加工中心换刀装置换刀位置的设置与高效率换刀方法的研究

CNC加工中心在加工过程中需要频繁换刀并设置换刀装置的换刀位置。换刀位置的设置和换刀方法的选择是加工中心加工中重要的操作内容,其精度和快速性将直接影响加工效率。为了提高加工中心的加工效率,从实用角度介绍了换刀点的设置方法和高效率换刀方法。     

立式加工中心空间误差验证及补偿

为提高某立式加工中心整机加工精度,借助旋量理论建立完备立式加工中心空间误差模型,在此基础上实现机床空间误差有效补偿.以旋量理论为基础推导并建立机床刀具运动链与工件运动链运动学正解,分析机床21项几何误差原理,在考虑21项几何误差的基础上建立该立式加工中心完备空间误差模型;利用九线法完成各项几何误差辨识;基于旋量运动学正解求解机床运动学逆解后得出运动轴实际运动路径,并通过体对角线实验对比补偿前后的效果.结果表明:所提补偿方法补偿效果显著,验证了机床空间误差模型的准确性,实现了提高机床加工精度的目的.     

5轴卧式加工中心回转工作台设计与应用

大型5轴卧式加工中心机床回转工作台是机床在加工过程中驱动工件回转的部件,回转精度及可靠性直接影响机床的整体性能。针对5轴机床在联动切削过程中B轴移动的特点,选用大型可调间隙蜗轮蜗杆进行传动,传动机构配置有减速机构以增加驱动力矩,回转机构通过大型轴向径向组合轴承支撑,通过高精度光栅尺进行位置反馈,得到快速进给5 r/min、定位精度为3″、重复定位精度为1.5″的高精度回转工作台。     

五轴并联机床刀具末端运动位姿自适应控制技术

高精尖领域对精密小零件的加工精度要求越来越高，五轴并联机床作为主要机械加工设备，严格控制机床刀具的运动位姿对于保证成品零件的加工精度具有极其重要的作用。在此背景下，研究一种五轴并联机床刀具末端运动位姿自适应控制技术。在五轴并联机床前方布置两个CCD 摄像头作为视觉系统，拍摄关于刀具末端的左右两张图像；在图像预处理、特征点提取与匹配等环节的基础上，求解位姿参数，包括位置三维坐标以及3个姿态数据。以刀具末端运动实际位姿为输入，利用神经网络获取与刀具末端紧密相连的五轴关节角度补偿量。利用PID控制器，通过补偿量计算位姿控制量不断纠正位姿误差，靠近理想位姿。结果表明：应用所提控制方法，与理想位姿之间的平均误差均更小，且平均误差的波动最小，三维位置坐标波动仅在-0.02~0.015、-0.01~0.02、-0.02~0.02 mm之间；横滚角、俯仰角以及方位角波动仅在-0.02°~0.03°、-0.01°~0.04°、-0.02°~0.01°之间，由此说明所提控制方法的精度更高，控制稳定性更高。     

Machining accuracy improvement in five-axis flank milling of ruled surfaces

The aim of this study is to develop a new adjustment method for improving machining accuracy of tool path in five-axis flank milling of ruled surfaces. This method considers interpolation sampling time of the five-axis machine tools controller in NC tool path planning. The actual interpolation position and orientation between G01 commands are estimated with the first differential approximation of Taylor expansion. The tool swept volume is modeled using the envelope surface and compared with the design surface to determine the deviation, which corresponds to the machining error induced by the linear interpolation. We propose a feedrate adjustment rule that automatically controls the tool motion at feedrate-sensitive corners based on a bisection method, thus limiting the maximum machining errors and improving the machining accuracy. Experimental cuts are conducted on different ruled surfaces to verify the effectiveness of the proposed method. The result shows that it can enhance the machining quality in five-axis flank milling in both simulation and practical operation.     

薄壁筒车削颤振稳定性预测

切削颤振是制约薄壁筒工件加工质量和效率的主要因素之一。采用半离散法对含有时滞项的动力学方程进行稳定性预测分析,结合薄壁筒工件切削振动试验,研究刀具、工件动力学参数匹配关系变化对切削加工稳定性的影响。通过仿真分析得出:随着刀具刚度或固有频率的提升,切削系统稳定性呈上升趋势,但过度提升刀具刚度并不会有效提升切削稳定性;在刀具与工件固有频率接近处,切削系统的稳定性较差;适当调整刀具动态特性参数有利于提高柔性工件切削加工的稳定性;切削过程中,时变的切削位置和工件尺寸会引起切削系统动态特性的变化。根据时变稳定性预测图,从稳定性分析角度解释了一次走刀切削试验中薄壁筒工件表面出现不同加工形貌的原因。     

