液压半自动化钻铣组合机床的设计【说明书+CAD】
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说明书+CAD
液压
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液压半自动化钻铣组合机床的设计【说明书+CAD】,说明书+CAD,液压,半自动,化钻铣,组合,机床,设计,说明书,CAD
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Dimensional Tolerance Allocation for New Type of Parallel Machine Tools1 INTRODUCTIONSThe prototype of a new type of parallel machine tools is a 6-6 Stewart platform mechanism. Compared to the traditional machine tool, a parallel machine tool possesses many advantages, such as high precision, high structural rigidity and high machining force/torque capacity, etc. Currently, many universities have research on this kind of machine tools. As shown in Fig.1, the parallel machine tool comprises the following four parts: a fixed platform, six in-parallel actuators, the length variation of each actuator is implemented by ball-screws and the ball-screws are driven by servo motors. The ball joints connect the fixed and moving platforms with ball-screws; a spindle is installed on moving platform bottom. When machining work is being carried out the variation of the moving platform allows a parallel machine tool to machine complicated curved work piece such as die, impeller etc.The developing tendency of modern NC machine tools is high speed and high precision, and the machine tool errors have direct effect on machine tool precision. In order to meet the precision specified, a scientific dimensional tolerance allocation is urgently required in the design of this kind of machine tool. The errors of the machine tool include the following parts: 1)transmission error in transmission chains, i.e. ,pitch cumulative error of the ball-screw in each actuator and dead domain error during starting or reversing operation;2)length transform error of the ball-screw due to the influence of ambient temperature;3)pares clearance error that connect fixed and moving platform;4)other errors, i.e. , positioning error caused by transmission rigidity, dynamic error caused by moving parts mass and velocity damping. The above errors will finally lead to error at the cutter head of the machine tool. This paper will carry out research on dimensional tolerance allocation based on a kind of statistical experiment method, Monte-Carlo method.2 SOLUTION OF INVERSE KINEMATICS FOR MACHINE TOOLThe kinematics solution for this parallel machine tool can be classified into two types. When the known quantity is the lengths of the six actuators and the orientation of the cutter head, this is called direct kinematics; On the contrary, when the known quantity is the position and the orientation of the cutter head and the unknown quantity is the lengths of the six actuators, this is called inverse kinematics. The solution of the direct kinematics of the machine tool is used to monitor while the solution of the inverse kinematics of the machine tool is used to control. As shown in Fig.2, a fixed coordinate frame O-XYZ is attached to the fixed platform and a moving coordinate frame O-XYZ is attached to the moving platform. The center of the joint connecting the itch actuator to the fixed platform will be denoted as whereas the center of the joint connecting the same leg to the moving platform will be denoted as , the Y-axis of the fixed coordinate frame is selected along the line which bisects the angle and the Y-axis of the moving coordinate frame is selected along the line which bisects the angle . Let the position of point O with respect to the origin of the fixed coordinate frame be denoted by vector , vectors will be defined as the position vectors of the moving coordinate frame, thus we can write the position vectors of the moving platform joints in fixed coordinate frame as : Where matrix is the rotation matrix describing the orientation of the moving platform with respect to the fixed platform, the elements of matrix are shown by RPY expression. Let be angles that the moving platform rotates about X-axis, Y-axis, Z-axis of the fixed coordinate frame, and then we can obtain:Let vectors be the position vectors