The investigation of mechanism of serrated chip formation under different cutting speeds

Chip type is determined by the coupled effects of workpiece material property, cutting speed, uncut chip thickness, feed rate, and tool edge geometry. The understanding of chip formation plays a critical role in studying surface integrity and optimization of machining process variables. Serrated chip, one of the major important chip type, is usually formed in hard cutting at high speed. In this study, a new analytical model has been proposed to better understand the formation of serrated chip, and the simulations have been acquired using ABAQUS/Explicit in machining AISI 1045 during different speeds (from 60 to 6000 m/min). The workpiece material property is modeled with the Johnson-Cook model, and the experiments have been conducted with AISI 1045 during speeds from 60 to 1200 m/min. It has been shown that flow stress is influenced simultaneously by the strain rate hardening and temperature softening. When the speed reaches very high, the temperature softening will fail, and the strain rate hardening will play a more important role. Also, it can be found that the hardening ratio increases when the cutting speed rises. The results of the simulations and experiments correlated well. The cutting force and thrust force both decrease as the cutting speed increases, and the difference between them will shrink when the machining speed reaches a high level.     

Slip-line modeling of built-up edge formation in machining

Extensive investigations on built-up edge (BUE) formation in machining have been conducted in the past. However, very little effort has been made to quantitatively predict the size of the BUE and its effect on chip flow and cutting forces under different machining conditions. This prediction is important because it is the key to predicting the fluctuation of cutting forces and provides better rationale for explaining various machining phenomena associated with BUE formation. A new slip-line model for machining with BUE formation and its associated hodograph are proposed in this paper. Consisting of four slip-line sub-regions, the new slip-line model meets both the stress equilibrium and velocity requirements of material flow. The new model simultaneously predicts the length and height of the BUE, cutting and thrust forces, chip up-curl radius, chip thickness, and tool–chip contact length. Dewhurst and Collins's matrix technique for numerically solving the slip-line problem is employed in the mathematical formulation of the model, with non-unique solutions being obtained. It is demonstrated that one of the four slip-line angles included in the new model directly governs the size and surface shape of the BUE. Compared with the well-known Lee and Shaffer's model, the new model predicts a much longer BUE covering a larger portion of the tool rake face. A small tool rake angle tends to generate a large BUE. The predicted trends of the variation of relevant machining parameters are consistent with experimental observations.     

Mechanistic and Finite Element Model for Prediction of Cutting Forces During Micro-Turning of Titanium Alloy

Titanium alloy Ti-6Al-4V is commonly used in biomedical applications due to its superior properties such as biocompatibility, high strength-to-weight ratio and corrosion resistance. To understand the mechanics of the micro-turning process of these alloys, a mechanistic model has been developed for predicting the cutting forces. A modified Johnson–Cook material model with strain gradient plasticity is used to represent the flow stress of work material. The micro-turning experiments were conducted to verify the cutting forces predicted by mechanistic model. A finite element model is also developed with different shear friction factors and calibrated using experimental results to confirm and interpret the results of mechanistic model. It is inferred that strain rate increases by increasing cutting speed, whereas it decreases with increase in the feed rate due to increase in adiabatic shear band spacing. Since Ti-6Al-4V has low thermal conductivity, when cutting speed increases, there is an increase in the tool-chip interface temperature that leads to decrease in cutting forces. When cutting speed increases, chip morphology changes from discontinuous to continuous, and there is significant deterioration in the surface finish. It is observed that the average cutting force prediction errors for mechanistic and finite element models are 9.69% and 11.45% respectively.     

金属切削过程的有限元法仿真研究

根据材料变形的弹塑性理论,建立了材料的应变硬化模型,采用有限元仿真技术,利用有限元软件ABAQUS对中碳合金钢40CrNiMo切削过程中剪切层及切屑的形成进行仿真,分析切削加工区域的应力、应变的分布。该方法比一般的试验法更省时省力,在研究金属切削理论、材料切削性能及开发刀具产品方面有着工程应用价值。     

