In 1946, the University of Pennsylvania in the United States successfully developed the world’s first electronic computer, which provided humans with tools that could enhance and partially replace mental labor, laying the foundation for the evolution of human society into the information age. In 1952, the Massachusetts Institute of Technology applied computer technology to a vertical milling machine, and successfully developed the world’s first CNC milling machine, and the world entered the CNC era.
Flexible manufacturing is based on numerical control processing technology and integrates major scientific achievements such as process design and control theory, system engineering theory, logistics control theory, etc. It is a combination of computer technology, information technology, inspection technology, quality control technology and production management technology. The advanced manufacturing technology embodies the deep integration of modern management and advanced manufacturing technology. At present, it has been widely used in the manufacturing industry of major developed countries such as the United States, Japan and Germany, and it also makes it possible to “unmanned factories” in the future.
On the whole, the manufacturing model of China’s equipment manufacturing industry lags behind developed regions such as the “Yangtze River Delta”, “Pearl River Delta”, Beijing-Tianjin, and Chengdu-Chongqing. It is still in the stage of large-scale production, that is, the rigid production stage, which is truly flexible. Production has not yet arrived. With the upgrading of consumption structure, new changes in personalized, customized, and time-sensitive market demand will force our industry to upgrade, and flexible production and flexible manufacturing that meet “diversified, small-scale, and controllable cycles” will become enterprises. The key to future survival and victory. This article intends to analyze the principles, modes, and trends of CNC machining and flexible manufacturing, hoping to provide a valuable reference for the transformation and upgrading of China’s equipment manufacturing industry
Concept and connotation
(1) CNC machining
- 1. Basic concepts. Numerical control processing refers to an advanced processing method in which the control system issues instructions to express the shape and size of the workpiece and the technical and processing requirements in the form of numbers and letters, so that the tool can perform various motions that meet the requirements. It is generally consistent with the traditional machine tool processing procedures. The most obvious change is the use of digital information to control the displacement of the workpiece and the tool, thereby realizing the automation, precision and efficiency of the processing process.
- 2. Components. CNC machine tool (hardware) is the abbreviation of digital control machine tool, which is an automatic machine tool equipped with a program control system. The control system can logically process programs with control codes or other symbolic instructions, and decode them, so that the machine tool can process according to the specified processing steps and processing actions. The controlled actions of the machine tool roughly include machine start and stop; spindle start and stop, rotation direction and speed conversion; feed movement direction, speed, mode; tool selection, length and radius compensation; tool replacement, coolant opening and closing, etc. When the processing program ends, the machine tool stops automatically.Numerical control system (software): It is the abbreviation of digital control system, which executes part or all of the numerical control functions according to the control program stored in the computer memory, and is equipped with a special computer system with interface circuit and servo drive device. Use digital commands composed of numbers, characters and symbols to realize the motion control of one or more processing equipment, and the control is usually the position, angle, speed and other mechanical quantities and switch quantities. The CNC system instructions are compiled by the programmer according to the workpiece material, processing requirements, machine tool characteristics, and the instruction format (numerical control language or symbols) specified by the system. The numerical control system sends out operation or termination information to the servo device and other functional components according to the program instructions to control various movements of the machine tool.
- 3. Features of CNC machining. One is to drastically reduce the number of tooling. No complicated tooling is required to process complex parts. If the shape and size of the part are changed, only the processing procedure needs to be modified, which saves preparation time. Second, the processing quality is balanced and stable. With high machining accuracy and high repeatability, it is very suitable for the machining requirements of high-end and super-difficult workpieces such as aircraft. The third is high automation and efficiency. It can meet the production requirements and construction period requirements of multiple varieties and small batches, and has strong market adaptability. The fourth is to meet the requirements of complex workpieces. It can process complex surfaces (such as curved surfaces, irregular shapes) that are difficult to process by conventional methods, and even process some unobservable machining parts (such as three-dimensional oblique holes).
