A simulated investigation on the machining instability and dynamic surface generation

In this paper, the authors propose the generic concept of machining instability based on the analysis of all kinds of machining instable behaviors and their features. The investigation covers all aspects of the machining process, including the machine tool structural response, cutting process variables, tooling geometry and workpiece material property in a full dynamic scenario. The paper presents a novel approach for coping with the sophisticated machining instability and enabling better understanding of its effect on the surface generation through a combination of the numerical method with the characteristic equations and using block diagrams/functions to represent implicit equations and nonlinear factors. It therefore avoids the lengthy algebraic manipulations in deriving the outcome and the solution scheme is thus simple, robust and intuitive. Several machining case studies and their simulation results demonstrate the proposed approach is feasible for shop floor CNC machining optimisation in particular. The results also indicate the proposed approach is useful to monitor the machining instability and surface topography and to be potentially applied in adaptive control of the instability in real time

FEA modeling of orthogonal cutting of steel: a review

Orthogonal cutting is probably the most studied machining operation for metals. Its simulation with the Finite Element Analysis (FEA) method is of paramount academic interest. 2D models, and to a lesser extent 3D models, have been developed to predict cutting forces, chip formation, heat generation and temperature fields, residual stress distribution and tool wear. This paper first presents a brief review of scientific literature with focus on FEA modelling of the orthogonal cutting process for steels. Following, emphasis is put on the building blocks of the simulation model, such as the formulation of the mechanical problem, the material constitutive model, the friction models and damage laws

Crack propagation in CBN insert in hybrid machining of RBSN ceramic

This research investigates the phenomenon that cryogenic cooling can significantly improve the cutting tool life and the workpiece quality in hybrid machining of advanced materials, such as Reaction Bonded Silicone Nitride (RBSN). Several finite element models are developed in order to analyze the temperature fields developed and the stress distributions in the Polycrystalline Cubic Boron Nitride (PCBN) cutting insert during a hybrid turning process. The analyses performed reveal that the hybrid machining with cryogenic cooling of the cutting tool leads to a decrease of stresses in the cutting insert, especially when micro-cracks are present in the insert exhibits. It is found that a decrease in cutting temperature from 1740°C to 597°C led to approximately 660 stress reduction at the tip of the micro-cracks on the flank face of the cutting tool, thus significantly reducing the wear rate of the cutting insert. The findings are consistent with previously published experimental data

Crystal-plasticity modelling of machining

A machining process is one of the most common techniques used to remove material in order to create a final product. Most studies on mechanisms of cutting are performed under the assumption that the studied material is isotropic, homogeneous and continuous. One important feature of material- its anisotropyis linked to its crystallographic nature, which is usually ignored in machining studies. A crystallographic orientation of a workpiece material exerts a great influence on the chip-formation mechanism. Thus, there is a need for developing fundamental understanding of material’s behaviour and material removal processes. While the effect of crystallographic orientation on cutting-force variation is extensively reported in the literature, the development of the single crystal machining models is somewhat limited. [Continues.

Analysis of orthogonal metal cutting processes

The orthogonal metal cutting process for a controlled contact tool is simulated using a limit analysis theorem. The basic principles are stated in the form of a primal optimization problem with an objective function subjected to constraints of the equilibrium equation, its static boundary conditions and a constitutive inequality. An Eulerian reference co-ordinate is used to describe the steady state motion of the workpiece relative to the tool. Based on a duality theorem, a dual functional bounds the objective functional of the primal problem from above by a sharp inequality. The dual formulation seeks the least upper bound and thus recovers the maximum of the primal functional theoretically. A finite element approximation of the continuous variables in the dual problem reduces it to a convex programming. Since the original dual problem admits discontinuous solutions in the form of bounded variation functions, care must be taken in the finite element approximation to account for such a possibility. This is accomplished by a combined smoothing and successive approximation algorithm. Convergence is robust from any initial iterate. Results are obtained for a wide range of control parameters including cutting depth, rake angle, rake length and friction. The converged solutions provide information on cutting force, chip thickness, chip stream angle and shear angle which agree well both in values and trend with the published data. But the available data represent only a small subset in the range of parameters exhaustively investigated in this paper.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/50099/1/1620340122_ftp.pd

