Optimisation of face-milling cutters by use of a computeraided milling-system simulator (Camss)

This paper focuses on the improvement of the cutting process in milling operations by the reduction of vibrations. For that purpose, a non-uniform chiploading pattern is considered as the design parameter, which is achieved by controlling individual insert geometry, namely axial rake and radial rake angles. In the development of the system model, a complete governing equation that describes the milling process is derived and formulated. The model can represent the closed-loop configuration that consists of the cutting process, structural dynamics and a feedback loop. Based on this model, a computer-aided milling-system simulator (CAMSS) is developed. It takes into account the forced-vibration criterion as well as the self-excited vibration to optimise the cutter geometry. Therefore, it should be able to investigate the effects of cutter geometry on chip loading and vibration. A series of comparisons showed that the optimal cutter was subject to fewer vibrations and lower cutting forces than the conventional one.     

Time domain coupled simulation of machine tool dynamics and cutting forces considering the influences of nonlinear friction characteristics and process damping

A higher machining ability is always required for NC machine tools to achieve higher productivity. The self-oscillated vibration called “chatter” is a well-known and significant problem that increases the metal removal rate. The generation process of the chatter vibration can be described as a relationship between cutting force and machine tool dynamics. The characteristics of machine tool feed drives are influenced by the nonlinear friction characteristics of the linear guides. Hence, the nonlinear friction characteristics are expected to affect the machining ability of machines. The influence of the contact between the cutting edge and the workpiece (i.e., process damping) on to the machining ability has also been investigated. This study tries to clarify the influence of the nonlinear friction characteristics of linear guides and ball screws and process damping onto milling operations. A vertical-type machining center is modeled by a multi-body dynamics model with nonlinear friction models. The influence of process damping onto the machine tool dynamics is modeled as stiffness and damping between the tool and the workpiece based on the evaluated frequency response during the milling operation. A time domain-coupled simulation approach between the machine tool behavior and the cutting forces is performed by using the machine tool dynamics model. The simulation results confirm that the nonlinear frictions influence the cutting forces with an effect to suppress the chatter vibration. Furthermore, the influence of process damping can be evaluated by the proposed measurement method and estimated by a time domain simulation.     

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.     

基于DEFORM-3D的高速车削加工仿真

DEFORM-3D是应用有限元方法(FEM)分析三维复杂加工过程的模拟工具,它不仅鲁棒性好,而且易于使用.借助于该模拟分析环境,能够对切削过程中刀具几何参数、切削条件以及加工过程中的其他因素产生的影响进行研究.应用DEFORM自带的切削仿真模型,模拟高速车削加工中工件及刀具的温度分布、切屑流动、应力、应变和切削力等.模拟结果对减少产品试验、降低开发成本、缩短开发新产品及新工艺的时间等方面都具有重大意义.DEFORM-3D对于研究刀具几何模型、切屑形成以及切削参数控制的刀具制造者和使用者来说,是一个较理想的工具.     

MODELING THE PHYSICS OF METAL CUTTING IN HIGH-SPEED MACHINING

Physical modeling of metal cutting was carried out to provide an understanding and prediction of machining process details. The models are based on finite element analysis (FEA), using a Lagrangian formulation with explicit dynamics. Requirements for material constitutive models are discussed in the context of high-speed machining. Model results address metal cutting characteristics such as segmented chip formation, dynamic cutting forces, unconstrained plastic flow of material during chip formation, and thermomechanical environments of the work-piece and the cutting tool. Examples are presented for aerospace aluminum and titanium alloys. The results are suited for analysis of key process issues of cutting tool performance, including tool geometry, tool sharpness, workpiece material buildup, and tool wear.     