6-Axis control cutting of overhanging curved grooves by means of non-rotational tool with application of ultrasonic vibrations

This paper presents a new machining method that efficiently cuts overhanging curve grooves on wall surfaces without causing a collision between the tool and the workpiece. It also describes the development of software for 6-axis control grooving and the effect of applied ultrasonic vibrations (USV) in cutting overhanging grooves (OHG). In general, rotational tools are used to produce grooves, thus resulting in long circular arc segments at the cutting end points, as well as placing restrictions on the manufacture of grooves with continuous change in curvature, while ensuring that they do not overshoot the side clearance angle of the cutting edge with the groove. The study aims at machining OHGs presently impossible to machine by conventional methods. From the experimental results, it was found that the new machining method, which is 6-axis control cutting using a non-rotational tool with the application of USV, is capable of cutting an OHG on wall surfaces correctly.     

数控机床位置伺服系统的无模型自适应迭代学习控制

数控机床位置伺服系统在加工过程中受负载、摩擦和电路系统响应特性等因素影响,很难精确建立其加工过程动力学模型。针对批量零件加工过程中的重复执行过程,设计了一种数据驱动的无模型自适应迭代学习控制方案。该方案借助沿迭代轴的动态线性化方法,将数控机床位置伺服系统加工动力学过程等价转化成一个虚拟的迭代数据模型,并根据设计的迭代学习控制律和参数估计律构建数控机床位置伺服系统的无模型自适应迭代学习控制方案。仿真结果表明:该迭代学习控制方案基于数控机床重复运行的特点,仅利用位置和电机电流信息,完成了对零件加工过程的改善,提高了加工精度。     

面向7075铝合金预拉伸厚板加工变形控制的变向迭代法

具有壁薄、尺寸大、加工精度高、材料去除量大等特点的航空整体结构件,其毛坯大多选用预拉伸铝合金厚板。在高速切削成形过程中,随着材料的大量去除,毛坯内初始残余应力势必发生释放,零件只有通过弯曲变形等行为才能达到新的平衡状态,导致加工精度得不到保证,甚至成为废品。因此,研究残余应力释放产生加工变形的演化机制,是控制和保证加工质量的核心所在,对于实现加工过程的高效化和精密化至关重要。首先,依据毛坯分解为去除材料和成形零件两部分,将初始残余应力分为释放应力和有效应力,利用静力平衡条件和弯曲变形理论建立加工变形的分析模型。然后采用有限元方法求解加工变形的分析模型,通过现场加工零件后经测试可知：加工变形的仿真值与测量值相比,无论是幅值水平还是变形曲线,尽管均具有具有很好的吻合性,但由于残余应力的测量误差使得两者亦存在10%左右的幅值误差。最后建立以最小加工变形为目标的零件加工位置优化模型。通过以一定步长正向从最小值开始选取加工位置,根据当前值与上一次取值的变形方向差异,确定下一次取值的步长及其方向,若变形方向相同则以相同步长继续正向取值,否则减小步长反向取值,直至步长的绝对值在阈值范围之内,提出求解加工位置优化模型的变向迭代方法。与企业实际使用的中间位置法相比,变向迭代方法能够使得加工变形减小99.79%。     

Study of machining accuracy in ultrasonic elliptical vibration cutting

The cutting speeds of the tool, the rake angle and clearance angle through the cycles of elliptical vibration cutting for separating type ultrasonic elliptical vibration cutting are defined initially in the present paper. Subsequently, a theoretical model of the thrust cutting force in ultrasonic elliptical vibration cutting is proposed, and the reason of the machining accuracy improvement by applying ultrasonic elliptical vibration is clarified theoretically. Finally, the effect of ultrasonic elliptical vibration cutting on machining accuracy is verified experimentally by utilizing an ultrasonic elliptical vibration cutting system.     

