Virtual machine tool modeling and machining process simulation

introduction

The modeling of the virtual manufacturing system is divided into three levels: the target system layer, the virtual manufacturing model layer and the model construction layer. The model construction layer is used to provide the basic model structure describing the manufacturing activities and their objects, in which the virtual machine tool is modeled and processed. The simulation is the most basic work, the most extensive, and the basis for constructing virtual factories and even virtual companies.

Virtual Machine Tool VMT (Virtual Machine Tool)

Virtual machine tools are mappings of index-controlled machine tools (such as machining centers) in a virtual environment. In pursuit of the immersive reality and the virtuality of “beyond reality”, virtual machine tools should meet the following requirements:

A comprehensive and realistic reflection of the real processing environment and processing;

It can provide alarm information for collisions and interferences occurring during processing;

Ability to evaluate the processability of the product and the rationality of the process specification;

Can evaluate and predict the processing accuracy of the product;

Must have the ability to handle multiple products and multiple processing techniques.

The virtual machine tool mainly consists of a machining environment model, a machining process model, a machining process simulation model, a virtual operation interface and a graphics processing module. The machining environment model includes three-dimensional geometric models of various physical objects such as blanks, medium-term product models, target product models, tools, fixtures, machine tools, and other manufacturing resources and environmental objects, and relationships between models. The machining process model includes thermal deformation, force deformation and vibration model in the cutting process corresponding to various machining methods.

Four-coordinate virtual machine

Modeling of virtual machining processes

2.1 Virtual Machine Modeling

The virtual machine tool is the carrier and core of the virtual machining process, and is composed of a geometric model and a motion model. Figure 1 shows a simplified geometric model of a four-coordinate horizontal machining center. In the virtual machine tool, there are two coordinate systems, one is the machine's absolute coordinate system X'Y'Z', and the other is the virtual environment's graphic display coordinate system XYZ. Since the tool change position of the machining center is fixed, the origin of the machine coordinate system is set at the center of the tool change position. Position the origin of the graphic coordinate system at the center of the work surface.

A moving part of a virtual machine corresponds to a coordinate. As shown in Figure 1, the table, parts 1, 2, and 3 correspond to coordinates B, X, Z, and Y, respectively. Each moving part and the bed body form a kinematic chain according to a certain rule, and the movement chain starts from the working table and terminates on the machine tool spindle, as shown in FIG. 2 . There is a contact relationship between adjacent components in the kinematic chain, and the bed is a non-moving member. The kinematic chain is divided into two segments, each segment has a hierarchical structure. That is, when moving away from the bed in each segment, the person far away from the bed will move with it. When component 2 moves along the Z coordinate, the table and component 1 will move along the Z coordinate; while component 3 moves along the Y coordinate, the spindle will follow.

The virtual motion of the virtual machine tool consists of the translation, rotation and linkage of the moving parts. The linkage of multiple moving parts can be converted into translation or rotation of a single moving part by using an interpolation algorithm. Therefore, the motion of the virtual machine can be achieved by shifting and rotating the components. The virtual motion speed is controlled by the pitch value of the translation and rotation. The modeling of the motion process is done using the object-oriented virtual reality modeling language VRML. Specifically, each moving component is defined as a moving object by using the VRML language, so that each moving object can be regarded as a node in the VRML language, and a node includes a set of events that it can accept and send, such as changing the position ( Pan or rotate) and change color events, etc. Each scene of the machining process can be defined as a set of nodes, and the complete machining process is a collection of scenes.

Figure 2 Virtual machine tool kinematics Figure 3 imaginary body modeling

2.2 Modeling of the process

The process model represents all physical models used to represent product behavior and manufacturing processes, including thermal deformation during the cutting process for various machining methods, rigid deformation of the system under cutting forces, and clamping deformation. The machining process model is the main basis for evaluating and predicting the machining accuracy and machinability of the products. Their establishment is related to the machining method, cutting conditions (tool structure shape, tool material, workpiece material, etc.) and cutting amount. Specific mathematical models can be established through experimental or finite element analysis modeling.

