高速铣削加工表面位置误差的频域分析

在高速铣削加工过程中,提高轴向切削深度和主轴转速可以获得较高的材料去除率,然而限制轴向切削深度提高的一个因素是加工颤振.高速铣削系统动态失稳可能导致加工零件的表面几何精度偏差.分析高速铣削的表面位置误差对表征切削过程、刀具寿命估算和加工优化都起着重要作用.因此,在不考虑再生颤振影响的前提下,提出了一种数值分析和加工实验相结合的方法来研究表面位置误差.首先,构建了高速铣削加工过程模型,然后建立了动态铣削力模型,并推导了表面位置误差的分析方法.通过数值分析和铣削实验相结合,得到了高速铣削加工的稳定性叶瓣图.接下来,研究了逆铣削加工过程的表面位置误差,并详细分析了主轴转速和轴向切削位置对表面位置误差的影响规律.最后,把稳定性叶瓣和表面位置误差数据组合在同一个图里得到了高速铣削加工的综合分析图.借助综合分析图,能预测表面位置误差和优化高速铣削的工艺条件.     

The form error reduction in side wall machining using successive down and up milling

In this paper, the form error reduction method is presented in side wall machining. Cutting forces and tool deflection are calculated considering surface profile generated by the previous cutting such as roughing and semi-finishing. Using the form error prediction from tool deflection curve, the effects of tool teeth numbers, tool geometry and cutting conditions on the form error are analyzed. The characteristics and the differences of generated surface shape in up and down milling are also discussed and over-cut free condition in up milling is presented. The form error reduction method through successive down and up milling has been suggested. The effectiveness and usefulness of the suggested method are verified from a series of cutting experiments under various cutting conditions. It is confirmed that the form error prediction from tool deflection in side wall machining can be used in proper cutting condition selection and real time surface error simulation for CAD/CAM systems. This research also contributes to cutting process optimization for the improvement of form accuracy in die and mold manufacture.     

The statistical modeling of surface roughness in high-speed flat end milling

Surface roughness is one of the most important requirements in machining process. The surface roughness value is a result of the tool wear. When tool wear increase, the surface roughness also increases. The determination of the sufficient cutting parameters is a very important process obtained by means of both minimum surface roughness values and long tool life. The statistical models were developed to predict the surface roughness.This paper presents the development of a statistical model for surface roughness estimation in a high-speed flat end milling process under wet cutting conditions, using machining variables such as spindle speed, feed rate, depth of cut, and step over. First- and second-order models were developed using experimental results of a rotatable central composite design, and assessed by means of various statistical tests. The highest coefficient of correlation (R_adj²) (88%) was obtained with a 10-parameter second-order model. Meanwhile, a time trend was observed in residual values between model predictions and experimental data, reflecting the probable effect of the tool wear on surface roughness. Thus, in order to enhance the estimation capability of the model, another independent variable was included into the model to account for the effect of the tool wear, and the total operating time of the tool was selected as the most suitable variable for this purpose. By inserting this new variable as a linear term into the model, R_adj² was increased to 94% and a good fit was observed between the model predictions and supplementary experimental data.In this study, it was observed that, the order of significance of the main variables is as X₅>X₃>X₄>X₁>X₂ (total machining time, depth of cut, step over, spindle speed and feed rate, respectively).     

Critical cutting speed for onset of serrated chip flow in high speed machining

The transition of continuously smooth chip flow to periodically serrated chip flow as the cutting speed increasing is one of the most fundamental and challenging problems in high speed machining. Here, an explicit expression of the critical cutting speed for the onset of serrated chip flow, which is given in terms of material properties, uncut chip thickness and tool rake angle, is achieved based on dimensional analysis and numerical simulations. It could give reasonable predictions of the critical cutting speeds at which chips change from continuous to serrated for various metallic materials over wide ranges of uncut chip thickness and tool rake angle. More interestingly, it is found that, as the turbulent flow is controlled by the Reynolds number, the transition of the serrated chip flow mode is dominated by a Reynolds thermal number. Furthermore, the influences of material properties on the emergence of serrated chip flow are systematically investigated, the trends of which show good agreement with Recht’s classical model.     