机床空间精度预测与NC代码补偿研究

以某立式加工中心为研究载体,提出一种空间精度补偿技术。以旋量理论为基础,在充分考虑机床切削点空间位置的基础上,建立包含全部几何误差的立式加工中心空间精度模型,同时输出空间精度显示预测模型。针对传统空间精度补偿不充分的局限性,将空间精度补偿思路转换为NC代码最优化问题,基于遗传算法求解该最优化问题,通过实验验证优化结果的有效性。结果表明：基于旋量理论的机床空间精度建模包含21项几何误差,空间精度预测结果较为准确;基于NC代码最优化的空间精度补偿技术使得机床空间定位精度最大补偿率为90.94%,验证了所提方法的有效性。     

基于数控指令修正的数控内圆磨床几何误差补偿

为减小各几何误差对机床加工精度的影响、提高机床加工精度,以数控精密内圆磨床为研究对象,基于多体系统理论及齐次坐标变换原理,得到磨床的空间运动误差模型,建立几何误差与运动位置之间的映射关系。对加工补偿点的确定方法及数控指令修改方法进行研究,得到精密加工数控指令;通过软件进行阶梯轴试件的加工仿真验证,分别得到补偿前后的数控指令,并选取5个补偿点;补偿前后到理想位置的空间误差分别从0.616、0.607、0.614、0.295、0.376 cm减小到0.354、0.398、0.376、0.188、0.255 cm,分别减小42.5%、34.4%、38.6%、36.3%、32.1%。结果表明：通过修改数控指令能够提高机床加工精度。     

A new method for monitoring micro-electric discharge machining processes

Micro-electric discharge machining (μ-EDM) is a very complex phenomenon in terms of its material removal characteristics since it is affected by many complications such as adhesion, short-circuiting and cavitations. This paper presents a new method for monitoring μ-EDM processes by counting discharge pulses and it presents a fundamental study of a prognosis approach for calculating the total energy of discharge pulses. For different machining types (shape-up and flat-head) and machining conditions (mandrel rotation and tool electrode vibration), the results obtained using this new monitoring method with the prognosis approach show good agreement between the discharge pulses number and the total energy of discharge pulses to the material removal and tool electrode wear characteristic in μ-EDM processes. On applying tool electrode vibration, the machining time becomes shorter, because it removes adhesion. The effect of tool electrode vibration in order to remove adhesion can be monitored with good results. In order to achieve high accuracy, the tool wear compensation factor has been successfully calculated, since the amount of tool electrode wear is different in each machining type and condition. Consequently, a deeper understanding of the μ-EDM process has been achieved.     

A new tool wear compensation method based on real-time estimation of material removal volume in micro-EDM

Owing to the reduced tool area and poor flushing conditions in deep holes, tool wear in micro-electrical discharge machining (EDM) is more significant than in macro-EDM. In micro-EDM drilling, the z-axis of the tool position is monitored as machining progresses. However, due to significant electrode wear, the machined hole depth is not identical to the programmed depth of the hole, and thus this will result in geometrical inaccuracy. This paper presents a new micro-EDM drilling method, in which the material removal volume is estimated as machining progresses. Compensation length is calculated and adjustment is made repeatedly along the tool path until the targeted material removal volume is reached. A real-time material removal volume estimator is developed based on the theoretical electro-thermal model, number of discharge pulse and pulse discrimination system. Under various energy input and machining depth settings, the experimental and estimated results are found to be in satisfactory agreement with average error lower than 14.3% for stainless steel, titanium, and nickel alloy work materials. The proposed drilling method can compensate the tool wear and produce more accurate micro-holes as compared to other methods. Experimental work also shows that the proposed method is more reliable as compared to the uniform wear method. In drilling micro-holes of 900 μm depth, the depth error can be reduced to 4% using the proposed method.     

Enhancement of ECDM efficiency and accuracy by spherical tool electrode

Electrochemical discharge machining (ECDM) is an emerging non-traditional processing technique that involves high-temperature melting and accelerated chemical etching under the high electrical energy discharged. However, there are still several obstacles to overcome. First, both machining time and hole entrance diameter were found to increase with increasing machining depth. In particular, the increase becomes drastic when machining depth exceeds 250 μm. In addition, achieving both high efficiency and accuracy in drilling a through hole in hard and brittle materials by ECDM poses even greater difficulty. To solve the above problems, this study proposed using a tool electrode with a spherical end whose diameter (150 μm) is larger than that of its cylindrical body (100 μm). Experimental results show that the curve surface of the spherical tool electrode reduces the contact area between the electrode and the workpiece, thus facilitating the flow of electrolyte to the electrode end, and enables rapid formation of gas film, resulting in efficient micro-hole drilling. Moreover, the curve surface does not cause excessive concentration of current density; and hence, bubbles grow at a more uniform speed; thus, increasing the discharge frequency. Comparison between machining depth of 500 μm achieved by conventional cylindrical tool electrode and the proposed spherical tool electrode shows that machining time was reduced by 83% while hole diameter was also decreased by 65%.     

