单刀片镗削加工时的力学建模

介绍镗削加工的力学模型.沿切削刃切屑厚度的分布,被作为刀具刃倾角、刀尖圆弧半径、切削深度和进给量的函数被建模.使用机械的和直角到斜角切削转变两种方法,建立该过程的切削力学模型。     

Thermomechanical modeling of oblique turning in relation to tool-nose radius

This article aims at predicting machining performances for oblique turning in relation to tool-nose radius. A new geometric analysis for the uncut chip area is proposed as function of depth of cut, feed rate, tool-nose radius, and edge direction angle. Cutting edge is discretized into increments and average uncut chip thickness, elementary direction angle and elementary depth of cut are determined for each one. A new thermomechanical model is developed for each increment which is supposed to be an oblique machining with single cutting edge. The predicted cutting force components are in good agreement with experimental data over a wide range of cutting conditions. In particular, the effect of tool-nose radius and cutting parameters on chip geometry, cutting temperature, and cutting force components are studied. It is underlined that tool-nose radius promotes the increase in radial force, however, its influence on the other parameters is negligible.     

PERFORMANCE PREDICTION MODELS FOR TURNING WITH ROUNDED CORNER PLANE FACED LATHE TOOLS. II. VERIFICATION OF MODELS

Extensive numerical simulation and experimental verification of the predictive force and surface roughness models for turning operations have been carried out encompassing the domain of cutting conditions recommended in practice. It has been shown that the three force models predicted very similar qualitative and plausible trends in the tangential, feed and radial force components as well as chip flow angle with changes in the many operation variables. The experimental testing involved about 500 turning cuts over a wide range of inclination angles, normal rake angles, major cutting edge angles, corner radius, feed and depth of cut. All the qualitative trends in the forces and chip flow angle noted in the simulation studies have been experimentally confirmed for all the three force models for both TiN coated and uncoated tools. The first two force models predictions correlated very well with the experimental results while only reasonable correlation was found with the third (approximate) model. The surface roughness predictive models were found to be adequate for turning with TiN coated HSS tools only while empirical equations were still necessary for reliable estimates of surface roughness for turning with the uncoated HSS tools. This investigation has shown that comprehensive and quantitatively reliable predictive models for the forces, power, chip flow angle and surface roughness can be established from fundamental cutting the- ory and analysis. In particular, the findings of this investigation provide further evidence in support of the generic nature of the 'unified mechanics of cutting approach' to technological performance prediction and the potential of the predictive surface roughness models for machining with coated tools.     

A FINITE ELEMENT ANALYSIS OF BURR FORMATION IN FACE MILLING OF A CAST ALUMINUM ALLOY

The research discussed in this article focuses on the effects of tool geometry (i.e., rake angle and cutting edge radius) and flank wear upon burr formation in face milling of a cast aluminum alloy. As to tool edge preparation, the use of a tool with variable cutting edge radius was investigated using FEM, and compared for its cutting performance (i.e., burr reduction and tool life) with a conventional tool with uniform cutting edge radius. In order to evaluate 3D face milling through 2D orthogonal cutting simulations, the cross-sections that consist in the cutting speed direction and chip flow direction were selected at different locations along the tool rounded corner. At these cross-sections, the local value of cutting edge radius and their associated tool rake angles as well as the effective uncut chip thickness were determined for 2D cutting simulations. In addition, 3D face milling simulations were conducted to investigate more realistic chip flow and burr generation. Comparisons were made for burrs produced from 3D simulations with a sharp tool, 3D simulations with a worn tool and face milling experiments. Finally, recommendations for cutting tool design are made to reduce burr formation in face milling.     

Influence of size effect on burr formation in micro cutting

Burr is an important character of the surface quality for machined parts, and it is even more severe in micro cutting. Due to the uncut chip thickness and the cutting edge radius at the same range in micro cutting process, the tool extrudes the workpiece with negative rake angle. The workpiece flows along the direction of minimum resistance, and Poisson burr is formed. Based on the deformation analysis and experiment observations of micro cutting process, the factor for Poisson burr formation is analyzed. It is demonstrated that the ratio of the uncut chip thickness to the cutting edge radius plays an important role on the height of Poisson burr. Increasing the uncut chip thickness or decreasing the cutting edge radius makes the height of exit burr reduce. A new model of micro exit burr is established in this paper. Due to the size effect of specific cutting energy, the exit burr height increases. The minimum exit burr height will be obtained when the ratio of uncut the chip thickness to the cutting edge radius reaches 1. It is found that the curled radius of the exit burr plays an important role on the burr height.     