A solid modeler based ball-end milling process simulation

A system for geometric and physical simulation of the ball-end milling process using solid modeling is presented in this paper. A commercially available geometric engine is used to represent the cutting edge, cutter and updated part. The ball-end mill cutter modeled in this study is an insert type ball-end mill and the cutting edge is generated by intersecting an inclined plane with the cutter ball nose. The contact face between cutter and updated part is determined from the solid model of the updated part and cutter solid model. To determine cutting edge engagement for each tool rotational step, the intersections between the cutting edge with boundary of the contact face are determined. The engaged portion of the cutting edge for each tool rotational step is divided into small differential oblique cutting edge segments. Friction, shear angles and shear stresses are identified from orthogonal cutting data base available in the open literature. For each tool rotational position, the cutting force components are calculated by summing up the differential cutting forces. The instantaneous dynamic chip thickness is computed by summing up the rigid chip thickness, the tool deflection and the undulations left from the previous tooth, and then the dynamic cutting forces are obtained. For calculating the ploughing forces, Wu's model is extended to the ball-end milling process [21]. The total forces, including the cutting and ploughing forces, are applied to the structural vibratory model of the system and the dynamic deflections at the tool tip are predicted. The developed system is verified experimentally for various up-hill and down-hill angles.     

Strategies for grinding of chamfers in cutting inserts

In order to increase tool life and improve workpiece quality, cutting processes with geometrically defined cutters demand inserts with a prepared cutting edge. Chamfers are widely used in many processes, since they can provide edge strengthening without damaging the chip flow. In order to achieve a stable and reliable cutting process, uniform chamfer geometry along the insert and high edge quality are necessary. For this, proper grinding strategies for chamfer manufacturing must be taken into account. With the objective of getting knowledge about the chamfer manufacturing process, strategies for grinding of chamfers are investigated in this paper. Chamfers were ground on PCBN, mixed ceramic and cemented carbide cutting inserts with a vitrified bond diamond grinding wheel. A single grain chip thickness model is used to characterize the process and different grinding strategies are analyzed in terms of reduction of chamfer geometry deviation. It was found that high insert rotational speeds increase the edge chipping and that the cutting insert material has a considerable influence on the chamfer geometry deviation.     

MODELING THE PHYSICS OF METAL CUTTING IN HIGH-SPEED MACHINING

ABSTRACT

Physical modeling of metal cutting was carried out to provide an understanding and prediction of machining process details. The models are based on finite element analysis (FEA), using a Lagrangian formulation with explicit dynamics. Requirements for material constitutive models are discussed in the context of high-speed machining. Model results address metal cutting characteristics such as segmented chip formation, dynamic cutting forces, unconstrained plastic flow of material during chip formation, and thermomechanical environments of the work-piece and the cutting tool. Examples are presented for aerospace aluminum and titanium alloys. The results are suited for analysis of key process issues of cutting tool performance, including tool geometry, tool sharpness, workpiece material buildup, and tool wear.     

Investigation and prediction of chip geometry in diamond turning

Management of the chips generated in diamond turning is often critical, because contact between chips and the workpiece can result in superficial damage to the finished surface. Controlling chip motion is not a trivial process as the proper positioning of an oil or air stream requires an understanding of the dynamics of a diamond turned chip and the machining parameters that affect it. Work has been performed to investigate the effects of cutting speed, depth of cut, tool geometry, tool wear, and workpiece material properties on chip motion and geometry. Utilizing radius of curvature data from cutting experiments, a parameter has been proposed that can be used to predict chip radius of curvature for a wide range of machining conditions. This chip curvature parameter, χ, exhibits a power law relationship with chip radius of curvature as a function of tool geometry, depth of cut, cutting speed, and both elastic and plastic properties of the workpiece material.     

FEM-based prediction of heat partition in dry metal cutting of AISI 1045

Thermal effects often limit the performance of cutting processes. The energy spent in cutting is almost completely converted into heat which partly flows to workpiece, chip, and tool during the process. Therefore, knowledge about this partition is valuable for the process, tool, and coolant system design or for the compensation of thermal deformations of the workpiece and machine tool. For this reason, a simulation model based on the finite element method was developed to analyze the heat partition in dry metal cutting. The model utilizes the coupled Eulerian-Lagrangian method to simulate the chip formation in orthogonal cutting and to calculate the temperature distribution within workpiece, chip, and tool. This distribution was used to compute the heat partition between workpiece, chip, and tool in dependence of relevant process parameters. Furthermore, the results were validated by orthogonal cutting experiments and summarized in a formula to calculate the rate of heat flow into the workpiece as a function of those parameters.     

