On tool wear and its effect on machined surface integrity

This paper presents the results of an investigation of induced residual stress, induced strain, and induced subsurface energy in machined surfaces due to the machining process. The influence of tool wear on residual stress, strain, and energy is also reported. The exact elasticity solution for a split ring was extended and used to calculate the residual stress in the machined surface by using ring dimension changes caused by the electrochemical removal of a thin layer of residually stressed surface. The strain distribution beneath the machined surface was determined by using the grid technique. The subsurface energy stored in the machined surface was then obtained from the data of residual stress and strain. For the materials studied, this investigation showed that such energy could not be neglected when establishing the total energy needed for machining a unit volume of material. Tool coatings having different surface roughness and tools having various magnitudes of flank wear were investigated. The experimental results show that tool wear is a dominant factor affecting the values of induced residual stress, strain, subsurface energy, and the quality of the machined surface. The increase of tool wear caused an increase of residual stress and strain beneath the machined surface. It was also found that the overall energy stored in the machined subsurface increases as the tool wear increases and as the tool surface gets rougher. When the cutting tool is severely worn, the machined surface not only becomes very rough, but also contains many partially fractured laps or cracks. This makes tool wear a key factor in controlling the quality of the machined surface.     

Production of multifluted drills on six-axis CNC tool-grinding machine

This paper presents a kinematic model to produce multifluted drills on a six-axis CNC tool-grinding machine. There are three special features of the proposed model. The first is the use of Denavit-Hartenberg notation to establish the ability matrix of a six-axis CNC tool-grinding machine. The second feature, the concept of a virtual hollow grinding wheel and virtual solid dresser, is invented to allow flutes and flanks to be machined by a single grinding wheel in one setup, thus improving manufacturing precision and lowering production costs. Thirdly, we present configuration matrices to express grinding wheel positions and orientations during machining of flutes and flanks. NC data equations are obtained by equating corresponding elements of the ability matrix and the configuration matrices of the grinding wheel. To verify the proposed methodology, a designed three-fluted drill was machined on a six-axis CNC tool-grinding machine. This paper combines the activities of drill design and manufacturing, making the production process more flexible, automatic, cost efficient and controllable.     

Greenhouse gases emitted in manufacturing a product—A new economic model

In this paper, we propose a machining microeconomic model that can optimize machining parameters and include all energy and environmental costs. A survey of microeconomic machining cost models is covered, with the result that a new cost model has been developed based on life cycle analysis (LCA) methodology. The scope includes the initial part production. Theoretical and actual experimental results are used to illustrate the model's implications with respect to carbon emissions and cost sensitivity. It is shown that for a manufacturing strategy, more certainty is required for inputs like carbon pricing to reduce financial risk. The limitations of the model, policy issues and future work are outlined.     

A quick method for evaluating the thresholds of workpiece surface damage in machining

This paper proposes a Pendulum-Based Cutting Test (PBCT) methodology which allows quick cutting tests for surface integrity evaluation along with providing cutting energies associated with particular level of workpiece surface damage, this is backed by an unified cutting energy model that links damage level of machined surface with energy partition in the cutting area. PBCT method could rapidly define the energy transferred to the workpiece that incurs particular magnitude of surface damage without using conventional machine tools and monitor the cutting process while only limited amount of materials is required. A demonstration of the proposed method is presented for Inconel718.     

Study of the Diffusion of Carbon, Its Sources, and Effect on Finishing Micro-EDM Performance of Cemented Carbide

Apart from the necessity of surface modification based on different applications, in most of the cases, diffusion of carbon or foreign particles on the workpiece surface during micro-electrodischarge machining (micro-EDM) is avoidable, especially in finishing micro-EDM. This study aims to investigate different sources of materials that migrate to the machined surface during fine-finishing of micro-EDM of cemented tungsten carbide (WC-Co). The machined surfaces have been examined under scanning electron microscope and energy dispersive x-ray to investigate the changes in chemical composition. It has been observed that during finishing of micro-EDM, the major source of materials' transfer to both the workpiece and electrode is the diffusion of carbon that comes from the decomposition of the hydrocarbon dielectric. In addition, materials from both workpiece and electrode transfer to each other based on machining conditions and discharge energy. The migration occurs more frequently at lower gap voltages during die-sinking with micro-EDM because of low spark gap and stationary tool electrode. Milling micro-EDM results in lower amount of carbon migration and fewer surface defects that improve the overall surface finish significantly.     

