Unit6 Computer Numerical Control

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Unit6 Computer Numerical Control

  1. 1. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/6/1 he he k k lic lic COMPUTER NUMERICAL CONTROL C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c UNIT 6 COMPUTER NUMERICAL CONTROL OBJECTIVES General Objective :To understand the concept and principles of computer numerical control (CNC) system. Specific Objectives : At the end of the unit you will be able to : Ø Understand the main components of the CNC system, Ø Understand the point-to-point system (positioning), Ø Understand the contouring system (continuous system), and Ø Write a simple CNC milling program. .
  2. 2. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/6/2 he he k k lic lic COMPUTER NUMERICAL CONTROL C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c INPUT 6.0 INTRODUCTION Computer numerical control is a system in which a control microcomputer is an integral part of a machine or a piece of equipment (onboard computer). The part programmes can be prepared at a remote site by programmer, and it may incorporate information obtained from drafting software packages and from machining simulations, in order to ensure that the part programme is bug free. The machine operator can, however, easily and manually programme onboard computers. The operator can be modify the programs directly, prepare programme for different parts, and store the programmes. Because of the availability of small computers having a large memory, microprocessor(s), and programme-editing capabilities, CNC systems are widely used today. The availability of low-cost programmable controllers also played a major role in the successful implementation of CNC in manufacturing plants. Numerical Control is a system where machine action is created from the insertion of Numeric Data. The Numeric Data is, in the beginning, written words in an easily understood code of letters and numbers (alphanumeric characters) known as a programme, which in turn is converted by the machine control unit (MCU) into the electrical signals used to control the machine movements.
  3. 3. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/6/3 he he k k lic lic COMPUTER NUMERICAL CONTROL C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c The relationship between the words "Numerical" and "Control" is shown below. NUMERICAL CONTROL An instructional expression, To control such machine in a language of numbers, actions as: which represents a series of Directing Altering commands for specific Commanding Timing Prescribing Ceasing machine tool movements Sequencing Guiding Initiating Two important points should be made about N.C. First, the actual N.C. machine tool can do nothing more than it was capable of doing before a control unit was joined to it. There are now new metal removing principles involved. N.C. machines position and drive the cutting tools, but the same milling cutters, drills, taps, feeds, and other tools still perform the cutting operations. Cutting speeds, feeds, and tooling principles must still be adhered to. Given this knowledge, what is the real advantage of numerical control? Primarily, the idle time or time to move into position for new cuts is limited only by the machine's capacity to respond. Because the machine receives commands from the machine control unit (MCU), it responds without hesitation. The actual utilisation rate or chip making rate is therefore much higher than on a manually operated machine. The second point is that numerical control machines can initiate nothing on their own. The machine accepts and responds to commands from the control unit. Even the control unit cannot think, judge, or reason. Without some input medium, e.g., punched tape or direct computer link, the machine and control
  4. 4. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/6/4 he he k k lic lic COMPUTER NUMERICAL CONTROL C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c unit will do nothing. The N.C. Machine will perform only when the N.C. tape is prepared and loaded and cycle start is initiated. 6.1. NC OPERATION CNC stands for Computer Numerical Control. An N.C. system in which a dedicated stored program computer is used to perform basic control functions. The functions of a CNC Controller are: 1. To read and store programme information. 2. To interpret the information in a logical command sequence. 3. To control the motion of the machines mechanical members. 4. To monitor the status of the machine. The interpretation of programme commands by a machine control unit and its conversion of those commands into machine motion is complex. The basic elements and operation of a typical NC machine are shown in Fig. 6.1. The functional elements in numerical control and the components involved follow: a. Data input: The numerical information is read and stored in the tape reader or in computer memory b. Data processing: The programmes are read into the machine control unit for processing. c. Data output: This is information is translated into commands (typically pulsed commands) to the servomotor (Fig. 6.2 and 6.3). The servomotor then moves the table (on which the work piece is mounted) to specific positions, through linear or rotary movements, by means of stepping motors, leadscrews, and other similar devices.
