机械专业英语董建国第22章.ppt

Unit 22 TEXT READING MATERIAL Unit 22 TEXT Fundamentals of Numerical Control Controlling a machine tool by means of a prepared program is known as numerical control, or NC. NC equipment has been defined the Electronic Industries Association (EIA) as“A system in which actions are controlled by the direct insertion of numerical data at some point. The system must automatically interpret at least some portion of this data”. In a typical NC system the numerical data which is required for producing a part is maintained on a punched tape and is called the part program. The part program is arranged in the form of blocks of information, where each block contains the numerical data required to produce one segment of the work-piece. The punched tape is moved forward by one block each time the cutting of a segment is completed. The block contains, in coded form, all the information needed for processing a segment of the work-piece: the segment length, its cutting speed, feed, etc. Dimensional information (length, width, and radii of circles) and the contour form (linear, circular, or other) are taken from an engineering drawing. Dimensions are given separately for each axis of motion (x, y, etc.). Cutting speed, feed-rate, and auxiliary function (coolant on and off, spindle direction, clamp, gear changes, etc.) are programmed according to surface finish and tolerance requirements. Preparing the part program for a NC machine tool requires a part programmer. The part programmer must possess knowledge and experiences in mechanical engineering. Part Programmers must be familiar with the function of NC machine tools and machining process and have to decide on the optimal sequence of operations. In NC machine tools, each axis of motion is equipped with a separate driving device which replaces the hand-wheel of the conventional machine. The driving device may be a DC motor, a hydraulic actuator, or a stepping motor. The type selected is determined mainly by the