Understanding and Characterizing the Drilling of Recycled Plastics

Recycled plastics are increasingly being used to manufacture planks used in large-volume applications, including decks, garden, and cloakroom chairs. These products, although manufactured near-to-net shape, often require drilling for assembly purposes. There are very limited data on the machining of plastic material. Manufacturers often rely on data and models established for metals. The machining of plastics, although limited to assembly purposes, or to the removal of excess materials, requires an understanding of the behavior of these materials during the machining in order to obtain better quality parts. It is even more important for recycled plastics, which are inhomogeneous, contain pores, and most often, are made with more than one type of product. This work analyzes the machining of recycled plastics in order to establish and compare their machining models with those traditionally used for metals, and to better understand the behavior of the plastics during machining. The workpiece is drilled at different process conditions and at different temperatures. The process performance indicators such as cutting forces, chip formation, and chip form are analyzed. The originality of this work resides in its study of chip formation and the effects of the preset workpiece temperature on the drilling mechanisms. It is found that there is a range of critical temperatures of transition for plastics similar to the Charpy impact ductile-brittle temperature separating the domain of low cutting force and long and spiral chip from that of high cutting force corresponding to the accordion-type of chip. A parameter describing this phenomenon is defined. It is also found that for low- to moderate-speed operations, the cutting speed has very little effect on the cutting forces, which depend mainly on the feed rate and the workpiece temperature. The relationship between the drilling forces and the feed rate established for metals remain valid, but the exponent of the feed rate for the thrust force is lower. The thrust force and the tangential force are proportional to the feed rate exponent 0.4 compared to 0.8 for metals when drilling workpiece at room temperature or below.     

Numerical analysis of chip formation and shear localisation processes in machining the Ti-6Al-4V titanium alloy

A finite element modelling was carried out to analyse the chip morphology and adiabatic shear banding localisation processes when high-speed machining refractory titanium alloys. A thermo-visco-plastic model for the machined material and a rigid with thermal behaviour for the cutting tool were assumed. The study tries to understand the effect of the material behaviour on the produced chip morphology. One of the main characteristics of titanium chips is a segmented shape for a wide range of cutting conditions. This kind of morphology was found only dependent on adiabatic shear banding without material damage effect in the shear zones (primary and secondary shear zones). The influence of the material characteristics (strain softening, thermal softening, etc.) and machining parameters on the cutting forces and chip morphology were analysed. Three flow-stress laws and different friction coefficients (low and high friction) at the tool-chip interface was particularly analysed to explain the different morphologies obtained for refractory titanium chips.     

ANALYTICAL MODELING OF MICRO END-MILLING FORCES WITH EDGE RADIUS AND MATERIAL STRENGTHENING EFFECTS

The study aims at developing a predictive analytical force model for the micro end-milling operation taking into account the material strengthening as well as the edge radius effects that come into play at the micro level. The mechanistic models for macro end-milling process have been extensively reported in literature and such models predominantly use milling force coefficients which are empirically determined from end-milling experiments. The proposed model for micro end-milling is based on determination of milling force coefficients from fundamental oblique cutting approach. The edge radius effect has been accounted by analyzing the rubbing action similar to the rolling of a cylinder over work surface. Johnson-Cook material model has been modified based on the strain gradient plasticity theory incorporating the increase in material strength with decreasing uncut chip thickness. From the micro orthogonal cutting experiments, a good agreement between the experimental and predicted shear strength values is observed. The force model is validated against measured forces in end-milling experiments carried out on the KERN Evo 5 axis micro machining center. The feed and lateral forces are predicted within 10% deviation on an average.     

MACHINING OF CORTICAL BONE: SIMULATIONS OF CHIP FORMATION MECHANICS USING METAL MACHINING MODELS

This paper investigates chip formation in the machining of cortical bone and the application of isotropic elastic-plastic material models with a pressure dependent yield stress and a strain path dependent failure strain law to finite element calculations to predict observed behaviour. It is shown that a range of models can be created that result in segmented chip formations and a range of specific cutting forces similar to those observed experimentally. Results from the simulations provide an explanation for differences in the ratio of thrust to cutting forces observed between previous experimental studies, namely that the cutting tools used may have had different edge sharpness or degree of damage induced by the material removal process. Measurements of edge profiles from one of these studies support that explanation and emphasize the importance of tool toughness in maintaining efficient cutting of bone.     

