feedrate and spindle speed while taking into account machine dynamics effect and the outcomes such as surface roughness generated, tool wear and material removal rate in the process. The machine dynamics has been investigated including a static analysis, machine tool-loop stiffness, modal analysis, frequency response function, etc, carried out for both the machine structure and the piezo-actuator device. The instrumentation implementation mainly includes the design of the desktop vibration assisted machining system and its control system. The machining system consists of a piezo-driven XY stage, air bearing spindle, jig, workpiece holder, PI slideway, manual slideway and solid metal table to improve the system stability. The control system is developed using LabVIEW 7.1 programming. The control algorithms are developed based on theoretical models developed by the author. The process optimisation of vibration assisted micro-milling has been studied by using design and analysis of experiment (DOE) approach. Regression analysis, analysis of variance (ANOVA), Taguchi method and Response Surface Methodology (RSM) have been chosen to perform this study. The effects of cutting parameters are evaluated and the optimal cutting conditions are determined. The interaction of cutting parameters is established to illustrate the intrinsic relationship between cutting parameters and surface roughness, tool wear and material removal rate. The predicted results are confirmed by validation experimental cutting trials. This research project has led to the following contribution to knowledge: (1) Development of a prototype desktop vibration assisted micro-milling machine. (2) Development of theoretical models that can predict the surface finish, tool wear and material removal rate quantitatively. (3) Establishing in depth knowledge on the use of vibration assisted machining principles. (4) Optimisation of cutting process parameters and conditions through simulations and machining trials for through investigation of vibration assisted machining.Financial support was obtained from Brunel University

Hard turning of martensitic AISI 440B stainless steel

Hard turning has been in use for some time to achieve close dimensional tolerances to eliminate time consuming and costly grinding operations. The most widely used cutting tools for finish machining of hardened steels under dry cutting conditions are the ceramics and PcBN cutting tools. The purpose of this study was to investigate the machinability of hardened martensitic AISI 440 B stainless steel (HRC 42-44) using commercially available cutting tools: alumina based ceramic and PcBN, by hard turning under different machining conditions, by providing an in-depth understanding of wear mechanisms of these cutting tools. The study also developed a serrated chip formation mechanism of the workpiece and provided a deep understanding of the chemical interaction between workpiece and cBN cutting tools, through microstructural analysis of the adhered layer on the worn cutting tool. Experimental studies on the effects of cutting parameters on the tool wear mechanism, cutting forces; surface roughness, dimensional accuracy, and chip formation mechanism were investigated. The characterization of the workpiece, cutting tools, chips and wear scars on the cutting tools was performed using an X-ray diffractometer, and optical, scanning and transmission electron microscopes, as well as an energy dispersive spectroscope (EDS). The cutting speeds selected for testing the cutting tools were in the range of 100 m/min and 600 m/min, depending on the type of parameter investigated. Two depths of cut, 0.1and 0.2 mm, and three feed rates, 0.05, 0.1 and 0.15 rev/min, were selected for the experiments. Experimental results showed that the flank wear in the PcBN cutting tool is lower than that of the mixed alumina, with PcBN showing better wear resistance at all cutting conditions (about five times longer in some instances). Apart from the cutting speed, the feed rate was found as a parameter that directly influences the flank wear rate of the cutting tool. The wear mechanism for the ceramic cutting tool is predominantly abrasive wear, and for PcBN tools it was adhesive wear and abrasive wear. The abrasive wear was caused by hard carbide particles in the workpiece material resulting in grooves formed on the flank face. There was formation of a transferred layer followed by plastic deformation of the workpiece material on the rake face of the PcBN tool when cutting at low cutting speed and feed rate. At much higher cutting speeds, some form of chemical wear preceded by adhesion and abrasion was the main tool wear resulting from the chemical affinity between the PcBN tool and the workpiece. Better surface finish (Ra) was recorded for mixed ceramics but with deteriorating surface topography. The increase in the cutting speed results for improvement in the surface finish produced by both cutting tools was investigated. The final part, using the PcBN cutting tool, produced better dimensional accuracy resulting from its better wear resistance at the flank face. The results also show that good dimensional accuracy can be achieved with cBN tools using a CNC machine with high static and dimensional stiffness coupled with high precision hard turning. The influence of cutting conditions on the chip formation showed production of continuous chip at a cutting speed of 100 m/min and segmented chip at higher cutting speeds above 200 m/min by both cutting tools. The increasing cutting speed affects the formation of shear localised chips with rapid increase in shear strain rate and degree of segmentation at cutting speeds higher than 200 m/min. The microstructure of the chip produced shows the distinct carbide grain in the martensite of the work material with intense shear localisation in the primary deformation zone of the cutting tool and formation of white layer in the secondary deformation zone. The microstructure of the crater of the worn PcBN cutting tool at cutting speeds of 100 m/min and 600 m/min were studied in detail. A situ lift-out technique, in a Focused Ion Beam/SEM instrument, was used to produce thin foil specimens, which were taken out of the crater face of the PcBN tool and observed using SEM and TEM. The SEM and TEM study showed evidence of chemical interaction between the work material and the PcBN tool. Fe from the work material was found in the vicinity of TiC and AlB grains of the PcBN tool, with TiC having greater affinity for Fe. Oxidation of the elements was common in all Fe-rich areas. The microstructure of the worn PcBN cutting tool at the cutting speed of 600 m/min showed deeper penetration of Cr and Fe into the cBN tool, which was not easily detected by SEM at the cutting speed of 100 m/min. The hard turning operations using the PcBN cutting tool for substituting traditional machining operations was successfully performed in the industrial environment. The overall surface finish and dimensional accuracy generated during the application of CBN-100 for machining within the industrial environment on specified mass produced shape showed a component acceptable tolerance range with good surface finish similar to that of the grinding operation