A new method for cutting force prediction in peripheral milling of complex curved surface

The instantaneous uncut chip thickness and entry/exit angle of tool/workpiece engagement vary with tool path, workpiece geometry and cutting parameters in peripheral milling of complex curved surface, leading to the strong time-varying characteristic for instantaneous cutting forces. A new method for cutting force prediction in peripheral milling of complex curved surface is proposed in this paper. Considering the tool path, cutter runout, tool type(constant/nonconstant pitch cutter) and tool actual motion, a representation model of instantaneous uncut chip thickness and entry/exit angle of tool/ workpiece engagement is established firstly, which can reach better accuracy than the traditional models. Then, an approach for identifying of cutter runout parameters and calibrating of specific cutting force coefficients is presented. Finally, peripheral milling experiments are carried out with two types of tool, and the results indicate that the predicted cutting forces are highly consistent with the experimental values in the aspect of variation tendency and amplitude.     

Cutting edge orthogonal contact-zone analysis using detailed tool shape representation

A new method for numerically controlled (NC)-simulation-based numerical analysis of the tool-workpiece contact area in cutting processes is presented. To gain enhanced knowledge about tool-workpiece interaction, determination of chip thickness, contact length, and resulting cross-section area of the undeformed chip is of major interest. Compared to common simulation approaches, where rotationally symmetrically constructed tool shape is used, the new method uses a detailed three-dimensional tool shape model for an extended and more accurate contact zone analysis. As a corresponding representation of the workpiece and its time-dependent change of shape, a multidexel model is used. To perform contact zone analysis, each cutting element and a multidexel model are intersected in discrete time steps corresponding to the tool rotation. Subsequently, the intersection point of each dexel is mapped on the local coordinate system of the cutting geometry. The parametric cutting geometry allows a direct computation of local cutting depth and contact length for each involved point. Based on the local values of contact length and cross section area of the undeformed chip, the characteristic values for the entire contact zone are calculated and used to predict mechanical loads caused by the cutting process. To demonstrate the application of the novel approach, a prediction of forces in slot milling and drilling of 1.1191 steel (C45EN) is presented.     

Finite element models of orthogonal cutting with application to single point diamond turning

Two computer models are described that treat the special case of orthogonal cutting. The models are based on the finite element method, which is used to discretize a portion of the workpiece in the vicinity of the cutting tool. From the models, the detailed stress and strain fields in the chip and workpiece, chip geometry and tool forces can be determined.

The first model is based on a specially modified version of a large deformation updated Lagrangian code developed at Lawrence Livermore National Laboratory called NIKE2D, which employs an elastic-plastic material model. The second model treats the region in the vicinity of the cutting tool as an Eulerian flow field. Material passing through the field is modeled as viscoplastic. Results obtained from both models show excellent agreement when compared with measured tool forces for slow speed cutting of aluminium 2024-T361.     

一种面向数控工艺参数优化的铣削过程动力学仿真系统研究

针对当前国内数控加工过程中工艺参数选择和优化存在的问题,在对国内外铣削过程动力学建模、仿真和优化进行深入研究的基础上,开发了一套面向数控铣削加工过程的动力学仿真优化系统。该系统以Matlab-GUIDE为软件开发平台,可以快速、有效地预测铣削加工过程中刀具的瞬时铣削力、主轴功率、主轴转矩等物理量,预测加工表面的形貌与刀具的振动情况。同时,借助于实验模态分析结果,可以准确地计算出整个系统的稳定切削区域,为数控机床工艺参数的合理、有效选择提供了有益的指导和理论依据。整个仿真系统的主要功能在多台数控加工中心上得到了成功的验证。     

Application of a variable flow stress machining theory to helical end milling

A methodology of modeling chip geometry of flat helical end milling based on a variable flow stress machining theory is presented in this article. The proposed model is concerned with the variation of the width of cut thickness. The nonuniform chip thickness geometry is discretized into several segments based on the radial depth of cut. The chip geometry for each segment is considered to be constant by taking the average value of the maximum and minimum chip thickness. The maximum chip thickness for each chip segment is computed based on the current width of cut, feed per tooth and the cutter diameter. The subsequent radial depth of cut is subtracted from the discretized size of the width of cut to obtain the minimum chip thicknesses. The forces for each segment are summed to obtain the total forces acting on the system of the workpiece and the tool. The cutting forces can be predicted from input data of work material properties, cutter configuration and the cutting conditions used. The validation of the proposed model is achieved by correlating experimental results with the predicted results obtained.     

