数字控制计算机数字控制与计算机数控机床的安全和维护 (2).doc

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1、苏州大学本科生毕业设计（论文） NC、CNC and Maintenance for CNC Machine1.1 The History of NC and CNC DevelopmentNumerical Control (NC) is any machining process in which the operations are executed automatically in sequences as specified by the program that contains the information for the tool movements .The NC conc

2、ept was proposed in the late 1940s by John Parsons of Traverse City ,Michigan .Parsons recommended a method of automatic control that would guide a milling cutter to produce a ”thru-axis curve ” in order to generate smooth profiles on work pieces.In 1949，The U.S. Air Force award Parsons a contract t

3、o develop a new type of machine tool that would be able to speed up production methods. Parsons commissioned the Massachusetts Institute of Technology (M.I.T) TO develop a practical implementation of his concept. Scientists and engineers at M.I.T built a control system for a two-axis milling machine

4、 that used a perforated paper tape as the input media .In a short period of time , all major machine tool manufacturers were producing some machines with NC , but it was not until the late 1970s that computer-based NC became widely used. NC matured as an automation technology when inexpensive and po

5、werful microprocessors replaced hard-wire logic-making computer-based NC systems.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 communication

6、s channels . When a programmer input some information in the program by tape and so on ,the computer calculates all necessary data to get the job done.On the first Numerically Controlled (NC) machines were controlled by tape ,and because of that ,the NC systems were known as tape-controlled machines

7、 .They were able to control a single operation entered into the machine by punched on magnetic tape .There was no possibility of editing the program on the machine. To change the program, new tape had to be made.Todays systems have computers to control data; they are called Computer Numerically Cont

8、rolled (CNC) machines .For both NC and CNC systems ,working principles are the same. Only the way in which the execution is controlled is different .Normally, new systems are faster ,more powerful ,and more versatile.1.2 The Applications of NC/CNCFig1-1 A modern center with a ten-station turret that

9、 accepts quick-change toolsSince its introduction ,NC technology has found many applications ,including laths and turning centers , milling machines and machining centers ,punches, electrical discharge machines (EDM) ,flame cutters, grinders ,and inspection equipment . The most complex CNC machine t

10、ools are the turning center ,shown in Fig.1-1(A modem turning center with a ten-station turret that accepts quick-change tools. Each tool can be positioned in Seconds with the press of a button).And the machining center shown in Fig.1-2(Vertical machining center ,the tool magazine is on the left of

11、the machine .The control panel on the right can be swiveled by the operator) and Fig.1-3(Horizontal machining center ,equipped with an automatic tool changer .Tool magazines can store 200 cutting tools ). Fig.1-2 A vertical spindle machining center Fig .1-3 A horizontal-spindle machining centerWhen

12、preparing a program for a particular operation ,the programmer must select all cutting data using recommendations for conventional machining. This includes proper selection of cutting speeds, federate, tools and tool geometry , and so on . When the programmer has chosen all of the necessary informat

13、ion properly, The operator loads the program into the machine and presses a button to start the cutting cycle. The CNC machine moves automatically from one machining operation to another ,changing the cutting tools and applying the coolant. In a surprisingly short time, the work piece is machined ac

14、cording to the highest quality standards . But that is not all .No matter how big the work series is ,all of the parts will be almost identical in size and surface finishing .At this time of advanced technology ,with its high demands for surface finishing and tolerances of components in ,for example

15、, aerospace, nuclear, and medical equipment manufacturing, only CNC machines provide successful results.1.3 The Advantages and Disadvantages of CNC Machines1.CNC machines have many advantages over conventional machines ,some of them are:(1)There is a possibility of performing multiple operations on

16、the same machine in one setup.(2)because of the possibility of simultaneous multi-axis tool movement special profile tools are not necessary to cut unusual part shapes.(3)The scrap rate is significantly reduced because of the precision of the CNC machine and lesser operator impact.(4)It is easy to i

17、ncorporate part design changes when CAD/CAM systems are used.(5)It is easier to perform quality assurance by a spot-check instead of checking all parts.(6)Production is significantly increased. 2.CNC machines also have some disadvantages:(1)They are quite expensive (2)They have to be programmed, set

18、up , operated. and maintained by highly skilled personnel.Obviously, CNC machines have more advantages than disadvantages. The companies that adopt CNC technology increase their competitive edge. As always with new technology, The cost per CNC unit is being cut further and more companies can afford

19、CNC equipment ,which enables them to answer the increasingly strong requirements for production speed and quality that competitive markets demand. In the future the broader use of CNC machines will be one of the best ways to enhance automation in manufacturing .1.4 The Construction of CNC MachinesCN

20、C machine tool are complex assemblies .However, in general ,any CNC machine tool consists of the following units:ComputersControl systemsDrive motorsTool changers According to the construction of CNC machines work in the following manner:(1)The CNC machine language, which is a programming language o

21、f binary notation used on computers ,not on CNC machines.(2)When the operator starts the execution cycle, the computer translates binary codes into electronic pulses which are automatically sent to the machines power units. The control units compare the number of pulses sent and received.(3)When the

22、 motors receive each pulse, they automatically transform the pulses into rotations that drive the spindle and lead screw, 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

23、.1. Computers The computer reacts on. As with all computers ,the CNC machine computer works on a binary principle using only two characters 1 and 0, for information processing precise time impulses from the circuit. There are two states that withvoltage,1,and a state without voltage,0. Series of one