Improvements in ultrasonically assisted turning of TI 15V3Al3Cr3Sn

Titanium alloys have outstanding mechanical properties such as high hardness, a good strength-to-weight ratio and high corrosion resistance. However, their low thermal conductivity and high chemical affinity to tool materials severely impairs their machinability with conventional techniques. Conventional machining of Ti-based alloys is typically characterized by low depth of cuts and relatively low feed rates, thus adversely affecting the material removal rates (MRR) during the machining process. Ultrasonically assisted turning (UAT) is an advanced machining technique, in which ultrasonic vibration is superimposed on a cutting tool. UAT was shown to improve machinability of difficult-to-machine materials, such as ceramics, glass or hard metals. UAT employment in the industry is, however, currently lacking due to imperfect comprehensive knowledge on materials‘ response and difficulties in obtaining consistent results. In this work, significant improvements in the design of a UAT system were performed to increase dynamic and static stiffness of the cutting head. Concurrent improvements on depth-of-cut controls allowed precise and accurate machining operations that were not possible before. Effects of depth of cut and cutting speed were investigated and their influence on the ultrasonic cutting process evaluated. Different cutting conditions -from low turning speeds to higher recommended levelwere analysed. Thermal evolution of cutting process was assessed, and the obtained results compared with FE simulations to gain knowledge on the temperatures reached in the cutting zone. The developed process appeared to improve dry turning of Ti-15-3-3-3 with significant reduction of average cutting forces. Improved surface quality of the finished work-piece was also observed. Comparative analyses with a conventional turning (CT) process at a cutting speed of 10 m/min showed that UAT reduced the average cutting forces by 60-65% for all levels of ap considered. Temperature profiles were obtained for CT and UAT of the studied alloy. A comparative study of surface and sub-surface layers was performed for CT- and UAT-processed work-pieces with notable improvements for the UAT-machined ones. Two- to three-fold reductions of surface roughness and improvements of other surface parameters were observed for the UAT- machined surfaces. Surface hardness for both the CT- and UAT-machined surfaces was investigated by microindentation. The intermittent cutting of the UAT-process resulted in reduction of hardening of the sub-surface layers. Optical and electronic metallographic analyses of cross-sectioned work-pieces investigated the effect of UAT on the grain structure in material‘s sub-surface layers. Backscatter electron microscopy was also used to evaluate the formation of α-Ti during the UAT cutting process. No grain changes or α-precipitation were observed in both the CT- and UAT-machined work-pieces

Selected area hot machining with a multi-tipped diamond tooling system.

