1 Introduction The parallel machine tool is a new type of numerical control machine tool which adopts the parallel mechanism as the main transmission mechanism. Its control strategy is obviously different from that of the series machine tool, but the research on this aspect is not deep enough. This paper takes the 6-DOF type 6-TPS parallel machine tool as the research object, introduces the general process of generating the parallel machine tool NC code from the tool location file, studies the preprocessing method of the tool post file in the post processing and develops the corresponding preprocessing. Module. Through the numerical control machining test of actual parts, the correctness and practicability of the method and system are verified.
Fig.1 Structure model and coordinate system of 6-TPS parallel machine tool
2 Structure model of the parallel machine tool The 6-DOF 6-TPS parallel machine tool model VAMT1Y is shown in Fig. 1. The main structure of the machine tool consists of a static platform, a movable platform and six retractable rods. In order to improve the motion sensitivity and avoid telescopic rod interference, the static and dynamic platforms adopt the upper and lower layered structure. Both ends of each telescopic rod are respectively connected with a static platform and a moving platform with a Hooke hinge and a ball hinge. Each telescopic rod is driven by AC servo motor and ball screw pair. The tool is mounted on a moving platform and is driven by a spindle motor. The moving platform and tool can achieve six degrees of freedom in space motion. Establish the coordinate system shown in Figure 1. The machine coordinate system {M} is located on the workbench, also known as the machining coordinate system; the dynamic coordinate system {T} is located on the moving platform; the basic coordinate system {B} is located on the static platform, and its three coordinate axes are oriented with {M} The same; workpiece coordinate system {C} is the reference coordinate system for workpiece programming, also known as the programming coordinate system. 3 Parallel machine tool control data generation Parallel machine tool control data generation process shown in Figure 2. After the system receives the tool location file from the CAM system, it first performs data preprocessing; then it acquires the machine tool structure parameters (including the mounting coordinates, reference vector and rotation range of the 12 hinges of the static and dynamic platforms, the movable platform and the six telescopic rods. Quality, basic length and retractable range of telescopic rod, diameter size and its movement speed, acceleration, driving force rating, etc.) and cutting condition parameters (including feed rate of tool, fast forward speed, etc.); With the principle of machining accuracy, the {C} positioning vector relative to {M} is determined, and a {M}-based tool location file is generated by code conversion; finally, Cartesian space interpolation calculation is performed to generate the coordinate space of the task space. At the same time, calculate the corresponding dynamic platform pose, and use the kinematic inverse solution algorithm to generate the actual servo axis control data.
Figure 2 Generation process of parallel machine tool control data
The process of generating a machine tool dynamic platform pose based on {M} from a tool location file relative to {C} can be expressed as MTCTCCq=MTT[0 0 -Lt l]T (1) MTCTCCk=MTT[0 0 1 0]T (2) where: Lt—tool length Cq—position vector of the tool relative to {C}, Cq=[qx qy qz 1]T Ck—direction vector of the tool relative to {C}, Ck=[kxky Kz 0]T TC——preprocessing matrix of tool location file MTC——positioning transformation matrix of work piece coordinate system, used to describe the relationship between {C} and {M} MTT——machine tool motion transmission matrix, relative to {T} It consists of the position vector MrT of {M} and the attitude matrix MRT. Its specific form is T=[ MRT MrT ] 0 1 (3) It can be obtained using the inverse mechanism algorithm of the mechanism based on the determined position and orientation of the moving platform MrT and MRT. The coordinates of each servo axis. The position vectors of the hinge points relative to {B} and {T} for the static and mobile platforms are BBi=[Xi,Yi,Zi]M and TTi=[xi,yi,zi]M, which are relative to the coordinates of {M} MBi=MrB+MRBBBi (i=1,2,...,6) (4) MTi=MrT+MRTTTi (i=1, 2,..., 6) (5) where MrB, MRB, and {B, respectively, are {B } Relative to the position and attitude matrix of {M}, determined by the machine structure parameters According to the definition of Figure 1, MRB is the identity matrix, so the vector of each servo branch is lili=MBi-MTi (i = 1,2,...,6 (6) where: li - the direction vector of the i th branch, vector li - the length of the i th branch, coordinates (6) for time, and the speed expression for V = Jv (7) Where: V - the joint space velocity vector v consisting of the speeds of the six telescoping poles - the six-dimensional velocity vector J of the tool in Cartesian space, the Jacobian matrix, which reflects the speed of the joint space to the