Finite element analysis of chip formation in grooved tool metal cutting

Machining simulations of orthogonal cutting are performed using the finite element method to predict chip formation in grooved tool cutting. A flat-faced cutting tool is first simulated to check the validity of the model, and the results obtained are used as a basis for comparison in grooved tool cutting simulation. Grooved tool simulations are modeled next to investigate the effect of a groove on the chip formation process. Groove depth and width were the two parameters changed for the purpose of analyzing the chip flow characteristics, stress and strain distributions found in the chip.;For all simulated cases, it is observed that a groove definitely impart more curl on the chip, and more tensile and/or compressive near the chip root, a condition conducive to chip breaking. More curling is observed when the groove radius of curvature is reduced by increasing the groove depth or decreasing the groove width. It is also observed that the increase in normal tensile stress at the chip root next to the tool tip is proportional to the amount of curl displayed in the chip. However, the increase in normal compressive stress is related to the increase in chip thickness, a variable that depends on the effective rake angle of cutting

Master of Science

thesisIn order to enable sustainable manufacturing, the indiscriminate use of cutting fluids in modern machining has to be tackled, given its environmental and economic impacts. A possible solution is the recent entrance of dry and near dry minimal quantity cooling and lubrication (MQC/L) machining. In order to evaluate the effectiveness and performance of MQC/L, however, further studies need to be done. The three major functions of cutting fluids are to perform cooling, lubrication, and chip removal from the cutting zone. The main objective of this work is to understand how the tribological aspects (cutting forces, chip flow, tool-chip contact area), chip morphology, and surface roughness and surface integrity (residual stresses) are influenced by i) the application of different cutting fluid combinations in minimal quantities ii) the direction of application of the cutting fluid in the facing of AISI 1045 steel using an uncoated flat-faced carbide tool. A Minimal Quantity Cutting Fluid (MQCF) dispensing system was tested and implemented to evaluate the effects of differing fluid dispensing rates and target directions. It was found that the effects of targeted cutting fluid combinations on the tribological aspects were significant when compared to dry machining, although the variation in the tribological aspects was marginal amongst the different cutting fluid combinations. In contrast, directing the coolant on the flank face of the tool revealed some interesting results. Compressive residual stresses were observed when coolant was directed to the tool flank face as opposed to other cases, which generated tensile residual stresses in the machined subsurface. This suggests that localized and carefully chosen cutting fluid target direction and combination can enhance product performance by enhancing machining performance and surface integrity. In summary, this thesis presents the significance of targeted minimal cutting fluid application in relation to machining performance (especially surface integrity) under the given cutting conditions and provides several recommendations for future work

