CNC Machining Technology for High-End Precision Manufacturing Introduction The application of CNC technology has radically changed the traditional manufacturing. Especially in recent years, the development of micro electronic technology and computer technology has brought new energy into CNC machining. Basically we could say that the CNC technology and CNC equipment are a significant basis of industrial modernization for a country.
CNC machine tools are the mainstream equipment in modern manufacturing and very necessary for precision machining. As a sign of modern machine tool technologies and modern machinery manufacturing, the CNC machine tools are actually strategic supplies related to national economy and people’s livelihood as well as top-end construction of national defense. Therefore, every country with a developed industry in the world is taking significant measures to develop its CNC technology and related industries.
CNC Machining CNC is the acronym of Computer Numerical Control, and CNC machining refers to machining with computer numerical control.
As an advanced machining technology in today’s machinery manufacturing, CNC machining features high efficiency, high accuracy and high flexibility as well as automation. A CNC program is made for the workpiece to be machined and then transferred to a CNC machine tool. Under the control of this program, the CNC machine tool automatically performs machining until a desired part is manufactured. With such parts manufactured by CNC machining, various beautiful products can be achieved.
Take moulds for example, all of their machining problems related to complexity, precision, multiple changes and small batches can be effectively resolved by CNC machining, which is fully suitable for modern production. Striving to develop CNC machining technologies has become an important path for China to accelerate its economic growth and improve its capability of independent innovation. At present, the application of CNC machine tools in China is getting more and more popular. The functions of a CNC machine tool can never be fully exerted without an operator who has mastered its programming.
The CNC machine tool is a typical mechatronic product with various advanced technologies combined, such as micro electronic technology, computer technology, measuring technology, sensor technology, automatic control technology and artificial intelligence technology. Integrated further with machining processes, the CNC machine tool can be called a new generation of technical equipment for machinery manufacturing.
Constitution of A CNC Machine Tool
The CNC machine tool is an automatic device comprised of a machine tool, a computer and a motor, with various technologies combined such as dragging technology, automatic control technology and detection technology. Basically, a CNC machine tool contains the control medium, a CNC device, a servo system, a feedback device, and its own body.
Control Medium The control medium is used to store the information on positions of all operative cutters required for CNC machining relative to the workpiece. Since it records the part machining programs, it can be considered as a carrier of transferring the part machining information to the CNC device. The control medium comes in different types such as punched tape, punched card, tape, and disk, depending on the type of the CNC device, As the CNC technologies develop, the punched tape and the punched card are almost out of date and obsolete. However, the method of programming with CAD/CAM and then direct transfer of program data to the CNC device though communication between the computer and the CNC system is in wider and wider application.
CNC Device As the core of a CNC machine tool, the CNC device is called the Central System as a figure of speech, which is necessary for every modern CNC machine tool. The CNC device, composed of an input unit, a central processing unit (CPU), and an output unit, is capable for the input, storage, conversion and interpolation operations of information as well as to achieve various control functions.
Servo System The servo system is the drive part which receives instructions from the CNC device and drives the machine tool to execute movements. It is composed of a spindle drive unit, a feed drive unit, a spindle motor and a feed motor. During operation, the servo system receives instruction information from the CNC system, and follows the requirements in the instruction information to compare position and speed feedback signals. And then it drives the moving unit or executive unit of the machine tool to operate and manufacture a part that meets requirements on the drawing.
Feedback Device The feedback device, composed of measuring components and their corresponding circuit, detects the speed and displacement and feeds information back to form closed-loop control. Some CNC machine tools without high accuracy requirement do not have the feedback devices, so it is called an open-loop system.
Machine Tool Body The machine tool body, as the substantial part of a CNC machine tool, is the mechanical part that performs actual machining such as cutting. It is usually composed of a bed, a foundation, a saddle, and a spindle.
Features of the CNC Machining Processes Machining rules must also be followed for the CNC machining processes, almost the same as the machining processes on an ordinary machine tool. Because it is a kind of automatic machining which combines computerized control with machining, the CNC machining features high efficiency and high accuracy. The CNC machining is unique with more complicated procedures and more detailed step arrangement. The CNC machining processes include cutter selection, determination of cutting parameters, and design of machining routes. As the foundation and core of CNC programming, the CNC machining processes must be reasonable enough to ensure efficient and quality CNC programs. The criterion for a good CNC program is manufacturing the most effective workpiece with the shortest machining time and the least cutter wear. As part of the whole workpiece machining process, CNC machining must be in good coordination with its previous and following processes, so as to finally meet the requirements of overall machine or mould assembling. Otherwise, it is impossible to produce up-to-standard parts. Generally, the CNC machining processes are divided into rough machining, medium&rough corner finishing, semi-fine machining and fine machining.