of the platform joints with respect to the fixed coordinate frame, then the length of each actuator can be written as:3 DIMENSIONAL TOLERANCE ALLOCATIONS WITH MONTE-CARLO METHOD2.1 Determination of Total Dimensional Chain Error of the Machine Tool Links In order to carry out dimension tolerance allocation, the variation feature between the errors of the in-parallel links and orientation error of the cutter head (or the center point of the moving platform) need to know. The direct kinematics is used to derive the cutter head error from the link errors of the machine tool. It is difficult to obtain analytical solution by direct kinematics. So an inverse error estimation method by which the link errors can be derived from the position and orientation errors of the cutter head is adopted.The key to the dimensional tolerance allocation is to determine the total dimensional chain error of the machine tool links. Monte-Carlo method is a numeral method to solve mathematics based on random sampling. In this parallel an evenly-distributed Monte-Carlo random simulation method on a certain position and orientation precision of the cutter head is used. By Esq. (1) the lengths of the machine tool links under the sample volume can be obtained. The maximum length error of each link can be obtained in different positions in the machine tool workspace. Take the minimum length errors as the total error for dimensional tolerance allocation of each link. Before random simulation by Monte-Carlo method, the machine tool precision should be estimated and the variation of the moving platforms position and orientation should be determined. Let the position and orientation of the center point of the moving platform be three transforming quantities and three rotating quantities. A simplified model of spindle system for parallel machine too is shown in Fig.3. From the figure we can see that point a and b represent cone bearings, segment bc represents cutter holder and segment cd represents the cutter, the cutter head point d bears machining force P. 4 CONCLUSIONSThe presented dimensional tolerance allocation method is the combination between machine tool precision and the actual dimension tolerances. The factors that influence on the cutter head errors of the parallel machine tool are analyzed first. Due to the multi-solution and the difficulty in obtainment of the analytical solution of the direct kinematics, the derivation of the cutter head error from the error of machine tool links is difficult. So a inverse error estimation method to derive errors of links from cutter head error is presented in this paper.To a certain spindle dimension of the parallel machine tool, the total dimensional tolerance of the machine tool link can be determined by Monte-Carlo random simulation method. The dimensional tolerance allocation for adjustable loop of the link is developed. If the allocated dimension tolerance can not meet the design or manufacturing requirement, the adjustments towards precision grade of the ball screw or the spindle dimension are desired for tolerance re-allocation. Example shows the presented tolerance allocation method is reasonable. This work provides a basis in the design stage of the parallel machine tool.一种新型并联机床的尺寸公差分配1介绍新类型的并联机床的原型是一个6-6斯图尔特平台机构。与普通机床相比，并联机床有许多优点，比如高的精度，高的结构刚度和高机制力量/转力矩能力等。现在，许多大学都对这种并联机床都加以研究。如图1所示。并联机床包括四个部分：一个固定的平台，六个平行的主动件，每个主动件的长度变化由球铰链来控制，而球铰链由伺服马达驱使。球关节用铰链连接固定部分和可动部分，转轴安装在移动平台底部。当机加工进行时，移动平台使并联机床能加工复杂的工件，如钢模的复杂弯工作块及其他的东西。 现代的控制母机的发展趋向是高速度和高精度，并且机床的误差应能直接影响机床的精度。为了解决这种精密的需要，一种尺寸公差分配方法需要应用于机床的设计中。机床的误差包括下列各项部分：（1）传输链的传输误差，也就是，在开始或颠倒操作的时候在每个主动件和死区中球铰链的累积误差（2）周围温度变化导致的球铰链长度变化误差（3）剥连固定和可动工作台的清除误差（4）其它的误差，也就是，由移动部分块和速度降低引起的动态误差。上述的误差最终会导致在工作母机的切削头上产生的误差。这篇论文将对一种基于统计的尺寸公差分配的实验方法蒙地卡罗方法加以研究。2 机床的逆向运动学的解决运动学解决这种并联机床的运动学解决方法可以分为两类。当已知量是六个主动件的长度和切削头的方向的时候，这叫做正向的运动学。另一方面，当已知量是位置和切削头的方向而未知量是六个主动件的长度的时候，这叫做逆向的运动学。并联机床的正向运动学用于解决检测问题，而逆向运动学用于解决控制问题。如图2所示，固定的坐标系附在固定的平台上，而移动的坐标系附在移动的平台上。连接主动件到固定的平台关节的中心被表示为，同样，连接相同的腿到那个移动平台的关节中心被表示为。固定坐标系的Y轴是按沿着角挑选的，而移动坐标系的Y轴是沿着角。让
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