三轴数控机床加工精度可靠性分析

机床作为机械制造业的基础,几何误差、热误差、装配误差等都会影响数控机床的加工精度,数控机床加工精度的高低直接决定产品的生产质量。为保证数控机床对产品的加工质量,需要对数控机床的加工误差数据处理,求得数控机床加工精度可靠性,而一次二阶矩法和蒙特卡罗法是常用的可靠性分析方法。以三轴数控机床为研究对象,针对给定的误差数据,运用一次二阶矩法和蒙特卡罗法分析出数控机床加工精度可靠性。此分析对提高数控机床加工精度及保证使用寿命具有重要指导意义和参考价值。     

高精度复杂型线刀具制造工艺与装备研究

以高精度复杂型线刀具制造工艺与装备为研究对象，以菌形叶片叶根型线加工专用刀具为切入点，通过分析叶根型线的特点及加工工艺的优劣，确认以单边两次成型加工作为刀具加工工艺方案；并在此基础上确定刀具的轮廓度精度、前后刀面表面粗糙度等各项参数及精度要求。通过分析专用刀具的加工要求，确定专用磨床的整体布局及结构方案，并对机床的关键部件磨削装置的结构做了具体设计与阐述。分析磨削点在B轴回转轴线延伸线上的结构优点可知，B轴回转的角度误差未使磨削点位置发生偏差，可有效提升刀具的加工精度。为验证设计的合理性，通过制造与装配机床，利用雷尼绍XK10激光校准仪检测可知，几何精度均达到设计要求。最后，采用设计的刀具加工工艺方案及专用磨床对某菌形型线刀具进行加工，Zoller检测仪检测结果表明：刀具轮廓公差为工作面±0.005 mm,其余面±0.015 mm。     

Smooth feedrate planning for continuous short line tool path with contour error constraint

In the machining program for free form surfaces, the tool path is usually represented as continuous short lines. For the computer numerical control, the feedrate profile for short line tool path should be smooth and optimized in order to achieve high machining quality and high speed. In high speed machining, the feedrate profile also has a strong influence on contour accuracy. This paper presents a new real-time smooth feedrate planning algorithm for short line tool path, in which the contour error constraint is included. To realize contour error control, the feedrate is adaptively adjusted based on the curvature radius of the tool path, which is directly estimated from the short lines. The 7-phase jerk-limited look-ahead planning is employed to generate a smooth feedrate profile. The target feedrate filter (TFF) and planning units merging techniques are developed to improve the smoothness of the feedrate profile and reduce the overhead on look-ahead. The advantage of the proposed algorithm is that it is not only convenient to achieve the balance among accuracy, smoothness and productivity by adjusting parameters, but also efficient in design, which makes it possible to be implemented on low cost hardware platforms. Experiment results demonstrate the feasibility of the proposed algorithm on smooth feedrate planning and contour error control for continuous short line tool path.     

Reduction of tool wear during hydrodynamic polishing: A ‘rock-and-roll’ polishing strategy

In this paper, a ‘rock-and-roll’ polishing strategy is proposed for reducing the influence of tool wear on machining rates during hydrodynamic polishing. An analytical study suggests that the radius of curvature of spherical tools changes rapidly during the tool wear process. In addition, variations in the tool radius have a significant effect on machining rates. Increasing the contact length of the tool rapidly decreases the variation in the tool's radius of curvature. A rock-and-roll polishing strategy is therefore proposed to increase the contact length of the tool. This strategy is proposed to design the tool's rocking motion with an appropriate dwelling-time distribution so as to increase the area (or contact length) of the wear zone and create a uniform wear depth, which will in turn reduce the variation in the tool radius. A separate volume removing analysis of the tool wear suggests that the dwelling time of a tool at a given position can be determined for a given wear depth and wear rate. Finally, an experimental study confirms that the proposed strategy can reduce the variation in the tool's radius of curvature and that the effect of tool wear on the machining rate can be suppressed.     

Optimal setup for five-axis machining

This paper presents a new optimization model designed to minimize kinematics error introduced by the initial setup of a five-axis milling machine. An initial setup consists of the position and orientation of the workpiece with respect to the mounting table and, optionally, the machine's initial configuration. Given a set of cutter contact points and tool orientations, a least-squares optimization procedure finds the optimal setup parameters. Since the set of optimization parameters depends on the machine's characteristics, three basic types of five-axis kinematics are introduced. The classification into types depends on the sequence of positions of the machine's rotary axes in its kinematics chain. For each type, sets of invariant and dependent variables are identified, a corresponding system of nonlinear equations is constructed, and then the system is solved numerically. The method is not only efficient, but also provides tangible accuracy increases in tests on practical machining problems.