Prediction of chatter in high speed milling including gyroscopic effects

Dynamic stability of machine tools during operations is dependent on many parameters including the spindle speed. In high and ultra high speed machining, the gyroscopic effect on the spindle dynamics becomes more pronounced and can affect the borders of stability of the rotating system. In this paper, a finite element based model of spindle, tool holder and cutting tool is presented which uses Timoshenko beam theory to obtain the frequency response of the system when gyroscopic terms are included. Using this response, the stability of a high speed spindle system in the presence of gyroscopic effect is investigated. It is shown that the gyroscopic effects lower the critical depth of cut in high speed milling.     

Prediction of multiple dominant chatter frequencies in milling processes

Chatter frequencies of milling operations and their strengths are analysed using the semi-discretization method. It is known that milling processes, being parametrically excited systems, are theoretically associated with infinitely many chatter frequencies that are given as a base frequency plus an integer multiplier of the tooth passing frequency (or the characteristic frequency of the cutter) according to the Floquet theory [1]. The dominant vibration frequencies that are usually associated with chatter frequencies are, however, hidden among the infinitely many harmonics. In this paper, it is shown that the amplitudes corresponding to the individual frequency harmonics can be determined in a simple way by analysing the eigenvectors of the Floquet transition matrix obtained by the semi-discretization method without increasing the computational cost. The method can be used to determine the dominant frequency components that help in the identification of the interactions between different modes and the spindle speed.     

复杂曲面多轴数控加工非线性误差理论分析及控制

针对复杂曲面通常采用的多轴数控加工方式，结合加工用刀具，建立了多轴数控加工的非线性误差的数学模型，分析其影响因素，并给出了有效减少非线性误差的控制方法。     

Real time monitoring of tool breakage in a milling operation using a digital signal processor

A monitoring system that can detect tool breakage and chipping in real time was developed using a digital signal processor (DSP) board in a face milling operation. An autoregressive (AR) model and a band energy method were used to extract the features of tool states from cutting force signals. Then, two artificial neural networks, which have a parallel processing capability, were embedded on the DSP board to discriminate different malfunction states from features obtained by each of the two methods of signal processing. In experiments, we found that feature parameters extracted by AR modeling were more accurate indicators of malfunctions in the process than those from the band energy method, although the computing speed is slower. By using the selected features, we were able to monitor malfunctions in real time.     

The analysis of correlative error in principal axis method for five-axis machining of sculptured surfaces

Correlative error is a kind of error between the cutter locations. This text presents a detailed analysis about correlative error in the principal axis method (PAM) that is a strategy of tool positioning for five-axis machining. It points out all the strategies based on linear projection cannot avoid this interference and cannot be disregarded especially in the strategies with large stepover. Accurate calculating is made individually on linear and curve feed and formulae are developed. The author also gave the available application range of PAM and the algorithm for estimating correlative error.     

高速闭式压力机曲轴的动、静态有限元分析

曲轴是压力机重要零件之一,其性能优劣直接影响到压力机的正常运转和使用寿命,尤其在高速压力机中,其强度和刚度分析是不可缺少的重要环节。为了更好地研究曲轴在工作过程中的动态特性,本文首先建立曲轴的数学模型进行理论计算,再对其进行静态和模态分析,得出曲轴的固有频率,分析振动对曲轴的影响,为曲轴的优化设计提供可靠的理论依据。     

Uncertainty analysis and variation reduction of three dimensional coordinate metrology. Part 1: geometric error decomposition

In this paper our study focuses on the uncertainty analysis and variation reduction of coordinate system estimation using discrete measurement data and is associated with the applications that deal with parts produced by end-milling processes and having complex geometry. This paper consists of three parts. Since the uncertainty of the estimated coordinate transformation arises from the geometric errors on a part surface, Part 1 is devoted to the study of surface geometric errors. In this study, according to the characteristics of end-milling processes the sampled geometric error is divided into two components, and a decomposition procedure is developed for geometric error decomposition. The results of surface geometric error decomposition will be used in Part 2 for uncertainty analysis and in Part 3 for variation reduction.     