An investigation of a novel ultra-precision tool is presented, the hot-milling multi-turret diamond tool. The tool implements micro-Laser Assisted Machining (μ-LAM) upon a multiple diamond fly cutter. Details of its design, both mechanical and optical, are presented- along with proposed modifications to Cranfield University’s Tetraform C machine to allow for its implementation. Relevant experimentation is presented- the modal analysis of the Tetraform C machine and the testing of laser heating on a silicon substrate. FE analyses are carried out and calculations presented to justify design choices. Analytical investigations predicted increased depths of cut by using laser, and indicated that at lower wavelengths of laser light and with lower feed rates that the depth of cut may be significantly increased. A novel method of selecting laser wavelength based upon change in material absorptivity with temperature is investigated which may offer substantial increases in μ-LAM performanceMRes in Manufacturin

An Iterative Size Effect Model of Surface Generation in Finish Machining

In this work, a geometric model for surface generation of finish machining was developed in MATLAB, and subsequently verified by experimental surface roughness data gathered from turning tests in Ti-6Al4V. The present model predicts the behavior of surface roughness at multiple length scales, depending on feed, nose radius, tool edge radius, machine tool error, and material-dependent parameters—in particular, the minimum effective rake angle. Experimental tests were conducted on a commercial lathe with slightly modified conventional tooling to provide relevant results. Additionally, the model-predicted roughness was compared against pedigreed surface roughness data from previous efforts that included materials 51CrV4 and AL 1075. Previously obscure machine tool error effects have been identified and can be modeled within the proposed framework. Preliminary findings of the model’s relevance to subsurface properties have also been presented. The proposed model has been shown to accurately predict roughness values for both long and short surface roughness evaluation lengths, which implies its utility not only as a surface roughness prediction tool, but as a basis for understanding three-dimensional surface generation in ductile-machining materials, and the properties derived therefrom

INFLUENCE OF SIZE EFFECTS ON SURFACE GENERATION DURING FINISH MACHINING AND SURFACE INTEGRITY IN TI-6AL-4V

Finish machining is an essential manufacturing process that is used to enhance the mechanical characteristics of critical components. The deformation that occurs at the tool and workpiece interface in finish machining significantly affects a host of component properties, commonly referred to as “surface integrity” properties. Surface roughness is a machining deformation-affected characteristic that is of high relevance in contemporary manufacturing. However, over recent decades it has been made clear that the material properties of the deformed surface layers are relevant to component performance as well. Predicting the overall surface quality of a machined component is of great relevance to the manufacturing industry. Current state-of-the-art predictive models in the area of machining-induced surface integrity are typically founded in two-dimensional F.E.M. analysis. These investigations frequently show the advantages of tool geometry manipulation. However, most efforts focus solely on the prediction of two-dimensional surface integrity qualities such as those found in orthogonal machining. Indeed, most recent models largely ignore three-dimensional properties such as surface roughness, and do not incorporate three-dimensional machining parameters that are highly relevant to the surface integrity state of typical finished components. In light of these shortcomings, the nature of surface integrity in three-dimensional machining is explored, and a physics-based geometric model of surface generation is applied to some areas of surface integrity prediction. The main focus of this work is to investigate and model the relationship between the more dominant parameters in finish turning (feed, nose radius, and edge geometry) and the surface generation phenomena that occur in the application of tools with varied geometries of this scope. The presented geometric model is derived from unique assumptions that allow for the close approximation of surface generation. The model is subsequently validated with experiments that utilize modified turning inserts of precise edge geometry, as well as pedigreed data from previous literature. Good agreement with experimental roughness results is obtained, thus verifying the validity of the surface generation assumptions. In addition, subsurface properties are found to correlate well with the geometry of ploughed areas predicted by the modeling methodology presented in this text

Hot ultrasonically assisted turning of Ti-15V3Al3Cr3Sn: experimental and numerical analysis