THE ELECTRICAL DISCHARGE MACHINING (EDM) HANDBOOK

Abstract

Recycled plastics are increasingly being used to manufacture planks used in large-volume applications, including decks, garden, and cloakroom chairs. These products, although manufactured near-to-net shape, often require drilling for assembly purposes. There are very limited data on the machining of plastic material. Manufacturers often rely on data and models established for metals. The machining of plastics, although limited to assembly purposes, or to the removal of excess materials, requires an understanding of the behavior of these materials during the machining in order to obtain better quality parts. It is even more important for recycled plastics, which are inhomogeneous, contain pores, and most often, are made with more than one type of product. This work analyzes the machining of recycled plastics in order to establish and compare their machining models with those traditionally used for metals, and to better understand the behavior of the plastics during machining. The workpiece is drilled at different process conditions and at different temperatures. The process performance indicators such as cutting forces, chip formation, and chip form are analyzed. The originality of this work resides in its study of chip formation and the effects of the preset workpiece temperature on the drilling mechanisms. It is found that there is a range of critical temperatures of transition for plastics similar to the Charpy impact ductile-brittle temperature separating the domain of low cutting force and long and spiral chip from that of high cutting force corresponding to the accordion-type of chip. A parameter describing this phenomenon is defined. It is also found that for low- to moderate-speed operations, the cutting speed has very little effect on the cutting forces, which depend mainly on the feed rate and the workpiece temperature. The relationship between the drilling forces and the feed rate established for metals remain valid, but the exponent of the feed rate for the thrust force is lower. The thrust force and the tangential force are proportional to the feed rate exponent 0.4 compared to 0.8 for metals when drilling workpiece at room temperature or below.     

AN ANALYSIS OF SURFACE CRACKING DURING ORTHOGONAL MACHINING OF GLASS

It has been proposed that a rough-semi-finish-finish strategy may be possible in the machining of glass and other brittle materials to achieve higher productivity than is realized through either grinding or ductile-mode machining, both of which have been studied extensively to date. A previously presented experimental study of orthogonal glass cutting exhibited clear transitions in machining modes as the uncut chip thickness is increased. One of those modes involves ductile-mode chip formation combined with surface damage in the form of surface cracks that protrude down into the machined surface and ahead of the cutting edge. Here, a model is formulated and exercised to better understand this surface-cracking damage. The finite element method is used with a custom written re-meshing subroutine employed under a commercial software package. The analysis focuses on the crack depth and lead (ahead of the tool) as a function of the normalized process force and the fracture toughness of the work material. It is found that load ratio, the ratio of the cutting (surface-tangential) force to the thrust (surface-normal) force, plays a significant role in the crack growth problem, as does the manner in which the thrust load is distributed relative to the cutting load. It is shown that point-wise application of the loads produces results far off from the experimental results, whereas distributed loads can produce results well aligned with the experiments. Given that ductile-mode chip formation occurs during surface cracking, the load distributions found to work well exhibit qualitatively the same characteristics that one would expect based on extending the well-known mechanics of metal cutting.     

Numerical modeling of the influence of process parameters and workpiece hardness on white layer formation in AISI 52100 steel

White layer formation is considered to be one of the most important aspects to take into account in hard machining. Therefore, a large number of experimental investigations have been carried out in recent times on the formation mechanisms and properties of the white layer. However, up to now, only very few studies have been reported on modeling of the white layer formation. This paper presents a finite element model which predicts the white layer formation during machining of hardened AISI 52100 steel. This numerical model was properly calibrated by means of an iterative procedure based on the comparison between experimental and numerical data. The empirical model was also validated for a range of cutting speeds, uncut chip thickness, and material hardness values. This study provides excellent results concerning cutting force, temperature, chip morphology, and white layer. From this study, it was also possible to properly analyze the influence of process variables on the white layer formation.     

PHYSICS-BASED SIMULATION OF HIGH SPEED MACHINING

Computer simulation of high speed machining processes can provide a unique insight and reduce the number of design iterations required to advance and optimize the process. Predictive modeling of high speed machining of exotic materials has been hindered by the nonlinear behavior of this type of materials at extremely high strain, strain rate, and temperatures. This paper presents a physics-based modeling technology that includes the change in the material constitutive equation and the friction characterization at cutting speeds up to 400 m min^-1. The dependence of the accuracy of the predicted parameters, such as the chip formation on cutting forces, chip/tool/workpiece interface temperature, stress and strain distributions are also discussed. The fundamentals of metal cutting were utilized to understand the effect of parameter changes in regimes that are outside current empirical knowledge databases.     