자동차용 판재성형 해석 적용을 위한 다중 스케일 마찰 모델 개발 및 평가

학위논문(박사) -- 서울대학교대학원 : 공과대학 재료공학부, 2022.2. 이명규.Sheet metal forming of advanced high strength steels (AHSS) has drawn significant attentions in automotive industry for their improved fuel efficiency by lightweightness and passenger safety by higher strength. However, the manufacturing of automotive parts with the AHSS accompanies inferior springback and formability compared to the conventional lower strength steels, which results in more time consuming trial and error in the tool design stage. To overcome this challenges in applying the AHSS to the automotive parts, finite element simulations have been commonly used as a numerical tool for predicting springback and formability of sheet metal parts prior to real try-out. Accurate modeling of finite element simulation in sheet metal forming process requires reliable numerical techniques, constitutive models, realistic boundary conditions, etc. Among these, the friction is one of important factors to determine the accuracy of the simulation, but it has been overlooked in most simulations. The frictional behavior in sheet metal forming is known to be very complex and depend on various parameters such as surface roughness, contact pressure, sliding velocity, lubrication condition, etc. However, it is a common practice to use the simplest Coulomb friction law in the finite element modeling. In the present study, a microscale asperity based friction model is further modified by imposing new model parameters for satisfying force equilibrium between contact surfaces. In addition, a geometrical shape model of the tool surface is newly proposed to determine the plowing effect of the friction. The tool geometry is modeled based on primary summits in tool height distribution determined by the measured wavelength, rather than the summits dependent on the resolution of surface measurement instrument. The friction models are required not only in the preceding boundary lubrication condition, but also in the mixed-boundary lubrication condition where sufficient lubrication exists in non-contacting surface valleys. The hydrodynamic friction model uses a load-sharing concept that considers the lubrication area and metal-to-metal contact separately. In this study, the hydrodynamic friction model is combined with the boundary lubrication friction model to account for the friction in the mixed lubrication domain. The lubricant film thickness, calculated as the volume of non-contacting surface valleys, is used to realize the coupling. The film lubrication behavior is implemented by the finite element coding of the Reynolds equation, which enables the calculation of the hydrodynamic pressure. To validate the boundary lubrication friction model, the calculated friction coefficient and the measured friction coefficient are compared according to the contact pressure under boundary lubrication conditions. Also, the boundary lubrication friction model is verified by the finite element simulation that is applied to the U-draw/bending process. Finally, the boundary lubrication friction model and the mixed boundary lubrication friction model are applied to the finite element simulation of the newly developed press-forming process, which represents the influence of various variables such as contact pressure, sliding speed and lubrication. The results of the validations show that the developed multi-scale friction models and their implementation can be efficiently used to the sheet metal forming simulations where the frictional behavior is critical for the quality of the automotive parts.AHSS(고장력강판)의 판금 성형은 경량화에 의한 연비 향상과 고강도화에 의한 승객 안전으로 자동차 산업에서 큰 주목을 받고 있습니다. 그러나 AHSS를 이용한 자동차 부품 제조는 기존의 저강도 강재에 비해 스프링백 및 성형성이 좋지않기에 툴 설계 단계에서 시행착오가 더 많이 발생하게 됩니다. 자동차 부품에 AHSS를 적용할 때 이러한 문제를 극복하기 위해 유한 요소 시뮬레이션은 실제 시험 전에 판재 성형 부품의 스프링백 및 성형성을 예측하기 위한 수치해석적 도구로 일반적으로 사용되었습니다. 판재 성형 공정에서 유한 요소 시뮬레이션의 정확한 모델링은 신뢰할 수 있는 수치해석적 기술, 구성 방정식, 정확한 경계 조건 등이 필요합니다. 이 중 마찰은 시뮬레이션의 정확도를 결정하는 중요한 요소 중 하나이지만 대부분의 시뮬레이션에서 간과되어 왔습니다. 판재 성형에서 마찰 거동은 매우 복잡하고 표면 거칠기, 접촉 압력, 미끄럼 속도, 윤활 조건 등과 같은 다양한 매개변수에 따라 달라지는 것으로 알려져 있습니다. 그러나, 대부분의 유한 요소 해석에서 가장 간단한 쿨롱 마찰 법칙을 사용하는 것이 일반적입니다. 본 연구에서는 접촉면 사이의 힘 평형을 만족시키기 위해 새로운 모델 매개변수를 부과하여 마이크로 스케일 돌기 기반 마찰 모델을 추가로 수정했습니다. 또한 마찰의 쟁기질 효과를 결정하기 위해 툴 표면의 기하학적 형상 모델이 새로 제안되었습니다. 툴 형상은 표면 측정 장비의 분해능에 의존하는 정점이 아니라 측정된 파장에 의해 결정되는 툴표면 높이 조도의 서밋을 기반으로 모델링됩니다. 마찰모델은 경계윤활조건뿐만 아니라 충분한 윤활이 존재하는 혼합경계윤활조건에서도 필요하다. 유체역학적 마찰 모델은 윤활 영역과 금속 대 금속 접촉을 별도로 고려하는 하중 공유 개념을 사용합니다. 본 연구에서는 유체역학적 마찰 모델을 경계 윤활 마찰 모델과 결합하여 혼합 윤활 영역의 마찰을 설명합니다. 비접촉 표면 밸리의 부피로 계산된 윤활유 필름 두께는 커플링을 구현하는 데 사용됩니다. 필름 윤활 거동은 유체역학적 압력의 계산을 가능하게 하는 Reynolds 방정식의 유한 요소 방법을 사용해 구현됩니다. 경계 윤활 마찰 모델을 검증하기 위해 경계 윤활 조건에서 접촉 압력에 따라 계산된 마찰 계수와 측정된 마찰 계수를 비교합니다. 또한 경계 윤활 마찰 모델은 U-draw/bending 과정에 적용된 유한 요소 시뮬레이션을 통해 검증되었습니다. 마지막으로 경계 윤활 마찰 모델과 혼합 경계 윤활 마찰 모델을 새로 개발된 프레스 성형 공정의 유한 요소 시뮬레이션에 적용했는데, 이는 접촉 압력, 미끄럼 속도 및 윤활과 같은 다양한 변수의 영향을 나타냅니다. 검증 결과는 개발된 다중 스케일 마찰 모델과 그 구현이 마찰 거동이 자동차 부품 품질에 중요한 판재 성형 시뮬레이션에 효율적으로 사용될 수 있음을 보여줍니다.1. Introduction 1 1.1. Sheet metal forming and deep drawing process 1 1.2. Motivation and objective 2 1.3. Literature review 5 1.3.1. Friction modeling on the boundary lubrication condition 6 1.3.2. Friction modeling on the mixed-boundary lubrication condition 22 2. Friction model in boundary lubrication 35 2.1. Framework of friction model in boundary lubrication 35 2.2. Statistical contact model for describing surface deformation 38 2.2.1. Assumptions for modeling 39 2.2.2. Flattening of workpiece asperity due to normal load 41 2.2.3. Flattening of workpiece asperity due to normal load and sliding 48 2.2.4. Flattening of workpiece asperity due to normal load and bulk strain 50 2.3. Friction model through a new approach 53 2.3.1. An elliptical paraboloid asperity model 53 2.3.2. A tool geometry model 56 3. Friction model in mixed-boundary lubrication 65 3.1. Overview of the mixed-boundary friction model (Hol [106]) 67 3.2. Finite element modeling for film fluid behavior 71 3.3. Verification of the developed finite element modeling 75 4. Application of boundary lubrication and mixed-boundary lubrication friction model to sheet metal forming process 82 4.1. Friction model parameters 82 4.1.1. Material properties 82 4.1.2. Surface data 83 4.1.3. Friction experiments 86 4.2. Application to sheet metal forming processes under non-lubrication conditions 91 4.2.1. Application to U-draw/bending simulation 94 4.2.2. Application to prototype press-forming process without lubricant 105 4.3. Application to sheet metal forming processes under lubrication conditions 116 5. Conclusions 129 Reference 134박