  5. 5. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/6/5 he he k k lic lic COMPUTER NUMERICAL CONTROL C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Computer: Input command, Processing, Output command Limit switches Position feedback Drive signal Figure 6.1. A schematic illustration of the major component of a computer numerical control machine tool Work table Pulse train Stepping motor Gear Lead screw Figure 6.2. An open-loop control system for a numerical-control machine
  6. 6. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/6/6 he he k k lic lic COMPUTER NUMERICAL CONTROL C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Work table Input Dc Comparator DAC servomotor Gear Feedback signal Lead screw Position sensor Figure 6.3. A closed-loop control system for a numerical-control machine 6.2. INDUSTRIAL APPLICATION 6.2.1. Metal Machining Lathes of all types Milling Machines of all types Drilling Machines Jig borers Electric Discharge Machining (including wire cut machines) Laser cutting machines Machining centres Turning centres All types of grinding machines Gear cutting machines 6.2.2. Metal Forming Punching and nibbling Guillotines Flame cut and profiling Folding
  7. 7. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/6/7 he he k k lic lic COMPUTER NUMERICAL CONTROL C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Pipe bending Metal spinning 6.2.3. Finishing Plating Painting 6.2.4. Assembly Joining - Pick and place robots, spot and seam welding machines and robots, riveting, looming of wires and assembly of components into printed circuit boards. 6.3. CNC AXIS CONVENTIONS CNC axis classification follows the three-dimensional Cartesian coordinate system and is established in BS 3635: 1972: Part 1. Fig. 5.3 shows the tree primary axes and the associated rotational axes. Most machines have two or three slide ways placed at right angles to one another. On CNC machines each slide is fitted with a control system, and is identified with either the letter X, Y or Z. Conventions have been adopted as to the naming of each axis. The axis of the main spindle, whether it is the axis of the tool spindle or the axis about which the work piece rotates is called the Z axis. The X axis is the motion of the largest travel of the primary movement (in case there is more than one).
  8. 8. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/6/8 he he k k lic lic COMPUTER NUMERICAL CONTROL C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c The Y axis then makes the third motion and is the shorter primary movement. In addition to these primary linear axes, there is provision for Rotary axes. They are designated A, B and C, with A rotary about the X axis, B rotary about the Y axis, and C rotary about the Z axis. It is often required to command a motion parallel to X, Y or Z axes within the realm of a secondary motion, or a tertiary motion within special automatic cycles such as describing the amount of finish allowance on a turned part, or to describe the distance of advancement of a drill during a drilling cycle etc. etc. Table 6.1. NC axes Linear Axes X Y Z Rotary Axes A B C Secondary Linear U V W Interpolation I J K Tertiary motion codes differ considerably, but the address characters variously used are P, Q, R, D, L, E, and H. The z-axis is parallel to the main spindle of the machine. It will be horizontal on a lathe or horizontal machining centre and vertical on a vertical machining centre. The x-axis is always horizontal and at 90o to z. The y-axis is at right angles to both the x and z axes.
  9. 9. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/6/9 he he k k lic lic COMPUTER NUMERICAL CONTROL C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c spindle rotation table z x y Figure 6.3. CNC axes 6.4. NC MACHINE SUB-UNIT We have already seen the many and varied applications of numerical control to the manufacturing and other industries, now we will look at the methods of controlling machines. There are three sub units to study: The machine tool itself. The control unit. The control system.
  10. 10. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/6/10 he he k k lic lic COMPUTER NUMERICAL CONTROL C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 6.4.1. The Machine Tool A machine tool is a device designed to cut away surplus material and leave a component of the required shape and size. To do this a machine tool must be capable of: - Holding the work piece securely - Holding the cutting tool securely and driving it with suitable power. - Moving the tool and work piece relative to one another precisely enough to achieve accuracy of size and surface finish. In addition, provision must be made for altering the spindle speed and feed rates, tool changing, supply of coolant etc. On a conventional machine an operator controls these functions and sets or alters them when he considers it necessary, the decision resulting from his training, skill and experience. Obviously, the machine settings may differ between operators as will the time taken to read scales, set positions, change tools, alter speeds and feeds, engage drives and set up the work piece etc. CNC Automatic Control can be applied to these functions and so result in consistent and reduced machining times through optimised cutting data, fast accurate positioning between cuts and fast automatic tool changing. 6.4.2. The Control Unit The CNC Machine Control Unit (MCU) has to read and decode the part programme, and to provide the decoded instructions to the control loops of the machine axes of motion, and to control the machine tool operations.
