Introduction CNC machine tools are devices that process workpieces according to pre-programmed machining procedures. These procedures must not only ensure the machined workpiece conforms to the drawing requirements but also maximize the functionality and performance of the CNC machine tool. Therefore, graphical simulation of the machining program's operation to verify its correctness is of great significance. Furthermore, CNC machining simulation training is receiving increasing attention because CNC machining requires operators to be familiar with CNC programming instructions and functions, as well as understand the machine tool's performance. These characteristics of CNC machining mean that traditional technical training methods are no longer sufficient. Traditional technical training involves operators first learning technical theory and operating procedures, then undergoing on-site training in a master-apprentice format. The main drawback of this method is that CNC machine tools are generally expensive equipment, and trainees are not allowed to arbitrarily start or stop them or cause accidents. Therefore, trainees have limited hands-on experience, resulting in a longer learning curve. In addition, the varying skill levels of instructors make it difficult to guarantee the quality of instruction. Therefore, CNC machining simulation plays a crucial role in both verifying the correctness of machining programs and training operators. This paper takes widely used CNC milling machines and machining centers as research objects, mainly discussing the use of object-oriented technology to analyze CNC machine tools, establish CNC machine tool object classes and simulation models. The CNC machine tools referred to in this paper are a general term for CNC milling machines and machining centers. 1 Object-Oriented Technology in Simulation Systems Object-oriented technology describes and processes objects in a way that closely resembles the real world. Its data abstraction, inheritance, and polymorphism mechanisms are particularly suitable for describing objects with complex internal mechanisms, and thus it has been rapidly and widely applied in various industries. 1.1 Object-Oriented Technology In object-oriented technology, objects with the same structure and processing mechanism are abstracted into classes, and an object is an instance of a class. Objects not only have states or attributes described by data, but also operations for changing the object's state (in object-oriented programming, these operations are called methods), realizing the combination of data and operations. Therefore, objects can simultaneously describe both the structure and behavior of things. Different objects communicate through message passing, and the internal data operations of an object are not visible to the outside world. This is called data encapsulation. Object inheritance in object-oriented programming allows the characteristics of a parent class to be passed to its child classes, providing the possibility for describing objects in the real world. Object inheritance is very useful for solving resource sharing among objects, thereby improving the reusability and extensibility of software. Reusability and extensibility have a significant impact on the maintainability of software products and the quality and productivity of updated products. In object-oriented technology, determining the derivation relationship between objects is the key to establishing the entire system. The entire system often has only one or a few most basic object base classes, and the others are derived classes of the base class. The derivation relationship in object-oriented technology allows us to directly obtain a large number of ready-made attributes and methods from existing classes, while its polymorphic attributes allow us to define attributes and methods different from those of the base class in derived classes. 1.2 Object-Oriented Analysis of CNC Machine Tools Although the structure of CNC machine tools is complex, they are all composed of relatively independent modules with a relatively fixed number of different functions. Using object-oriented methods to analyze CNC machine tools, specific CNC machine tools and modules are all regarded as objects. That is, CNC machine tools are composed of various independent objects. Figure 1 shows the object model of a CNC machine tool. In the object model, a CNC machine tool consists of a spindle, column, bed, worktable, control panel, tool changer, and pallet exchanger. Its composition differs slightly from the actual simulation object. Some components irrelevant to the simulation, such as hydraulic and electrical systems, can be omitted from the simulation model to simplify it. The black spheres “·” and “+” in the diagram are multiple symbols, representing many instances of one class related to instances of another class. The hollow sphere “O” represents 0 or 1 instances of one class related to instances of another class. As can be seen from the object diagram, the spindle, column, bed, worktable, and control panel are essential components of every CNC machine tool. The tool changer and pallet exchanger are optional components, but for machining centers, they are also essential. 1.3 Checking Exclusion Relationships A CNC machine tool is not an arbitrary combination of all components; there are certain exclusion relationships between the selection and combination of components. The simulation system must be able to describe these exclusion relationships to ensure that the simulation model reflects the real-world CNC machine tool. For example, in a tool changer, the robotic arm is an optional component, while the tool magazine is a necessary component. However, once a chain-type tool magazine is selected, a robotic arm must also be chosen. A machining center can have multiple tool magazines, but they must work in conjunction with a robotic arm to complete the tool change action. Similarly, there is an incompatibility between the worktable and the pallet device; a rectangular worktable cannot be equipped with a pallet device on the same CNC machine tool. The movement mode of the indexing table base determines the matching relationship between the indexing table and the pallet device type; a fixed-base indexing table can only be used with a rotary pallet device, and so on. Because in the simulation system, only components have an incompatibility relationship, and the number of component types is limited (but within each type, the component models are unlimited), each component type can be pre-assigned a specific ID number. According to CNC machine tool specifications, the number of CNC machine tool types is relatively small, and the simulation system can enumerate all ID numbers with incompatibility relationships for each type of CNC machine tool. Exclusion relationships are described using sets. Let A and B be two sets, where a1, a2, ..., an represent the ID numbers of CNC machine tool components and are elements in set A, and b1, b2, ..., bm also represent the ID numbers of CNC machine tool components and are elements in set B. That is, A = {a1, a2, ..., an} and B = {b1, b2, ..., bm}. Then, the exclusion relationship in the simulation system can be expressed as: set A excludes set B, or set B excludes set A. That is, if all elements of sets A and B exist in a certain simulation model, it indicates that there is component exclusion in the model, and the component type needs to be reselected. 