Local toolpath smoothing for five-axis machine tools

When five axis CNC machine tools follow series linear toolpath segments, the drives experience velocity, acceleration and jerk discontinuities at the block transition points. The discontinuities result in fluctuations on machine tool motions which lead to poor surface quality. This paper proposes to insert quintic and septic micro-splines for the tool tip and tool-orientation, respectively, at the adjacent linear toolpath segments. Optimal control points are calculated for position and orientation splines to achieve C³ continuity at the junctions while respecting user-defined tolerance limits. The geometrically smoothed corners are traveled at a smoothly varying feed with cubic acceleration trajectory profile. The proposed method is experimentally demonstrated to show improvements in motion smoothness and tracking accuracy in five-axis machining of free-form surfaces found in dies, molds and aerospace parts.     

EUCLID软件在汽车覆盖件模具制造上的应用

本文论述了ＥＵＣＬＩＤ软件系统的总体结构，重点讨论了汽车覆盖件模具典型同的造型特点，并在此基础上提出了采用ＥＵＣＬＩＤ软件构造这些曲面的方法；还讨论了加工模具表面所需刀具运动轨迹的生成方法；最后介绍了ＥＵＣＬＩＤ软件进行发动机罩中加强板曲在造型及刀具运动轨迹生成的实例。     

On the use of fractal methods for the tool flank wear characterization

Fractal theory is widely used to analyze the topography of machined surfaces, but the relationship between fractal dimensions and tool flank wear has hardly been reported. In this paper, the fractal dimensions of tool flank wear are described based on the surface roughness R_a rather than the conventional worn width VB to evaluate tool wear, thus providing better fractal identification in evaluating tool performance.     

Improving endmilling surface finish by workpiece rotation and adaptive toolpath spacing

Free-form surfaces are being used in a growing number of engineering applications, especially in injection molding of consumer products. Decreasing the manufacturing cost and time of these molds will improve the efficiency of manufacturing injection molded consumer products. This paper is motivated by the need for simple strategies to improve the quality of and decrease the time required to machine free-form surfaces. We present two methods to improve the surface finish of parts finished with ball-nose endmilling. In the first method the surface finish is improved by finding an optimal orientation angle for the workpiece relative to the machining axis. In the second method finish is improved by adaptively varying the step-size when using raster toolpaths. The adaptive variation is controlled by a user-defined finish improvement factor, where the spacing density is increased only if the improvement in surface finish is greater than the finish improvement factor. These methods are implemented using an analytical model of the workpiece surface finish based on the mean scallop height of the machined surface. Results from the analytical model are verified with machining experiments, and we show that the adaptive spacing strategy can improve the surface quality by more than 50% with a small increase in the toolpath length. To achieve a similar improvement in surface quality by uniformly decreasing the path spacing results in a much larger increase in the toolpath length. The strategies discussed in this paper allow process planners intuitive control over the toolpath layout and spacing and improve the efficiency of machining high quality parts.     

Prediction of cutting forces in three and five-axis ball-end milling with tool indentation effect

Simulation of multi-axis ball-end milling of dies, molds and aerospace parts with free-form surfaces is highly desirable in order to optimize the machining processes in virtual environment ahead of costly trials. This paper presents a mechanics model that predicts the cutting forces in feed (x), normal (y) and axial (z) directions by modeling the chip thickness distribution, and cutting and indentation mechanics. The shearing forces are based on commonly known cutting mechanics models. The indentation of the cutting edge into the work material is modeled analytically by considering elasto-plastic deformation of the work material pressed by a rigid cutting tool edge with a positive or negative rake angle. The distribution of chip thickness and geometry of indentation zone are evaluated by considering five-axis motion of the tool along the toolpath. The proposed model has been experimentally validated in plunge indentation, as well as in three and five-axis ball-end milling of free-form surfaces. The prediction of axial (z) cutting forces is shown to be improved significantly when the proposed indentation model is integrated into the mechanics of ball-end milling.     

