COMBINED EFFECTS OF FLANK AND CRATER WEAR ON CUTTING FORCE MODELING IN ORTHOGONAL MACHINING—PART II: BAYESIAN APPROACH-BASED MODEL VALIDATION

Flank and crater wear are the primary tool wear patterns during the progressive tool wear in metal cutting. Cutting forces may increase or decrease, depending on the combined contribution from the flank and/or crater wear. A two-dimensional (2D) slip-line field based analytical model has been proposed to model the force contributions from both the flank and crater wear. To validate the proposed force model, the Bayesian linear regression is implemented with credible intervals to evaluate the force model performance in orthogonal cutting of CK45 steels. In this study, the proposed analytical worn tool force model-based predictions fall well within the 75% credible intervals determined by the Bayesian approach, implying a satisfactory modeling capability of the proposed model. Based on the parametric study using the proposed force model, it is found that cutting forces decrease with the increasing crater wear depth but increase with the increasing flank wear length. Also, the predicted cutting forces are affected noticeably by the friction coefficients along the rake and flank faces and the ratio of crater sticking region to sliding region, and better knowledge of such friction coefficients and ratio is expected to further improve worn tool force modeling accuracy. Compared with the finite element approach, the proposed analytical approach is efficient and easy to extend to three-dimensional worn tool cutting configurations.     

A NEW MECHANISTIC MODEL FOR PREDICTING WORN TOOL CUTTING FORCES

An enhanced model for predicting worn tool cutting forces in metal cutting without the need for any worn tool calibration tests is presented in this paper. The new model utilizes a previously developed slip-line field approach in conjunction with a mechanistic force model to predict the shear flow stress and shear angle for a range of cutting conditions with only a minimal number of sharp tool calibration tests. The shear flow stress and shear angle values are then used as inputs into a worn tool force model to predict the cutting forces due to tool flank wear. Predictions of worn tool cutting forces from the new model have been compared to experimental data from both a steel and a ductile iron workpiece. Ductile iron tests are significant because previous shear flow stress and shear angle models require chip measurements which cannot be made with the chips produced by iron workpieces. Model predictions are also compared to literature data obtained using an aluminum workpiece. An excellent comparison between the model predictions and the experimental data is found for all of the materials considered.     

A study of deformation of the machined workpiece and tool under different low cutting velocities with an elastic cutting tool

To understand the effects of cutting velocity, tool elastic deformation generated by high normal stresses during metal cutting processing and artificial tool flank wear on the cutting process, an iterative mathematical model for calculating the tool–workpiece contact problem was developed in this paper under the assumption of elastic cutting tools. In this model, the finite element method is used to simulate cutting of mild steel by the P20 cutting tool with constant artifical tool flank wear under the condition of three different cutting velocities. The results obtained in the simulation were found to match the experimental data reported by related studies. The simulation results also indicate that the thrust and the cutting forces are functions of cutting velocity. Besides, both the normal stress on the tool rake face and the residual stress of machined workpiece generally decrease with increase in cutting velocity. According to the findings in this study, though the residual stress of the machined workpiece decreases as the cutting velocity increases, its value is still higher than that in ordinary conditions due both to the influence of tool flank wear and tool elastic deformation. Also, the phenomenon of curvature at the workpiece end easily occurs.     

A NEW CONCEPT FOR DECOUPLING THE CUTTING FORCES DUE TO TOOL FLANK WEAR AND CHIP FORMATION IN HARD TURNING

Determining the temperature field in metal cutting when the tool flank is progressively worn requires the knowledge of the forces due to tool flank wear and that due to chip formation. In the past, these forces have been computed from data experimentally measured with a dynamometer, under the assumption that the chip formation configuration remained unaltered when the tool flank is progressively worn. This approach has been used in the literature even though there has been evidence that it is not correct. The error introduced by this doubtful assumption in computing the maximum surface temperature in the work-piece can be significant.