CNC Programming
CNC programming refers to the whole process of acquiring the CNC machining program based on the part drawing, with computing the Cutter Location (CL) point during feeding for machining as its main task. Usually, the point of intersection between the cutter axis line and the cutter surface is taken as the cutter location point. During multi-axis machining, the cutter-axis vector also needs to be given.
Following workpiece drawing requirements and based on machining processes, the CNC machine tool makes a program sheet for the amounts of movement, speeds and action order of the cutters used and each part, and spindle rotation speed, spindle rotation direction as well as operations like cutter bit clamping, loosing and cooling, in the form of specified CNC codes which will be inputted to a dedicated computer. Next, the CNC system performs compilation, operation and logical processing based on entered instructions and then outputs various signals and instructions to control each part to follow specified movements and sequential actions, so as to manufacture workpieces of different shapes. Therefore, programming has a very big influence on the functioning of a CNC machine tool. A CNC machine tool must input the instruction codes representing different functions to the CNC device in the form of programs. The CNC device performs computing and processing, and then transmits pulse signals to control the operation of each moving part of the CNC machine tool, so as to complete cutting of a workpiece. Presently, the CNC program follows two standards: International Standardization Organization (ISO) and the USA’s Electronic Industries Association (EIA)
The former is used in China. As technologies develops, the commercial CAD/CAM software is used more and more, instead of manual programming, for 3D CNC programming. As the core of the computer-assisted programming system, CAD/CAM provides the various functions, including data input/output, machining path computation&compilation, process parameter setting, machining simulation, CNC program postprocessing, and data management. In China, Mastercam, UG, Cimatron, PowerMILL, and CAXA are currently popular and favored by users for their powerful CNC programming functions. All of the software uses the same CNC programming principles, graphic processing methods, and machining methods, expect some slight differences.
CNC Machining Steps Analyze the drawing of a part to have a rough understanding (including its geometry, materials and process requirements). Determine CNC machining processes (content and route) for the part Perform necessary numerical calculation (coordinate calculation for the base point and node). Write a program sheet. (Follow the user manual, because different machine tools use different program sheets.) Check the program. (Input the program to the machine tool and conduct graphical simulation to verify the program for correctness.) Perform machining on the workpiece (Good process control can save time and improve machining quality very well.) Carry out workpiece inspection and quality error analysis. (Inspect the workpiece. If it is OK, go to the next process; if it is not OK, conduct quality analysis to find out the error reason and the correction method.) CNC machining, precision CNC machining
History of CNC Machine Tool Development After World War II, most of the work in the manufacturing industry was manually performed. Workers read the drawings and then operated the machine tools to manufacture parts. Production with this method leads to high costs and low efficiency, without quality guaranteed. At the end of 1940s, an American engineer named John Parsons conceived a method of punching on a piece of hard paper card to indicate the geometry of the part to be machined. Controlling a machine tool’s actions with just a hard card was only a conception at the time. In 1984, John Parsons demonstrated his idea to the US air force which showed great interest. The US air force was looking for an advanced machining method to fix the problem of contour prototype machining for airplanes. Because of complex shapes and high accuracy requirements, it is difficult to solve with ordinary equipment. The US air force promptly entrusted and sponsored Massachusetts Institute of Technology (MIT) for research and development of this machine tool that is controlled by hard paper cards. Finally, in 1952, in cooperation with Parsons Corporation, MIT successfully manufactured the first demonstration machine. By 1960, simpler and more economic drilling machines with point to point control and CNC milling machines with straight line control had been developing rapidly, allowing CNC machine tools to be gradually used in all sectors of the manufacturing industry. CNC machining has been in the world for more than half a century, and the NC system has developed from the earliest analog signal circuit control to extremely complex integrated machining system. The programming has also developed from manual programming to intelligent powerful CAD/CAM integration system. The development of CNC technologies in China is relatively slow. In most workshops, the equipment is not advanced and the technicians are on a lower technical level, which leads to low machining quality and efficiency, so the delay of delivery occurs quite often. The first generation of NC system was introduced in 1951, whose control unit was mainly composed of various valves and analog circuits. In 1952, when it was born, the first CNC machine tool was already a machining center developed from a milling or turning machine tool and became the key equipment of modern manufacturing. The second generation of NC system was born in 1959 and mainly composed of individual transistor and other components. The third generation of NC system was introduced in 1965, which used PCB for the first time. Actually, the fourth generation of NC system was developed in 1964, that is, the Computer Numeric Control (CNC) system that we are very familiar with.