基于响应曲面法的钛合金干式车削工艺参数优化

为了提高钛合金干式车削加工质量,采用响应曲面法对主要车削工艺参数进行了优化,以工件表面粗糙度Ra和刀具磨损量VC作为评价指标,设计了切削速度、背吃刀量和进给量三因素的Box-Behnken实验模型。利用方差和拟合残差概率分布分析三因素的显著性及交互作用,并结合实验检验所建表面粗糙度和刀具磨损二阶响应预测模型的有效性。响应曲面法优化后的最佳工艺参数为:切削速度20 m/min、背吃刀量0.1788 mm、进给量0.1 mm/r,此时得到的表面粗糙度和刀具磨损量为1.031μm和155.6μm,与预测值的误差分别为:9.93%和1.58%。结果表明:基于响应曲面法的钛合金干式车削表面粗糙度和刀具磨损量预测模型准确有效。     

Correlating surface roughness,tool wear and tool vibration in the milling process of hardened steel using long slender tools

High speed milling is an operation frequently used in finishing and semi-finishing of dies and molds. However, when it is necessary to produce molds with deep cavities and/or with small corner radius, long tools with small diameters are required. This represents a challenge for manufacturing professionals: how to minimize tool vibration using a tool with such low rigidity and obtain good workpiece surface quality and long tool lives. This paper attempts to answer this question. Milling experiments on hardened AISI H13 steel were carried out using integral and indexable insert tools with different tool overhangs and different diameters. Tool wear, workpiece surface roughness and cutting forces were measured and these parameters were correlated with the frequency response function (FRF) obtained with the tools fixed in the machine tool. The main conclusion of this study is that good workpiece surface roughness allied to long tool lives for long tools with small diameters can be achieved, provided the tooth passing frequency used in the milling process (and its harmonics) does not produce high FRF values.     

基于HPSO优化BP神经网络的刀具磨损状态识别

针对BP神经网络容易陷入局部极值导致识别精度低的问题,文章提出了一种基于混合粒子群算法(HPSO)的BP神经网络优化算法。在刀具磨损监测实验过程中,采集刀具切削的声发射(AE)信号,利用小波包分解算法对AE信号进行滤波,并进行特征提取。将频带能量特征和切削参数分别作为主特征和辅助特征,并对其对归一化处理。采用混合粒子群优化算法(HPSO)对BP神经网络预测模型进行优化,利用优化后的模型对测试样本进行模式识别,结果表明,优化后的HPSO-BP模型能够有效地降低神经网络陷入局部极值的情况,提高刀具磨损识别精度。     

基于MasterCAM X6的高速切削优化

针对传统零件加工刀具容易磨损、影响加工效率、零件表面质量差等缺点。本文介绍应用MasterCAM X6高速加工刀具路径的参数优化点,可以一定地提高加工效率、同时相对传统刀具路径有很大的优化,减少刀具磨损,延长刀具寿命。     

Toolpath selection based on the minimum deflection cutting forces in the programming of complex surfaces milling

In this paper, a new methodology for the selection of the milling toolpaths on complex surfaces that minimize dimensional errors due to tool defection is presented. In this way, an improvement on the accuracy of milled surfaces is achieved. The methodology can be applied to both three and five axes milling. In the three axes case, it is based on the calculation of the minimum cutting force component that is related with the tool deflection. This component has been previously defined as that perpendicular to the tool axis and contained on the plane defined by the tool axis and the normal vector to the workpiece surface.Cutting forces are calculated for each 15° sense on the tangent plane to the milled surface, in a grid of control points defined by the user, both for dowmilling and upmilling. With this information there are two possibilities. First, select a general toolpath direction that minimizes the mean value of the tool deflection force on the surface, and bearing this in mind, the CAM operator can produce a CNC program which leads to an accuracy improvement. The second option is the selection of different milling directions at each control point. Joining these points, the minimum force lines are defined on the workpiece surface. These can be used as the master guides for the toolpath programming of a complete surface.In the case of five axes milling, the approach is different, because in this case the tool-axis orientation with respect to the workpiece surface may be changed using the two rotary axes. Therefore, for each workpiece area both tool-axis orientation and machining direction can be selected to keep tool deflection force below a threshold value.Some case studies of both techniques and in-deep discussion of results are presented. Applying this approach, in three axes milling dimensional errors fall down from 30 μm to below 4 μm. In five axes milling errors can be kept below 15 μm in most of the cases.