Of late there has been considerable interest in employing hard turning as the final finishing process in place of grinding and superfinishing. Consequently, the ability to accurately predict the maximum surface temperature and its distribution in the workpiece is now most desirable, for avoiding thermal damage to the machined surface. This paper discusses a new method based on the thickness of the microstructural change in chips to decouple the tool-flank forces for predicting the maximum surface temperature and its distribution in the workpiece.     

Tool wear estimation in machining based on the flank wear inclination angle changes using the FE method

Abstract

In this study, a FEM-based tool wear approach with a focus on the geometry of the worn tool, especially the changes of flank wear land inclination angle, was developed. The relationship between the variables of the wear rate equation and the average nodal temperature on the flank wear land through integrating FE-simulations of the cutting process and Response Surface Methodology (RSM) was determined in order to define the temperature as a function of wear rate model parameters. Then, that data was used to calibrate the wear rate equation which was obtained by establishing the relationship between the Usui wear rate equation and the geometry of the worn tool, using a MATLAB program. This approach was validated by comparing the predicted flank wear rates and experimental measurements. The estimated flank wear shows some improvement compare to the model with a constant inclination angle.     

ESTIMATION OF TOOL WEAR OF CARBIDE TOOL IN ORTHOGONAL CUTTING USING FEM SIMULATION

In metal cutting, tool wear on the tool-chip and tool-workpiece interfaces (i.e. flank wear and crater wear) is strongly influenced by the cutting temperature, contact stresses, and relative sliding velocity at the interface. These process variables depend on tool and workpiece materials, tool geometry and coatings, cutting conditions, and use of coolant for the given application. Based on the predicted temperatures and stresses on the tool face from the finite element analysis (FEA) simulation, tool wear may be estimated with acceptable accuracy by incorporating an empirical wear model.

The overall objective of this study is to develop a methodology to predict the tool wear evolution and tool life in orthogonal cutting using FEM simulations. To approach this goal, the methodology is proposed with three different parts. In the first part, a tool wear model for the specified tool-workpiece pair is developed via a calibration set of tool wear cutting tests in conjunction with cutting simulations. In the second part, modifications are made to the commercial FEM code used to allow for tool wear calculation and tool geometry updating. The last part includes the validation of the developed methodology. This paper is mainly focused on the modifications made to the commercial FEM code in order to make reasonable tool wear estimates (the second part).     

ESTIMATION OF TOOL WEAR OF CARBIDE TOOL IN ORTHOGONAL CUTTING USING FEM SIMULATION

In metal cutting, tool wear on the tool-chip and tool-workpiece interfaces (i.e. flank wear and crater wear) is strongly influenced by the cutting temperature, contact stresses, and relative sliding velocity at the interface. These process variables depend on tool and workpiece materials, tool geometry and coatings, cutting conditions, and use of coolant for the given application. Based on the predicted temperatures and stresses on the tool face from the finite element analysis (FEA) simulation, tool wear may be estimated with acceptable accuracy by incorporating an empirical wear model.

The overall objective of this study is to develop a methodology to predict the tool wear evolution and tool life in orthogonal cutting using FEM simulations. To approach this goal, the methodology is proposed with three different parts. In the first part, a tool wear model for the specified tool-workpiece pair is developed via a calibration set of tool wear cutting tests in conjunction with cutting simulations. In the second part, modifications are made to the commercial FEM code used to allow for tool wear calculation and tool geometry updating. The last part includes the validation of the developed methodology. This paper is mainly focused on the modifications made to the commercial FEM code in order to make reasonable tool wear estimates (the second part).     

Analytical prediction of cutting tool wear

An analytical method is presented which enables the crater and flank wear of tungsten carbide tools to be predicted for a wide variety of tool shapes and cutting conditions in practical turning operations based only on orthogonal cutting data from machining and two wear characteristic constants. A wear characteristic equation is first derived theoretically and verified experimentally. An energy method is developed to predict chip formation and cutting forces in turning with a single-point tool from the orthogonal cutting data. Using these predicted results, stress and temperature on the wear faces can be calculated. Computer simulation of the development of wear is then carried out by using the characteristic equation and the predicted stresses and temperatures upon the wear faces. The predicted wear progress and tool life are in good agreement with experimental results.     

