三轴机床微细异型电极加工与尺寸误差分析

采用直线插补与圆弧插补算法,在三轴卧式微细电火花机床上加工出圆锥台、四棱台以及带有圆弧曲面的微细异型电极,并对加工结果尺寸误差及导致误差产生的原因进行了全面分析。结果表明,微细异型电极的尺寸误差均在2%以内,而放电间隙的随机性、机床运动精度、尺寸测量精度等均是导致误差形成的原因。     

数控机床加工非圆曲线的程序编制方法

对在数控机床上不能直接进行插补加工的非圆曲线、列表函数，提出了用三次样条函数插值、用双圆弧逼近加工轮廓线的方法，并给出了在计算机上予以实现的流程图。     

High speed CNC system design. Part I: jerk limited trajectory generation and quintic spline interpolation

Reference trajectory generation plays a key role in the computer control of machine tools. Generated trajectories must not only describe the desired tool path accurately, but must also have smooth kinematic profiles in order to maintain high tracking accuracy, and avoid exciting the natural modes of the mechanical structure or servo control system. Spline trajectory generation techniques have become widely adopted in machining aerospace parts, dies, and molds for this reason; they provide a more continuous feed motion compared to multiple linear or circular segments and result in shorter machining time, as well as better surface geometry. This paper presents a quintic spline trajectory generation algorithm that produces continuous position, velocity, and acceleration profiles. The spline interpolation is realized with a novel approach that eliminates feedrate fluctuations due to parametrization errors. Smooth accelerations and decelerations are obtained by imposing limits on the first and second time derivatives of feedrate, resulting in trapezoidal acceleration profiles along the toolpath. Finally, the reference trajectory generated with varying interpolation period is re-sampled at the servo loop closure period using fifth order polynomials, which enable the original kinematic profiles to be preserved. The proposed trajectory generation algorithm has been tested in machining a wing surface on a three axis milling machine, controlled with an in house developed open architecture CNC.     

Stability-based spindle speed control during flexible workpiece high-speed milling

High-speed machining (HSM) is a technology used to increase productivity and reduce production costs. The prediction of stable cutting regions represents an important issue for the machining process, which may otherwise give rise to spindle, cutter and part damage. In this paper, the dynamic interaction of a spindle-tool set and a thin-walled workpiece is analysed by a finite element approach for the purpose of stability prediction.The gyroscopic moment of the spindle rotor and the speed-dependent bearing stiffness are taken into account in the spindle-tool set finite element model and induce speed-dependent dynamic behaviour. A dedicated thin-walled workpiece is designed whose dynamic behaviour interacts with the spindle-tool set. During the machining of this flexible workpiece, chatter vibration occurs at some stages of machining, depending on the cutting conditions and also on the tool position along the machined thin wall.By coupling the dynamic behaviour of the machine and the workpiece, respectively, dependent on the spindle speed and the relative position of both the systems, an accurate stability lobes diagram is elaborated.Finally, the proposed approach indicates that spindle speed regulation is a necessary constraint to guarantee optimum stability during machining of thin-walled structures.     

数控加工中变坐标问题的研究

针对多轴加工中心中机床无法加工死角或者干涉面的情况,添加能够变角度加工的刀具以实现不加装其他旋转轴或者翻转平面加工垂直刀具无法接触到的加工面,从而达到无需改变机床结构就可以增大其加工范围和适应性的效果,并能减少工件重复装夹次数,提高加工精度和效率。通过TCL语言对该后处理进行定制,达到自动判断实际加工时需要的G17/G18/G19平面,并且进行稳定的圆弧插补、直线插补、进刀和退刀,保证加工的安全性和准确性;除此以外,还加入了人性化的刀具列表和参数错误报警,为操作技师提供了很好的辅助参考;最后通过仿真实验证明该定制后处理可以在侧铣头加工中稳定、高效的使用,并获得较好的加工效果。     

加工中心几何精度检验的探讨

本文简要的论述了加工中心整机几何精度检验合理的实施方法， 分析了新颁国际标准（ 草案） ＩＳＯ／ＤＩＳ１０７９１ 的几何精度部分同原机械工业部部颁标准ＪＢ／ＧＱ１１４０ － ８９ 加工中心精度部分二者的差异与劣优， 差重指出ＩＳＯ／ＤＩＳ１０７９１ 新标准仍有漏检或易引起争议的一些尚待商榷的问题， 提出了制订我国加工中心几何精度检验新标准的具体建议。     