Surface topography and roughness in hole-making by helical milling

Helical milling is used to generate holes with a cutting tool traveling on a helical path into the workpiece in which the diameter of the hole can be adjusted through that of the helical path. Based on an improved Z-map model, a 3D surface topography simulation model is established to simulate the surface finish profile generated after a helical milling operation using a cylindrical end mill. The surface topography simulation model incorporates the effects of the relative motion between the cutting tool and the workpiece, in which the effect of the insert runout error of the cutting tool is considered. Furthermore, the roughness parameters are deduced from simulations of the 3D surface topography. The experimental result shows that the proposed simulation algorithm can predict well the surface roughness in a helical milling operation. The surface topography simulation model is used to study the effects of cutting conditions such as the tangential feedrate, the diameter of the cutting tool and the hole, the insert runout error of the cutting tool, as well as the revolution of the cutting tool around the axis of the hole on the surface finish profile. It is found that the surface quality can be improved by optimization of the cutting conditions. As a result, the proposed model will be helpful in determining the cutting conditions to meet surface finish requirements in helical milling operation.     

三切削刃BTA深孔钻钻削过程研究

通过试验对3个切削刃BTA深孔钻削过程进行了研究,主要分析了在特定的深孔条件下切屑变形和钻削力的情况.通过对测试系统所得到的试验数据进行评估和证实,阐述了BTA深孔钻轴向力的组成和切屑变形、刀具磨损以及钻削力之间的关系.研究结果表明,钻头的内刃在切削过程中产生的切屑变形最大,3个切削刃(内刃、中间刃、外刃)的切削力和切屑变形的总体变化趋势是相同的.     

Molecular dynamics modeling of a single diamond abrasive grain in grinding

In this paper the nano-metric simulation of grinding of copper with diamond abrasive grains, using the molecular dynamics (MD) method, is considered. An MD model of nano-scale grinding, where a single diamond abrasive grain performs cutting of a copper workpiece, is presented. The Morse potential function is used to simulate the interactions between the atoms involved in the procedure. In the proposed model, the abrasive grain follows a curved path with decreasing depth of cut within the workpiece to simulate the actual material removal process. Three different initial depths of cut, namely 4 ?, 8 ? and 12 ?, are tested, and the influence of the depth of cut on chip formation, cutting forces and workpiece temperatures are thoroughly investigated. The simulation results indicate that with the increase of the initial depth of cut, average cutting forces also increase and therefore the temperatures on the machined surface and within the workpiece increase as well. Furthermore, the effects of the different values of the simulation variables on the chip formation mechanism are studied and discussed. With the appropriate modifications, the proposed model can be used for the simulation of various nano-machining processes and operations, in which continuum mechanics cannot be applied or experimental techniques are subjected to limitations.     

Investigation of the direction of chip motion in diamond turning

Management of the chips generated in diamond turning is often critical since contact between chips and the workpiece can result in superficial damage to the finished surface. Controlling chip motion is not a trivial process as the proper positioning of an oil or an air stream requires an understanding of the dynamics of a diamond turned chip and the machining parameters that affect it. Previous work [1] introduced the chip curvature parameter, χ, which is useful in predicting chip radius of curvature over a wide range of cutting speeds, depths of cut, tool geometries and workpiece material properties. To control chip motion, however, an understanding of the direction chips leave the tool/workpiece interface must also be obtained. Cutting experiments were performed investigating the influence of cutting speed, depth of cut, feed rate, tool path angle, tool geometry and tool orientation on the directional characteristics of the motion of diamond turned chips. Flow angle measurements obtained during cutting were found to remain within ± 10° of predictions from a simple geometrical model originally proposed for conventional machining.