Since the first numerical control (NC) machine tool was born in 1952, NC technology has developed very rapidly, and NC systems have evolved from the original hard-wired NC to today's computer numerical control (CNC). However, modern production has increasingly higher requirements for CNC. Issues such as incompatibility between systems, programming difficulties, and low levels of intelligence greatly restrict the development of modern production and NC technology itself. Meanwhile, people gradually realized that the G and M codes (ISO 6983) that NC systems have always used can no longer meet the needs of modern production and technological development. This NC program, which is oriented towards motion and switch control, limits the openness and intelligent development of CNC systems. It also creates a bottleneck between CNC and CAX technologies, seriously hindering the development of the mechanical manufacturing industry. In 1997, the European Community developed an object-oriented data model in compliance with the STEP standard through the OPTIMAL program. It redefined the object-oriented data model and the programming interface for milling processes, proposing the concept of STEP-NC. STEP-NC extends the product data conversion standard STEP to the CNC field, redefining the interface between CAD/CAM and CNC. It requires CNC systems to directly use CAD three-dimensional product data models compliant with the STEP standard (ISO 10303), including geometric data, design and manufacturing features, along with process information and tool information, to directly generate machining programs to control machine tools. Subsequently, STEP-NC has become a hot topic of research in industrialized countries around the world, with more representative research projects including Europe's STEP-NC project, America's Super Modal project, and Japan's Digital Master project. Currently, research on the STEP-NC project has made substantial progress. According to predictions by America's STEP Tools company, STEP-NC controllers will emerge in the first decade of this century, and at that time, people will witness a new scene of automated manufacturing.

     STEP-NC is a data standard redefining CNC systems, which connects product design information and manufacturing information in an object-oriented form based on STEP. STEP-NC defines a new STEP application protocol (AP-238, still being refined) as the data exchange specification between CAM and CNC. AP-238 covers all informa tion required throughout the entire process from product concept to finished part (component), including three-dimensional geometric information (AP-214), process information (such as milling, turning, electrical discharge machining, etc.), and inspection information (AP-219). The current STEP-NC standard draft (ISO-DIS-14694) has been formulated. Relevant basic rules and standards for milling processing have been completed, including basic concepts and rules (Part 1), general standards (Part 10), numerical control milling process (Part 11), milling cutters (Part 111), etc. STEP-NC standards currently under development include: numerical control turning (Part 12), electrical discharge machining (Part 13), wood and glass processing (Part 14), inspection (Part 15), etc. As a simplified model of generic data, it includes two parts: the workpiece and the work plan. The workpiece refers to the final finished product, and the areas on the workpiece where material needs to be removed are defined by a series of machining features. The work plan links several work steps (such as planes, complex surfaces, holes, etc.) with specific operations. Here, the operation itself is also a concept defined in ISO-14649, involving the design of machining methods, tools, guides, and process strategies, etc.

    The STEP-NC based CNC program abandons the traditional approach of directly programming coordinate axes and tool movements in CNC programs. Instead, it adopts the ISO-10303 data format and feature-oriented programming principles. It uses work steps as the basic unit of the machining process, linking features with technical information. Each work step defines only one operation (such as 'what to do' and 'how to do it'), but can only use one tool and one strategy.

   The program itself also adopts the file format specified by ISO-10303. Structurally, it can be divided into two parts: the file header and the data segment. The file header is marked with \"HEADER\" and mainly explains the filename, programmer, date, and comments. The data segment starts with \"DATA\" and contains all the information and operation tasks required for machining parts. According to the regulations, it first requires a PROJECT statement, and the subsequent content can be divided into three parts: work plan and executable statements (including work steps, general NC functions such as information display, and flow control), technical description (tools, machine tool functions, machining strategies, etc.), and geometric description (geometric data, machining features, etc.).

   The development and design of STEP-NC have extended the STEP standard to the underlying equipment of automated machining, establishing a high-speed highway for the entire manufacturing network. It can be foreseen that in the future, CNC systems will undergo fundamental changes in terms of structure, function, and their status within manufacturing systems. Such changes will inevitably affect the development of related CAX technologies (such as CAD, CAPP, CAM, CAE, PED, ERP), cutting tools, machine tool bodies, fixtures, and the implementation of advanced production models. Based on current research achievements, the more direct foreseeable impacts mainly include the following aspects:

1) CNC Programming Interface: Replacing ISO-6983 with ISO-14649 has greatly improved the programming interface, making on-site programming more convenient and replacing easily reusable programs. When certain features of the workpiece to be machined change slightly, only the geometric description of the relevant features needs to be modified, and other elements do not need to be changed. In addition, unified programs can be run directly on different models of machine tools.

2) Openness of CNC systems: Currently, due to the narrow coverage of ISO-6983, CNC manufacturers have had to develop their own extended instructions. Therefore, CAM and CNC must use the same set of codes; otherwise, specific post-processing programs must be selected. For STEP-NC controllers, their data format (AP-238) is exactly the same. It tells the CNC 'what to process' rather than specific actions, so post-processing programs are not needed. The specific actions are determined by the CNC, resulting in good interoperability and portability of the program.

3) Intelligence of numerical control systems: As the current interface between CAM and CNC, the formation process of G and M codes causes a large amount of useful information to be lost, which is also one of the main reasons for the low intelligence level of current CNC systems. In contrast, STEP-NC numerical control includes all the information required for machining products, providing the basic conditions for CNC systems to perform autonomous machining based on a comprehensive understanding of the product.

4) Redefinition of functions between CMA/CNC: CNC has a better understanding of machine tool operation than CAM or CAPP systems. Specific process handling performed within CNC (such as tool selection, compensation, and determination of tool paths) is more likely to achieve optimal machining results. Therefore, future CNC systems will perform part of the functions of CAM systems and may be equipped with embedded CAM systems to directly process based on CAD data models.

5) Processing Quality and Efficiency: The introduction of STEP-NC has changed the current status of CNC systems as passive executors of machining tasks. Strengthening CNC functions can also improve the efficiency of upstream processes. Research by STEP Tools Company shows that the application of STEP and STEP-NC can reduce production data preparation in the CAD stage by 75%, machining process planning (CAM) time by 35%, and machining time (CNC five-axis high-speed milling) by 50%.

6) Data Sharing and Network Manufacturing: The development of STEP-NC has enabled bidirectional seamless connectivity between STEP-NC-based CNC systems and all STEP-based CAX systems (for example, CAD systems can directly read geometric information from STEP-NC data in the CNC system), creating conditions for network-based manufacturing models and technologies.

   Closing remarks: STEP-NC is both an evolving CNC interface standard and a technology to enhance the implementation of modern CNC systems. It provides broad development space for the openness and intelligence of CNC, while also addressing the core issue of bidirectional seamless connection between CNC and CAX. It will have an unpredictable profound impact on future automated manufacturing. Compared with the previously proposed open CNC, the significance of STEP-NC controllers (CNC based on STEP) is greater (as they focus on seamless integration throughout the product lifecycle), and their implementation technology is more practical and effective (starting from data models). Currently, although research on STEP-NC controllers is still in its early stages, it is developing very rapidly (Siemens has successfully developed a prototype). The prospects are very promising. This is also an excellent opportunity for our country to narrow the gap, develop domestic CNC systems, and comprehensively upgrade the level of automated manufacturing in our country.