(2) Flexible manufacturing
- 1. Basic concepts. Flexible automated manufacturing technology is referred to as flexible manufacturing technology. It was first proposed by the British Molins in 1967. It refers to an automated manufacturing system that can adapt to the transformation of processing objects. It consists of a unified information control system, a material storage and transportation system and a set of digitally controlled processing equipment. That is: a group of machines arranged in order, connected by automatic loading and unloading and conveying machines, and integrated by a computer system. Raw materials and parts to be processed are loaded and unloaded on the parts transmission system. After the parts are processed on one machine, they are automatically transferred to the next. One machine, each machine accepts operating instructions and automatically loads and unloads the required tools without manual intervention.
- 2. Type of technology. At present, the commonly used flexible manufacturing technologies mainly include flexible manufacturing systems, flexible manufacturing units and flexible automatic production lines.Flexible Manufacturing System (FMS) is a production system based on CNC machine tools or machining centers and equipped with material conveying devices. The system is automatically controlled by an electronic computer, which can meet the needs of multi-variety processing without stopping the machine. The flexible manufacturing system is suitable for processing parts with complex shapes, many processing procedures and large batches. Its processing and material transfer flexibility is large, but the personnel flexibility is low.Flexible manufacturing cell (FMC) is a processing unit composed of one or several CNC machine tools or machining centers. The unit can automatically change tools and fixtures as needed to process different workpieces. The flexible manufacturing unit is suitable for processing complex shapes, simple processing procedures, long processing hours, and small batches of parts. It has greater equipment flexibility, but low personnel and processing flexibility.Flexible automatic production line (FML) is a production line composed of multiple adjustable machine tools (mostly dedicated machine tools) connected together with automatic conveying devices. The production line can process parts of different specifications in larger batches. The flexible automatic production line with low flexibility is close to the automatic production line for mass production in performance; the flexible automatic production line with high flexibility is close to the flexible manufacturing system for small batch and multi-variety production.
- 3. Flexible manufacturing characteristics. One is the high utilization of equipment. After a group of equipment is incorporated into the flexible manufacturing system, the output is several times higher than that in the case of discrete stand-alone operations, reducing the number of equipment and plant area; reducing the inventory of blanks and products in progress, reducing the product in process by about 80%, and shortening the production cycle; Under human supervision, 24-hour continuous “unmanned production” can be realized. Second, the ability is relatively stable. The automatic processing system is composed of one or more machine tools. When a fault occurs, it has the ability to degrade operation. The material conveying system also has the ability to bypass the faulty equipment by itself. The third is high product quality. During the processing of the product, the loading and unloading is completed at one time, with high processing precision and stable processing form. The fourth is flexible operation. The inspection, installation and maintenance of some flexible manufacturing systems can be completed in the first shift, and the second and third shifts can be normally produced without supervision. In an ideal flexible manufacturing system, its monitoring system can also deal with unforeseen problems such as tool wear and replacement, logistics blockage and dredging. Fifth, the product has great adaptability. Due to the adjustability of cutting tools, fixtures and material transportation devices, the layout of the process flow and manufacturing system is reasonable, which facilitates the addition and reduction of equipment and can meet the needs of market changes.
2. The status quo and analysis of foreign development
(1) CNC machining
1. CNC machine tools.
In 1948, entrusted by the U.S. Air Force, the U.S. Parsons Corporation (Parsons) developed an aircraft propeller blade profile prototype processing equipment. Due to the complex shape and high precision of the model, the idea of using a computer to control the machine tool was proposed. In 1949, the Massachusetts Institute of Technology was commissioned by the US Air Force to combine the Cincinnati milling machine with the Parsons concept to cooperate with it to research numerical control systems. In 1952, a large-scale vertical profiling milling machine was successfully converted into a three-axis CNC milling machine, also known as the first generation of CNC machine tools.
In the early stage of development, CNC machine tools were dominated by continuous trajectory control. Continuous trajectory control CNC machine tools, also known as contour control CNC machine tools, require the tool to move on a prescribed trajectory relative to the part. Not only control the start and end coordinates of the moving parts of the machine tool, but also control the speed and displacement of each point of the entire processing contour, and finally process the workpiece into the required contour shape. The commonly used CNC lathes, CNC milling machines, and CNC grinders are typical contour control CNC machine tools.
