1、数控专业外文翻译数控机床的组成部分精品附录英文原文N/C Machine Tool Elements FromEnglish for Machinery N/C machine tool elements consist of dimensioning systems, control systems, servomechanisms and open-or closed-loop systems. It is important to understand each element prior to actual programming of a numerically controlled
2、 port.The term measuring system in N/C refers to the method a machine tool uses to move a part from a reference point to a target point. A target point 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 ab
3、solute 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
4、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 16.2 show X and Y values using an incremental measuring system. Notice that with this syst
5、em, 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 commonly used on N/C equipment: point-to-point a
6、nd continuous path. A point-to-point 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. Point-to-point systems
7、are generally found on drilling and simple milling machine where hole location and straight milling jobs are performed. Point-to-point 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 mac
8、hined 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 continuous-path or contouring. These machines are used for machining arcs, radii, circles, and angles of any size in two or there dimensio
9、ns. Continuous-path machines are more expensive than point-to-point systems and generally require a computer to aid programming when machining complex contours.N/C servomechanisms are devices used for producing accurate movement of a table or slid along an axis. Two types of servos are commonly used
10、 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 high-torque power servos and mounted directly to a lead screw of a table or tool slide. Most stepping motors are actuated by magneti
11、c 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 10-pitch lead screw allows 0.0005-in. movements to be made. Hydraulic servos produce a fluid pressure that flows through gears or p
12、istons to effect shaft rotation. Mechanical motion of lead screws and slides is accomplished 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 open-loop system contain no
13、-feedback 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 1.000 in, the servo unit generally moves the table 1.000 in. There is no means for comparing the actual table movement with the input signal, however,
14、 The only assurance that the table has actually moved 1.000 in. is the reliability of the servo system used. Open-loop systems are, of course, less expensive than closed-loop systems. A closed-loop system compares the actual output with the input signal and compensates for any errors. A feedback uni
15、t actually compares the amount the table has been moved with the input signal. Some feedback units used on closed-loop systems are transducers, electrical or magnetic scales, and synch. Closed-loop systems greatly increase the reliability of N/C machines.Machining CentersMany of todays more sophisti
16、cated 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 combination turret lathe and milling machine. Additional features are some
17、times 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
18、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 ste
19、p 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
20、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 overcome the limitation of human operators, and it has done so. Numerical control mac
21、hines are more accurate than manually operated machines, they can produce parts more uniformly, they are faster, and the long-run tooling costs are lower. The development of NC led to the development of several other innovations in manufacturing technology:Electrical discharge machining.Laser cuttin
22、g.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 complex machining processes. Numerical control has a
23、llowed 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 conc
24、ept 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 horizon
25、tal 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 problem led to the development in 1959 of the Automatically Programmed Tools (APT) language. This is a special program
26、ming language for NC that uses statements similar to English language to define the part geometry, describe the cutting tool configuration, and specify the necessary motions. The development of the APT language was a major step forward in the further development of NC technology. The original NC sys
27、tems were vastly different from those used today. The machines had hardwired logic circuits. The instructional programs were written on punched paper, which was later to be replaced by magnetic plastic tape. A tape reader was used to interpret the instructions written on the tape for the machine. To
28、gether, all of this represented a giant step forward in the control of machine tools. However, there were a number of problems with NC at this point in its development.A major problem was the fragility of the punched paper tape medium. It was common for the paper tape containing the programmed instr
29、uctions to break or tear during a machining process. This problem was exacerbated by the fact that each successive time a part was produced on a machine tool, the paper tape carrying the programmed instructions had to be rerun through the reader. If it was necessary to produce 100 copies of a given
30、part, it was also necessary to run the paper tape through the reader 100 separate times. Fragile paper tapes simply could not withstand the rigors of a shop floor environment and this kind of repeated use.This led to the development of a special magnetic plastic tape. Whereas the paper tape carried
31、the programmed instructions as a series of holes punched in the tape, the plastic tape carried the instructions as a series of holes punched in the tape, the plastic tape carried the instructions as a series of magnetic dots. The plastic tape was much stronger than the paper taps, which solved the p
32、roblem of frequent tearing and breakage. However, it still left two other problems.The most important of these was that it was difficult or impossible to change the instructions entered on the tape. To make even the most minor adjustments in a program of instructions, it was necessary to interrupt m
33、achining operations and make a new tape .It was also still necessary to run the tape through the reader as many times as there were parts to be produced. Fortunately, computer technology became a reality and soon solved the problems of NC associated with punched paper and plastic tape.The development of a concept known as direct numerical
