1、CNC numerical control machine toolsComputer digital control is a cover all kinds, with different size, shape and function of the machines term ，But the most simple methods of understanding is CNC is a kind of controlled by computer in the cube material processing out useful objects machine. CNC mach
2、ine can be divided into two categories: the nc lathe and nc milling machine. Numerical control lathe usually consists of clamping workpiece high-speed rotating device and a can cut to workpiece excess material tool composition (tool can be in workpiece up to and from sports, until get the desired sh
3、ape of the). Nc milling machine is a kind of can in the X, Y, Z axis three directions of milling processing device. the nc lathe and nc milling machine does not necessarily need computer to produce a queue to control. Numerical control technology and computer technologies can allow people to first o
4、f all on the computer design products, and then define how nc machine tool processing the product. Product design is on the computer generates a computer aided design documents, then need people define how NC machine tool processing the products and, in the end, it step produced a computer aided man
5、ufacturing file (or G code files, or NC file, this type of file have many name). The computer aided processing file contains all of the nc machine tools must follow the steps machining information. Computer and CNC numerical control machine tool communications tell it how to explain these processing
6、 document to carry on the processing. Nc machine tool by: procedures, input/output device, CNC unit, servo system, position feedback system, machine body composition. 1. program storage medium, also called program carrier 1) Punched tape (outdated, eliminated); 2) Cassette tape (outdated, eliminated
7、); 3) Floppy disk, disk, usb flash disk, 4) Communication. 12. input/output devices 1) For punched tape, with photoelectric reading machine; (out of date, eliminated); 2) For cassette tape, with recorders, (out of date, eliminated); 3) For the floppy disk, with floppy disk drive and drive card; 4) M
8、odern nc machine tool, but also through the manual way (MDI style); 5) DNC communication network, RS232 serial communication. 3. CNC unit CNC unit is the core of the nc machine tools, CNC unit is composed of information input, processing and output of three parts. CNC unit to accept digital informat
9、ion, through numerical control device of control software and logic circuit decoding, interpolation, logic processing, will be all kinds of instruction information output servo system, servo drive execution parts feed movement. The other and main moving parts of variable speed, reversing and start-s
10、top signal. Choice and exchange tool cutting tool command signal, cooling and lubrication of the start-stop, workpiece and machine tool parts loosen, clamping, indexing table inversion and auxiliary command signal, etc. Prepare function: G00, G01, G02, G03, Auxiliary function: M03, M04 Tools, feed s
11、peed, spindle: T, F, S 4. servo system By drive, drive motor composition, and with the execution of the machine parts and mechanical transmission parts of nc machine tool feed system. Its role is the numerical control device from the pulse signal into the movement of the machine moving parts. For st
12、epping motor for, each pulse signal make motor turned an Angle that, in turn, drives a machine moving parts move a small distance. Each movement of feed execution component has a corresponding servo drive system, the whole machine performance mainly depends on the servo system. Such as triaxial link
13、age of machine tools has three sets of drive system. Pulse equivalent: each pulse signal make machine moving parts moving displacement. Commonly used pulse equivalent to 0.001 mm/pulse. 5. position feedback system (detection feedback system) Servo motor Angle displacement feedback, nc machine tools
14、actuator (table) displacement feedback. Including grating, rotary encoder, laser range finder, the magnetic grid, etc. Feedback device to test results into electrical signal feedback to numerical control device, by comparison, calculating the actual position and instruction position deviation betwee
15、n the, and sent out the deviation command control executive part of the feed movement. Feedback system including half closed loop and closed loop two system. 6. machine tool of mechanical components 1) Main moving parts 2) Feed unit (table, head) 3) Foundation support (bed, column, etc.) 4) Auxiliar
16、y parts, such as hydraulic, pneumatic, cooling and lubrication part, etc 5) Reserve tool knife library, automatic tool changer (ATC) For machining center type of nc machine tools, and store tool knife library, exchange cutter manipulator etc. Numerical control machine mechanical parts of the composi
17、tion and common machine tool similar, But for the transmission structure more simple, in precision, rigidity, earthquake resistance etc requirements higher, And the drive and speed control system is more easy to realize automatic extension. Servo drive and position detection device Machine bed The m
18、ain motion mechanism Feed motion mechanism Auxiliary action mechanism Nc device CNCInput device programming Auxiliary control and high voltage control device 图1-1 数控机床的组成In the numerical control machine parts processing by the following steps: (1) preparation phase: According to the processing parts
19、 drawings, determine the relevant processing data (tool path coordinate point, processing cutting dosage, cutter size information, etc.). According to the process scheme, selection of fixture, tool, such as the type of choice about other auxiliary information. The workpiece and tool relative motion
20、trajectory size parameters (feed execution component feed size) Machining process parameter (the Lord movement and feed movement speed, cutting depth, etc.) All sorts of auxiliary operation (the Lord movement speed, cutting tool transformation, cooling and lubricating fluid start-stop, workpiece cla
21、mping loosen) (2) programming stage According to process information, a machine tool numerical control system can identify language writing nc machining program (for machining process description), and fill in the program list. (3) for the information carrier According to the compiled good program l
22、ist, will program stored in information carrier (perforated tape, magnetic tapes, disks, etc.), Through the information carrier will all processing information to the numerical control system. If the numerical control machine tool and computer networking, information can be directly in the numerical
23、 control system. (4) processing stage When the implementation procedure, machine tool numerical control system (CNC) will processing program statement decoding, operation, Convert drive all moving parts of action instruction, in the system under the uniform coordination drive all moving parts of tim
24、ely movement, automatic completion of workpiece processing. On the whole, numerical control machine tool is and machining parts related information, the provisions of the words, Numbers and symbols of code, according to certain format to compile the program list, will processing program through the