In 1975, the NC system used powerful MPU, as the fifth generation of NC system. The sixth generation of NC system uses current Integrated Manufacturing System (IMS) + DNC + Flexible Machining System (FMS). CNC machining, precision CNC machining
Trend of CNC Machine Tool Development High Speed As various industries such as automobile, national defense, aviation, and aerospace develop rapidly and new materials including aluminum alloy are used, the needs for high-speed CNC machining are getting higher and higher. 1. Spindle speed: The machine tool uses a motorized spindle (built-in spindle motor), with a top spindle speed of 200,000r/min. 2. Feed rate: When the resolution is 01µm, the maximum feed rate reaches 240 m/min, and precision machining for complex shapes can be achieved. 3. Computing speed: The rapid development of MPUs provides guarantee for the CNC system to evolve towards high speed and high precision. The CNC system with a 32 bit or even 64 bit CPU has been developed, with frequency increased to several hundred or even over one thousand MHz. The great improvement of computing speed allows a 24-240m/min speed even when the resolution is 1 µm or 0.01 µm. 4. Cutter changing speed: At present, the cutter changing time on advanced machining centers in foreign countries is usually about 1 second, and can even be 0.5 second on some ones. The Chiron Group from Germany has designed the cutter magazine into a basket style, with cutters arranged around the spindle, to achieve 0.9 s cutter changing time.
High Accuracy Nowadays, the accuracy requirement for a CNC machine tool is not limited to static geometric accuracy. More and more importance are attached to the kinematic accuracy, thermal deformation, and vibration monitoring and compensation of a machine tool. 1. The CNC system’s control accuracy is improved. The high-speed interpolation technology is used to achieve continuous feed with tiny program segments, so as for fine operation of the CNC control unit. The high-resolution position detecting device is used to improve the position detection accuracy. The position servo system uses methods such as feed forward control and non-linear control. 2. The error compensation technology is used. Technologies like reverse backlash compensation, error compensation for screw rods and thread pitches, and cutter error compensation, are used for comprehensive compensation for the thermal deformation error and space error of the equipment. 3. The grid decoder is used to check and improve the motion trail accuracy of the machining center. Simulation is performed to predict the machine tool’s machining accuracy, so as to guarantee its positioning accuracy and repeated positioning accuracy. This ensures that the machine tool has long-term sable performances and is capable of finishing multiple types of machining tasks under different operation conditions, with part machining quality guaranteed. Compound Functions A complex machine tool refers to a type of machine tool on which multiple kinds of machining can be achieved as much as possible to turn a part from blank to finished product. By structural characteristics, there are two types: process compounding and procedure compounding. By performing multiple procedures such as turning, milling, drilling, hobbing, grinding, and laser heat treatment, all machining can be achieved on one machining tool. As the modern machining requirements get higher and higher, a large amount of multi-axis motion CNC machine tools are favored by big companies. Intelligent Control As the artificial intelligence technologies develop, in order to meet the development requirements of flexible production and automatic manufacturing in the manufacturing industry, the intelligentization level of CNC machine tools is getting higher and higher, which is reflected in the following aspects: self-adaptive controltechnology for the machining process; intelligent optimization and selection of machining parameters; intelligent fault self-diagnosis and auto-repair technology; intelligent fault playback and fault simulation technology; intelligent AC servo driving device Intelligent 4M CNC system, with Measuring, Moulding, Machining, and Machine operating (4M) integrated into one system.
Open System 1. Open to future technologies: Because recognized standard protocols are followed for both software and hardware interfaces, the system can adopt, absorb and be compatible with a new generation of common hardware and software. 2. Open to special customer requirements: Products are updated, functions are extended, and various combinations of software and hardware products are provided to meet special application requirements. 3. Establishment of CNC standards: A standard programming language is easy for the users to use and reduces labor consumption directly related to operation efficiency. Drives in Parallel Multiple functions such as multi-coordinate linkage CNC machining, assembling and measuring can be achieved, to better satisfy the machining of complex special parts. The parallel machine tool is considered “the most significant progress in the machine tool industry since CNC technologies were invented” and “the new generation of CNC machining equipment in the 21st century”. Extremalization(Large and Micro) The development of national defense, aviation and aerospace as well as basic industries like energy requires large equipment, which needs the support of large CNC machine tools with good performances. As the strategic technologies of the 21st century, both the ultraprecision machining technology and micro&nano technologies require new manufacturing processes and equipment suitable for microminiature size and micro&nano machining accuracy. Information interaction network This achieves not only network resource sharing but also remote monitoring and control as well as remote diagnosis and maintenance of CNC machine tools. Green Machining Process In recent years, more and more machine tools that do not use or use less coolant and achieve dry cutting and half-dry cutting show up. The grand trend of green manufacturing is driving the accelerated development of all kinds of green machine tools. Application of Multimedia Technologies The multimedia technologies integrate computer technology with video and audio and communication technologies, allowing the computer capable of processing sound, text, image and video information in a comprehensive way. CNC machining, precision CNC machining Presently, the CNC machine tools develop with each passing day along the tread and in the direction of high speed, high accuracy, compounding, intelligence, openness, parallel driving, networking, extremalization, and environmental friendliness.
