【正文】 oint may be a certain locating for drilling a hole, milling a slot, or other machine operation. The two measuring systems used on N/C machines are the absolute and incremental. The absolute measuring system uses a fixed reference point. It is on this point that all positional information is based. In other words, all the locations to which a part will be moved must be given dimensions relating to that original fixed reference point. Figure shows an absolute measuring system with X and Y dimensions, each based on the origin. The incremental measuring system has a floating coordinating system. With the incremental system, the time the part is moved. Figure show X and Y values using an incremental measuring system. Notice that with this system, each new location bases its values in X and Y from the preceding location. One disadvantage to this system is that any errors made will be repeated throughout the entire program, if not detected and corrected.There are two types of control systems monly used on N/C equipment: pointtopoint and continuous path. A pointtopoint controlled N/C machine tool, sometimes referred to as a positioning control type, has the capability of moving only along a straight line. However, when two axes are programmed simultaneously with equal values a 45 angle will be generated. Pointtopoint systems are generally found on drilling and simple milling machine where hole location and straight milling jobs are performed. Pointtopoint systems can be utilized to genetate arcs and angles by programming the machine to move in a series of small steps. Using this technique, however, the actual path machined is slightly different from the cutting path specified.Machine tools that have the capability of moving simultaneously in two or more axes are classified 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 more expensive 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 magnetic pulses from the stator and rotor assemblies. The net 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 magnetic scales, and synch. Closedloop systems greatly increase the reliability of N/C machines.Machining CentersMany 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 ControlOne 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:Electrical discharge machining.Laser cutting.Electron beam welding.Numerical control has also made machine tools more versatile than their manually operated predecessors. An NC machine tool can automatically produce