FUNDAMENTALS OF CNC MACHINESCNC Technology: • If various functions of machine tools are controlled by means of prepared program, which consists of letters, numbers and symbols, then the machine tools are called Numerical Control of machine tools. The NC program consist of the following criteria: 1. Methodology o manufacture 2. The movement of machine tools 3. What tool is to be used? 4. At what speed? 5. At what feed? 6. To move from which point to which point in what path? Refer the Fig • • • • • The basic information are given to the. NC system in the form of part drawing along with the cutting process parameters and the cutting tools used. By using these, the part programs are written in the form of letters, numbers and symbols. This part program is entered in the punched tape. The program is then read by the punched tape reader. This numerical codes are translated by the Machine Control Unit (MCU) into a form so that the machine can understand and the motion of the machine tool are controlled. With the availability of powerful computers, the functions of machine control units are done by the computers. 1 • • • The part programs are written as a software using high level Languages. Since computer has enhanced memory capacity, the NC m operations are controlled completely by computer. This NC system controlled by the computers are known as Computer Numerical control (ON Here the punched tapes are replaced by floppy (or) CD ROM disc. To manufacture complicated and accurate parts in less time, CNC is used. The following are advantages of CNC machines over conventional machines 1. Accuracy is more and it is repeatable.. i.e. accuracy is kept in all ranges of speeds and feeds. . 2. Production time is less. 3. Complicated part can be manufactured 4. Highly skilled and experienced operator is not necessary. Since the operator can have more free time,. he can look after other machine operations also. Disadvantages 1. The price of CNC machine is very high. 2. Maintenance is complicated. Special care should be taken by specially trained people. 3. Well trained software professionals are needed for writing part programs. CNC control in machine tools: The principle of operation of CNC machine tools is shown in Figure. • • • In the CNC system, part program’s which is required for producing part is maintained in a floppy (or) CD ROM. The .separate software — part program — is developed for each part. Once this part program is loaded in the computer, it will control the operations of the machines. The part program contains all the information needed for processing a segment .of the work piece. The dimensional information like length, width 2 and radii of circles and the profile of linear and circular form will be taken from the part drawing. • Instructions are given separately for each axis of rotation (X, Y and Z). • The cutting speed, feed and auxiliary function like coolant on and off, spindle direction, gear change etc. are programmed according to surface finish and tolerance requirements. Basic components of CNC system 1. Program of instructions 2. Computer (Controller Unit) 3. Machine tool • • • • • • The program of instruction known as part program is developed by a part programmer. The part programmer must have knowledge and experience in mechanical engineering field with software developing skill. He must have the knowledge of tools, cutting fluids, fixture design techniques and various processes of engineering. He must be familiar with functions of CNC processes. He must be a skilled person in developing software in high level languages like C, C++ and special software like APT language and manual part programming language FANUC etc., This program should be made available in a floppy or CD ROM and it should be copied in the hard disc of a computer of CNC system. The computer reads the part program and decode it. The decoded instructions control the machine axis of motion and machine tool actions. So the computer interprets the program of instructions and convert it into mechanical actions of the machine tool. The computer controller unit includes 1. Data Processing Unit (DPU) with memory, Refer Fig (a). 2. Control loops unit which are connected to the servo motors to control the machine tool operation Refer Fig. (b) 3. Feed back device to make sure that the instructions have been properly executed by the machine. This feed back is sent back to the computer controller by feed back signals.. 3 • • • The control loop unit provides signals stating that the previous operation is completed and the DPU has to read new set of data. The control loop unit controls the driving motors of the machine lead screws (or) any other motion and receives feed back signals about the actual position and velocity of each one of the axes. The control loop unit circuit is shown in figure. The following automatic functions are carried out by CNC machines. • • • • • • • • • • • • • • The spindle of machine is started and stopped automatically. Spindle speed is controlled automatically. The tools are positioned at desired locations and guided along automatically. Feed rate of tool is controlled automatically. Tools in the spindle are automatically changed. Standard Axes The cutting tool movement with respect to work piece is known as axis of motion. This axis of motion is achieved by the movement of the machine tool slides. The X, Y and Z are the main three axes of motion. The main axes to be designated are the rectangular axes and rotary axes. The right handed coordinate, system is shown in fig. This is the coordinate system approved by International Standard 150fR841 to designate the axes of, their machines. The Z axis motion is set parallel to the spindle axis in the machines like drilling, vertical milling boring etc. In shapers and planers, the Z axis is set perpendicular to the surface of the work holding table. If the tool is moving away from the