EDM and the resulting hydrogen embrittlement of maraging steel

The use of electrical discharge machining (EDM), or spark erosion as it is sometimes called, introduces hydrogen into maraging steel 250 such that brittle fracture surfaces result from embrittlement accelerated by slow-strain-rate tensile tests. Brittle fracture features indicate potential premature failure due to hydrogen embrittlement. Experimental control was provided by test specimens machined by abrasive waterjet, a machining technique that does not evolve hydrogen as a part of the machining process. Hydrogen concentration measurements indicate the hydrogen concentration of specimens fabricated by EDM is approximately 0.5 ppm, while that of specimens machined by abrasive waterjet is approximately 0.1 ppm. On the basis of constant-load tests, the time to failure of test specimens machined by EDM and loaded to 50% of the yield strength is estimated to be a minimum of 30 years.     

An Optimum Two-tool Solution for Milling 2½D Features from Technological and Geometric Viewpoints

This paper describes a procedure to determine the optimum pair of tools that can machine a milling feature with soft and/or hard boundaries. The optimum cutting conditions, as well as the actual distances traversed by the two tools, are used in the determination of the total machining cost. In addition to technological constraints such as machine tool power, geometrical constraints including minimum concave radius, bottleneck width and entry distance are determined from the Voronoi diagram. The paper also describes a novel method to determine the stock machined by the larger tool. An example is included to illustrate the method.     

涂层硬质合金刀片车削典型CFRP的工程技术研究

用相同几何结构、不同材质的2种涂层硬质合金刀片车削碳纤维增强复合材料T800H棒料,通过多分量力学传感器、压力传感器和高速摄影机监测车削过程中的切削载荷和刀具工作状态,通过光学扫描系统和数码显微系统观测分析工件已加工表面和切屑的形貌特征。结果表明:车削T800H棒料时,每种刀具都存在一个临界切削速度v_cr,在相同进给量f和恒定切削深度a_p的情况下,切削速度v_c对工件已加工表面质量影响小。实验还表明:采用硬度更高的刀具车削碳纤维,能获得更好的表面质量。      

The optimal cutting-parameter selection of production cost in HSM for SKD61 tool steels

The main purpose of this study was to construct an investigation of optimal cutting parameters for minimizing production cost on the rough machining of high speed milling operation. A machining model is constructed based on a polynomial network. The polynomial network can learn the relationships between cutting parameters (cutting speed, feed per tooth, and axial depth of cut) and tool life through a self-organizing technique. Once the material removal volume for machined parts and various time and cost components of the high speed milling operations are given, an optimization algorithm using a simulated annealing method is then applied to the polynomial network for determining optimal cutting parameters. The optimal cutting parameters are subjected to an objective function of minimum production cost with the feasible range of cutting parameters.     

外圆磨削参数优化模型及其应用

磨削加工工艺成本很高,磨削用量的选择意义重大。分析磨削用量及其对加工精度和生产率的影响,针对外圆磨削用量的选择,建立了以磨削多工序成本组成的工艺成本最低为目标的磨削参数优化模型。该模型以表面粗糙度、磨削工件刚度、磨床功率、磨床参数、磨削余量作为约束条件,以磨削余量、纵向进给量、横向进给量3个磨削参数为优化变量。以某型号电机轴为应用案例,运用MATLAB语言对其磨削参数进行了优化求解,与根据磨削手册和经验选择的磨削参数进行比较表明:优化后的外圆磨削工艺成本可降低近18%,从而验证了模型的有效性。     

Microscopic phase-dependent residual stresses in the machined surface layer of two-phase alloy