After 1960, point-control CNC machine tools developed rapidly. Point position control refers to the precise positioning of the tool or the worktable to move from one point to another, and then the fixed-point processing is carried out. The path between points does not need to be controlled. Because point-controlled CNC systems are much simpler than contour-controlled CNC systems, point-controlled CNC milling machines, punching machines, coordinate boring machines and other CNC machine tools have developed rapidly.
In 1959, marked by the successful research and development of “Machining Center” by Carne-Treck of the United States, CNC machine tools developed to the second generation. This is a numerical control machine tool with automatic tool changer that uses transistor elements and a printed circuit board numerical control device, which can realize multi-process processing of workpieces at one time. The tool magazine is equipped with tools such as taps, drills, reamers, milling cutters, etc. The tools can be automatically selected according to instructions, and the tools can be replaced by a manipulator, which shortens the time for parts loading and unloading and tool replacement. Machining centers include not only vertical and horizontal boring and milling processing for box parts, but also turning centers and grinding centers that can be used for turning integral parts.
2. Numerical control system. Since the application of computer technology to machine tools, the CNC system has gone through two stages and six generations of development.
The first stage, the numerical control (NC) stage (1952-1970). Due to the low computing speed and large structure of early computers, they had little impact on scientific calculation and data processing at that time, but they did not meet the requirements of real-time control of machine tools. People have to use digital logic circuits to “build” a machine tool dedicated computer as a numerical control system, which is called hardware-connected numerical control, or numerical control (NC) for short. With the development of components, this stage has gone through three generations, namely: 1952, the first generation-electronic tubes; 1959, the second generation-transistors; in 1965, the third generation-small-scale integrated circuits.
The second stage, the computer numerical control (CNC) stage (after 1970). In 1970, with the appearance of small computers and mass production. So it was transplanted as the core component of the CNC system, and thus entered the CNC stage, which is the fourth generation of the CNC system. In 1971, Intel first integrated the two core components of a computer-arithmetic unit and controller-on a single chip using large-scale integrated circuit technology, called a microprocessor (also called CPU). In 1974, the microprocessor was used. In the CNC system. Although the speed and function of early microprocessors were not high enough, they could be solved by multi-processor structure. Because the microprocessor is the core component of a general-purpose computer, it is still called computer numerical control, that is, the fifth-generation numerical control system. In 1990, the performance of PCs (called microcomputers in China) has developed to a very high level, which can meet the requirements of the core components of the CNC system. The CNC system has since entered the PC stage, which is the sixth generation of the CNC system.
(2) Flexible manufacturing
Flexible Manufacturing System (FMS). Two years after the emergence of the third-generation integrated circuit numerical control device, in 1967, the British Molins Company first proposed the basic concept of FMS. With the passage of time, FMS has developed greatly in terms of technology and quantity. In the practical stage, FMS composed of 3-5 devices is the most, but there are also larger-scale systems that have been put into use.
Flexible manufacturing cell (FMC). In 1976, FANUC exhibited a flexible manufacturing cell (FMC) composed of machining centers and industrial robots, which provided an important form of equipment for the development of FMS. The flexible manufacturing cell (FMC) is generally composed of 12 CNC machine tools and a material conveying device. It has an independent workpiece storage station and unit control system. It can automatically load and unload workpieces on the machine tool, and even automatically detect workpieces, which can realize continuous production with limited processes. It is suitable for small batch production and application of multiple varieties.
Flexible automatic production line (FML). In 1967, the White Sunstrand Company of the United States built the Omniline I system, which consists of eight machining centers and two multi-axis drilling machines. The workpieces are mounted in the fixtures on the pallet, and they are in a fixed sequence in a certain cycle between the machine tools. Transport and process. This kind of flexible automation equipment is suitable for use in small variety and mass production, and is similar in form to traditional automatic production lines, so it is also called flexible automatic production line. In 1982, Japan’s FANUC (FANUC) built an automated motor processing workshop, which is composed of 60 flexible manufacturing units (including 50 industrial robots) and a three-dimensional warehouse. There are also two automatic guided trolleys to transfer blanks and workpieces. There is an unmanned motor assembly workshop, which can operate continuously for 24 hours.