workpiece, it is considered as positive Z axis. So, in case of drilling machine, if the drill moves into the workpiece, then it is moving the desired path 4 in (— Z) negative direction. This set up will reduce the accidents if the part program is wrongly typed in the coordinate signs. • For other machines, the positive + Z motion increases the clearance between the work piece and tool holder. • The X axis and Y axis are set perpendicular to each other and they both together are perpendicular to Z axis i.e. X and Y axes are horizontal. • The X axis is normally parallel to the work holding surfaces wherever possible. • The X axis is from left to right when operator is facing the ONC machine. The positive X axis is towards right. • But for the turning machines like CNC lathe, it is parallel to the cross slide i.e. X is positive when the tool moves away from the axis of rotation of the work piece. • The Y axis is set perpendicular to both X and Z axes. • The Y axis motion is from front to back. The Y axis moving away from the operator is ± Y direction. Rotary motions • Refer Fig. (c) A, B and C are the positive rotary motions about X axis, Y axis and Z axis respectively. • These are positive in (— Z) negative direction. This set up will reduce the accidents if the part program is wrongly typed in the coordinate signs. • For other machines, the positive + Z motion increases the clearance between the workpiece and tool holder. • The X axis and 1’ axis are set perpendicular to each other and they both together are perpendicular to Z axis i.e. X and Y axes are horizontal. • The X axis is normally parallel to the work holding surfaces wherever possible. • The X axis is from left to right when operator is facing the CNC machine. • The positive (+) X axis is towards right. But for the turning machines like CNC lathe, it is parallel to the cross slide i.e. X is positive when the tool moves away from the axis of rotation of the work piece. 5 • • • The Y axis is set perpendicular to both X and Z axes. The Y axis motion is from front to back. The Y axis moving away from the operator is ± Y direction. In CNC machine tools, each axis of motion is rotated by a separate driving motors which replace the hand wheel in conventional. The driving motor can be a dc motor, a stepping motor or operated by hydraulic system. Feed Back Device • The product dimensions are checked by a feed back devices which are the part of the CNC system. • In conventional machines, the parts are produced by moving a cutting tool around a work piece by means of hand wheels, which are guided by the operator. • The complicated • contour cuttings are done by the expert operator with his experience and skill. But in case of CNC machine tools, skilled operators are not at all necessary. The semi skilled operators are sufficient to monitor only the operation of the machine, to insert the floppy (or) CD and usually to replace the workpiece. Functions of CNC The important functions of CNC are: 1. To control the machine tool. 2. To do the in-process compensation 3. To improve the programming and operating features 4. To diagnose the machine tool. 1. The Control of machine tool CNC system’s main function is to control the machine tool. This is done by decoding the part program into an understandable form of machine tool and carry out the machine tool motions with the aid of computer and servo system. 2. In process compensation In-process compensation means dynamic correction of the machine tool motions for any change or errors which occur during processing. 3. Improved programming and operating features: • • • • • • • The part program can be edited at any time. We can verify the tool path in the graphic display terminal. Circular, parabolic, cubic and linear interpolations can be done Old units and new units (S.1 unit) are supported. Specially written subroutines are used Data can be entered manually using Manual Data Input feature. More than one part program can be stored. 4. Diagnostics: • Diagnostics features are used in CNC machines in order to assist in maintaining and repairing the system. Due to this feature, 6 • • • • • The down time is minimised. The possible failures are felt. Some unreliable components are stacked as spares and these are known as redundant components. When any one of these components fails, the diagnostics will automatically disconnect the faulty component from the system, and activate the redundant component in place of faulty one. So, repairs can be corrected without any breaks in normal operations of the CNC machine. After getting corrected, the redundant com withdrawn from the system automatically so that newly installed component can perform its function. Classification of CNC systems: The CNC systems are classified based on the following. 1. Based an. Feed back control, it is classified as open loop and closed loop systems. 2. Based on Motion. Control, it is classified as positional(Point to point), paraxial and continuous path (Contouring) systems. 3. Based on Power Drive, it is classified as hydraulic, electric and pneumatic systems. 4. Based on Circuit Technology, it is classified as Analog and Digital systems. 5. Based on Positioning system, it is classified as absolute positioning and Incremental positioning systems 6. Based on axis identification, it is classified as 2-axis, 3-axis, 4-axis and 5-axis systems. Classification based on feed back control: The position and velocity of a machine slide is controlled by Servo Mechanism (servo motor). One data for each axis of machine motion is passed to servo system in order to drive the machine slides. Open - loop control system • If there is no feed back, device to compare the actual position of tool slide with the desired one, then it is known as open loop control system. • A control system in which the output value is not checked against the desired value, is known as open loop control system. • In this system, if the command signal to servo-motor is stopped, then it means that the desired location is reached. 