25、control medium input to the numerical control device, the numerical control device after processing, send all and processing program corresponding signal and command control machine tool automatic processing. The first benefit offered by all forms of CNC machine tools is improved automation. The ope
26、rator intervention related to producing workpieces can be reduced or eliminated. Many CNC machines can run unattended during their entire machining cycle, freeing the operator to do other tasks. This gives the CNC user several side benefits including reduced operator fatigue, fewer mistakes caused b
27、y human error, and consistent and predictable machining time for each workpiece. Since the machine will be running under program control, the skill level required of the CNC operator (related to basic machining practice) is also reduced as compared to a machinist producing workpieces with convention
28、al machine tools.The second major benefit of CNC technology is consistent and accurate workpieces. Todays CNC machines boast almost unbelievable accuracy and repeatability specifications. This means that once a program is verified, two, ten, or one thousand identical workpieces can be easily produce
29、d with precision and consistency.A third benefit offered by most forms of CNC machine tools is flexibility. Since these machines are run from programs, running a different workpiece is almost as easy as loading a different program. Once a program has been verified and executed for one production run
30、, it can be easily recalled the next time the workpiece is to be run. This leads to yet another benefit, fast change over. Since these machines are very easy to set up and run, and since programs can be easily loaded, they allow very short setup time. This is imperative with todays just-in-time (JIT
31、) product requirements. Motion control - the heart of CNCThe most basic function of any CNC machine is automatic, precise, and consistent motion control. Rather than applying completely mechanical devices to cause motion as is required on most conventional machine tools, CNC machines allow motion co
32、ntrol in a revolutionary manner2. All forms of CNC equipment have two or more directions of motion, called axes. These axes can be precisely and automatically positioned along their lengths of travel. The two most common axis types are linear (driven along a straight path) and rotary (driven along a
33、 circular path).Instead of causing motion by turning cranks and handwheels as is required on conventional machine tools, CNC machines allow motions to be commanded through programmed commands. Generally speaking, the motion type (rapid, linear, and circular), the axes to move, the amount of motion a
34、nd the motion rate (feedrate) are programmable with almost all CNC machine tools.A CNC command executed within the control tells the drive motor to rotate a precise number of times. The rotation of the drive motor in turn rotates the ball screw. And the ball screw drives the linear axis (slide). A f
35、eedback device (linear scale) on the slide allows the control to confirm that the commanded number of rotations has taken place3.Though a rather crude analogy, the same basic linear motion can be found on a common table vise. As you rotate the vise crank, you rotate a lead screw that, in turn, drive
36、s the movable jaw on the vise. By comparison, a linear axis on a CNC machine tool is extremely precise. The number of revolutions of the axis drive motor precisely controls the amount of linear motion along the axis.How axis motion is commanded - understanding coordinate systemsIt would be infeasibl
37、e for the CNC user to cause axis motion by trying to tell each axis drive motor how many times to rotate in order to command a given linear motion amount4. (This would be like having to figure out how many turns of the handle on a table vise will cause the movable jaw to move exactly one inch!) Inst
38、ead, all CNC controls allow axis motion to be commanded in a much simpler and more logical way by utilizing some form of coordinate system. The two most popular coordinate systems used with CNC machines are the rectangular coordinate system and the polar coordinate system. By far, the more popular o
39、f these two is the rectangular coordinate system.The program zero point establishes the point of reference for motion commands in a CNC program. This allows the programmer to specify movements from a common location. If program zero is chosen wisely, usually coordinates needed for the program can be
40、 taken directly from the print. With this technique, if the programmer wishes the tool to be sent to a position one inch to the right of the program zero point, X1.0 is commanded. If the programmer wishes the tool to move to a position one inch above the program zero point, Y1.0 is commanded. The co
41、ntrol will automatically determine how many times to rotate each axis drive motor and ball screw to make the axis reach the commanded destination point . This lets the programmer command axis motion in a very logical manner.All discussions to this point assume that the absolute mode of programming i
42、s used6. The most common CNC word used to designate the absolute mode is G90. In the absolute mode, the end points for all motions will be specified from the program zero point. For beginners, this is usually the best and easiest method of specifying end points for motion commands. However, there is
43、 another way of specifying end points for axis motion.In the incremental mode (commonly specified by G91), end points for motions are specified from the tools current position, not from program zero. With this method of commanding motion, the programmer must always be asking How far should I move th
44、e tool? While there are times when the incremental mode can be very helpful, generally speaking, this is the more cumbersome and difficult method of specifying motion and beginners should concentrate on using the absolute mode.Be careful when making motion commands. Beginners have the tendency to th
45、ink incrementally. If working in the absolute mode (as beginners should), the programmer should always be asking To what position should the tool be moved? This position is relative to program zero, NOT from the tools current position.Aside from making it very easy to determine the current position
46、for any command, another benefit of working in the absolute mode has to do with mistakes made during motion commands. In the absolute mode, if a motion mistake is made in one command of the program, only one movement will be incorrect. On the other hand, if a mistake is made during incremental movem
47、ents, all motions from the point of the mistake will also be incorrect.Assigning program zeroKeep in mind that the CNC control must be told the location of the program zero point by one means or another. How this is done varies dramatically from one CNC machine and control to another8. One (older) m
48、ethod is to assign program zero in the program. With this method, the programmer tells the control how far it is from the program zero point to the starting position of the machine. This is commonly done with a G92 (or G50) command at least at the beginning of the program and possibly at the beginning of each tool.Another, newer and better way to assign program zero is through some form of offset. Refer to fig.4. Commonly machining center control manufacturers call offsets used to assign program zero fixture offsets. Turning center
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