Numerical-Control数控技术大学毕业论文外文文献翻译及原文

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　　翻译日期： 2017.02.14

　　Numerical Control

　　Numerical control (N/C) is a form of programmable automation in which the processing equipment is controlled by means of numbers, letters and other symbols. The numbers, letters, and symbols are coded in an appropriate format to define a program of instructions for a particular workpart or job. When the job is changed, the program of instructions must be changed. The capability to change the program is what makes N/C suitable for low-volume and medium-volume production. It is much easier to write programs than to make major alterations of the processing equipment.

　　There are two basic types of numerically controlled machine tools: point—to—point and continuous—path (also called contouring). Point—to—point machines use unsynchronized motors, with the result that the position of the machining head Can be assured only upon completion of a movement, or while only one motor is running. Machines of this type are principally used for straight—line cuts or for drilling or boring.

　　The N/C system consists of the following components: data input, the tape reader with the control unit, feedback devices, and the metal—cutting machine tool or other type of N/C equipment.

　　Data input, also called “man—to—control link”, may be provided to the machine tool manually, or entirely by automatic means. Manual methods when used as the sole source of input data are restricted to a relatively small number of inputs. Examples of manually operated devices are keyboard dials, pushbuttons, switches, or thumbwheel selectors. These are located on a console near the machine. Dials ale analog devices usually connected to a synchronization-type resolver or potentiometer. In most cases, pushbuttons, switches and other similar types of selectors are digital input devices. Manual input requires that the operator set the controls for each operation. It is a slow and tedious process and is seldom justified except in elementary machining applications or in special cases.

　　In practically all cases, information is automatically supplied to the control unit and the machine tool by cards, punched tapes, or by magnetic tape. Eight—channel

　　punched paper tape is the most commonly used form of data input for conventional N/C systems. The coded instructions on the tape consist of sections of punched holes called blocks. Each block represents a machine function, a machining operation, or a combination of the two. The entire N/C program on a tape is made up of an accumulation of these successive data blocks, Programs resulting in long tapes all wound on reels like motion-picture film. Programs on relatively short tapes may be continuously repeated by joining the two ends of the tape to form a loop. Once installed, the tape is used again and again without further handling. In this case, the operator simply loads and unloads the parts. Punched tapes ale prepared on type writers with special tape—punching attachments or in tape punching units connected directly to a computer system. Tape production is rarely error-free. Errors may be initially caused by the part programmer, in card punching or compilation, or as a result of physical damage to the tape during handling, etc. Several trial runs are often necessary to remove all errors and produce an acceptable working tape.

　　While the data on the tape is fed automatically, the actual programming steps ale done manually, Before the coded tape may be prepared, the programmer, often working with a planner or a process engineer, must select the appropriate N/C machine tool, determine the kind of material to be machined, calculate the speeds and feeds, and decide upon the type of tooling needed. The dimensions on the part print are closely examined to determine a suitable zero reference point from which to start the program. A program manuscript is then written which gives coded numerical instructions describing the sequence of operations that the machine tool is required to follow to cut the part to the drawing specifications.

　　The control unit receives and stores all coded data until a complete block of information has been accumulated. It then interprets the coded instruction and directs the machine tool through the required motions.

　　The function of the control unit may be better understood by comparing it to the action of a dial telephone, where, as each digit is dialed, it is stored. When the entire number has been dialed, the equipment becomes activated and the call is completed.

　　Silicon photo diodes, located in the tape reader head on the control unit, detect

　　light as it passes through the holes in the moving tape. The light beams are converted to electrical energy, which is amplified to further strengthen the signal. The signals are then sent to registers in the control unit, where actuation signals are relayed to the machine tool drives.

　　Some photoelectric devices are capable of reading at rates up to 1000 characters per second. High reading rates are necessary to maintain continuous machine—tool motion; otherwise dwell marks may be generated by the cutter on the part during contouring operations. The reading device must be capable of reading data blocks at a rate faster than the control system can process the data.

　　A feedback device is a safeguard used on some N/C installations to constantly compensate for errors between the commanded position and the actual location of the moving slides of the machine tool. An N/C machine equipped with this kind of a direct feedback checking device has what is known as a closed-loop system. Positioning control is accomplished by a sensor which, during the actual operation, records the position of the slides and relays this information back to the control unit. Signals thus received ale compared to input signals on the tape, and any discrepancy between them is automatically rectified.

　　In an alternative system, called an open—loop system, the machine is positioned solely by stepping motor drives in response to commands by a controllers. There are three basic types of NC motions, as follows:

　　Point-to-point or Positional Control In point-to-point control the machine tool elements (tools, table, etc.) are moved to programmed locations and the machining operations performed after the motions are completed. The path or speed of movement between locations is unimportant; only the coordinates of the end points of the motions are accurately controlled. This type of control is suitable for drill presses and some boring machines, where drilling, tapping, or boring operations must be performed at various locations on the work piece. Straight-Line or Linear Control Straight-Line control systems are able to move the cutting tool parallel to one of the major axes of the machine tool at a controlled rate suitable for machining. It is normally only possible to move in one direction at a time, so angular cuts on the work

　　piece are not possible, consequently, for milling machines, only rectangular configurations can be machined or for lathes only surfaces parallel or perpendicular to the spindle axis can be machined. This type of controlled motion is often referred to as linear control or a half-axis of control. Machines with this form of control are also capable of point-to-point control.

　　Continuous Path or Contouring Control In continuous path control the motions of two or more of the machine axes are controlled simultaneously, so that the position and velocity of the can be tool are changed continuously. In this way curves and surfaces can be machined at a controlled feed rate. It is the function of the interpolator in the controller to determine the increments of the individual controlled axes of the machines necessary to produce the desired motion. This type of control is referred to as continuous control or a full axis of control.

　　Some terminology concerning controlled motions for NC machines has been introduced. For example, some machines are referred to as four-or five-or even six-axis machines. For a vertical milling machine three axes of control are fairly obvious, these being the usual X, Y, Z coordinate directions. A fourth or fifth axis of control would imply some form of rotary table to index the work piece or possibly to provide angular motion of the work head. Thus, in NC terminology an axis of control is any controlled motion of the machine elements (spindles, tables, etc). A further complication is use of the term half-axis of control; for example, many milling machines are referred to as 2.5-axis machine. This means that continuous control is possible for two motions (axes) and only linear control is possible for the third axis. Applied to vertical milling machines, 2.5axis control means contouring in the X, Y plane and linear motion only in the Z direction. With these machines three-dimensional objects have to be machined with water lines around the surface at different heights. With an alternative terminology the same machine could be called a 2CL machine (C for continuous, L for linear control). Thus, a milling machine with continuous control in the X, Y, Z directions could be termed be a three-axis machine or a 3c machine. Similarly, lathes are usually two axis or 2C machines. The degree of work precision depends almost entirely upon the accuracy of the lead screw and the

　　The original N/C used the closed—loop system. Of the two systems, closed and open loop, closed loop is more accurate and, as a consequence, is generally more expensive. Initially, open—loop systems were used almost entirely for light-duty applications because of inherent power limitations previously associated with conventional electric stepping motors. Recent advances in the development of electro hydraulic stepping motors have led to increasingly heavier machine load applications.