FINITE ELEMENT MODELING OF MACHINING: FROM PROOF-OF-CONCEPT TO ENGINEERING APPLICATIONS

For the past fifty years researchers have developed various machining models to improve cutting performance. Several approaches have been taken including analytical techniques, slipline field solutions, empirical approaches and finite element techniques. Of these, the finite element approach provides the most detailed information on chip formation and chip interaction with the cutting tool. Finite element models have been developed for calculating the stress, strain, strain-rate, and temperature distributions in both the chip and the workpiece. In addition, tool temperatures, machining forces and cutting power requirements can be determined. This information is extremely, useful for developing more fundamental understanding of complex machining problems. This paper presents a critique of finite element approaches used for simulating machining processes. Several applications of the finite element technique for simulating various machining problems are also reviewed. A new application for determining diffusion wear rates in cutting tools is described, and future directions for finite element modeling of machining processes are discussed.     

MANUFACTURING PROCESSES AND EQUIPMENT

ABSTRACT

For the past fifty years researchers have developed various machining models to improve cutting performance. Several approaches have been taken including analytical techniques, slipline field solutions, empirical approaches and finite element techniques. Of these, the finite element approach provides the most detailed information on chip formation and chip interaction with the cutting tool. Finite element models have been developed for calculating the stress, strain, strain-rate, and temperature distributions in both the chip and the workpiece. In addition, tool temperatures, machining forces and cutting power requirements can be determined. This information is extremely, useful for developing more fundamental understanding of complex machining problems. This paper presents a critique of finite element approaches used for simulating machining processes. Several applications of the finite element technique for simulating various machining problems are also reviewed. A new application for determining diffusion wear rates in cutting tools is described, and future directions for finite element modeling of machining processes are discussed.     

Effect of very high cutting speeds on shearing, cutting forces and roughness in dry turning of austenitic stainless steels

Behavior of austenitic stainless steels has been studied at very high cutting speeds. Turning tests were carried out using the AISI 303 austenitic stainless steel. In particular, the influence of cutting speed on tool wear, surface quality, cutting forces and chip geometry has been investigated. These parameters have been compared when performing machining at traditional cutting speeds (lower than 350?m/min) versus high cutting speeds. The analysis of results shows that the material undergoes a significant change in its behavior when machining at cutting speeds above 450?m/min, that favors the machining operation. The main component of cutting forces reaches a minimum value at this cutting speed. The SEM micrographs of the machined surfaces show how at the traditional cutting speeds the machined surfaces contain cavities, metal debris and feed marks with smeared material particles. Surfaces machined at high cutting speeds show evidence of material side flow, which is more evident at cutting speeds above 600?m/min. Tool wear is located at the tool nose radius for lower cutting speeds, whereas it slides toward the secondary edge when cutting speed increases. An analysis of chips indicates also an important decrement in chip thickness for cutting speeds above 450?m/min. This study concludes that there is an unexplored range of cutting speeds very interesting for high-performance machining. In this range, the behavior of stainless steels is very favorable although tool wear rate is also significant. Nevertheless, nowadays the cost of tool inserts can be considered as secondary when comparing to other operation costs, for instance the machine hourly cost for high-end multitasking machines.     

Finite element simulation of high-speed machining of titanium alloy (Ti�C6Al�C4V) based on ductile failure model

A Johnson?CCook material model with an energy-based ductile failure criterion is developed in titanium alloy (Ti?C6Al?C4V) high-speed machining finite element analysis (FEA). Furthermore, a simulation procedure is proposed to simulate different high-speed cutting processes with the same failure parameter (i.e., density of failure energy). With this finite element (FE) model, a series of FEAs for titanium alloy in extremely high-speed machining (HSM) is carried out to compare with experimental results, including chip morphology and cutting force. In addition, the chip morphology and cutting force variation trends under different cutting conditions are also analyzed. Using this FE model, the ductile failure parameter is modified for one time, afterword, the same failure parameter is applied to other conditions with a key modification. The predicted chip morphologies and cutting forces show good agreement with experimental results, proving that this ductile failure criterion is appropriate for titanium alloy in extremely HSM. Moreover, a series of relatively low cutting speed experiments (within the range of HSM) were carried out to further validate the FE model. The predicted chip morphology and cutting forces agree well with the experimental results. Moreover, the plastic flow trend along an adiabatic shear band is also analyzed.     

