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1、英文资料翻译题目CNCCNCCNC stands for Computerized Numerical Control and has been around since the early1970s. prior to this, it was called NC,for numerical control. While people in most walks of life have never heard of this term, CNC has touched almost every form of manufacturing process in one way or anot
2、her. If youll be working in manufacturing, its likely that youll be dealing with CNC on a regular basis.Before CNCWhile there are exceptions to this statement,CNC machines typically replace (or work in conjunction with) some existing manufacturing processes. Take one of the simplest manufacturing pr
3、ocesses,drilling holes,for example.A drill press can of course be used to machine holes. A person can place a drill in the drill chuck that is secured in the spindle of the drill press. They can then (manually) select the desired speed for rotation (commonly by switching belt pulleys), and activate
4、the spindle. Then they manually pull on the quill lever to drive the drill into the workpiece being machined.As you can easily see, there is a lot of manual intervention required to use a drill press to holes. A person is required to do something almost every step along the way! While this manual in
5、tervention may be acceptable for manufacturing companies if but a small number of holes workpieces must be machined, as quantities grow, so does the likelihood for fatigue due to the tediousness of the operation. And do note that weve used one of the simplest machining operations(drilling) for our e
6、xample. There are more complicated machining operations that would require a much higher skill level (and increase the potential for mistakes resulting in scrap workpieces) of the person running the conventional machine tool. (We commonly refer to style of machine that CNC is replacing as the conven
7、tional machine.)By comparison, the CNC equivalent for a drill press (possibly a CNC machining center or CNC drilling & tapping center) can be programmed to perform this operation in a much more automatic fashion. Everything that the drill press operator was doing manually will now be done by the CNC
8、 machine, including:placing the drill in the spindle, activating the spindle,positioning the workpiece under the drill, machining the hole, and turning off the spindle.How CNC worksAs you might already have guessed,everything that an operator would be required to do with conventional machine tools i
9、s programmable with CNC machines. Once the machine is setup and running, a CNC machine is quite simple to keep running. In fact CNC operators tend to get quite bored during lengthy production runs because there is so little to do. With some CNC machines, some of the specific programmable functions.M
10、otion controlAll CNC machine types share this commonalty: They all have two or more programmable directions of motion called axes. An axis of motion can be linear(along a straight line) or rotary(along a circular path). One of the first specifications that imply a CNC machines complexity is how many
11、 axes it has. Generally speaking, the more axes, the more complex the machine.The axes of any CNC machine are required for the purpose of causing the motions needed for the manufacturing process. In the drilling example, these axes would position then tool over the hole to be machined (in two axes)
12、and machine the hole (with the third axis). Axes are named with letters.Common linear axis named X,Y,and Z. Common rotary names are A,B,and C. There are related to the coordinate system.Programmable accessoriesA CNC machine wouldnt be very helpful if all it could only move the workpiece in two or mo
13、re axes. Almost all CNC machines are programmable in several other ways. The specific CNC machine type has a lot to do with its appropriate programmable accessories. Again,any required function will be programmable on full-blown CNC machine tools. Here are some examples for one machine type(machinin
14、g centers).Automatic tool changerMost machining centers can hold many tools in a tool magazine. When required, the required tool can be automatically placed in spindle for machining.Spindle speed and activation The spindle speed(in revolutions per minute) can be easily specified and the spindle can
15、be turned on in a forward or reverse direction.It can also,of course, be turned off.CoolantMany machining operations require coolant for lubrication and cooling purposes. Coolant can be turned on and off from within the machine cycle.The CNC program Think of giving any series of step-by-step instruc
16、tions. A CNC program is nothing more than another kind of instruction set. Its written in sentence-like format and the control will execute it in sequential order,step by step.A special series of CNC words are used to communicate what the machine is intended to do. CNC words begin with letter addres
17、s(like F for feedrate,S for spindle speed,and X,Y,and Z for axis motion). When placed together in a logical method, a group of CNC words make up a command that resemble a sentence.The CNC controlThe CNC control will interpret a CNC program and active the series of commands in sequential order. As it
18、 reads the program, the CNC control will activate the appropriate machine functions, cause axis motion, and in general, follow the instructions given in the program.Along with interpreting the CNC program, the CNC control has several other purposes. All current model CNC controls allow programs to b
19、e modified(edited) if mistakes are found. The CNC control allows special verification functions(like dry run) to confirm the correctness of the CNC program. The CNC control allows certain important operator inputs to be specified separate from the program, like tool length values. In general, the CN
20、C control allows functions of the machine to be manipulated.What is a CAM system?For simple applications (like drilling holes),the CNC program can developed manually. That is ,a programmer will sit down to write the program armed only with pencil,paper, and calculator. Again, for simple applications
21、,this may be the very best way to develop CNC programs.As applications get more complicated, and especially when new programs are required on a regular basis, writing programs manually becomes much more difficult.To simplify the programming process,a computer aided manufacturing (CAM) system can be
22、used. A CAM system is a software program that runs on a computer(commonly a PC) that helps the CNC programmer with the programming process. Generally speaking, a CAM system will take the tediousness and drudgery out of programming.In many companies the CAM system will work with the computer aided de
23、sign(CAD) drawing developed by the computers design engineering department.This eliminates the need for redefining the workpiece configuration to the CAM system .The CNC programmer will simply specify the machining operations to be performed and the CAM system will create the CNC program(much like t
24、he manual programmer would have written) automatically.What is a DNC system? Once the program is developed (either manually or with a CAM system), it must be loaded into the CNC control. Tough the setup person could type the program right into the control, this would be like using the CNC machine as
25、 a very expensive typewrite. If the CNC program is developed with the help of a CAMsystem, then it is already in the form of a text file.If the program is written manually,it can be typed into any computer using a common word processor (though most companies use a special CNC text editor for this pu