Residual stresses in the machined surface layer, which affect fatigue crack nucleation and stress corrosion cracking especially in aerospace engines and gas turbines for power generation, depend on microstructures in case of machining a multiple-phase alloy. Hence, the microscopic phase-dependent residual stresses should be known when a machined part is used under critical stress conditions and circumstances. In the present paper, finite element modeling of machining two-phase alloys has been developed for obtaining the residual stresses in the machined surface layer. Iron and steels, which consist of different volume fractions of ferrite and eutectoid pearlite, were selected as work materials to be machined. First, it was confirmed that the calculated results agree well in chip formation and cutting forces with experimental ones. Then, residual stresses in the machined surface layer were obtained for different carbon contents and regular/random arrangements of microstructure. As a result, it is found that the microstructure of the workpiece has a great influence on the residual stress distribution on the machined surface and that tensile surface residual stress on pearlite is much larger than that on ferrite. Finite element machining of the work material with stripe arrangement of ferrite and pearlite revealed that the peak of residual stress would be reduced by decreasing the width of stripes of ferrite and pearlite.     

Theoretical Estimation of Machined Surface Profile Based on Cutting Edge Movement and Tool Orientation in Ball-nosed End Milling

This paper presents the theoretical estimation method of machined surface profile without actual machining in ball-nosed end miling. The fundamental simultaneous equations of identifying the cusp height at any point of a workpiece in the simulated surface have been successfully derived from the geometric relationship between the cutting edge movement and the normal line at the point. By the numerical calculation, the machined surface profile can be estimated and illustrated graphically. It was found that the maximum and minimum cusp heights exist in a narrow range of tool orientation less than 3 degrees near the normal direction.     

Fabrication of deep micro-holes in reaction-bonded SiC by ultrasonic cavitation assisted micro-EDM

Ultrasonic vibration was applied to dielectric fluid by a probe-type vibrator to assist micro electrical discharge machining of deep micro-holes in ceramic materials. Changes of machined hole depth, hole geometry, surface topography, machining stability and tool material deposition under various machining conditions were investigated. Results show that ultrasonic vibration not only induces stirring effect, but also causes cloud cavitation effect which is helpful for removing debris and preventing tool material deposition on machined surface. The machining characteristics are strongly affected by the vibration amplitude, and the best machining performance is obtained when carbon nanofibers are added into the vibrated dielectric fluid. As test pieces, micro-holes having 10 μm level diameters and high aspect ratios (>20) were successfully fabricated on reaction-bonded silicon carbide in a few minutes. The hybrid EDM process combining ultrasonic cavitation and carbon nanofiber addition is demonstrated to be useful for fabricating microstructures on hard brittle ceramic materials.     

High-speed grooving with applying MQL

The performance of minimum quantity lubrication (MQL) in high-speed cutting was evaluated in grooving 0.45%C carbon steel with a carbide tool coated with TiC/TiCN/TiN triple coating layers. MQL with supplying vegetable oil at a small and constant rate of 7 ml/h reduced the corner and flank wears more effectively than a solution type of cutting fluid at high cutting speeds of 4 and 5 m/s. In MQL grooving, the wears decreased drastically with increasing the pressure of air supply. This suggested that the air supply took an important role in transporting the oil mist to the interface between the flank wear land and machined surface. Then, a controlled oil mist direction (COD) tool was devised and its performance was proved to be high at a reduced rate of oil supply.     

Powder solders for hard soldering of aluminium and its alloys

In this paper, the possibilities for forming non-circular pulleys using laser cutting techniques and cutting techniques using water jets with abrasive materials are described. Research on laser cutting with use of gases for formation of carbon steel and acid-resistant steel pulleys is also discussed. The advantages of water jet cutting of non-circular pulleys are shown, and the methodology for measuring machined pulleys with a coordinate measuring technique is also presented. Use of computer-aided design systems with a range of software enabling automation of CNC contour saw control and optimization processes for formed elements spacing are all also discussed.     