Optimizing the geometric parameters of chamfered edge for rough machining Fe–Cr–Ni stainless steel

Geometry of cutting edge has great influence on performance and reliability of modern precision cutting tools. In this study, two-dimensional finite element model of orthogonal cutting of Fe–Cr–Ni stainless steel has been built to optimize the geometric parameters of chamfered edge. A method to measure the chip curl radius has been proposed. The effect of cutting edge geometric parameters on tool stress and chip curl radius has been analyzed. Then, the chamfered edge parameters have been optimized based on numerical simulation results. It finds that, keeping the equal material removal rate, the optimal geometric parameters of chamfered edge for rough machining Fe–Cr–Ni stainless steel are that the rake angle is from 16° to 17°, and the chamfer length is from 60 to 70 μm. Small (large) rake angle combined with small (large) chamfer length is more reasonable to reduce the tool stress. When the length of land is approximately equal to undeformed chip thickness and the rake angle is larger than 15°, the chip curl radius is minimal. The groove type with large radio of width to depth should be used in the chip breaking based on the optimization results.     

A study on critical depth of cuts in micro grooving

Ultra precision diamond cutting is a very efficient manufacturing method for optical parts such as HOE, Fresnel lenses, diffraction lenses, and others. During micro cutting, the rake angle is likely to become negative because the tool edge radius is considerably large compared to the sub-micrometer-order depth of cut. Depending on the ratio of the tool edge radius to the depth of cut, different micro-cutting mechanism modes appear. Therefore, the tool edge sharpness is the most important factor which affects the qualities of machined parts. That is why diamond, especially monocrystal diamond which has the sharpest edge among all other materials, is widely used in micro-cutting. The majar issue is regarding the minimum (critical) depth of cut needed to obtain continuous chips during the cutting process. In this paper, the micro machinability near the critical depth of cut is investigated in micro grooving with a diamond tool. The experimental results show the characteristics of micro-cutting in terms of cutting force ratio (Fx/Fy), chip shape, surface roughness, and surface hardening near the critical depth of cut.     

基于钝圆尺寸效应的微细切削机理试验研究

通过微细车削试验,研究了微细切削加工参数对切削力、表面质量、切屑形成的影响,发现切削厚度与刃口半径的比值是影响微细切削的关键因素,当该比值过小时,刃口尺寸效应作用极其显著,导致切削比能迅速增大,表面质量恶化,切屑形成困难。根据这一结论可确定微细切削加工参数选择的下限范围,从而为微细切削加工参数选择提供理论依据。     

PERFORMANCE PREDICTION MODELS FOR TURNING WITH ROUNDED CORNER PLANE FACED LATHE TOOLS. I. THEORETICAL DEVELOPMENT

In this paper the need for reliable quantitative machining performance information for efficient and effective use of machining operations is discussed, as are the recent developments of predictive models for forces and power in practical machining operations based on the 'unified mechanics of cutting approach'. This investigation is aimed at extending this mechanics of cutting approach to turning with rounded corner plane faced lathe tools. Three predictive models for the forces, power and chip flow angle based on the 'unified mechanics of cutting approach1 have been developed while the surface roughness models have been based on the feed marks generated on the machined surface allowing for the precise tool corner profile. The first force model is based on the modified mechanics of cutting analyses for single edge tools while the two alternative models are based on the generalised mechanics of cutting analyses for single edge and multi-edge form tools for the turning cut as a whole. The predictive force models incorporate the effects of the major tool geometrical variables including the corner radius, the cutting conditions as well as the effect of TiN coating. This first paper will outline the development of the models while the proposed models will be numerically tested and experimentally verified qualitatively and quantitatively in the subsequent parts of this investigation.     

Fabrication of clean cut surface on high strength steel using a new shaving die design

A shaving process is commonly applied to achieve a smooth cut surface thorough the workpiece thickness and a square cut-edge, also known as a finishing operation. However, this process is rarely successful for high-strength steel sheets, which is a major problem. In the present study, finite element method (FEM) simulation was used to clarify the main causes of this problem by comparing the shaving mechanisms between medium carbon steel grade SPCC (JIS) and high-strength steel grade SPFH 590 (JIS). Results show that in the case of SPFH 590 based on material flow, stress distribution, and strain distribution analyses, the shaved chip was difficult to form by sliding along the punch face. Moreover, the tensile stress generated in the shearing zone was increased and readily generated cracks. The shaving process was developed in the present study by generating the cutting-edge angle and rake radius on the punch. The cutting edge angle was designed to generate high compressive stress in the cutting-edge vicinity and shearing zone, and the rake radius was designed to tear a shaving allowance off and move it along the rake radius instead of moving downward along the punch movement direction, thereby decreasing the tensile stress in the shearing zone. Under these mechanisms, the increases in the generated tensile stress in the shearing zone could be delayed, and cracks could thus be prevented. The effect of the punch geometry on the cut surface characteristics and cutting forces were also investigated. Laboratory experiments were performed to validate the FEM simulation results. Experimental results agreed well with the FEM simulation results. Therefore, a smooth cut surface thorough the workpiece thickness of high-strength steel sheets could be successfully achieved by using the developed shaving process.

