纤维夹角和铣削参数对CFRP铣削力的影响

为探索CFRP铣削加工中出现的分层、崩边等表面缺陷形成机理,对CFRP进行铣削加工实验。基于单因素实验法获得了纤维夹角对CFRP铣削力的影响规律,基于中心复合曲面设计,获得了硬质合金刀具铣削CFRP过程中铣削速度、每齿进给量和铣削深度对铣削力的影响规律,并构建了铣削力的预报模型。实验结果表明:纤维夹角在0°～90°,铣削力随纤维夹角的增大而降低,而在90°～180°,铣削力随纤维夹角的增大而增大。f_z和a_e对三个方向铣削力影响都较为显著。v_c对y向和z向铣削力影响较为显著,而对x向铣削力影响不显著。铣削力随三个铣削参数的升高而增大,其中每齿进给量对铣削力影响最大。     

基于加工表面完整性的TB6精铣参数优选

TB6高强度钛合金因其优良的比强度和热变形性能被广泛应用于航空领域,是制造直升机旋翼系统主承力结构件和起落架的主要材料.为了使零件获得较佳的疲劳性能,基于加工后的表面完整性指标对其精铣加工参数进行了优选.试验结果表明,每齿进给量fz对加工后的表面粗糙度Ra和表面残余压应力σH有较大影响,随着每齿进给量fz的不断增大,表面粗糙度Ra近似线性增大,表面残余压应力的幅值也随之增大;线速度vc主要影响加工表面显微硬化率,显微硬化率随线速度的增大而降低;径向切宽ae对加工表面残余应力也有较大的影响,随着ae的增大,加工表面残余压应力的幅值逐渐降低.因此,从提高零件疲劳寿命的方面考虑,在满足例如零件表面质量要求和生产效率等各项生产要求的前提下,应尽可能降低铣削线速度,减小径向切宽,提高每齿进给量,适当选择轴向切深.     

γ-TiAl合金铣削加工表面残余应力研究

为了探索高温理想结构材料γ-Ti Al合金铣削表面残余应力特性,结合正交试验数据处理方法中的极差分析法,建立各工艺因素情况下不同水平对表面残余应力的关系曲线,并对其工艺过程中的热力影响机理进行分析;同时建立面向残余应力的各因素灵敏度数学模型,进而确定以获取较大残余压应力为目标的工艺参数优选区间。研究表明:在试验参数限定范围内,步进方向以及进给方向均呈现为残余压应力,主要原因是冷塑性变形占主导作用。通过对灵敏度数学模型进行分析,发现残余应力对设计变量中的铣削速度变化最敏感,对每齿进给量、铣削深度和铣削宽度的敏感程度近似相同,为Ti Al金属间化合物铣削表面残余应力控制研究提供了理论依据。     

A review of chatter vibration research in milling

Chatter is a self-excited vibration of parts in machining systems. It is widely present across a range of cutting processes, and has an impact upon both efficiency and quality in production processing. A great deal of research has been dedicated to the development of technologies that are able to predict and detect chatter. The purpose of these technologies is to facilitate the avoidance of chatter during cutting processes, which leads to better surface precision, higher productivity, and longer tool life. This paper summarizes the current state of the art in research regarding the problems of how to arrive at stable chatter prediction, chatter identification, and chatter control/suppression, with a focus on milling processes. Particular focus is placed on the theoretical relationship between cutting chatter and process damping, tool runout, and gyroscopic effect, as well as the importance of this for chatter prediction. The paper concludes with some reflections regarding possible directions for future research in this field.     

Time-domain modeling of a cutter exiting a workpiece in the slot milling process

In a milling operation, there must be processes of a cutter entering and exiting the work-piece boundary. The cutter exit is usually in the feed direction and the dynamic response is different from that in the normal cutting process. This paper presents a new time-domain modeling of mechanics and dynamics of the cutter exit process for the slot milling process. The cutter is assumed to exit the workpiece for the first time with one tooth right in the feed direction. The dynamic chip thickness is summed up along the feed direction and compared with the remaining workpiece length in the feed direction to judge whether the cutter is ready to exit the workpiece or not. The developed model is then used for analyzing the cutting force and machining vibration in the cutter exit process. The developed mathematical model is experimentally validated by comparing the simulated forces and vibrations against the measured data collected from real slotting milling tests. The study shows that stable cutting parameters cannot guarantee stable cutting in a cutter exit process and further study can be performed to control the vibration amplitude in this specific process.     

基于真实刀位轨迹的铣削厚度建模

瞬时未变形切削厚度是决定铣削加工切削力预测精度的一个重要参数.现有铣削力预测方法中,通常采用圆形轨迹逼近铣刀刀齿的运动轨迹并进行未变形切削厚度的计算.铣刀刀齿的实际运动轨迹为摆线轨迹,采用圆形轨迹近似必然造成在计算未变形切削厚度时存在误差.为获得更精确的铣削力预测结果,本文提出了一种基于刀齿真实轨迹的未变形切削厚度计算方法.通过计算铣刀刀齿的真实摆线运动轨迹,推导出未变形切削厚度计算的超越方程.通过对该超越方程的数值求解,得到了准确的未变形切削厚度.最后,通过算例分析及对比验证了所提方法的有效性.     