Influences of tool pin profile and welding speed on the formation of friction stir processing zone in AA2219 aluminium alloy

AA2219 aluminium alloy has gathered wide acceptance in the fabrication of light weight structures requiring a high strength to weight ratio. Compared to the fusion welding processes that are routinely used for joining structural aluminium alloys, friction stir welding (FSW) process is an emerging solid state joining process in which the material that is being welded does not melt and recast. This process uses a non-consumable tool to generate frictional heat in the abutting surfaces. The welding parameters and tool pin profile play major roles in deciding the weld quality. In this investigation, an attempt has been made to understand the effect of welding speed and tool pin profile on FSP zone formation in AA2219 aluminium alloy. Five different tool pin profiles (straight cylindrical, tapered cylindrical, threaded cylindrical, triangular and square) have been used to fabricate the joints at three different welding speeds. The formation of FSP zone has been analysed macroscopically. Tensile properties of the joints have been evaluated and correlated with the FSP zone formation. From this investigation it is found that the square pin profiled tool produces mechanically sound and metallurgically defect free welds compared to other tool pin profiles.     

Experimental and numerical analysis of solidification cracking behaviour in fibre laser welding of 6013 aluminium alloy

Abstract

Experimental observation and numerical modelling were employed to investigate the solidification cracking behaviour during fibre laser welding of 6013 aluminium alloy. The solidification cracking initiation location and propagation path were studied using a high speed camera system and via metallurgical analysis. A three-dimensional thermomechanical finite element model of fibre laser welding of aluminium alloys was developed, which considered cylindrical volumetric heat source, temperature dependent material properties, solidification shrinkage and stress relaxation in the weld molten pool. The transient evolution and distribution of mechanical strain in the brittle temperature range (BTR) were analysed in detail to find the factors which drove the crack initiation and propagation. The results showed that the solidification cracking initiated near the fusion line and then propagated along the centreline of the weld, which was the result of the strain distribution characteristic in BTR.     

机床沿曲线高速进给加工的最大安全恒定进给速度的确定

分析了曲线、曲面轮廓数控加工中,刀具路径曲线的几何特性与机床运动学特性的关系,建立了机床沿曲线恒速进给的运动学方程;基于虚功原理推导出刀具路径几何特性、机床特性及电机特性等之间关系的动力学方程.并用解析法,计算了在各坐标轴加速度、伺服电机额定转矩及额定转速约束条件下,机床沿曲线进给时的最大安全恒定进给速度.在最大安全恒定进给速度范围内,选择合适的进给速度,既能使加工表面粗糙度均匀,保证加工表面质量,同时又能获得较高的高速加工效率.     

Role of chlorides in hot corrosion of a cast Fe–Cr–Ni alloy. Part I: Experimental studies

High alloy stainless steels are often used in corrosive, high temperature applications because they form a protective, adherent Cr₂O₃ scale. When the environment in such applications includes condensed molten salts, especially alkali sulfates and alkali chlorides, these alloys are likely to undergo hot corrosion, even at moderate temperatures compared to their typical maximum application temperature. The chemical (or electrochemical) reactions and transport modes for hot corrosion in a system involving a multi-component alloy and a multi-component salt are complex, but some insight can be gained with the help of a multi-component thermochemical model to identify major reactions. The present work consists of two parts: (a) experimental measurements of hot corrosion rates and characterization of corrosion products on a commercial, cast super-duplex stainless steel (HH), which result from exposure to thin deposits of a mixture of alkali sulfates and alkali chlorides, and (b) an evaluation of possible corrosion reactions by a multi-component thermochemical model. In laboratory hot corrosion tests alloy coupons experienced rapid oxidation rates, penetration of the oxide scale, scale blistering, and scale spallation (on cooling). Compared to simple air oxidation, alkali sulfate deposits increased the corrosion rate by a factor of about 200, but mixtures of alkali sulfate and alkali chloride increased the rate by about 20,000 times. A principal goal of the study was to identify the role of alkali chlorides in accelerating hot corrosion.