EXPERIMENTAL STUDIES AND MODELING OF HEAT GENERATION IN METAL MACHINING

Heat generation in the cutting zones due to plastic deformation and friction in the cutting region governs insert wear, tensile residual stresses on the machined component surface and may give rise to undesired tolerances and short component life. Therefore, it is crucial that the heat generation is kept under control during metal cutting. In this study an analytical model for prediction of heat generation in the primary and secondary deformation zones is compared with results from finite element simulations and temperature measurements using IR-CCD camera. The used cutting data are altered to study the temperature influence from tool geometry and feed when machining stainless steel SANMAC316L and low carbon steel AISI 1045.     

涂层刀具前后刀面摩擦下界面应力分析

基于任意拉格朗日欧拉方法(ALE)建立金属正交切削加工的热力耦合的有限元模型,获得不同速度下切削稳定时涂层刀具前后刀面的接触应力、剪应力以及温度场。通过对涂层刀具施加已获得的刀具表面的应力场和温度场,分析了不同速度下摩擦分界点变化的规律以及对涂层基体界面应力的影响。结果表明,随着速度的增大,摩擦分界点逐渐有向前刀面移动的趋势,表明磨损的方式开始从后刀面磨损向前刀面月牙湾磨损转变,这与切削试验结果一致。同时随着速度的增大,涂层界面应力突变更加显著,表明高速条件下涂层更容易破坏,且速度越高,前刀面涂层破坏的几率越大。     

Effect of coating layer loss on the wear rate change of coated carbide tools in turning process

This paper reports an experimental study of flank wear on TiN- and TiAlN-coated carbide tools in the turning of AISI 1045, AISI 4135, ductile cast iron, and Inconel 718, and it was conducted with the purpose of showing the relationship between the change in wear rate and the loss of coating layer on the cutting edge. It was found that the relation between cutting distance and flank wear in log-log scale clearly shows the change in wear rate, thus providing a straightforward way to determine the relation between worn out coating layer and increase in wear rate. This relation was confirmed by analyzing the presence of coating layer before and after the inflection point appears by means of scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) photographs. It was observed that the coating layer on the flank face is worn away and finally is worn out. However, even if the layer on the flank face is worn out, tool wear is suppressed as long as the coating layer on the cutting edge exists. On the other hand, when the coating layer on the cutting edge is worn out, the wear resistance of the tool depends on the substrate; thus, the wear rate increases. According to the results, as the cutting speed increases, the change in wear rate appears in a shorter cutting distance, making flank wear to be high. High pressure and high temperature act on the rake face; thus, thermal stability of the coating layer in the cutting edge is important. A low cutting speed decreases cutting efficiency, but a high cutting speed causes flank wear to be high; therefore, in order to optimize machining cost, an acceptable cutting speed, from the standpoint of tool wear, should be selected.     

UNDERSTANDING OF FINITE ELEMENT ANALYSIS RESULTS UNDER THE FRAMEWORK OF OXLEY'S MACHINING MODEL

We introduce an accurate coupled thermo-mechanical finite element analysis (FEA) of machining using the Arbitrary Lagrangian Eulerian (ALE) analysis capability of ABAQUS/Explicit. This analysis provides detailed information about the cutting forces, chip thickness, contact length, the extent of the primary and secondary shear zones as well as the distribution of strain, strain rate and temperature in the deformation zones. This information has to be viewed under the framework of an analytical model for it to lead to better understanding of the physics of machining. We use the best available analytical model, namely, Oxley's machining model, for this purpose and the FEA results are compared with the assumptions and predictions of Oxley's analysis. The strain rate in the primary shear zone, the hydrostatic pressure variation along the shear plane, the distribution of normal and shear stresses along the tool-chip interface and the shape of the secondary shear zone are the quantities compared. Due to the key role of temperature in the prediction of tool wear, the fraction of heat conducted away into the workpiece, the maximum temperature along the tool-chip interface and the maximum temperature along the flank face are also compared. The comparison reveals that Oxley's model captures the physics of machining quite well. However, some details such as the heat partition module and the assumptions on stress and temperature distribution at the tool-chip interface need to be revisited.     

