【文章內(nèi)容簡介】 annels are connected to the servomotors and other controls in the machine tool. Through these channels, the instructions are sent to the machine tool from the controller unit. To make certain that the instructions have been properly executed by the machine, feedback data are sent back to the controller via the feedback channels. The most important function of this return loop is to assure that the table and workpart have 7 $ been properly located with respect to the tool. Most NC machine tools in use today are provided with position feedback controls for this purpose and are referred to as closedloop systems. However, in recent years there has been a growth in the use of openloop systems, which do not make use of feedback signals to the controller unit. The advocates of the openloop concept claim that the reliability of the system is great enough that feedback controls are not needed and are an unnecessary extra cost. Sequence controls coordinate the activities of the other elements of the controller unit. The tape reader is actuated to read data into the buffer from the tape, signals are sent to and from the machine tool, and so on. These types of operations must be synchronized and this is the function of the sequence controls. Another element of the NC system, which may be physically part of the controller unit or part of the machine tool, is the control panel. The control panel or control console contains the dials and switches by which the machine operator runs the NC system. It may also contain data displays to provide information to the operator. Although the NC system is an automatic system, the human operator is still needed to turn the machine on and off, to change tools (some NC systems have automatic tool changers), to load and unload the machine, and to perform various other duties. To be able to discharge these duties, the operator must be able to control the system, and this is done through the control panel. Machine tool The third basic ponent of an NC system is the machine tool or other controlled process. It is the part of the NC system which performs useful work. In the most mon example of an NC system, one designed to perform machining operations, the machine tool consists of the worktable and spindle as well as the motors and controls necessary to drive them. It also includes the cutting tools, work fixtures, and other auxiliary equipment needed in the machining operation. Transfer Machines The highest degree of automation obtainable with specialpurpose, multifunction machines is achieved by using transfer machines. Transfer machines are essentially a 8 bination of individual workstations arranged in the required sequence, connected by work transfer devices, and integrated with interlocked controls. Workpieces are automatically transferred between the stations, which are equipped with horizontal, vertical, or angular units to perform machining, gagging, workpiece repositioning, assembling, washing, or other operations. The two major classes of transfer machines are rotary and inline types. An important advantage of transfer machines is that they permit the maximum number of operations to be performed simultaneously. There is relatively no limitation on (the number of workpiece surfaces or planes that can be machined, since devices can be interposed in transfer machines at practically any point for inverting, rotating, or orienting the workpiece, so as to plete the machining operations. Work repositioning also minimizes the need for angular machining heads and allows operations to be performed in optimum time. Complete processing from rough castings or fings to finished parts is often possible. One or more finished parts are produced on a transfer machine with each index of the transfer system that moves the parts from station to station. Production efficiencies of such machines generally range from 50% for a machine producing a variety of different parts to 85% for a machine producing one part, in high production, depending upon the workpiece and how the machine is operated (materials handling method, maintenance procedures, etc.) All types of machining operations, such as drilling, tapping, reaming, boring, and milling, are economically bined on transfer machines. Lathetype operations such as turning and facing are also being performed on inline transfer machine, with the workpieces being rotated in selected machining stations. Turning operations are performed in lathetype segments in which multiple tool holders are fed on slides mounted on tunneltype bridge units. Workpieces are located on centers and rotated by chucks at each turning station. Turning stations with CNC are available for use on inline transfer machines. The CNC units allow the machine cycles to be easily altered to acmodate changes in workpiece design and can also be used for automatic tool 9 adjustments. Maximum production economy on transfer lines is often achieved by assembling parts to the workpieces during their movement through the machine. Such items as bushings, seals, Welch plugs, and heat tubes can be assembled and then machined or tested during the transfer machining sequence. Automatic nut torturing following the application of part subassemblies can also be carried out. Gundrilling or reaming on transfer machines is an ideal application provided that proper machining units are employed and good bushing practices are followed. Contour boring and turning of spherical seats and other surfaces can be done with tracer controlled singlepoint inserts, thus eliminating the need for costly special form tools. Inprocess gaging of reamed or bored holes and automatic tool setting are done on transfer machines to maintain close tolerances. Less conventional operations somet