Evolution of key technologies of CNC machine tools

Introduction: The machine that makes the machine

Machine tools refer to machines used to manufacture machines, also known as "machine tools" or "machine tools". Early machine tools appeared as early as the 15th century. In 1774, the barrel boring machine invented by the British Wilkinson was considered the world's first real machine tool, which solved the cylinder processing problem of Watt's steam engine. By the 18th century, various types of machine tools appeared and developed rapidly, such as thread lathes, gantry machine tools, horizontal milling machines, gear hobbing machines, etc., laying the foundation for manufacturing tools for the industrial revolution and the establishment of modern industry. In 1952, the world's first numerical control (NC) machine tool was launched at the Massachusetts Institute of Technology in the United States, marking the beginning of the era of numerical control of machine tools. CNC machine tools are machine tools equipped with a digital control system (abbreviated as "NC system"). The CNC system includes two major parts: the numerical control device and the servo device. The current numerical control device is mainly implemented by electronic digital computers, also known as computerized numerical control (CNC) devices.

CNC machine tools can be classified according to processing technology, motion mode, servo control mode, machine tool performance, etc. From the process characteristics of the surface formation of the processing object (parts), CNC machine tools are traditionally divided into two categories: CNC metal cutting machine tools and CNC metal forming machine tools. In recent years, due to the increasing application of new materials in complex products (such as aircraft, automobiles, aero engines, etc.), the materials of CNC machine tool parts are no longer limited to metal materials, but have been expanded to non-metallic materials such as composite materials and ceramic materials, and the processing technology also includes special processing methods. In addition, from the perspective of function and performance, CNC machine tools can be divided into three categories: economical, mid-range (or popular) and high-end. At present, there is no clear and unified definition of high-end CNC machine tools. The author believes that high-end CNC machine tools are CNC machine tools with high performance, intelligence and high value characteristics and meet the corresponding functional and performance technical indicators. High-end CNC machine tools are typical representatives of the technical level of the CNC machine tool industry and the competitiveness of the equipment manufacturing industry.

Evolution of CNC machine tools

As a "working machine", machine tools have accompanied the development of industrialization throughout the process. After the Industrial Revolution in the 18th century, machine tools have evolved with the development of different industrial eras and presented the technical characteristics of each era. As shown in Figure 1, corresponding to the era of Industry 1.0~Industry 4.0, machine tools have developed from mechanical drive/manual operation (machine tool 1.0), electric drive/digital control (machine tool 2.0) to computer digital control (machine tool 3.0) and are evolving towards cyber-physical machine tools (Cyber-physical machine)/cloud solutions (machine tool 4.0).

Figure 1 Industrialization and the evolution of machine tools

The development history of CNC machine tools has experienced several important turning points.

In 1952, the world's first CNC machine tool was successfully developed at the Massachusetts Institute of Technology in the United States, which was a revolutionary leap in manufacturing technology. CNC machine tools use digital programming, program execution, servo control and other technologies to realize the automatic control of the trajectory movement and operation of machine tools according to the digital processing program compiled according to the part drawings. Since then, NC technology has made the development of machine tools inseparable from electronics, computers, control, information and other technologies. Subsequently, in order to solve the problem of automation of NC program compilation, the use of computers to replace manual automatic programming tools (APT) and methods has become a key technology, and computer-aided design/manufacturing (CAD/CAM) technology has also been rapidly developed and popularized. It can be said that the digitalization of manufacturing began with the birth of CNC machine tools and their core digital control technology.

It is precisely because of the several major characteristics of CNC machine tools and CNC technology at the beginning of their birth - digital control ideas and methods, the combination of "software-hardware", and the multidisciplinary intersection of "mechanical-electronic-control-information", that the subsequent major progress of CNC machine tools and CNC technology has been directly related to the development of electronic technology and information technology (Figure 2).

The earliest CNC devices used electronic vacuum tubes to form computing units. Transistors were invented in the late 1940s, integrated circuits were introduced in the late 1950s, and electronic digital computers using integrated circuits and large-scale integrated circuits appeared in the early 1960s. The breakthrough progress of computers in computing processing power, miniaturization and reliability brought the first turning point in the development of CNC machine tool technology - from digital control (NC) based on discrete components to computer digital control (CNC), and CNC machine tools also began to enter actual industrial production applications.

