【正文】 have power drive for all tool movements except on the pound monly range in size from 305 to 610 mm(12 to 24 inches)swing and from 610 to 1219 mm(24 to 48 inches)center distances, but swings up to 1270 mm(50 inches)and center distances up to 3658mm(12 feet)are not have chip pans and a builtin coolant circulating engine latheswith swings usually not over 330 mm(13 inches)–also are available in bench type, designed for the bed to be mounted on a bench on a bench or engine lathes are versatile and very useful, because of the time required for changing and setting tools and for making measurements on the work piece, thy are not suitable for quantity the actual chipproduction tine is less than 30% of the total cycle addition, a skilled machinist is required for all the operations, and such persons are costly and often in short , much of the operator’s time is consumed by simple, repetitious adjustments and in watching chips being , to reduce or eliminate the amount of skilled labor that is required, turret lathes, screw machines, and other types of semiautomatic and automatic lathes have been highly developed and are widely used in 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 ere manually operated and the many limitations associated with manual control machine tools, perhaps none is more prominent than the limitation of operator manual control, the quality of the product is directly related to and limited to the skills of the control represents the first major step away from human control of machine control means the control of machine tools and other manufacturing systems through the use of prerecorded, written symbolic than operating a machine tool, an NC technician writes a program that issues operational instructions to the machine a machine tool to be numerically controlled, it must be interfaced with a device for accepting and decoding the programmed instructions, known as a control was developed to overe the limitation of human operators, and it has done control machines are more accurate than manually operated machines, they can produce parts more uniformly, they are faster, and the longrun tooling costs are development of NC led to the development of several other innovations in manufacturing technology: Electrical discharge machining,Laser cutting,Electron beam control has also made machine tools more versatile than their manually operated NC machine tool can automatically produce a wide of parts, each involving an assortment of widely varied and plex machining control has allowed manufacturers to undertake the production of products that would not have been feasible from an economic perspective using manually controlled machine tolls and so many advanced technologies, NC was born in the laboratories of the Massachusetts Institute of concept of NC was developed in the early 1950s with funding provided by the its earliest stages, NC machines were able to made straight cuts efficiently and , 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 shorter the straight lines making up the steps, the smoother is the curve, Each line segment in the steps had to be problem led to the development in 1959 of the Automatically Programmed Tools(APT) is a special programming language for NC that uses statements similar to English language to define the part geometry, describe the cutting tool configuration, and specify the necessary development of the APT language was a major step forward in the fur ther development from those used machines had hardwired logic instructional programs were written on punched paper, which was later to be replaced by magnetic plastic tape reader was used to interpret the instructions written on the tape for the , all of this represented a giant step forward in the control of machine , there were a number of problems with NC at this point in its major problem was the fragility of the punched paper tape was mon for the paper tape containing the programmed instructions to break or tear during a machining 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 it was necessary to produce 100 copies of a given part, it was also necessary to run the paper tape through the reader 100 separate paper tapes simply could not withstand the rigors of a shop floor environment and this kind of repeated led to the development of a special magnetic plastic the paper carried the programmed instructions as a series of holes punched in the tape, the plastic tape carried the instructions as a series of magnetic plastic tape was much stronger than the paper tape, which solved the problem of frequent tearing and , it still left two other most important of these was that it was difficult or impossible to change the instructions entered on the made even the most minor adjustments in a program of instructions, it was necessary to interrupt machining operations and make a new was also still necessary to run the tape through the reader as many times as there were parts to be , puter technology became a reality and soon solved the problems of NC associated with punched paper and plastic development of a concept known as direct numerical control(DNC)solved the paper and plastic tape problems associated with numerical control by simply eliminating tape as the medium for carrying the programmed direct numerical control, machine tools are tied, via a data transmission link, to a host for operating the machine tools are stored in the host puter and fed to the machine tool an needed via the data transmission numerical control represented a major step forward over punched tape and plastic , it is subject to the same limitations as al