     

Investigation on ball end milling of P20 die steel with cutter orientation

The generation mechanism of machining-induced residual stresses is a complex nonlinear and thermal–mechanical coupling problem. The cutting forces and cutting temperature produced in machining process must be considered simultaneously. The influence of cutter orientation and feed per tooth on the cutting speed, cutting forces, cutting temperature, and residual stresses is discussed in the present study. Effective cutting speed in accordance with the inclination angle in feed direction is analyzed. The cutting forces are gained by milling experiment, and the cutting temperature is obtained by finite element method. Moreover, the influence of the effective cutting speed on the cutting forces and cutting temperature is stated, and the relationship among the cutting forces, cutting temperature, and residual stresses is discussed. The experimental and numerical methods are both adopted in this study to give a better understanding of the milling process. After analysis of the phenomenon, several conclusions are made. The inclination angle in feed direction affects the effective cutting speed, and then the cutting forces, cutting temperature, and residual stresses are affected. Priority selection of inclination angle in feed direction is suggested from 5° to 30° in order to reduce the cutting forces. The overall trend of the workpiece temperature presents the parabolic shape, while the chip temperature increases with the increasing inclination angle in feed direction. Residual stress in feed direction almost increases with the increasing feed per tooth, which is not obvious in the general scope of the feed rate. The inclination angle of 5° and 15° is the priority in order to produce residual compressive stresses in cross feed direction.     

Finite element analysis of the influence of tool edge radius on size effect in orthogonal micro-cutting process

The size effect in metal cutting is evident in the nonlinear scaling phenomenon observed in the specific cutting energy with decrease in uncut chip thickness. It has been argued by many researchers that this scaling phenomenon is caused mainly by the cutting tool edge radius, which purportedly affects the micro-cutting process by altering the effective rake angle, enhancing the plowing effect or introducing an indenting force component. However, the phenomenological reasons why the tool edge radius causes size effect and the relationship between the tool edge radius and the characteristic length scale associated with the size effect in micro-cutting has not been sufficiently clarified. In this paper, a strain gradient plasticity-based finite element model of orthogonal micro-cutting of Al5083-H116 alloy developed recently is used to examine fundamentally the influence of tool edge radius on size effect. The applicability of two length scales—tool edge radius and the material length scale l in strain gradient plasticity—are also examined via analysis of data available in the literature.     

Thermoviscoplastic modelling of oblique cutting: forces and chip flow predictions

A modelling of oblique cutting for viscoplastic materials is presented. The thermomechanical properties and the inertia effects are accounted for to describe the material flow in the primary shear zone. At the tool–chip interface, a temperature-dependent friction law is introduced to take account of the extreme conditions of pressure, velocities and temperature encountered during machining. The chip flow angle is calculated by assuming that the friction force is collinear to the chip flow direction on the tool rake face. Due to the temperature dependence of the friction law at the tool–chip interface, the chip flow angle predicted by the model, is affected by the cutting speed, the undeformed chip thickness, the normal rake angle, the edge inclination angle and the thermomechanical behavior of the work material. This dependence and the trends predicted by the present approach are confirmed by experimental observations. Effects of cutting conditions on the cutting forces are also presented and compared to experiments.     

径向跳动对球面铣刀切削力的影响研究

研究了径向跳动对刀齿的实际切削半径、切屑形状以及切屑厚度的影响机理。研究了各刀齿沿刀刃螺旋线的切削微元实际切削半径的数学表示和变化规律,实际切削半径的变化改变了刀齿的切削路径,使各刀齿上切屑形状分布不均匀。建立了三维切削下切屑厚度的数学表示,提出了递延累加切屑厚度计算算法。实验验证表明,计算的切削力与测量结果能很好地吻合,瞬时切削力、切削力峰值、平均切削力的预测精度达到85%以上。     