(3) Development trend
- 1. Numerical control system. One is high-speed and high-precision. Speed and accuracy are two important technical indicators of the CNC system, which are directly related to product quality and processing efficiency. For the numerical control system, high-speed, first of all, the computer numerical control system is required to process the displacement of the servo motor at a high speed after reading the processing instruction data, and the servo motor is required to respond at a high speed. In addition, in order to achieve high-speed production systems, it is also necessary to achieve high-speed operation of various key components such as spindle speed, feed rate, tool exchange, and pallet exchange. High precision is generally achieved by reducing the control error of the numerical control system and using compensation technology. The second is to open the system. The traditional numerical control system is a kind of special closed system. The products of various manufacturers and the general-purpose computer are not compatible, and it is difficult to maintain and upgrade. It is increasingly difficult to meet the requirements of the market for numerical control systems. In response to this situation, people The concept of open CNC system is proposed. Huazhong CNC system, Huazhong “Century Star” is an open, networked CNC system based on industrial PCs, and is the representative of my country’s high-end CNC systems. The third is multifunctional. A multi-functional numerical control system can maximize the utilization of equipment. The numerical control machining center is this kind of numerical control machine tool that can display multi-process processing. It adopts a multi-system hybrid control method, and uses different cutting methods to process different parts of the part at the same time. The number of axes of modern numerical control systems is as many as 15 axes, and simultaneous 6-axis linkage is realized. Fourth, the system is intelligent. The important goal of high-tech CNC system is intelligence, which is mainly reflected in: adaptive control system for processing, intelligent 4M (modeling, processing, measurement, machine operation) control system, intelligent optimization and selection of processing parameters, intelligent failure Diagnosis function, intelligent AC servo drive device, pattern recognition technology. The fifth is the high reliability of the system. Numerical control systems are more expensive, and users expect good investment benefits and require reliable equipment. Especially for CNC systems that run in unmanned operating environments for a long time, reliability has become the most concerned issue.
- 2. Flexible manufacturing technology. One is that flexible manufacturing units will become a hot investment for small and medium-sized enterprises. Because flexible manufacturing cell (FMC) investment is much less than flexible manufacturing system (FMS), and the economic benefits are similar, it is more suitable for small and medium-sized manufacturing enterprises with limited financial resources. At present, many foreign manufacturers list FMC as the development focus. The second is to develop more efficient flexible automatic production lines. Flexible automatic production line (FML) is more suitable for multi-variety, high-volume production enterprises, such as large-scale operations in automobile and engineering machinery manufacturing. It will be the development trend of FML to use low-cost dedicated CNC machine tools to replace general-purpose machining centers. The third is the development of flexible manufacturing systems in a multi-functional direction. The flexible manufacturing system (FMS) has been further developed from a simple processing type to a variety of functions including welding, assembly, inspection and sheet metal processing, as well as casting, forging and other manufacturing processes. At present, it has been widely used in the fields of automobiles, engineering machinery, home appliances, machinery and electronics, textiles and clothing.
Concluding Remarks
At present, developed countries such as the United States, Europe and Japan have taken the lead in completing the industrial upgrading of CNC machine tools and have an advantage in the high-end field. As the software of numerical control processing, the numerical control system is bound to continue to progress with the development of information technology. The world’s machine tool industry has entered the era of “mechanical and electrical intelligence” integration with digital and intelligent manufacturing technology as the core.
“Made in China 2025” clearly puts forward the “three-step” strategic goal of my country’s realization of a manufacturing powerhouse, and emphasizes “the development of a batch of precision, high-speed, high-efficiency, flexible CNC machine tools and basic manufacturing equipment and integrated manufacturing systems”, which is also flexible manufacturing. It was mentioned for the first time at the national strategic level. In the next ten years, China’s manufacturing industry will usher in the great development of CNC machining and flexible manufacturing, and the boundary between flexible manufacturing and intelligent manufacturing will become increasingly blurred. We must pay close attention to it and take precautions.
