【正文】 N/C equipment: pointtopoint andcontinuous path. A pointtopoint controlled N/C machine tool, sometimes referred to as apositioning control type, has the capability of moving only along a straight line. However, whentwo axes are programmed simultaneously with equal values a 45 angle will be systems are generally found on drilling and simple milling machine where holelocation and straight milling jobs are performed. Pointtopoint systems can be utilized togeate arcs and angles by programming the machine to move in a series of small steps. Usingthis technique, however, the actual path machined is slightly different from the cutting pathspecified. Machine tools that have the capability of moving simultaneously in two or more axes areclassified as continuouspath or contouring. These machines are used for machining arcs, radii,circles, and angles of any size in two or there dimensions. Continuouspath machines are moreexpensive than pointtopoint systems and generally require a puter to aid programming when machining plex contours. N/C servomechanisms are devices used for producing accurate movement of a table or slid along an axis. Two types of servos are monly used on N/C equipment: electric stepping motors and hydraulic motors. Stepping motor servos are frequently used on less expensive N/C equipment. These motors are generally hightorque power servos and mounted directly to a lead screw of a table or tool slide. Most stepping motors are actuated by magic pulses from the stator and rotor assemblies. The result of this action is that one rotation of the motor shaft produces 200 steps. Connection the motor shaft to a 10pitch lead screw allows . movements to be made. Hydraulic servos produce a fluid pressure that flows through gears or pistons to effect shaft rotation. Mechanical motion of lead screws and slides is acplished through various values and controls from these hydraulic motors. However, they are more expensive and noisy. Most larger N/C machines use hydraulic servos. N/C machines that use an openloop system contain nofeedback signal to ensure that a machine axis has traveled the required distance. That is, if the input received was to move a particular table axis in, the servo unit generally moves the table in. There is no means for paring the actual table movement with the input signal, however, The only assurance that the table has actually moved in. is the reliability of the servo system used. Openloop systems are, of course, less expensive than closedloop systems. A closedloop system pares the actual output with the input signal and pensates for any errors. A feedback unit actually pares the amount the table has been moved with the input signal. Some feedback units used on closedloop systems are transducers, electrical or magic scales, and synchros. Closedloop systems greatly increase the reliability of N/C machines. Machining Centers Many of today’s more sophisticated lathes are called machining centers since they are capable of performing, in addition to the normal turning operations, certain milling and drilling operations. Basically, a machining center can be thought of as being a bination turret lathe and milling machine. Additional features are sometimes included by manufacturers to increase the versatility of their machines. Numerical Control One of the most fundamental concepts in the area of advanced manufacturing technologies is numerical control (NC). Prior to the advent of NC, all machine tools were manually operated and controlled .Among the many limitations associated with manual control machine tools, perhaps none is more prominent than the limitation of operator skills. With manual control, the quality of the product is directly related to and limited to the skills of the operator. Numerical control represents the first major step away from human control of machine tools. Numerical control means the control of machine tools and other manufacturing systems through the use of prerecorded, written symbolic instructions. Rather than operating a machine tool, an NC technician writes a program that issues operational instructions to the machine tool. For a machine tool to be numerically controlled, it must be interfaced with a device for accepting and decoding the programmed instructions, known as a reader. Numerical control was developed to overe the limitation of human operators, and it has done so. Numerical control machines are more accurate than manually operated machines, they can produce parts more uniformly, they are faster, and the longrun tooling costs are lower. The development of NC led to the development of several other innovations in manufacturing technology: 1. Electrical discharge machining. 2. Laser cutting. 3. Electron beam welding. Numerical control has also made machine tools more versatile than their manually operated predecessors. An NC machine tool can automatically produce a wide variety of parts, each involving an assortment of widely varied and plex machining processes. Numerical control has allowed manufacturers to undertake the production of products that would not have been feasible from an economic perspective using manually controlled machine tools and processes. Like so many advanced technologies, NC was born in the laboratories of the Massachusetts Institute of Technology. The concept of NC was developed in the early 1950s with funding provided by the U. S. Air force. In its earliest stages, NC machines were able to make straight cuts efficiently and effectively. However, curved paths were a problem because the machine tool had to be programmed to undertake a series of horizontal and vertical steps to produce a curve. The shorter is the straight lines making up the steps, the smoother is the curve. Each line segment in the steps had to be calculated. This prob