  11. 11. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/6/11 he he k k lic lic COMPUTER NUMERICAL CONTROL C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c The main grouping of parts of a control could be considered to be: The Control Panel. The Tape Reader, The Processors The first part of the control panel is the human interface that allows various modes of machine or control operation to be initiated, from switching on and homing, to programme loading and editing, to setting work positions and tool offsets, manually controlled movements and commencing the automatic cycling of a programme. Information about machine status and condition is available to the operator via VDU screens, gauges, meters, indicator lights and readouts. The tape reader is the device used to transfer the programme information contained on a programme tape into the control unit. Most tape readers are of the photo-electric type which offers high speed reading with reliability and accuracy providing the tape is in good condition and the reader is kept clean and free of paper dust particles. The processors within a control are the electronic circuits that permit conversion of part programme data into machine motions and they may be classified into two main sections. The data processing unit and the axis control processor. The function of the data processor is to decode the commands of the part program, process it and provide data to the axis control processor which then operates the slide drives and receives feedback signals on the actual position and velocity of each axis.
  12. 12. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/6/12 he he k k lic lic COMPUTER NUMERICAL CONTROL C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c The Data Processing Unit includes the following functions: i. The input device, such a tape reader. ii. Reading circuits and parity checking logic. iii. Decoding circuits for distributing data to the controlled axes iv. An interpolator to supply velocity commands to the axes, either singly or in combination. The axis control processor consists of the following circuits: i. Position control loops for each and all axes. ii. Velocity control loops. iii. Deceleration and backlash take up circuits. An MCU is adaptable to virtually any machine, the differing control motions and codes being a result of the way the control has been programmed. This permanent resident program is known as an executive programme and resides in the read only memory (ROM) of the control, whereas the N.C. programme resides in the Random Access Memory (RAM). RAM allows external access and alteration if necessary, while ROM is programmed by the manufacturer and cannot be accessed through the control keyboard. 6.4.3. Control System There are two types of control systems used on NC machines. The point-to-point system and the continuous-path system. Point-to-point systems are not so common these days, but they operate only in straight lines, which are suitable for positioning moves on a drilling machine or limited use on a lathe or milling machine, where at best 45% cuts are possible with two axes running continuous path controls allow angular
  13. 13. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/6/13 he he k k lic lic COMPUTER NUMERICAL CONTROL C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c path and radius motion because the control interpolator has the ability to move the axis drive motors at varying velocities. The point-to-point controls were NC controls, while the continuous path controls could be NC or CNC controls. NOTE: NC is a general term used for Numerical Control and is also a term used to describe controls that run directly off tape. CNC is a specific term for Computer Numerical Control. CNC Machines are all NC machines, but NC controlled machines are not CNC machines. 6.5. PROGRAM INPUT Programmes can be produced and entered (loaded) by any of the following methods where available. a. Punched Tape b. Computer c. Direct Input 6.5.1. .Punched Tape Punched tapes may be made of paper or plastic (Mylar) and have a standard width of one inch (25.4mm) for the eight track (8 bit) format used in numerical control. A tape punch unit is connected to a teletype or similar keyboard and produces a punched tape during typing of the program. Alternatively, a punch unit can be connected to a personal computer (P.C.) and the completed programme punched. The holes punched in the tape form
