1. Introduction

The manufacturing process has undergone significant change during the past 50 years. The major contribution to this change has been programmable automation, made possible by machines that can be automatically controlled to perform a variety of tasks.

From humble beginnings in the late1940s, numerical control has progressed to the forefront of automated machine development. In more than five decades, numerical control has evolved from simple automatic positioning machines, controlled by instructions on perforated tape, to sophisticated machine systems integrated within a computer controlled manufacturing network.

2. The Development of NC Technology

The development of numerical control was a peculiar break through in machine tool industry and marked the beginning of the qualitatively new period of advancements in this field. NC machines combine the high productivity specific to special-purpose machines with the flexibility of universal equipment. These features have made NC machines quite effective means of automation of large-lot production.

The use of high-strength, difficult to machine steels led to the development of new processing methods. Ceramal tools made it possible to increase the cutting speeds to such values that it became difficult or, in many case, impossible to control manually machine tools. The need for automatic control was obvious. One more factor that spurred the development of numerical control was the demand of machine building and aircraft industry for machine tools capable of multi-coordinate processing of complex geometry parts which are practically impossible to work on conventional machine.

The advances in electronics and computer engineering facilitated the development of NC machines. The commercial production of semiconductor elements and then integrated circuits offered the possibility of designing small-size high reliability NC systems.

In the late 1950s and early 1960s the first universal transistor computer began to make appearance and the program control devices based on semiconductor elements became available. The 1970s saw the wide introduction of integrated circuit elements. The scale of integration grew very rapidly and NC systems using MSI and LSI circuits came into being. It is interesting that in the 1970s the rates of growth rates in many fields of industry, including machine building.

The use of universal computers for part programming increased the efficiency of NC machines and led to the improved organization of the manufacturing process as a whole. The method of specifying control programs in numerical form created the need to introduce changes in both the organization of production and the procedure of preparing and transmitting the control information on the part geometry and manufacturing steps.

From the first NC machine developed by MIT in 1952, numerical control technology has evolved through five distinct phases to its current level of sophistication.

(1) The first Numerical control machine was developed by retrofitting a Cincinnati Milling Machine Co. vertical Hydro-Tel milling machine (conventional tracer mill) that had been donated by the Air Force of USA from surplus equipment The controller combined analog and digital components, consisted of 292 vacuum tubes, and occupied a floor area greater than the machine tool itself. The prototype successfully performed simultaneous control of three-axis motion based on coordinate-axis data on punched binary tape. This experimental machine was in operation by March 1952.

(2) In the 1950s, industry began the commercial production of control systems based on discrete transistors, using the control programs prepared manually at the offices of plants.

(3) The wide introduction of program controlled machined machine tool in many branches on industry began in the late 1960s.Program control devices based on integrated circuit elements became popular and the first computer-assisted programming systems appeared, which enabled the automatic calculation of tool path from the part drawing.

(4) In the 1970s DNC and CNC systems became available.

The first attempt to use a digital computer to drive machine tool was DNC (Direct Numerical Control ) .This was in the late 1960s before the advent of CNC. As initially implemented, DNC involved the control of a number of machine tools by a single (mainframe)computer through direct connection and in real time. Instead of using a punched tape reader to enter the part program into the MCU, the program was transmitted to the MCU directly from the computer, one block of instructions at a time. The DNC computer provided instruction blocks to the machine tool on demand; when a machine needed control commands, they were communicated to it immediately. As each block was executed by the machine . The next block was transmitted. As far as the machine tool was concerned, the operation was no different from that of a conventional NC machine. The general configuration of a DNC system is depicted in Figure 7.1.The system consisted of three components:①central computer,②NC machine tools, and③data transmission interface. In operation, the computer sent the required part program to the designated machine tool. This procedure was replicated for all machine tools under direct control of the computer. In addition to transmitting data to the machines, the central computer also received data back from the machines to indicate operating performance in the shop (e.g., number of machining cycles completed, machine utilization, and breakdowns).Thus, a central objective of DNC was to achieve two-way communication between the machine and the central computer.