26、rpose). Either way, the program is in the form of a text file that can be transferred right into the CNC machine. A distributive numerical control (DNC) system is used for this purpose.A DNC system is nothing more than a computer that is networked with one or more CNC machines. Until only recently,
27、rather crude serial communications protocol (RS-232C) had to be used for transferring programs. Newer controls have more current communications capabilities and can be networked in more conventional ways (Ethernet, etc.). Regardless of methods , the CNC program must of course be loaded into the CNC
28、machine before it can be run. When Numerical Control is performed under computer supervision, it is called Computer Numerical Control (CNC). Computers are the control units of CNC machines. They are built in or linked to the machines via communications channels. When a programmer inputs some informa
29、tion in the program by tape and so on, the computer calculates all necessary data to get the job done. Todays systems have computers control data, so they are called Computer Numerically Controlled Machines. For both NC and CNC systems, work principles are the same. Only the way in which the executi
30、on is controlled is different. Normally, new systems are faster, more powerful, and more versatile unit. The Construction of CNC Machines CNC machine tools are complex assemblies. However, in general, any CNC machine tool consists of the following units: computers, control systems, drive motors and
31、tool changers. According to the construction of CNC machine tools, CNC machines work in the following manner: (1) The CNC machine language, which is a programming language of binary notation used on computers, is not used on CNC machines. (2) When the operator starts the execution cycle, the compute
32、r translates binary codes into electronic pulses that are automatically sent to the machines power units. The control units compare the number of pulses sent and received. (3) When the motors receive each pulse, they automatically transform the pulses into rotations that drive the spindle and lead s
33、crew, causing the spindle rotation and slide or table movement. The part on the milling machine table or the tool in the lathe turret is driven to the position specified by the program. 1.Computers As with all computers, the CNC machine computer works on binary principle using only two characters 1
34、and 0, for information processing precise time impulses from the circuit. There are two states, a state with voltage, 1, and a state without voltage, 0. Series of ones and zeroes are the only states that the computer distinguishes are called machine language, and it is the only language the computer
35、 understands. When creating the program, the programmer does not care about the machine language. He or she simply uses a list of codes and keys in the meaningful information.Special built-in software compiles the program into the machine language and the machine moves the tool by its servomotors. H
36、owever, the programmability of the machine is dependent on whether there is a computer in the machines control. If there is a minicomputer programming, say, a radius (which is a rather simple task), the computer will calculate all the points on the tool path.On the machine without a minicomputer, th
37、is may prove to be a tedious task, since the programmer must calculate all the points of intersection on the tool path. Modern CNC machines use 32-bit processors in their computers that allow fast and accurate processing of information. 2.Control systems There are two types of control systems on NC/
38、CNC machines: the open loop and the closed loop. The type of control loop used determines the overall accuracy of the machine. The open-loop control system does not provide positioning feedback to the control unit. The movement pulses are sent out by the control and they are received by a special ty
39、pe of servomotor called a stepper motor.The number of pulses that the control sends to the stepper motor controls the amount of the rotation of the motor. The stepper motor then proceeds with the next movement command. Since this control system only counts pulses and cannot identify discrepancies in
40、 positioning, the machine will continue this inaccuracy until somebody finds the error. The open-loop control can be used in applications in which there is no change in load conditions, such as the NC drilling machine.The advantage of the open-loop control system is that it is less expensive, since
41、it does not require the additional hardware and electrics needed for positioning feedback. The disadvantage is the difficulty of detecting a positioning error. In the closed-loop control system, the electronic movement pulses are sent from the control to the servomotor, enabling the motor to rotate
42、with each pulse. The movements are detected and counted by a feedback device called a transducer. With each step of movement, a transducer sends a signal back to the control, which compares the current position of the driven axis with the programmed position. When the number of pulses sent and recei
43、ved matches, the control starts sending out pulses for the next movement. Closed-loop systems are very accurate. Most have an automatic compensation for error, since the feedback device indicates the error and the control makes the necessary adjustments to bring the slide back to the position. They
44、use AC, DC or hydraulic servomotors. Position measurement in NC machines can be accomplished through direct or indirect methods. In direct measuring systems, a sensing device reads a graduated scale on the machine table or slide for linear movement. This system is more accurate because the scale is
45、built into the machine and backlash (the play between two adjacent mating gear teeth) in the mechanisms is not significant. In indirect measuring systems, rotary encoders or resolves convert rotary movement to translation movement. In this system, backlash can significantly affect measurement accura
46、cy. Position feedback mechanisms utilize various sensors that are based mainly on magnetic and photoelectric principles. 3.Drive Motors The drive motors control the machine slide movement on NC/CNC equipment. They come in four basic types: stepper motors, DC servomotors, AC servomotors and fluid ser
47、vomotors. Stepper motors convert a digital pulse generated by the microcomputer unit (MCU) into a small step rotation. Stepper motors have a certain number of steps that they can travel. The number of pulses that the MCU sends to the stepper motor controls the amount of the rotation of the motor.Ste
48、pper motors are mostly used in applications where low torque is required. Stepper motors are used in open-loop control systems, while AC, DC or hydraulic servomotors are used in closed-loop control systems. Direct current (DC) servomotors are variable speed motors that rotate in response to the appl
49、ied voltage. They are used to drive a lead screw and gear mechanism. DC servomotors provide higher-torque output than stepper motors. Alternative current (AC) servomotors are controlled by varying the voltage frequency to control speed. They can develop more power than a DC servomotor. They are also used to drive a lead screw and gear mechanism. Fluid or hydraulic servomotors are also variable speed motors. They are able t