文献翻译一种新型并联机床的尺寸公差分配Word格式.docx

1、The 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 univ

2、ersities 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

3、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 d

4、eveloping 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

5、 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

6、 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 machi

7、ne 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 know

8、n 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 invers

9、e 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 a

10、ttached 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 b

11、isects 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

12、, 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 m

13、oving 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

14、 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

15、） 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

16、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