A basic concept in advanced manufacturing technology for precision parts processing is digital control (NC). Before the advent of numerical control technology, all machine tools were manipulated and controlled manually. Among the many limitations associated with manually controlled machine tools, the skill of the operator is probably the most prominent issue. The use of manual control is that the quality of the product is directly related to the skill of the operator. Digital control represents the first step from manual control of machine tools. Digital control means using pre-recorded and stored symbolic instructions to control machine tools and other manufacturing systems. The job of a CNC technician is not to operate the machine tool, but to write a program that can issue machine tool operating instructions. For a CNC machine tool, an interface device called a reader must be installed on it to receive and interpret programming instructions. The development of numerical control technology is to overcome the limitations of human operators, and it does accomplish this job. Digitally controlled machines are more accurate than manually operated machines, produce better parts with better consistency, faster production speed, and lower cost of long-term process equipment. The development of numerical control technology has led to several other new inventions in the manufacturing process:
EDM technology, laser cutting, and digital control of electron beam welding also make machine tools more versatile than their predecessors with manual operations.
A CNC machine tool can automatically produce many types of parts, and each part can have a different and complex machining process. Numerical control can enable manufacturers to undertake product production tasks that are economically uneconomical for machine tools and processes that adopt manual control. Like many advanced technologies, CNC was born in the laboratory of the Massachusetts Institute of Technology. The concept of numerical control was put forward in the early 1950s with funding from the US Air Force. In its initial price range, CNC machine tools can economically and efficiently perform straight cutting. However, the curve trajectory has become a problem for machine tool processing, and a series of horizontal and vertical steps should be used to generate the curve during programming. The shorter each line segment that makes up the steps, the smoother the curve. Every line segment in the steps must be calculated. Prompted by this problem, the Automatic Programming Tool (APT) language was born in 1959. This is a programming language specifically suitable for CNC, using sentences similar to English to define the geometric shape of the part, describe the shape of the cutting tool and specify the necessary movement. The research and development of APT language is a great progress in the further development of CNC technology. The numerical control system used today under the original numerical control system is very different. At that time, there were only hard-wired logic circuits in machine tools. The instruction program is written on perforated paper tape (it was later replaced by plastic tape), and the instructions written on the paper tape or tape are translated to the machine using a tape reader. All these together constitute a huge advancement in the digital control of machine tools. However, there are still many problems in this stage of CNC development.
A major problem is the fragility of the punched tape. In the machining process, it is common for the paper tape containing the programming instruction information to be broken and torn. Every time a part is processed on the machine tool, the paper tape containing the programming instructions needs to be put into the reader to run it again. Therefore, this problem becomes very serious. If 100 parts of a certain kind need to be manufactured, the paper tape should be passed through the reader 100 times. The fragile paper tape obviously cannot withstand the strict workshop environment and this kind of repeated use.
This led to the development of a special plastic tape. The paper tape uses a series of small holes to carry the programming instructions, and the plastic tape uses a series of magnetic dots to carry the programming instructions. The strength of plastic tape is much higher than that of paper tape, which can solve the common tear and break problems. However, it still has two problems. One of the most important problems is that it is very difficult or impossible to modify the instructions entered into the belt. Even if the smallest adjustment is made to the instruction program, processing must be interrupted and a new tape made. Moreover, the number of times the belt passes through the reader must be the same as the number of parts to be processed. Fortunately, the practical application of computer technology quickly solved the problems related to perforated paper tape and plastic tape in numerical control technology.
After the concept of direct digital control (DNC) is formed, paper tape or plastic tape can no longer be used as the carrier of programming instructions, thus solving the related problems. In direct digital control, several machine tools are connected to a host computer through data transmission lines. The programs needed to operate these machine tools are stored in this host computer. When needed, it is provided to each machine tool through the data transmission line. Direct digital control is a big improvement on the basis of punched paper tape and plastic tape. However, it dares to have the same limitations as others relying on host computer technology. When the main computer fails, all machine tools controlled by it will stop working. This problem prompted the emergence of computer digital control technology.
The development of microprocessors prepares for the development of programmable logic controllers and microcomputers. These two technologies laid the foundation for the development of computer numerical control (CNC). After adopting CNC technology, each machine tool has a programmable logic controller or microcomputer for digital control. This allows programs to be entered and stored inside each machine tool. It can also compile programs outside the machine tool and download it to each machine tool. Computer numerical control solves the problem caused by the failure of the main computer, but it creates another problem called data management. The same program may be loaded into ten microcomputers that have no communication links with each other. This problem is currently being solved, it is through the use of local area network to connect each microcomputer, in order to obtain better data management.