working space General data preprocessing of 4 position tool files For some parts with symmetrical structure, in order to reduce the part programming workload, CAD/CAM system is generally not required to generate all the tool position files of the part. This requires the system to have a tool position. Mirroring function member or rotation processing, i.e., cutter location data for the pre-determined formula (1) and (2) the TC. For this purpose, a data preprocessing module was designed and developed in the postprocessor of VAMT1Y. Its functions include: 1 normal format conversion; 2X-Y plane mirror conversion; 3Y-Z plane mirror conversion; 4Z-X plane Mirror transformation; 5 (X = Y) - Mirror transformation of the Z plane; 6 (X = - Y) - Mirror transformation of the Z plane; 7 Rotation transformation around the X axis; 8 Rotation transformation around the Y axis; 9 Around the Z axis Rotation transformation. The conversion form is Cq1=TCCq (8) Ck1=TCCk (9) Where: Cq1 - the converted tool position Ck1 - the direction vector of the tool position after the conversion The tool position file preprocessing matrix TC has the following forms: Normal Format TC=I XZ plane image TC=TX-Z= [1 0 0 0 ] 0 -1 0 0 0 0 1 0 0 0 0 1 YZ plane image TC=TY-Z= [ =1 0 0 0 ] 0 1 0 0 0 0 1 0 0 0 0 1 XY plane mirror TC=TX-Y= [1 0 0 0 ] 0 1 0 0 0 0 -1 0 0 0 0 1 (X=Y)-Z plane mirror can be considered first Rotate around the Z axis by 45°, then mirror the XZ plane, and rotate the coordinate system around the Z axis by -45°. The transformation matrix is ​​TC = Rot(Z, p/4) TX - ZRot(Z, -p/ 4)=[ 0 1 0 0 ] 1 0 0 0 0 0 1 0 0 0 0 1 (X=-Y)-Z plane image can be considered to rotate around Z-axis first - 45°, then mirror with respect to XZ plane, Finally, rotate the coordinate system by 45° around the Z axis. The transformation matrix is ​​TC=Rot(Z, p/4) TX-ZRot(Z, -p/4)= [ 0 -1 0 0 ] -1 0 0 0 0 0 1 0 0 0 0 1 Rotate around the X axis q TC = Rot(X, q) = [ 1 0 0 0 ] 0 Cq -Sq 0 0 Sq Cq 0 0 0 0 1 Rotate around the Y axis q TC = Rot (Y , q) = [ Cq 0 Sq 0 ] 0 1 0 0 -Sq 0 Cq 0 0 0 0 1 Rotate around the Z axis q TC = Rot(Z, q) = [ Cq - Sq 0 0 Sq Cq 0 0 0 0 1 0 0 0 0 1
Figure 3 CNC machining test
5 Numerical control machining test To verify the correctness of the cutter location file preprocessing algorithm and the practicability of the pretreatment module, a tapered column part with a circular top end and an elliptic bottom cross section shown in Fig. 3 was performed. CNC machining test. The tool file is first generated by ProE modeling. Due to the symmetrical structure of the part, it is only necessary to generate a tool bit file under one quadrant. The generated partial location file is as follows: $S* Pro/CLfile Vers 1.0 MACHIN/AIMILL, M0001UNITS/MM LOADTL/1 $$ -> CUTTER/12.000000 MULTAX/ON COOLNT/ON SPINDL/RPM, 800.000000, CLW $$ SETSTART/ 0.0000000, 0.0000, 700.00000, $ 0.0000, 0.0000, 1.00000 RAPID GOTO/0.00000, -28.43108, 80.00000, 0.00000, -0.17365, 0.98481 GOTO/0.00000, -28.43108, 13.78731, 0.00000, -0.17365, 0.98481 FEDRAT/200. GOTO/0.00000 , -24.95811, -5.90885, 0.00000, -0.17365, 0.98481 FEDRAT/200. GOTO/26.11525, 0.00000, 79.78946, 0.04197, 0.00000, 0.99912 RAPID GOTO/26.11525, 0.00000, 120.00000, 0.04197, 0.00000, 0.99912 $$ -> END/ After the FINI has been converted into the four formats of normal format, XZ plane mirroring, YZ plane mirroring, and Z-axis rotation, the offset in the Z-axis direction is increased, and the four-quadrant moving platform pose file is obtained, where the first quadrant is obtained. The following files are as follows: % N2 H136.000000 N4 S800.000000 N6 G92 X0.0000000 Y0.0000 Z700.000000 A0.000000B 0.000000 C0.000000 N8 G00 X0.0000000 Y0.0000 Z464.000000 A0.000000B 0.000000 C - 0.235619 N10 G00 X0 .000000 Y-28.431080 Z164.000000 A0.000000B0.174520 C - 0.235619 N12 G00 X0.000000 Y - 28.431080 Z97.787310 A0.000000B0.174520 C - 0.235619 N14 F200. N16 G01 X0.000000 Y - 24.958110 Z78.091150 A0 .000000B0.174520 C - 0.235619 N592 G01 X26.115250 Y0.000000 Z163.789460 A-1.570796B-0.04195 C1.335177 N594 G00 X26.115250 Y0.000000 Z204.000000A-1.570796B-0.04195 C1.335177 N596 M02 This file After working space inspection, speed control, interpolation calculation, and virtual-real mapping transformation, real axis control commands are obtained and input to the servo control system. Although the part's CNC machining program consists of more than one thousand lines of program in four blocks, the tool runs smoothly and the speed is uniform during machining. The surface precision of the machining sample reaches the level of an ordinary five-axis milling machine. 6 Conclusion This article has focused on the six-degrees of freedom 6-TPS parallel tool location tool file preprocessing method, the development of the pre-processing module has a tool file for a variety of mirroring and rotation processing functions, can enhance the machine tool data reception ability. The numerical control machining experiment of the actual parts verifies the correctness of the preprocessing algorithm and the practicability of the preprocessing module.
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