7 • • • • • • • • • • The open loop systems are usually of digital type. They are the stepping motors to drive the machine slide. A stepping motor rotates, through a fixed angle in response to an input signal. It converts the digital electrical signals into proportional movement. For each pulse signal the stepper motor rotates exactly by 1.8 degree. So, 200 ‘electric pulse signals’ are required to complete one full revolution of motor i.e. 360°. Since there i no check on the machine slide position, the system accuracy solely depends on the stepping motor’s ability to step through exact number of steps provided at the input. Applications of open loop control system It is best suited for the following applications, where light duty machinery is required. when less money can be invested. when there is no need of much accuracy. where ‘less maintenance is required due. to absence of monitoring devices. Disadvantages of open loop control system • Due to wear and tear, back lash errors are found in lead screw. • The operator may feel that the slide has moved (say) 200mm, but actually, it may be only 199.8 mm. • To compensate the change in drive mechanism (or) to compensate the error, a periodical adjustments are required. . • It is less accurate. • Some of the above drawbacks can be avoided by introducing ‘SLO—SYN’ stepping mater which will run in exact synchronism with its input pulse signal. 8 Closed loop control system • If there is a feed back device to compare and correct the actual position of tool slide with the desired one, it is known as closed loop control system.. • • • • • • • • • A control system in which movement of the machine slide is checked and corrected with respect to input signals and mainly by the signals from feed back devices, is closed loop control system. There are two signals to drive a stepper motor. One signal is a command signal by which the servomotor is driven. Once the machine slide starts moving, another signal is generated by the position sensor known as Transducer. Transducer is used to indicate whether, desired position is attained or not. The above two signals are compared, in the Comparator Device known as Differential Analyzer and it produce new electrical signal proportional to the difference between the two. This new signal is sent to the servo motor through an amplifier to move the machine slide in a direction to reduce the difference’ (‘to reduce the error). This loop is repeated (‘do-looped’) until the difference between the input signal and feed back signal to be zero. At this point, the comparator output becomes zero and the machine slide occupies the correct position. The inertia effects of machine slide is also taken care o. by this closed ioop control systems.. 9 • • • • • • • In case of contouring system (continuous path system), another feed back transducer is used to control the velocity of the motor. It is required to control the cutter speed and feed so that the cutter follows the right path to get the required profile. These control systems have capabilities of up to 0.0001 mm and speeds up to 10 m/mm. So the closed loop control system measures the actual position of slide and its velocity x compares them it with the desired position and desired velocity respectively. The difference between the actual and the desired values is the error and this control system eliminates the error. This closed loop control system is used for obtaining higher degree of accuracy. Most of the contouring (continuous path) systems are controlled by this closed loop. Classification based on motion Control system The CNC machines are classified 1. Point to Point (Positional) motion control system 2. Straight-line motion (or) Paraxial control system. 3. Contouring (or) continuous path motion control system 4. Combined motion control system. Point to Point (Positional) control system (PTP) • Point to Point (PTP) is also called as positioning system. In PTP, the importance is given to move the cutting tool to a predefined location with respect to the work piece (or) to move the workpiece to a predefined location with respect to the cutting tool. • The speed (or) path of the cutting tool is not important here. Once the cutting tool 10 2: Flame cutting (or) torching and (3) Fabrication of intricate die templates. This also known as C-type motion control system. Interpolators: • • • • In any profile, the coordinates of points vary continuously. So each and every point of path of the profile should be defined; This tedious work is done by a subsystem of, computer namely Interpolator. Hence the interpolator will do the interpolation work. - he method of calculating intermediate points and speed of the motor is known as interpo1ation There are three types of interpolation: 1. Linear Interpolation 2. Circular Interpolation 3. Parabolic Interpolation • • By knowing extreme end points of straight line, calculating the intermediate points of straight line is known as linear interpolation. This is used for machining tapers. • For forming circles and arcs, circular interpolation is used. 11 • By knowing ‘end points and centre of arc’ (or) ‘radius and centre of circle’, calculating the intermediate points of arc (or) circle is known as circular interpolation. Absolute positioning system • If all the measurements are taken from a fixed origin with coordinates • X =0 ; Y= 0 and Z =0, then the system is known as Absolute Positioning system. • So all the position commands are given as absolute distances from the origin which is known a Zero Point. The origin can be defined in two ways: 1. Fixed origin and 2. Floating origin. • Fixed Origin is a fixed poir4t at the lower left corner of the worktable. • Here all the measurements are positive with respect to origin. Refer the following fig. 12 • • Floating Origin will be useful for symmetrical parts. Here, the centre of symmetrical part will be taken as origin and a the right side and upside measurements are taken as positive and left side and downward measurements are taken as negative. • • In this absolute coordinate system, the origin O(G,O,O) is stored in the computer memory and the control system returns the table to origin by simply pressing ‘Zero Reset’ button on the control panel. Here all the dimensions are independent of each other. This system is mostly used in PTP system. Incremental Coordinate system • In an incremental system: all measurements for the next position are calculated in the form of increments of distance from the previous position. 