基于机床旋转轴角加速度的五轴加工刀轴矢量局部优化

针对现有刀轴矢量光顺方法缺少对刀轴矢量进行局部超限修正的不足,提出一种在机床坐标系下对刀轴矢量进行局部光顺的方法。通过四元数插值法,进行整个加工轨迹上的初次刀轴矢量规划,得到工件坐标系下光顺的初始刀轴矢量。以机床旋转轴角度变化最小为目标,以机床运动学性能限制为约束,求解得到超限区域。针对超限区域,以角加速度最小为目标进行局部修正。通过叶轮加工实例验证所提刀轴矢量局部光顺算法。结果表明：所提算法能够在保持原有刀轴矢量优良加工特性的同时,避免局部超限,从而达到对刀轴矢量进行局部光顺的目的。     

高速切削技术的发展与研究

介绍了高速切削技术的发展，总结了该技术的特点；分别对高速切削机理、高速切削机床、刀具以及高速切削工艺等关键技术进行了阐述；根据高速切削技术目前的发展现状，分析并提出了该技术未来研究发展需着重解决的问题。     

In-process high-speed photography applied to orthogonal turning

Material behaviour understanding is a basic pillar for the building of predictive models applied to machining processes and the majority of the formulated material flow rules that are intimately associated to strain and strain rate. The use of high-speed filming allows observing a sequence of frozen images focused on the chip formation area when machining steel in orthogonal turning tests. This article presents the set-up and images acquired over square grid marked tube work-pieces on 42CrMo4 steel. Variables such as chip geometry, shear angle, strain, strain rate, chip thickness, and tool vibration amplitude are measured. Information acquired by the displacement of flow patterns allows measuring of plastic variables. Strain and strain rate results are calculated and compared to analytical modelling results. Industrial machining speeds and feeds are analysed by means of short shutter times, high image acquisition rates, and high magnifications achieving a good compromise between image quality, recording continuity, and cost of the equipment and experiments.     

Laser-assisted high-speed finish turning of superalloy Inconel 718 under dry conditions

Inconel 718 (IN718) is used in aerospace applications due to its superior mechanical properties. This study investigates the high-speed machinability of this material under laser-assisted machining (LAM) and dry conditions. Finish turning tests were performed for cutting speeds up 500 m/min and feeds up to 0.5 mm/rev, using focused Nd:YAG laser beam and ceramic tool (SiAlON). At optimum machining conditions, nearly eight-fold increase in material removal rate and significant improvement in the tool life and surface finish were achieved, compared to conventional machining. The mechanisms of tool failure were identified. SEM analysis and microstructure examination of machined surfaces revealed the improvement in the surface integrity under LAM conditions.     

An accurate,efficient envelope approach to modeling the geometric deviation of the machined surface for a specific five-axis CNC machine tool

Geometric deviation, defined as the difference between the nominal surface and the simulation model of the machined surface, is the fundamental concern of five-axis tool path planning. Since the machined surface is part of the cutter envelope surface generated by the cutter motion, it is necessary to calculate the envelope surface in order to obtain the geometric deviation. In the stage of tool path planning, current approaches calculate the cutter envelope surface by using the cutter motion along the given tool path. However, the cutter motion of practical machining on a specific five-axis CNC machine tool is different from the given tool path. Moreover, the computation is very challenging when the accurate cutter motion of practical machining is applied to calculate the envelope surface. To overcome these two problems, a geometric envelope approach with two major distinctions is proposed in this paper. First, the envelope surface of the cutter undergoing a general motion is efficiently obtained as a closed-form vector expression. Second, the accurate cutter motion, which is determined by machine kinematic and interpolation scheme in practical machining, can be easily applied to calculate the accurate envelope surface. With the envelope surface, the geometric deviation is calculated to estimate the overcut or undercut in five-axis milling. An example is given to demonstrate the validity of the proposed method.     