FE-Simulation of Metal Cutting Processes

Machining a new component or a new material requires the selection of the cutting conditions, the tool material and the tool geometry. The selections should be also optimized for the existing components and materials to improve the quality of the produced components and reduce the cost of production. Selecting the most suitable and optimum conditions for the metal cutting processes can be done by performing finite element (FE) simulations which provide more in-depth and detailed information about the cutting processes and also reduce the experimental effort compared to trial-and-error approach.In this thesis, the challenges and complexities that are needed to be considered in FE simulations of cutting processes are addressed. Firstly, the type of FE simulation should be selected according to the purpose of performing the simulation. Different types of FE simulations of metal cutting such as chip forming, heat transfer and material flow simulations are discussed while explaining their purpose and advantages. These simulations are also combined with semi-analytical methods and machine learning approaches to improve the performance of the simulations in terms of both accuracy and time consumption. Secondly, the selection of the suitable material model for the workpiece and the identification process of the material model parameters are crucial to obtain realistic results from FE simulations. In this aspect, an efficient and robust method of inverse identification of the material model parameters is presented in the scope of the thesis to improve the results of the metal cutting simulations. This identification approach is also implemented to identify the parameters of different material models to find the best-suited model to represent the behavior of the presented carbon steel workpiece material under different cutting conditions. In addition, different effects such as elastic, plastic, viscous and damage behaviors in the material modeling are also discussed throughout the thesis while touching upon their indicators in metal cutting.There are many more effects and parameters that can be implemented in FE simulations which make the simulations more in-depth and accurate in exchange for computational time. That is why finding the optimum point between the accuracy and time consumption for metal cutting simulations is of interest to many researchers and engineers. The aim of this thesis is to accomplish this while assessing the different aspects of FE simulations of metal cutting processes and discussing the mentioned challenges and complexities in more detail