24、s and zeroes are the only states that the computer distinguishes a called machine language, it is the only language the computer 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 informati

25、on. Special built-in software compiles the program into machine language and the machine moves the tool by its servomotors. However, 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

26、a rather simple task), the computer will calculate all the points on the tool path. On the machine without a minicomputer, this may prove to be a tedious task, since the programmer must calculate all the points of intersection the tool path .Modern CNC machines use 32-bit processors in their compute

27、rs that allow fast and accurate processing of information. 2. Control Systems There are two types of control systems on NC/CNC machines :open loop and closed loop .The type of control loop used determines the overall accuracy of the machine.The open-loop control system does not provide positioning f

28、eedback to the control unit .The movement pulses are sent out by the control and they are received by a special type 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 proceed

29、s with the next movement command. Since this control system only counts pulses and cannot identify discrepancies in 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 ,

30、such as the NC drilling machine. The advantage of the open-loop control system is that it is less expensive, since it does not require the additional hardware and electronics needed for positioning feedback. The disadvantage is the difficulty of detecting a positioning error(Fif.1-4 a)In the closed-

31、loop control system, the electronic movement pulses are sent from the control to the servomotor enabling the motor to rotate with each pulse. The movements are detected and counted by a feedback device called a transducer. With each step of movement ,a transducer and signal back to the control ,whic

32、h compares the current position of the driven axis with the programmed position .When the number of pulses sent and received match ,the control stats sending out pulses for the next movement.Closed-loop systems are very accurate. Most ha an automatic compensation for error, since the feedback device

33、 indicates the error and the control makes the necessary adjustments to bring the slide back to the position. They use AC,DC or hydraulic servomotors(Fif.1-4b) Fig1-4 Typical control systems Position measurement in NC machines can be accomplished trough direct or indirect methods. In direct measurin

34、g systems ,a sensing device reads a graduated scale on the machine table or slide for linear movement(Fig.1-5a).this system is the more accurate because the scale is built into the machine and backlash(the play between two adjacent mating gear teeth) in the mechanisms is not significant. In indirect

35、 measuring systems, rotary encoders or revolvers (Fig.1-5 b and c) convert rotary movement to translation movement. In this system, backlash can significantly affect measurement accuracy. Position feedback mechanisms utilize various sensors that are based mainly on magnetic and photoelectric princip

36、les.3. Drive Motors The drive motors the machine slide movement on NC/CNC equipment. They come in four basic types:(1)Stepper motors (2)DC servomotors(3)AC servomotors(4)Fluid servomotorsStepper motors convert a digital pulse generated by the microcomputer unit (MCU) into a small step rotation . Ste

37、pper 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. Stepper motors are mostly used in applications where low torque is required.Stepper motors are used in open-loop control sys

38、tems. 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 applied voltage. They are used to drive a lead screw and great mechanism. DC Servomotors provide higher-torque output than stepp

39、er 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

40、 to produce more power, or more speed in the case of pneumatic motors than electric servomotors. The hydraulic pump provides energy to valves that are controlled by the MCU.4. TOOL Changers Most of the time ,several different cutting toolsare used to produce apart. The tools must be replaced quickly

41、 for the next machining operation. For this reason, the majority of NC/CNC machine tools are equipped with automatic tool changers, such as magazines on machining centers and turrets on turning centers. They allow tool changing without the intervention of the operator. Typically ,an automatic tool c

42、hanger grips the tool in the spindle ,pulls it out and replaces it with another tool. On most machines with automatic tool changers, the turret or magazine can rotate in either direction ,forward or reverse (Fig.1-6).Tool changers may be equipped for either random or sequential selection. In random

43、tool selection ,there is no specific pattern of tool selection. On the machining center, when the program calls for the tool, it is automatically indexed into waiting position ,where it can be retrieved by the tool-handling device. On the turning center, the turret automatically rotates ,bringing th

44、e tool into position.1.5 Safety Notes for CNC machine operationsSafety is always a major concern in a metal-cutting operation. CNC equipment is automated and very fast, and consequently it is a source of hazards. The hazards have to be located and the personnel must be aware of them in order to prev

45、ent injuries and damage to the equipment. Main potential hazards include: rotating parts, such as the spindle , chuck, part in the chuck, and the turret with the tools and rotating clamping devices; movable parts, such as the machining center table ,lathe slides, tailstock center, and tool carousel;

46、 errors in the program such as improper use of the G00 code in conjunction with the wrong coordinate value, which can generate an unexpected rapid motion ;an error in setting or changing the offset value ,which can result by unqualified changes in a proven program. to minimize or avoid hazards, try

47、the following preventive action:(1)Keep all of the original covers on the machine as supplied by the machine tool builder.(2)Wear safety glasses ,gloves ,and shoes.(3)Do not attempt to run the machine before you are familiar withits control. (4)Before running the program, make sure that the part is

48、clampedproperly.(5)When proving running the program, follow these safety procedures.Run the program using the machine lock function to check the program for errors in syntax and geometry.Slow down rapid motions using the RAPID OVERRIDE switch or dry run the program.Use a single-block execution to co

49、nfirm each line in the program before executing it.When the tool is cutting ,slow down the feed rate using the FEED OVERRIDE switch to prevent excessive cutting conditions.(6)Do not handle chips by hang and do not use chip hooks to break long curled chips .program different cutting conditions for better chip control .stop the mac