Titanium alloys have outstanding mechanical properties such as high hardness, a good strength-to-weight ratio, excellent fatigue properties and high corrosion resistance. However, several inherent properties including their low thermal conductivity and high chemical affinity to tool materials impairs severely their machinability with conventional machining techniques. Conventional machining of Ti-based alloys is typically characterized by low depths of cuts and relatively low feed rates, thus adversely affecting the material removal rates during the machining process. Recently, a non-conventional machining technique known as ultrasonically assisted turning (UAT) was introduced to machine modern alloys, in which low-energy, high-frequency vibration is superimposed on the movement of a cutting tool during a conventional cutting process. This novel machining technique results in a multi-fold decrease in the level of cutting forces with a concomitant improvement in surface finish of machined modern alloys. Also, since the late 20th century, machining of wear resistant materials that soften when heated has been carried out with hot machining techniques. In this work, a new hybrid machining technique called Hot Ultrasonically Assisted Turning (HUAT) is introduced for processing of a Ti-based alloy Ti-15V3Al3Cr3Sn. In this technique, UAT is combined with a traditional hot machining technique to gain combined advantages of both schemes for machining of intractable alloys. HUAT of the studied alloy was analysed experimentally and numerically to demonstrate its benefits in terms of reduction in cutting forces over a wide range of industrially relevant speed-feed combinations. Thermal evolution in the cutting process was assessed, and the obtained results were compared with FE simulations to gain knowledge of temperatures reached in the cutting zone. The developed novel turning process appeared to improve dry turning of the Ti alloy with significant reduction of average cutting forces without any substantial metallurgical changes in the workpiece material. Nano-indentation, light microscopy and SEM studies were performed to get an insight into the development of hardness in a zone near the machined surface in the workpiece. Backscatter electron microscopy was also used to evaluate the formation of α-Ti during the novel HUAT. No grain changes or α-precipitation were observed in machined workpieces in conventional and hybrid turning processes. 3D elasto-plastic thermomechanically coupled finite-element models for the orthogonal turning process were developed for conventional turning (CT), hot conventional turning (HCT), UAT and HUAT, followed by a more realistic novel 3D finite-element model for the oblique turning process. These 3D models were used to study the effects of cutting parameters (cutting speed, feed rate and depth of cut, ultrasonic vibration, ultrasonic frequency, rake angle and tool nose radius) on cutting forces, temperature in the process zone and stresses. The later model was used to analyse the effect of vibration and heat on the radial and axial components of cutting forces in HUAT, which was not possible with the developed 3D orthogonal-turning model. Comparative studies were performed with the developed CT, HCT, UAT and HUAT finite-element models and were validated by results from experiments conducted on the in-house prototype and in literature. The HUAT for the Ti-15333 was analysed experimentally and numerically to demonstrate the benefits in terms of a significant reduction in the cutting forces and improvement in surface roughness over a wide range of industrially relevant speed-feed combinations

Understanding nanoscale material behaviour for improved precision machining of shape memory alloys; testbed study on elliptical vibration assisted cutting of CuZr SMA.

The field of ultra-precision machining has gained significant importance in the manufacture of components for the electronic, optical and medical industry. Two crucial factors that play a key role in the machinability of materials are the machining parameters and the material’s physical properties. Certain materials such as hardened steel or nickel-based superalloys are difficult-to-machine but innovations in the field of precision machining have developed a technique known as elliptical vibration assisted machining, which enables to improve the machinability of these materials. CuZr high-temperature shape memory alloy is categorized as a difficult-to-cut material and although EVAM has been applied to a wide range of metals it hasn’t yet been studied in CuZr HTSMA. In this context, the purpose of this thesis is twofold: On the one hand, to characterise the mechanical properties of CuZr SMA using Molecular Dynamics and, on the other hand, to explore the nanoscale mechanism of material removal of CuZr shape memory alloy (SMA) during elliptical vibration assisted machining (EVAM). The conclusions of this thesis can be summarized as follows. To characterise the mechanical properties of Cu₅₀Zr₅₀, Cu₂Zr and Cu₅Zr, a tensile and shear test were carried out using MD. Tensile test was done with crystal orientation and direction of tensile pulling as . The results showed that Cu₅₀Zr₅₀ and Cu₂Zr exhibited a phase transformation (pseudoelasticity) during loading. However, Cu₅Zr showed dislocation nucleation as the main plastic deformation mechanism followed by fracture. Shear tests were done in the same phases with crystal orientation and direction of shear pulling as . Interestingly, the shear test results showed no phase transformation for Cu₅₀Zr₅₀ and Cu₂Zr but the Cu₅Zr composition did show phase transformation during loading. It is important to highlight that all three phases of CuZr binary alloy that we have tested showed a different plastic response during the tensile test and the shear test. As far as machining is concerned, we observed indications that EVAM shows improved machinability compared with conventional machining. Although cutting forces were lower in EVAM, the stresses on the workpiece were slightly higher and both techniques showed the same mechanism of plasticity during machining. Neither dislocation nucleation or martensitic transformation was exhibited in either of the two machining techniques and instead, amorphisation was observed as the main plastic deformation mechanism in both cases. Interestingly, amorphisation has been previously observed by Saitoh and Kubota (2010) during loading NiTi SMA [1]; however, it didn’t show up in every crystal orientation confirming that NiTi shows significant changes in response to loading in different lattice directions. One of the main outcomes from this thesis is that CuZr SMA exhibits different modes of plastic deformation; namely amorphisation, dislocation nucleation and martensitic transformation during loading. The governing mechanism that arises during loading highly depends in the lattice direction in which the load is being applied. These findings can potentially enable reliable predictions and provide guidelines of the microstructural design of CuZr SMA systemsPhD in Manufacturin