  14. 14. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/6/14 he he k k lic lic COMPUTER NUMERICAL CONTROL C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c certain patterns and the pattern represents a value when read by the tape reader on the MCU. The pattern is a code in itself, and complies to a standard, either E.I.A. ( Electronics Industries Association ) which uses an odd number of holes or I.S.O. ( International Standards Organization ) which uses an even number of holes for each character. The I.S.O. code is most commonly used in Australia. The E.I.A. code is known as an odd parity system, and the I.S.O. code as an even parity system. One of the tracks of each is assigned as a parity track and a hole is sometimes punched there automatically to maintain the parity. The purpose of parity is to check during tape reading for errors caused by unpunched holes, dirt or oil spots etc. 6.5.2. Computers Personal computers can be used to type the programme in its entirety while being visible on the screen, so mistakes can easily be spotted and corrected before the programme is loaded into the machine. The programme can be down loaded to the machine via a connecting link (interface cable) or via punched tape if a punch unit is connected to a computer. 6.5.3. Direct Input Programming at the machine may eliminate the need for tape punching equipment and computers, but the machine is usually non- productive during this time. Programming can be done by several methods, such as ;
  15. 15. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/6/15 he he k k lic lic COMPUTER NUMERICAL CONTROL C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c - by typing the program directly into the memory of the M.C.U. through the edit function. - programmes produced in this way can include all functions available, are stored in the machine ready for use at any time and can be output to a punch unit or computer for external storage. 6.5.4. By Manual Data Input (M.D.I.) M.D.I. may not allow all programming functions to operate, the programme can only be used once, and as it is not stored in the main memory cannot be output to an external device. Additionally, some controls allow only one line (block) of programme to be entered and executed at a time. 6.5.5. Interactive Programming. Some controls allow programming by a method that may be simpler and speedier than conventional N.C. program language. These methods usually take up considerable memory space and so fewer programmes could be stored in the M.C.U. than if they were prepared by N.C. language. Most of the controls allow for external storage of these programs providing the necessary devices are at hand. Interactive programming can also be known as "conversational", "symbolic", "blueprint","IGF" etc.
  16. 16. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/6/16 he he k k lic lic COMPUTER NUMERICAL CONTROL C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 6.5.6. . External Devices. A programme can be loaded directly from a P.C. as described, or from other specialised units that have been designed as a portable device for loading, transferring or storing N.C. programs. The devices may store the information on floppy discs, hard discs, magnetic tape, or through solid state circuitry. Those with discs or magnetic tape would be no larger than a briefcase or perhaps a tissue box, down to pocket calculator size for solid state devices. NOTE: Where punched tape was once the only practical way for most programmers to transfer (or load) their programs into the controls, it is presently being overtaken by personal computers connected directly via interface cable. Other popular and convenient methods are simply through the edit mode or by interactive (conversational) programming. Companies specialising in complex die work probably use DNC more often now. Inputting through M.D.I. mode is common providing the restrictions noted are adhered to. 6.6. NC PROGRAMMING 6.6.1. Job Planning 1. Sketch the part. Add incremental or absolute dimensions. 2. Ascertain fixturing. Select fixtures which have minimal projections above the part. 3. Identify a set-up point. Locate the set-up point near: 1. A corner of the part 2. A spot above the fixture
  17. 17. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/6/17 he he k k lic lic COMPUTER NUMERICAL CONTROL C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Consider space requirements for: 1. Part loading and unloading 2. Tool change. 4. Plan operation sequence Mark sequence pattern of sketch. Test program data for accuracy. 5. Record necessary data for each movement of the table and tool on the program sheet. 6. Record instructions for Identify, specific: the machine operator. 1. Tools needed. 2. Speed and feed data 3. Tool change points 4. Console switch setting 6.6.2. Incremental The word "incremental" may be defined as a dimension or a movement with respect to the preceding point in a prescribed sequence of points. Each positioning move is described quantitatively in distance and in direction from a previous point rather than from a fixed zero reference point.
  18. 18. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/6/18 he he k k lic lic COMPUTER NUMERICAL CONTROL C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c In incremental mode all moves are with respect to the last position reached. N10 G91 Y N15 G01 X10.Y10.F300. N20 Y10. N25 X20. 40 N30 X10.Y20. 30 N35 X20-Y-30. 20 N40 X-10.Y-10. 10 N45 X-50. X N50 M02 10 20 30 40 50 60 6.6.3. Absolute The data in the absolute system describes the next location always in terms of its relationship to the fixed zero point. The zero point when used as a programme datum is known as the programme origin. The G90 code sets the control up in absolute mode. All moves are performed with respect to the axes zero.

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