2.3 Simulation Process Modeling

The machining in the virtual machine is controlled by the NC command to control the movement of the tool. During the execution of each NC command, the tool moves from a starting position to another position. In this process, the tool sweeps through a certain volume in space, and the space shape enveloped by the tool during the movement can be called “Swept Volume Solid”. As shown in Fig. 3, the cylindrical milling cutter (cutting edge portion) of Fig. 3a moves along the tool path AB to form a virtual body as shown in Fig. 3b. Relative to the actual shape, it is not a real shape, but it can be treated as a real shape when calculating and modeling. The imaginary body is inextricably linked to the geometry of the tool itself, the trajectory of the tool motion, and the starting position of the tool motion. Since the imaginary body is formed by the static object during the motion, the imaginary body can be modeled by the method of forming the envelope surface by the static object (tool) boundary surface in motion. The intersection of the imaginary body and other static shapes can be used to check the collision and interference during the machining process. The difference between the blank and the imaginary body can simulate the material removal process. Therefore, the simulation of the machining process can be realized by modeling the imaginary shape.

Virtual machining process simulation

3.1 Establishment of simulation environment

The establishment of the simulation environment mainly includes the definition of virtual machine tools, tool magazines and tools, installation fixtures, blanks and workpieces.

Defining the magazine includes selecting each tool in the order of machining and defining the tool parameters.

Defining a virtual machine includes determining the machine type, number of coordinates, control system, machine coordinate origin, graphic coordinate origin, and programming origin according to machining requirements. By default, the origin of the graphic coordinate system is used as the programming origin.

The initial installation of the fixture, blank and workpiece on the workbench can be achieved by overlapping the origin of the product coordinate system with the origin of the graphical coordinate system. The actual installation position can be obtained by translational transformation of the product relative to the origin of the graphical coordinates. The coordinate system is generally fixed when the product is modeled, and the coordinate system is different for different machine tools. Therefore, when the fixture or blank is installed on the workbench, its position in the graphic coordinate system is as follows. Transform to determine:

(x,y,z,1)=A*B*(x1,y1,z1,1)

(x1, y1, z1) is a point in the product coordinate system, and (x, y, z) is the corresponding point in the graphic coordinate system in the virtual environment.

A11~a33 is determined by selecting a coordinate plane of the product coordinate system, such as one of XOY, XO-Y, XOZ, -XOZ, YOZ, -YOZ, and overlapping with the coordinate base surface XOY plane of the graphic coordinate system. (x0, y0, z0) is the coordinate value of the center of the table mounting surface in the machine coordinate system.

3.2 Process Simulation

The simulation of the machining process takes three forms:

Tool trajectory simulation, at this time only the tool moves around the blank according to the machining trajectory, the purpose is to visually verify the rationality of the tool trajectory.

The machine motion process simulation, at this time the workpiece is mounted on the machine table, the tool motion trajectory is decomposed into the movement of the moving parts of the machine tool, the purpose is to visually check the collision and interference between the tool and the machine tool parts and machine parts.

The material removal process is simulated. At this time, the tool cuts the blank according to its movement trajectory. The purpose is to simulate the actual cutting process and generate a product processing result model to evaluate the machining accuracy and machinability. In this simulation process, by estimating the cutting force, clamping force and cutting heat, the relative displacement between the tool and the workpiece caused by thermal deformation and deformation of the process system is superimposed on the theoretical movement trajectory of the tool, so that the generated product is generated. The processing result model can reflect the influence of dynamic factors on the processing quality.

3.3 Evaluation of processing error

The machining error model can be obtained by comparing the theoretical model of the product part with the machining result model obtained by removing the blank material. In the machining error model, the machining area is represented by different colors according to the error. By visually checking it, the processing error and its causes can be analyzed and judged, and the product can be evaluated for processability. in accordance with.

in conclusion

Virtual machine tools not only lay the foundation for building a virtual manufacturing system in the future, but also play an active role in the following aspects:

Train NC code programmers and machine operators;

CNC equipment selection;

Evaluate machining accuracy;

Verify the NC code;

Assess the processability of the product;

Evaluate the rationality of the process specification.