As the number of CNC machine installations grew during the 1970s and 1980s,DNC emerged once again, but in the form of a distributed computer system, or distributed numerical control (DNC).The configuration of the new DNC is very similar to that shown in Figure 7.1 except that the central computer is connected to MCUs, which are computers themselves. This permits complete part programs to be sent to the machine tools, rather than one block at a time. It is also permits easier and less costly installation of the overall system, because the individual CNC machines can be put into service and the distributed NC can be added later. Redundant computers improve system reliability compared with the original DNC. The new DNC permits two-way communication of data between the shop floor and the central computer, which was one of the important features included in the old DNC. However, improvements in data collection devices as well as advances in computer and communications technologies have expanded the range and flexibility of the information that can be gathered and disseminated. Distributed NC system can take on a variety of physical configurations, depending on the number of machine tools included, job complexity security requirements, and equipment availability and preferences. There are several ways to configure a DNC system. We illustrate two types in Figure 7.2(a) switching network and (b) LAN.

The switching network is the simplest DNC system to configuration. It uses a dataswitching box to make a connection from the central computer to a given CNC machine for downloading part programs or uploading data. Transmission of programs to the MCUs is accomplished through a RS-232-C connection. (Virtually all commercial MCUs include the RS-232-C or compatible device as standard equipment today.) Use of a switching box limits the number of machines that can be included in the DNC system. The limit depends on factors such as part program complexity, frequency of service required to each machine, and capabilities of the central computer. The number of machine in the DNC system can be increased by employing a serial link RS-232-C multiplexer.

Local area networks have been used for DNC since 1980s. Various network structures are used in DNC system, among which is the centralized structure illustrated in Figure 7.2 (b). In This arrangement, the computer system is organized as a hierarchy, with the central computer coordinating several satellite computers that are each responsible for a number of CNC machines. Alternative LAN structures are possible, each with its relative advantages and disadvantages. Local area networks in different sections and departments of a plant are often interconnected in plant-wide and corporate-wide networks.

Numerical control technology was in its second decade before computers were employed to actually control machine tool motions. In the mid-1960s, the concept of DNC was developed, In DNC, individual machine tools were controlled by a mainframe computer located remotely from the machine. Mainframe computers represented the state of the technology in the mid-1960s. There were no personal computers or microcomputers at that time. But the trend in computer technology was toward the use of integrated circuits of increasing levels of integration, which resulted in dramatic increases in computational performance, at the same time that the size and cost of the computer were reduced. At the beginning of 1970s, the economics were right for using a dedicated computer as the MCUs. This application came to be known as computer numerical control (CNC). At first minicomputer were used as the controllers.

(5) In mid-1970s, microcomputer began used as MCUs, and the performance and size trend continued. At present, the CNC systems are all based on microcomputers and singlechip microcomputers.

3. Automation of manufacturing

Automation is the technology by which a process or procedure is accomplished without human assistance. It is implemented by using a program of instructions combined with a control system that executes the instructions. To automate a process, power is required, both to drive the process itself and to operate the program and control system. Although automation can be applied in a wide variety of areas, it is most closely associated with the manufacturing industries. It was in the context of manufacturing that the term was originally coined by an engineering manager at Ford Motor Company in 1946 to describe the variety of automatic transfer devices and feed mechanisms that had been installed in Ford's production plants. It is ironic that nearly all modern applications of automation are controlled by computer technologies that were not available in 1946.

Automated systems operate in the factory on the physical product. They perform operations such as processing, assembly, inspection, or material handling, in some cases accomplishing more than one of these operations in the same system. They are called automated because they perform their operations with a reduced level of human participation compared with the corresponding manual process. In some highly automated systems, there is virtually no human participation. Examples of automated manufacturing systems include:

(1) Automated machine tools that process parts

(2) Transfer lines that perform a series of machining operations

(3) Manufacturing systems that use industrial robots to perform processing or assembly operations

(4) Automatic material handing and storage systems to integrate manufacturing operations

(5) Automatic inspection systems for quality control

Automated manufacturing systems can be classified into there basic types: ① fixed automation, ② programmable automation, and ③flexible automation.

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