Wear behaviour of cryogenically treated tungsten carbide inserts under dry and wet turning conditions

Cryogenic treatment has been acknowledged as a means of extending the life of tungsten carbide inserts but no study has been reported in open literature regarding the effect of coolant on the performance of cryogenically treated tungsten carbide inserts in turning. In order to understand the effect of coolant, a comparative investigation of the wear behaviour of cryogenically treated tungsten carbide inserts in dry and wet orthogonal turning has been carried out in this study. The commercially available uncoated square-shaped tungsten carbide inserts with chip breakers were procured and cryogenically treated at ?196 °C and the cutting tests were executed in accordance to the International Standard Organisation, ISO 3685-1993 for continuous and interrupted machining mode. The criterion selected for determining the tool life was based on the maximum flank wear (0.6 mm) and the selection of cutting conditions was made to ensure the significant wear at a suitable time interval. The results showed that cryogenically treated tungsten carbide inserts performed significantly better in wet turning conditions under both continuous and interrupted machining modes especially at higher cutting speeds. A considerable increase in tool life was also recorded in interrupted machining mode as compared with continuous machining mode.     

Corner optimization for pocket machining

The aim of this paper is to propose a pocketing tool path improvement method by adapting the geometry of the tool path to the kinematic performance of high speed machining machine tools. The minimization of the machining time is a major objective, which should be taken into account for the tool path computation. In this way the tool path length can be reduced or the real feedrate increased. The described method proposes modification of the values of the corner radii in order to increase real feedrate. In the same way, this method checks the radial depth of cut variations along the tool path. The computed tool path presents a smaller length and the machine tool produces a higher average feedrate at the same time. In addition, the use of Bspline for the tool path computation is a significant improvement compared with straight lines and circle arcs for the machining time reduction. Several tests are realized on various machine tools in order to quantify the benefits: the proposed method can reduce the machining time by approximately 25% compared with classical tool paths computed using a CAM system.     

Investigation of heat partition in high speed turning of high strength alloy steel

High Speed Machining (HSM) is now recognised as one of the key processes in advanced machining technology for automotive, die and mould, and aerospace industries. Machining of metals at high cutting speeds produces high temperatures in the primary shear zone, which induces plasticity in the workpiece and hence decreases the cutting forces. This investigation is concerned with the estimation of the amount of heat flowing into the cutting tool in high speed turning of BS 970-709M40EN19 (AISI/SAE-4140) high strength alloy steel. The aim is to characterise the thermal field in the cutting zone and thus understand the mechanics of HSM. Experimental results are presented of temperature measurements on the tool rake face during orthogonal cutting at cutting speeds ranging between 200 and 1200 m/min. These measured temperatures are compared with temperature fields in the cutting tool obtained from a finite element transient thermal analysis. It is shown that the tool–chip contact area, and hence the proportion of the secondary heat source conducting into the tool, changes significantly with cutting speed; it decreases with the cutting speed in the conventional and the transition regions but increases in the HSM region approaching 65% at 1200 m/min. These results are relevant to the study of thermal expansion of the cutting tools and the cutting edge wear in HSM operations.     

Influence of cutting edge radius on cutting forces in machining titanium

The performance of machining titanium can be enhanced by using cutting tools with rounded cutting edges. In order to better understand the influence of rounded cutting edges and to improve the modelling of the machining process, their impact on active force components including ploughing forces and tool face friction is analysed. This paper presents experimental results of orthogonal turning tests conducted on Ti-6Al-4V with different cutting edge radii and changing cutting speeds and feeds. As an accurate characterisation method for the determination of the cutting edge radius is prerequisite for this analysis, a new algorithm is described which reduces uncertainties of existing methods.     

A comparative study of the cutting forces in high speed machining of Ti–6Al–4V and Inconel 718 with a round cutting edge tool

Titanium alloy Ti–6Al–4V and nickel-based superalloy Inconel 718 have been widely employed in modern manufacturing. The published literature on high speed machining (HSM) of the two materials often involves different machining set-up, which makes it difficult to directly apply the research findings from one material to the other to select the most appropriate tool geometry and cutting conditions. A comparative experimental study of HSM of Ti–6Al–4V and Inconel 718 is conducted in this paper using the same machining set-up. The scope of this study is limited in high speed finish machining, where the tool edge geometry plays a significant role. The experimental set-up and the methods of measuring the cutting forces and the tool edge radius are introduced. A total of 40 orthogonal high speed tube-cutting tests were performed, involving five levels of cutting speeds and four levels of feed rates. Based on extensive experimental data, the similarities and differences between HSM of Ti–6Al–4V and Inconel 718 are quantitatively compared and qualitatively explained in terms of four quantities: (1) the cutting force F_c, (2) the thrust force F_t, (3) the resultant force R, and (4) the force ratio F_c/F_t. A total of 12 empirical regression relationships are obtained.     