大刃倾角切削的变形研究

以刃倾角大于４５°的斜角切削为基础,通过获取切屑根部标本,分析了大刃倾角切削的切屑外形和金相显微组织的变形情况,给出了刃倾角与变形系数和剪应变之间的关系曲线。     

A study of the effect of tool cutting edge radius on ductile cutting of silicon wafers

Ductile mode cutting of silicon wafers can be achieved under certain cutting conditions and tool geometry. An experimental investigation of the critical undeformed chip thickness in relation to the tool cutting edge radius for the brittle-ductile transition of chip formation in cutting of silicon wafers is presented in this paper. Experimental tests for cutting of silicon wafers using diamond tools of different cutting edge radii for a range of undeformed chip thickness are conducted on an ultra-precision lathe. Both ductile and brittle mode of chip formation processes are observed in the cutting tests. The results indicate that ductile cutting of silicon can be achieved at certain values of the undeformed chip thickness, which depends on the tool cutting edge radius. It is found that in cutting of silicon wafers with a certain tool cutting edge radius there is a critical value of undeformed chip thickness beyond which the chip formation changes from ductile mode to brittle mode. The ductile-brittle transition of chip formation varies with the tool cutting edge radius. Within the range of cutting conditions in the present study, it has also been found that the larger the cutting edge radius, the larger the critical undeformed chip thickness for the ductile-brittle transition in the chip formation.     

NANOSCALE CUTTING OF MONOCRYSTALLINE SILICON USING MOLECULAR DYNAMICS SIMULATION

It has been found that the brittle material, monocrystalline silicon, can be machined in ductile mode in nanoscale cutting when the tool cutting edge radius is reduced to nanoscale and the undeformed chip thickness is smaller than the tool edge radius. In order to better understand the mechanism of ductile mode cutting of silicon, the molecular dynamics (MD) method is employed to simulate the nanoscale cutting of monocrystalline silicon. The simulated variation of the cutting forces with the tool cutting edge radius is compared with the cutting force results from experimental cutting tests and they show a good agreement. The results also indicate that there is silicon phase transformation from monocrystalline to amorphous in the chip formation zone that can be used to explain the cause of ductile mode cutting. Moreover, from the simulated stress results, the two necessary conditions of ductile mode cutting, the tool cutting edge radius are reduced to nanoscale and the undeformed chip thickness should be smaller than the tool cutting edge radius, have been explained.     

Modeling of the effect of tool edge radius on surface generation in elliptical vibration cutting

The elliptical vibration cutting (EVC) technique has been found to be a promising technique for ultraprecision machining of various materials. During the EVC process, two-dimension vibration movement of the cutting tool generates consecutively overlapping EVC cycles. In each cycle, the tool position relative to the workpiece gets continuously varied, and meanwhile, cusps are left along the nominal cutting direction. Such vibration marks, which have never been found in conventional cutting process, are considered to be a critical characteristic for the EVC technique. In order to analyze this unique characteristic, an analytical model based on geometrical relationships in the EVC process was developed to calculate the theoretical roughness, where the tool edge is assumed to be perfectly sharp. However, the effect of tool edge radius is probably significant, especially in the situation where the tool edge radius is comparable to the vibration amplitudes. Hence, in the present research, an analytical surface generation model for the EVC process is developed to better understand the surface generation process and predict the surface roughness. The tool edge radius is considered and investigated in detail in this new approach. Mathematical evaluation shows that the surface roughness value along the nominal cutting direction decreases with the increase of the edge radius. In order to validate the proposed model, a series of EVC grooving tests on soft and hard work materials were conducted using a polycrystalline diamond (PCD) tool by applying the ultrasonic EVC technique. The results show that the predicted roughness based on the proposed model correlates well with the experimental results measured by a white light interferometer, and the model considering the tool edge radius performs significantly better than the one without considering the edge radius in predicting the roughness along the nominal cutting direction.     

Selection of the Machining Inclination Angle in High-Speed Ball End Milling

High-speed machining is a key issue for current die and mould manufacturing. Though this technology has great potential for high productivity, tool wear accelerated by high cutting speeds and hard materials is a barrier. In this work, we attempted to reduce tool wear by controlling the machining inclination angle between the tool and the workpiece. To do this, the range of cutting angles engaged in the cutting using a ball end cutter is obtained from the boundary lines describing machined sculptured surfaces represented by the cutting envelop condition and the geometric relationships of successive tool paths. Then, the chip cross-sectional area and chip length can be obtained from the calculated cutting edge and the rotational engagement angle. The simulation results showed that a machining inclination angle of 15° was good enough from the point view of machineability, and this value was verified by a cutting experiment using high-speed ball end milling.