A New Slip-Line Field Modeling of Orthogonal Machining with a Rounded-Edge Worn Cutting Tool

In this study, a new slip-line field model and its associated hodograph for orthogonal cutting with a rounded-edge worn cutting tool are developed using Dewhurst and Collins's matrix technique. The new model considers the existence of dead metal zone in front of the rounded-edge worn cutting tool. The ploughing force and friction force occurred due to flank wear land, chip up-curl radius, chip thickness, primary shear zone thickness and length of bottom side of the dead metal zone are obtained by solving the model depending on the experimental resultant force data. The effects of flank wear rate, cutting edge radius, uncut chip thickness, cutting speed and rake angle on these outputs are specified.     

UNDERSTANDING OF FINITE ELEMENT ANALYSIS RESULTS UNDER THE FRAMEWORK OF OXLEY'S MACHINING MODEL

ABSTRACT

We introduce an accurate coupled thermo-mechanical finite element analysis (FEA) of machining using the Arbitrary Lagrangian Eulerian (ALE) analysis capability of ABAQUS/Explicit. This analysis provides detailed information about the cutting forces, chip thickness, contact length, the extent of the primary and secondary shear zones as well as the distribution of strain, strain rate and temperature in the deformation zones. This information has to be viewed under the framework of an analytical model for it to lead to better understanding of the physics of machining. We use the best available analytical model, namely, Oxley's machining model, for this purpose and the FEA results are compared with the assumptions and predictions of Oxley's analysis. The strain rate in the primary shear zone, the hydrostatic pressure variation along the shear plane, the distribution of normal and shear stresses along the tool-chip interface and the shape of the secondary shear zone are the quantities compared. Due to the key role of temperature in the prediction of tool wear, the fraction of heat conducted away into the workpiece, the maximum temperature along the tool-chip interface and the maximum temperature along the flank face are also compared. The comparison reveals that Oxley's model captures the physics of machining quite well. However, some details such as the heat partition module and the assumptions on stress and temperature distribution at the tool-chip interface need to be revisited.     

Slip-line solution for orthogonal cutting with a chip breaker and flank wear

A slip-line field model for orthogonal cutting with chip breaker and flank wear has been developed. For a worn tool, this slip-line field includes a primary deformation zone with finite thickness; two secondary shear zones, one along the rake face and the other along the flank face; a predeformation zone; a curled chip; and a flank force system. It is shown that the cutting geometry is completely determined by specifying the rake angle, tool-chip interface friction and the chip breaker constraint. The chip radius of curvature, chip thickness, and the stresses and velocities within the plastic region are readily computed. Grid deformation patterns, calculated with the velocity field determined, demonstrate that the predicted effects of changes in frictional conditions at the tool-chip interface and of the rake angle on chip formation are in accord with experimental observations. The calculated normal stress distribution at the tool-chip interface is in general agreement with previously reported experimental measurements. The model proposed predicts a linear relationship between flank wear and cutting force components. The results also show that non-zero strains occur at and below the machined surface when machining with a worn tool. Severity and depth of deformation below the machined surface increases with increasing flank wear. Forces acting on the chip breaker surface are found to be small and suggest that chip control for automated machining may be feasible with other means.     