13 • This incremental system needs no predetermined datum point. This system is mostly used in CNC lathes. • • The coordinates of the workpiece in the absolute system is calculated in incremental system in the above fig. The coordinates of all the six holes calculated by both the systems are tabulated here. CNC controllers Machine Control Unit (MCU) is known as ONC controllers. The powerful computer has enhanced the capability of the machine control unit. In Olden days, MCU is a bigger unit but present days MCU is very smaller unit. By having tremendous computing power of computers, MCU become versatile for doing following functions easily and successfully. 1. Complicated Graphics profile can be machined and the corresponding part ptogram can’ be developed and edited. • • • • 14 2, Since computer has got more memory capacity, large part program of more than 1 MB can be stored. 3. Advanced part programming like ‘English Statements’ can be developed. 4. In case of parabolic and helical (or) even complicated profile defined by mathematical equation can be done by using complex interpolations. 5. Do loops are used for repetitive part programming; 6. Subroutines and macros are used. 7. Use of parameters in part programming. 8. There are help topics for operators. 9. Special geometric calculation facilities. 10. Better interfaces to outside world with serial and parallel communications. 11. Diagnostic facilitates. 12. Direct Numerical Control (DNC) functions are possible with links to factory networks. 13. Using Programmable Logic Control(PLC), a better shop floor control is possible. 14. Adaptive control, lead screw pitch error compensation, thermal l are possible proving enhanced the machine control. 15. Contouring control system with multiple axis machining simultaneously can be easily controlled. So the CNC controller — ie CNC Machine Control Unit (MCU) has number ‘of subsystem to perform the above functions. Communication (Butter memory) • MCU memory will not be sufficient when the part program is very large. • In such situations, a buffer memory in the DNC computer will store the part program and communication will be flowing from DNC to ON Macros Tool Management • Tool Management is very much important in running the CNC machines to have great accuracy of the parts machined and to have high productivity. Visual Display Unit (Monitor) • Monitors are very much useful for having interaction with the computers. • We can view the graphical displays, part programs etc. 15 Diagnostics • It is necessary to check the functions of the machine tool continuously to make the machine utilization high. • Hence, a number of sensors are normally inserted in the machine tools. These sensors will send the information to the controller so that any fault alreadypresent (or) likely to come in future will be identified and corrective action can be taken. • Such diagnosis is communicated as alarms to operator through Video Display Unit (VDU). • In some CNC controllers, a built in modem and a telephone line will send such information to the service people for corrective action. External Inputs: • This unit is useful for CNC to have interaction with the outside world. • The computer in CNC can be linked with DNC to down load part program, to have remote datum, and to link with Robots and Automated Guided Vehicles (AGV) etc. External Outputs • For sending output from CNC to outside world ie for works is uploading the generated or corrected programs, sending information to the controller such as certain PLO register status, diagnostic information etc. Graphic Proving • To verify the accuracy of the part generated, the machining process can be simulated on the ‘video •display unit’ of the MOU. Most modem controls provide a solid simulation process where the workpiece and the tool are actually shown in 3D shaded image: • When the tool moves through the work piece, the material is removed layer by layer. • The layer removed is shown in different colour to have a realistic, feel of material removal process. • This process is able to simulate uncut material at the corners because of tool radius, burr information etc more realistically. • By changing speeds and feeds, the surface finish generation process also can be simulated to get a feel of final surface finish which will be obtained on the component. Part programming Like English statement (Conversational Part Programming) • Part programs are developed in such a way that it can be read as English statement so that everyone can understand easily. • This type of part programming is as conversational part programming. Many of the controllers are available, with conversational part programming facility built in. FAPT-TURN is a conversational part program which uses vector drawing technique for making the turning part contour in 2D 16 Advanced Management • The modern CNC controllers provide additional facilities to carry out the activities like axis calibration, Adaptive control, thermal compensation and Lead screw pitch error compensation. Control Program • The control program controls the various elements of the CNC machine tool. • This program is useful for interactions between the control system