Influence mechanism of machining angles on force induced error and their selection in five axis bullnose end milling

In the machining of complicated surfaces, the cutters with large length/diameter ratios are used widely and the deformation of the machining system is one of the principal error sources. During the process planning stage, the cutting direction angle, the cutter lead and tilt angles are usually optimized to minimize the force induced error. It may lead to a low machining efficiency for bullnose end mills, as the material removal rates are different largely for different machining angles. In this paper, the influence mechanism of the machining angles on the force induced error is studied based on the models of the instantaneous cutting force when the cutter flute traveling through the cutting contact point and the stiffness of the machining system. In order to evaluate the machining angles, the force induced error/efficiency indicator (FEI) is defined as the division of the force induced error and the equal volume sphere of the removed material. FEI is dimensionless, with the lower FEI, the lower force induced error and the higher machining efficiency. For optimal selection of the machining angles, the critical FEI is calculated with the constraint of force induced error and the desired material removal rate, and the critical FEI separate the set of the machining angles into two subsets. After the feed rate scheduling process, the machining angles in the optimal subset would have higher machining accuracy and efficiency, while the machining angles in the other subset have lower machining accuracy and efficiency. Through the machining experiment of five axis machining and freeform surface machining, the effectiveness and superiority of the proposed FEI method is verified with a bullnose end mill, which can improve the machining efficiency with the constraint of force induced error.     

Influences of milling and grinding on machined surface roughness and fatigue behavior of GH4169 superalloy workpieces

Surface topography of superalloy GH4169 workpieces machined by milling and grinding is different significantly. Meanwhile, surface roughness, as one of the main indicators of machined surface integrity, has a great influence on the fatigue behavior of workpieces. Based on analyzing the formation mechanism and characteristics of surface roughness utilizing different machining processes and parameters, the machined surface roughness curve can be decoupled into two parts utilizing frequency spectrum analysis, which are kinematic surface roughness curve and stochastic surface roughness curve. The kinematic surface roughness curve is influenced by machining process,parameters, geometry of the cutting tool or wheel, the maximum height of which is expressed as R'_z.By subtracting the kinematic part from the measurement curve, the stochastic surface roughness curve and its maximum height R'_zcan be obtained, which is influenced by the defects of cutting tool edge or abrasive grains, built-up edges(BUE), cracks, high frequency vibration and so on. On the other hand, the results of decoupling analysis of surface roughness curves indicate that Raand Rz values of milling GH4169 are 2–5 times and 1–3 times as high as those of grinding, while R'_zvalue of milling is 13.85%–37.7% as high as that of grinding. According to the results of fatigue life tests of specimens machined by milling and grinding, it can be concluded that fatigue behavior of GH4169 decreases with the increase of R'_zmonotonically, even utilizing different machining processes.     

SiC_p/Al复合材料高速铣削切削力模型建立

使用聚晶金刚石(PCD)刀具并采用正交试验设计法,对含不同体分比的碳化硅颗粒增强铝基复合材料(SiCp/2009Al)进行高速铣削试验。在测量切削力和切屑厚度的基础上,建立了剪切角、剪切应力和摩擦角的预测模型,并结合金属切削基本理论公式建立了切削力的预测模型。该模型包含铣削速度、每齿进给量、径向切宽、增强颗粒体分比等重要参数,模型对进给方向最大铣削力预测值的平均误差为5.9%,对铣刀径向切深方向最大铣削力预测值的平均误差为9.2%,皆高于普通经验公式的预测精度,从而可对SiCp/2009Al复合材料高速铣削时的铣削力进行有效预测。     

Effects of in-situ TiB2 particles on machinability and surface integrity in milling of TiB2/2024 and TiB2/7075 Al composites

In-situ ceramics particle reinforced aluminum matrix composites are favored in the aerospace industry due to excellent properties. However, the hard ceramic particles as the reinforcement phase bring challenges to machining. To study the effect of in-situ TiB₂ particles on machinability and surface integrity of TiB₂/2024 composite and TiB₂/7075 composite, milling experiments were performed, and compared with conventional 2024 and 7075 aluminum alloys. In-situ TiB₂ particles clustered at the grain boundaries and dispersed inside the matrix alloy grains hinder the dislocation movement of the matrix alloy. Therefore, the milling force and temperature of the composites are higher than those of the aluminum alloys due to the increase of the strength and the decrease of the plasticity. In the milling of composites, abrasive wear is the main wear form of carbide tools, due to the scratching of hard nano-TiB₂ particles. The composites containing in-situ TiB₂ particles have machining defects such as smearing, micro-scratches, micro-pits and tail on the machined surface. However, in-situ TiB₂ particles impede the plastic deformation of the composites, which greatly reduces cutting edge marks on the machined surface. Therefore, under the same milling parameters, the surface roughness of TiB₂/2024 composite and TiB₂/7075 composite is much less than that of 2024 and 7075 aluminum alloy respectively. Under the milling conditions of this experiment, the machined subsurface has no metamorphic layer, and the microhardness of the machined surface is almost the same as that of the material. Besides, compared with 2024 and 7075 aluminum alloy, machined surfaces of TiB₂/2024 composite and TiB₂/7075 composite both show tensile residual stress or low magnitude of compressive residual stress.