Residual stress distribution caused by milling

The effects of cutting speed, feed rate and depth of cut on the residual stress distribution in the machined surface region caused by milling of five different materials are determined using an electrolytic etching-deflection technique. The analysis of the experimental data is carried out using response surface methodology (RSM).The results show that the residual stress is low tensile at the machined surface and increases with an increase in depth beneath the surface reaching a maximum tensile, then decreases with a further increase in depth, eventually becoming vanishingly small. The peak residual stress is found to be strongly dependent on both milling conditions and tensile strength of work material. A mathematical model correlating the process input parameters and their interactions with the residual stress is proposed.     

细长轴车削与磨削参数优化研究

针对细长轴车削和磨削加工参数的选择问题,建立以工件刚度、刀片强度、刀杆刚度、机床进给机构强度、机床功率、机床参数、加工表面粗糙度、加工余量等作为约束条件,以切削速度、进给量、背吃刀量、工件转速、磨削余量、径向进给量等参数为优化变量,以车、磨多工序成本最低为目标的切削参数优化模型。以某型号电机轴为案例,运用MATLAB模式搜索工具箱对其车削和磨削参数进行寻优,与切削参数的传统选择方法比较表明,工序成本可降低35%以上,从而验证了优化模型的有效性。     

Investigations of White Layer Formation During Machining of Powder Metallurgical Ni-Based ME 16 Superalloy

Surface integrity of machined parts made from the advanced Ni-based superalloys is important for modern manufacturing in the aerospace industry. Metallographic observations of the ME 16 alloy microstructure were made using optical metallography and a high-resolution scanning electron microscope with energy dispersive x-ray spectrometer (HR SEM/EDS). Tool life of cemented carbide inserts with TiAlN coating during machining (finishing turning operation) of ME 16 superalloy has been studied and wear patterns of the cutting tools were identified. Surface integrity of the machined part after completion of the turning operation was investigated. The morphology of machined parts has been examined and cross-sections of the machined surfaces have been analyzed. The formation of white layer on the surface of the machined part was studied for varied machining conditions. It was found that a 2-4 μm thick white layer forms during turning of the ME 16 superalloy. This layer was investigated using EDS and XRD. The studies show that the white layer is an oxygen-containing layer with a high amount of aluminum, enriched by chromium and tungsten. Under specific cutting conditions, the structure of white layer transforms into a γ-alumina. Formation of this thermal barrier ceramic white layer on the surface of the machined part negatively affects its surface integrity and cutting tool life.     

A semi-empirical approach for residual stresses in electric discharge machining (EDM)

High residual stresses are developed on the surfaces of electric discharge machined parts. In this study, layer removal method is used to measure the residual stress profile as a function of depth beneath the surface caused by die sinking type EDM. Cracking and its consequences on residual stresses are also studied on samples machined at long pulse durations. A modified empirical equation is developed for scaling residual stresses in machined surfaces with respect to operating conditions. In this model, a unit amplitude shape function representing change in curvature with respect to removal depth is proposed. The proposed form is found to be a special form of a Gauss Distribution. It is the sum of two Gaussian peaks, with the same amplitude and pulse width but opposite center location. The form can be represented by three constant coefficients. These coefficients depend on the released energy by a power function.     

5-Axis adaptive flank milling of flexible thin-walled parts based on the on-machine measurement

Deformation of the part and cutter caused by cutting forces immediately affects the dimensional accuracy of manufactured parts. This paper presents an integrated machining deviation compensation strategy based on on-machine measurement (OMM) inspection system. Previous research attempts on this topic deal with deformation compensation in machining of geometries in 3-axis machine tools only. This paper is the first time that concerned with 5-axis flank milling of flexible thin-walled parts. To capture the machined surface precision dimensions, OMM with a touch-trigger probe installed on machine׳s spindle is utilized. Probe path is planned to obtain the coordinate of the sampling points on machined surface. The machined surface can then be reconstructed. Meanwhile, the cutter׳s envelope surface is calculated based on nominal cutter location source file (CLSF). Subsequently, the machining error caused by part and cutter deflection is calibrated by comparing the deviation between the machined surface and the envelope surface. An iteration toolpath compensation algorithm is designed to decrease machining errors and avoid unwanted interference by modifying the toolpath. Experiment of machining the impeller blade is carried out to validate the methodology developed in this paper. The results demonstrate the effectiveness of the proposed method in machining error compensation.