components, the machine tool mechanics, tooling, material changes during machining etc. • This program reads and studies the part program a few lines ahead so that the speed of tool can be adjusted to cater to the future difficulties in the tool path like sharp changes in direction. • This control program is fully integrated with the servo and feed back systems involved. Direct Numerical Control (DNC system) • • • When several CNC machines are used, it is a common practice to use a Central Computer to connect all the CNC machines for preparing part programs at a central place and to transfer the same to the CNC system whenever required. This feature is called DNC system. So Direct Numerical control (DNC) can be defined as a type of manufacturing system in which several CNC machines are controlled remotely by a main frame computer. It is similar to many computers are controlled by server. In DNC system, one large computer can be used to control more than 100 separate CNC machines In a DNC system, the processing and post processing of part program is performed in a central computer. This enables the programmer to modify and edit the part programs easily. The tape reader is eliminated in DNC. Instead, the part program is transmitted to the machine tool directly from the computer memory. The DNC computer provides instruction to each CNC machines on demand. The 2 way communication interface option (11S232C) enables the user to immediately access any program from the library of programs in the central computer. Also., this link permits transmission of messages and other critical data. So DNC also involves data collection and processing from the machine tool back to the central computer. • • • • • • Component of DNC system 1. Central computer (or) Main frame computer 2. Bulk memory to store part programs 3 Communication Network. 4: CNC machine tools. • When the’ number of CNC machines are more and the computational requirements are more, it is necessary to make use of satellite computers. • These satellite computers relieve some of the burden off the central computer. • The part programs are received from the central computer and stored in the satellite computer and they are delivered to individual CNC machines whenever required. 17 • • Feed back data from the machines are also stored in the satellite computer and then delivered to central computer. So, the communication network connects the central computer with the lengthy cables to the CNC machines’ directly Refer fig (a), (or) it connects the central computer with each satellite ‘computer at each individual CNC machine. Refer fig (b). Types of DNC There are two types of DNC, depending upon the communication network between the computer and the CNC machine tool. 1. Special Machine Control Unit 2. Behind— Tape — Reader (BTR) system Special Machine Control Unit • In this configuration, the regular CNC controller is eliminated and it is replaced by a special MCIJ. This special MCU facilitates the communication between the CNC and computer. 18 • • • This type is very much useful in circular interpolation of the cutter path. It achieves superior accuracy of interpolation with fast metal removal rates. The special MCU is soft wired with flexibility. • • • • • In this system the central computer is directly linked with the GNU controller unit. Since the tape reader, is replaced by telecommunication lines, the name BTR has given to this system. The central computer is connected with the CNC controller by by-passing the tape reader (i.e. without tape reader) - i.e. behind the tape reader; The CNC controller has two storage buffers. One receives block of instruction from the DNC computer and convert them into machine actions and another one receives data from the CNC machine and it will be sent to DNC computer. BTR system cost is less since minor changes are enough to convert conventional NC into DNC. However, this BTR advantage is not a permanent one.- Because, most of the NC machines are nowadays CNC machines computer numerical control machines. So the CNC controller will serve the same purpose as a special MCU when incorporated with DNC system. • • • • Advantages of DNC 1. The punched tapes and tape readers are eliminated. 2. Enhanced computational capability and flexibility are possible in DNC. Part programming can be modified and edited at any time. Heavy computational work and data processing functions are performed efficiently. 3. Part programmes can be stored as a file in the computer memory. 4. The computer can give commands to many CNC machines simultaneously. 5. The production performance data from the CNC machines can be collected, processed, analysed and reported for further actions. 19 6. The DNC system makes the production• plant fully automated factory with the use of computers. 7. The central computer can be located any where. Combined DNC/CNC system: 1. The combination of DNC/CNC system eliminates the use of punched tape and tape reader. 2. The DNC computer downloads the part program directly tq the CNC computer memory. This reduces the amount of communication required between the central computer and each CNC machine tool. 3. The CNC controller unit has sufficient storage capacity to store the entire part program. 4. This combination creates redundancy. If the central computer (DNC) fails, the individual CNC machine need not be down. It is possible to provide necessary backup to make the CNC machines to operate on a stand-alone basis. 5. This backup capability consists of duplicate part’ programs contained in the DNC computer files. 6. This combined DNC/CNC system improves communication between the central computer and the shop floor. With the computer located at both ends of the factory, many of the constraints in- the factory managements information systems are removed. It becomes easy for computers to communicate with other computers than with hard wired devices 20