The monitoring of flank wear on the CBN tool in the hard turning process

In precision hard turning, tool flank wear is one of the major factors contributing to the geometric error and thermal damage in a machined workpiece. Tool wear not only directly reduces the part geometry accuracy but also increases the cutting forces drastically. The change in cutting forces causes instability in the tool motion, and in turn, more inaccuracy. There are demands for reliably monitoring the progress of tool wear during a machining process to provide information for both correction of geometric errors and to guarantee the surface integrity of the workpiece. A new method for tool wear monitoring in precision hard turning is presented in this paper. The flank wear of a CBN tool is monitored by feature parameters extracted from the measured passive force, by the use of a force dynamometer. The feature parameters include the passive force level, the frequency energy and the accumulated cutting time. An ANN model was used to integrate these feature parameters in order to obtain more reliable and robust flank wear monitoring. Finally, the results from validation tests indicate that the developed monitoring system is robust and consistent for tool wear monitoring in precision hard turning.     

Tool wear monitoring in ramp cuts in end milling using the wavelet transform

Tool wear identification and estimation present a fundamental problem in machining. With tool wear there is an increase in cutting forces, which leads to a deterioration in process stability, part accuracy and surface finish. In this paper, cutting force trends and tool wear effects in ramp cut machining are observed experimentally as machining progresses. In ramp cuts, the depth of cut is continuously changing. Cutting forces are compared with cutting forces obtained from a progressively worn tool as a result of machining. A wavelet transform is used for signal processing and is found to be useful for observing the resultant cutting force trends. The root mean square (RMS) value of the wavelet transformed signal and linear regression are used for tool wear estimation. Tool wear is also estimated by measuring the resulting slot thickness on a coordinate measuring machine.     

An analytical approach to cutting force prediction in milling of carbon fiber reinforced polymer laminates

ABSTRACT

A prediction model of cutting force for milling multidirectional laminate of carbon fiber reinforced polymer (CFRP) composites was developed in this article by using an analytical approach. In the predictive model, an equivalent uniform chip thickness was used in the case of orthogonal plane cutting, and the average specific cutting energy was taken as an empirical function of equivalent chip thickness and fiber orientation angle. The parameters in the model were determined by the experimental data. Then, the analytical model of cutting force prediction was validated by the experimental data of multidirectional CFRP laminates, which shows the good reliability of the model established. Furthermore, the cutting force component of flank contact force was correlated with the surface roughness of workpiece and the flank wear of tool in milling UD-CFRP composites. It was found that surface quality as well as flank wear has a co-incident varying trend with the flank contact force, as confirmed by the observations of the machined surfaces and tool wear at different fiber orientations. So, it can be known that low flank contact force be required to reduce surface damage and flank wear.     

Notch wear detection in cutting tools using gradient approach and polynomial fitting

Cutting tool wear is well known to affect the surface finish of a turned part. Various machine vision methods have been developed in the past to measure and quantify tool wear. The two most widely measured parameters in tool wear monitoring are flank wear and crater wear. Works carried out by several researchers recently have shown that notch wear has a more severe effect on the surface roughness compared to flank or crater wear. In this work, a novel gradient detection approach has been developed to detect the presence of micro-scale notches in the nose area of the cutting tool. This method is capable of detecting the location of the notch accurately from a single worn cutting tool image.     

AN EXPERIMENTAL AND FEM STUDY ON DRY TURNING OF ROTOR STEEL 26NiCrMoV145 USING MULTILAYER COATED CEMENTED CARBIDE TOOL

Rotor steel 26NiCrMoV145 has been widely employed in aerospace and power industry because of its unique properties. As to this new material that is used widely in important places, it is worth studying its machinability. An experiment and finite element (FE) simulation has been carried out to study the cutting forces, cutting temperature and tool wear mechanism during high speed dry turning of 26NiCrMoV145 using multilayer TiCN + Al₂O₃ coated carbide inserts. The orthogonal test was chosen and cutting forces were measured to validate the FE simulations. SEM analysis has been carried out on worn tools to determine the tool wear mechanism. Crater wear and coating peeling could be found at every cutting speed. Flank face wear appears smooth when cutting speed increases. Analyses of tool stress and temperature distribution during simulated cutting process were also taken out to help in understanding tool wear mechanism. At last, an optimization of the cutting conditions was suggested when considering material remove rate, cutting forces and tool wear.