The development of PCs brought the second turning point to CNC machine tool technology. In the 1980s, IBM launched personal computers (PCs) with 16-bit microprocessors, which led to the development of CNC devices (including hardware and software) by dedicated manufacturers in the past, and moved towards general PC-based computer numerical control. At the same time, open-structure CNC systems also came into being, promoting the development of CNC technology to a higher level of digitalization and networking. On this basis, new technologies such as high-speed machine tools, virtual axis machine tools, and compound processing machine tools were rapidly iterated and applied.

Since the 21st century, the third turning point of CNC machine tools has become clear. Intelligent CNC technology has also begun to sprout. With the development of the new generation of information technology and the new generation of artificial intelligence technology, new technologies such as intelligent sensing, the Internet of Things, big data, digital twins, cyber-physical systems, cloud computing and artificial intelligence are deeply integrated with CNC technology. CNC technology will usher in a new turning point and may even be a new leap - a new generation of intelligent CNC that integrates cyber physics.

Figure 2 Development history and important turning points of CNC machine tools

In this process, the machining efficiency and machining accuracy of machine tools have been continuously improved. The continuous progress and application of advanced manufacturing technology have greatly shortened the machining time and improved the machining efficiency. Figure 7a is a widely cited curve chart showing the development of advanced manufacturing technology and the progress of machining time (efficiency). From the development trend, on the one hand, from 1960 to 2020, the total machining time (including cutting time, auxiliary time and preparation time) in manufacturing production has been reduced to 16% of the original machining time, that is, the machining efficiency has been significantly improved; on the other hand, the proportions of "cutting time, auxiliary time, and preparation time" have gradually become consistent. Therefore, in the future, to improve machining efficiency, we must not only focus on optimizing and improving process methods and increasing the degree of automation, but also effectively shorten the waiting time from the perspective of digitalization, networking and intelligence of production management. Figure 7b is a prediction of the machining accuracy that can be achieved by different machine tools by 2020 given by Taniguchi in the 1980s (the dotted line of accuracy improvement from 2000 to 2020 in the figure is added by the author). It can be seen that the development of various machining processes and machine tool (or equipment) technologies has brought about a continuous improvement in machining accuracy. However, the field of mechanical machining is different from the field of integrated circuit manufacturing. There is no Moore's Law that can be effective in a short period of time (the number of transistors that can be accommodated on an IC doubles every 18 to 24 months). Its accuracy improvement is a long-term process of technological accumulation and continuous iteration (for example: it takes more than 20 years for precision machining to improve the accuracy by one order of magnitude).

Figure 3 Progress in machining efficiency and machining accuracy

Key Technology Evolution: Machine Tool Structure

The key technologies of CNC machine tools can be divided into machine tool structure, spindle, servo drive device, CNC device and interpolation technology.

The structure of a machine tool mainly consists of two parts: the fixed parts of the machine tool (such as the base, bed, column, headstock, etc.) and the moving parts that carry the workpiece and the tool. These two parts are now commonly referred to as the machine tool base and functional parts.

Taking common turning and milling as examples, the typical evolution process of CNC machine tool structure is shown in Figure 4. The structure of CNC turning machine tools has evolved from the early classic structures such as 2-axis feed flat bed and 2-axis feed inclined bed to the machining center structure for high-efficiency turning of rotating parts such as 4-axis feed and double tool holder, multi-spindle and multi-tool, and further developed into a multifunctional turning and milling composite machining center structure that can adapt to the "one clamping, all completed (done in one)" of complex parts. The structure of CNC milling machining machine tools has evolved from the early classic vertical/horizontal milling machine structure that mainly realizes the coordinate axis linkage and spindle motion functions to the 3-axis linkage vertical/horizontal milling machining center structure with tool magazine and automatic tool change mechanism, and the vertical/horizontal milling machining center structure with exchange worktable. In order to meet the high-efficiency processing requirements of complex structural parts, 4-axis linkage and 5-axis linkage milling machining center structures have emerged. Subsequently, the multifunctional milling and turning composite machining center structure with milling/boring processing as the main function and turning/drilling processing functions has been rapidly developed and applied. In the development process of 5-axis linkage, the concept of parallel virtual axis from robots was introduced into CNC machine tools, and parallel or serial-parallel combination of 5-axis linkage appeared, but the actual application was limited. At present, the new structure machine tool for additive and subtractive hybrid processing that realizes the functions of "additive processing + cutting processing" on the same CNC machine tool has entered the practical development stage.