基于ABAQUS的钛合金铣削力的模拟分析

为了研究钛合金在铣削过程中切削力随着切削参数的变化规律,建立了三维斜角切削有限元模型。通过对材料本构模型,刀—屑接触摩擦模型和切屑分离准则等关键环节建模,采用通用有限元求解器ABAQUS/Ex-plicit对钛合金Ti6Al4V的斜角切削过程进行了模拟,获得了切削速度v、切削深度ap和每齿进给量fz对切削力的变化趋势及影响程度。模拟结果表明:切削力随着切削深度ap和每齿进给量fz的增大而增大,而随着切削速度增大切削力波动很小。切削深度对切削力的影响最大,进给量次之,切削速度对切削力的影响最小。该模型可以为切削参数的合理选择提供参考。     

Prediction and experimental validation of micro-milling cutting forces of AISI H13 steel at hardness between 35 and 60 HRC

This paper presents prediction and validation of micro-milling cutting forces of AISI H13 steel at hardnesses between 35 and 60 HRC. The cutting forces are predicted based on an approach considering the full kinematics of the cutting tool including the run-out effect, effects of the cutting velocity and tool geometry, ploughing and chip formation phenomena and the hardness of the AISI H13 steel. A plane strain dynamic thermo-mechanical finite element (FE) model of orthogonal cutting is used to predict the cutting forces where the geometry of the cutting tool edge is modelled based on scanning electron microscope measurements. A constitutive elastic–plastic isotropic material model describing the relationship between stresses, strains, strain rates and hardnesses is modelled and implemented into ABAQUS/Explicit FE code by the user-defined subroutine VUMAT. Finite element analyses (FEA) are employed to obtain the relationship between cutting forces, uncut chip thickness, cutting velocity and material hardness. Numerous FEA are performed at different uncut chip thicknesses (0–20?μm), cutting velocities (104.7–4,723?mm/s) and hardnesses (35–60 HRC) using the FE model of orthogonal cutting. The full kinematics of the cutting tool including the run-out effect and the FE-predicted cutting forces are incorporated to predict the micro-milling cutting forces. The predicted micro-milling cutting forces have been experimentally validated at hardness of 43.2 HRC at different feed rates and spindle speeds. The result showed that the cutting forces and cutting temperatures increase by increasing the hardness of the AISI H13 while the stability limits of the process decrease by increasing the hardness.     

An explicit finite element model to study the influence of rake angle and friction during orthogonal metal cutting

A fundamental understanding of the tribology aspects of machining processes is essential for increasing the dimensional accuracy and surface integrity of finished products. To this end, the present investigation simulates an orthogonal metal cutting using an explicit finite element code, LS-DYNA. In the simulations, a rigid cutting tool of variable rake angle was moved at different velocities against an aluminum workpiece. A damage material model was utilized for the workpiece to capture the chip separation behavior and the simultaneous breakage of the chip into multiple fragments. The friction factor at the cutting tool–workpiece interface was varied through a contact model to predict cutting forces and dynamic chip formation. Overall, the results showed that the explicit finite element is a powerful tool for simulating metal cutting and discontinuous chip formation. The separation of the chip from the workpiece was accurately predicted. Numerical results found that rake angle and friction factor have a significantly influence on the discontinuous chip formation process, chip morphology, chip size, and cutting forces when compared to the cutting velocity during metal cutting. The model was validated against the experimental and numerical results obtained in the literature, and a good agreement with the current numerical results was found.     

Numerical experiments on the influence of material and other variables on plane strain continuous chip formation in metal machining

Finite element simulations of metal machining chip formation have been carried out with model materials that have been given a range of thermal softening and strain hardening behaviours. For materials that are approximately perfectly plastic, predictions of slip-line field theory regarding the dependence of chip/tool normal contact stress distribution on the combination of shear plane angle, friction angle and tool rake angle are reproduced. But it has not proved possible to generate the full range of non-unique fields predicted by slip-line theory. The introduction of strain hardening causes chips to thicken but with deviations at high hardening rates from the behaviour proposed by Oxley. These observations are generally in agreement with previously published physical test data. A study of the effect of increasing the cutting edge radius confirms the important effect of that, particularly on tool thrust forces. By continually comparing the results to expectations from more simple modelling, and asking the question ‘Is that expected?’, a general problem of creating a friction law applicable to both plastically flowing high stress conditions and to more lightly loaded elastic conditions has been recognised and is the subject of continuing work.