Mechanism of minimum quantity lubrication in high-speed milling of hardened steel

The rapid wear rate of cutting tools due to high cutting temperature is a critical problem to be solved in high-speed machining (HSM) of hardened steels. Near-dry machining such as minimum quantity lubrication (MQL) is regarded as one of the solutions to this difficulty. However, the function of MQL in HSM is still uncertain so far which prevents MQL from widely being utilized in the machining of hardened steels. In this paper, the mechanism of MQL in HSM of hardened steel is investigated more comprehensively. Comparing with dry cutting, the tool performance can be enhanced by MQL under all cutting speeds in this study. It is found that MQL can provide extra oxygen to promote the formation of a protective oxide layer in between the chip–tool interface. This layer is basically quaternary compound oxides of Fe, Mn, Si, and Al, and is proved to act as diffusion barriers effectively. Hence, the strength and wear resistance of a cutting tool can be retained which leads to a significant improvement of tool life. It is found that there exists an optimal cutting speed at which a stable protective oxide layer can be formed. When cutting speed is lower than this speed, there is less oxide layer and the improvement of tool life is less apparent. As the cutting speed is far beyond the optimal value, the protective layer is absent and the thermal cracks are apt to occur at the cutting edge due to large fluctuation of temperature. Resultantly, application of MQL is inappropriate in the extreme high-speed cutting condition irrespective of its little increase in tool life. Based on this study, it is concluded that the tool life can be effectively improved by MQL in HSM of NAK80 hardened steels when cutting parameters are chosen properly.     

A strategy for identifying static deviations in universal spindle head type multi-axis machining centers

This research was conducted in order to investigate the effectiveness of the checking method specified in ISO 10791-6 and to propose an additional method for identifying the geometric deviations inherent to five-axis machining centers with a universal spindle head. The specified method is not sufficient for assessing the influence of individual geometric deviations. Therefore, additional measurements are needed in order to use this checking method effectively. The spherical motion in the additional measurements is a modified version of the ISO standard. Some of the deviations are presumed based on the result of the additional measurements, and so the method of identifying the remainder of the geometric deviations was designed by analyzing the link geometry of the machine. The identification procedure consists of three steps. In the first and second steps, after performing two measurements, six out of ten deviations are identified by means of an observation equation. In the third step, four of the ten deviations are identified by means of the link geometry. The exactness of the proposed procedure for identifying the deviations inherent to the five-axis machine is confirmed through simulations.     

Efficient Tool Paths and Part Orientation for Face Milling

High speed machining is pushing the limits of feeds and speeds. A different approach for high throughput is described here. The focus is on the maximum feed that can be obtained for a segment; the feed rate losses due to sharp changes in tool path are minimized. The interdependency of individual axis drive speeds for a tool path segment are analyzed. There exists an optimum work angle relative to the axes that reduces losses and increases allowable feeds for particular segments, saving valuable cycle time and balancing feed drive loads. Face milling and roughing steps of end milling are the most attractive application areas.     

A PC–NC milling machine with new simultaneous 3-axis control algorithm

Increasing demands on precision machining have necessitated that the tool move not only with a position error as small as possible, but also with smoothly varying feed rates. In this paper, a 3-axis PC–NC milling system, which is capable of synchronized simultaneous 3-dimensional (3D) machining, is developed. To achieve the synchronous 3D linear and circular motions, new interpolation algorithms based on the intersection criteria are presented. A real-time reference-pulse 3D linear and circular interpolator is developed using a PC to implement in the framework of the PC–NC milling machine reconfigured in this research. The performance test via computer simulation and actual machining have shown that the developed PC–NC milling system is useful for the machining of arbitrary lines and circles in 3D space.