1.4 Establishment of object classes
Because there are many models of CNC machine tools, and their functions and structures are different, it is a time-consuming and laborious task to analyze the structural characteristics and functional characteristics of each CNC machine tool and establish a simulation model. From the history of the development of CNC machine tools, CNC machine tools have obvious inheritance. As far as the machining center is concerned, it is developed on the basis of CNC boring and milling machine. In simple terms, the machining center = CNC boring and milling machine ten tool changer. Therefore, it is necessary to use the object-oriented method to establish the hierarchical relationship of object classes and realize the reuse of simulation model information through inheritance by using the similar characteristics of the structure and functional characteristics of CNC machine tools.
CNC machine tools are classified according to their structural form and can be divided into the following three categories: horizontal CNC machine tools (HNCMT), vertical CNC machine tools (VNCMT), and gantry CNC machine tools (GNCMT). These three types of CNC machine tools can be abstracted into three basic CNC machine tool object classes, which form a layer in the CNC machine tool library structure. These three types of CNC machine tool object classes also have some common properties, such as the name of the CNC machine tool, the model, the spindle speed range, etc., all of which have the properties of all CNC machine tools, so you can further abstract a new class - NC The machine tool (NCMT) class, which is at the top of the numerical control machine tool hierarchy, is the base class of the entire CNC machine tool class. Taking into account the actual situation of CNC machine tools, the structure and function properties of CNC machine tools are analyzed in more detail. HNCMT class, VNCMT class and GNCMT class can be further classified according to 3 coordinates, 4 coordinates and 5 coordinates. They are in CNC machine tools. The lower layer of the hierarchy. In this way, the hierarchical structure of the entire CNC machine tool class is established, and the hierarchical structure of the CNC machine tool object class is shown in Figure 2.
2 The overall structure of the simulation system
The CNC machine tool machining system consists of CNC machine tools, tools, workpieces and fixtures. The process of numerical control machining is the process in which the CNC machine tool drives the tool to cut the workpiece fixed by the fixture on the worktable under the driving of the NC code. To achieve simulation of the CNC machining process, first establish a geometric model of the CNC machine tools, tools, workpieces and fixtures. Based on the geometric model, the motion model of the CNC machine tool is established. Finally, the simulation of the CNC machining process is realized. The overall structure of the CNC machining simulation system is shown in Figure 3.
3 simulation model
The simulation model of NC machining simulation system includes geometric model and motion model of CNC machine tools, tools, workpieces and fixtures. This paper only discusses the geometric model of CNC machine tools. The geometric models of tools, workpieces and fixtures are simple. Similar to CNC machine tools, simulation system The motion model is described in [3]. The geometric model of a CNC machine tool is actually an assembly model, which is an assembly in which the assembly units are combined according to certain constraints. The assembly unit is a part or a component. The following mainly discusses the implementation of the assembly model and the description of the positional relationship between the sub-components inside the assembly.
3.1 The basic form of the assembly model
The basic form of the assembly model is the hierarchical tree and graph. The hierarchical tree can clearly express the composition relationship of the assembly, and is also conducive to the assembly sequence planning solution, but it is not easy to express the cooperation relationship between the parts. Contrary to the hierarchical tree structure, the structure of the graph is easier to express the relationship between the parts, and provides a path to find the relationship between one part directly from one part in the assembly, especially suitable for the analysis of tolerances and kinematic chains, but very Difficult to represent hierarchical composition, the structure is more complicated, difficult to maintain, and the operation is more difficult [2]. Geometric models differ from the requirements of assembly models for other systems (such as CAD/CAM systems) and can be summarized as: (1) description of assembly geometry; (2) description of assembly topological relationships; and (3) hierarchical construction of parts. Since the geometric model can be considered regardless of the tolerance fit, and the motion relationship also has a hierarchical relationship, a hierarchical structure can be employed.
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