1.4 Object Class Establishment Since there are many models of CNC machine tools with different functions and structures, analyzing the structural and functional characteristics of each CNC machine tool and establishing a simulation model is a time-consuming and laborious task. From the historical development of CNC machine tools, it's clear that they exhibit significant inheritance. For example, machining centers evolved from CNC milling machines; simply put, a machining center = CNC milling machine + tool changer. Therefore, leveraging the similarities in structure and function of CNC machine tools, it's essential to establish a hierarchical structure of object classes using object-oriented methods and to reuse simulation model information through inheritance. CNC machine tools can be relatively easily classified into three categories based on their structural form: 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, forming one layer in the CNC machine tool class library structure. These three types of CNC machine tool object classes also share some common attributes, such as the name, model, and spindle speed range of the CNC machine tool—attributes common to all CNC machine tools. Therefore, a new class—the CNC Machine Tool (NCMT) class—can be further abstracted upwards. This class is at the top level of the CNC machine tool class hierarchy and is the base class for the entire CNC machine tool class. Considering the actual situation of CNC machine tools, a more detailed analysis of their structural and functional attributes is conducted. The HNCMT, VNCMT, and GNCMT classes can be further subdivided according to 3-axis, 4-axis, and 5-axis coordinates, respectively. These classes are located at the lower levels of the CNC machine tool class hierarchy. Thus, the hierarchical structure of the entire CNC machine tool class is established, as shown in Figure 2. 2. Overall Structure of the Simulation System The CNC machine tool machining system consists of CNC machine tools, cutting tools, workpieces, and fixtures. The process of CNC machining is the process in which the CNC machine tool drives the cutting tool to cut the workpiece fixed on the worktable by the fixture under the drive of NC code. To realize the simulation of the CNC machining process, the geometric model of the CNC machine tool, the cutting tool, the workpiece and the fixture must first be established. The motion model of the CNC machine tool is established on the basis of the geometric model. 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 the CNC machining simulation system includes the geometric model and motion model of the CNC machine tool, the cutting tool, the workpiece and the fixture. This paper only discusses the geometric model of the CNC machine tool. The geometric models of the cutting tool, the workpiece and the fixture are relatively simple and similar to the CNC machine tool. The motion model of the simulation system is referred to in reference [3]. The geometric model of the CNC machine tool is actually an assembly model, which is an assembly of assembly units combined according to certain constraints. The assembly unit is a part or 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 Basic Forms of Assembly Models The basic forms of assembly models are hierarchical trees and graphs. Hierarchical trees can clearly express the compositional relationships of an assembly and are also conducive to solving assembly sequence planning, but they are not easy to express the fit relationships between parts. In contrast to the hierarchical tree structure, the graph structure is easier to express the relationships between parts and provides a path to find the connection between one part and another part in the assembly. It is particularly suitable for tolerance and kinematic chain analysis, but it is difficult to express hierarchical compositional relationships, the structure is more complex, it is not easy to maintain, and it is also more difficult to operate [2]. The requirements of the geometric model are different from those of the assembly model of other systems (such as CAD/CAM systems), which can be summarized as: (1) description of assembly geometric relationships; (2) description of assembly topology relationships; (3) construction of part hierarchy. Since the geometric model does not need to consider tolerance fit and the kinematic relationship also has a hierarchical relationship, a hierarchical structure can be adopted. 3.2 Simulation Model Establishment In this system, an assembly is represented as a binary tree, as shown in Figure 4. The root node represents the final assembly required by the user, and non-leaf nodes represent sub-assemblies. Sub-assemblies are composed of parts or other sub-assemblies. Parts are represented by the lowest-level leaf nodes in this tree structure. The left child of a non-leaf node is designated as the base unit; its position remains unchanged, and the transformation matrix only affects its right child. Only assembly units at the same level have "positional constraints," while assembly units at different levels have "subordinate relationships." By transforming the local coordinate system of the assembly unit with the coordinate system of the assembled assembly unit, and performing corresponding movements and rotations, the assembly unit is positioned at its theoretical assembly location. This model is characterized by its simple description, convenient operation, easy maintenance, and small storage requirements. Conclusion The development trend of modern simulation technology is object-oriented. Introducing object-oriented technology into the research and development of CNC machining process simulation systems will inevitably improve the quality of CNC machining process simulation system research and promote the application and development of CNC machining process simulation systems. Designing a correct simulation object is the foundation for the implementation of the entire simulation system. This paper analyzes the structure and functional characteristics of CNC machine tools, pointing out that CNC machine tools are composed of a relatively independent number of modules with different functions. These modules, in assembling the CNC machine tool, not only have geometrical constraints but also selection exclusions. Therefore, an object-oriented method for defining CNC machine tools is proposed, and a CNC machine tool class library is established based on this method. The paper also establishes the overall structure of a CNC machining process simulation system.