A Textbook of Production Technology PDF

716 A Textbook of Production Technology 3. Data Output. The control unit sends command signals to the drive units of machine tool and also to the Electrical control cabinet called “Magnetic Box”. Command signals sent to the drive units...

716 A Textbook of Production Technology 3. Data Output. The control unit sends command signals to the drive units of machine tool and also to the Electrical control cabinet called “Magnetic Box”. Command signals sent to the drive units of the machine tool, control the lengths of travel and the feed rates, while the command signals sent to the magnetic box control other functions such as : spindle motor starting and stopping, selecting spindle speeds, actuation of tool change, coolant supply etc. It is clear from above that MCU basically consists of two elements : Data Processing Unit (DPU) and the Control Loops Units (CLU). The DPU processes the data (in coded form) read from the tape or anyother source and passes the information regarding the various controls to CLU. NC is used to provide the following controls: — all cutting speeds — complete path and feed rates of a cutter in relation to the workpiece or fixture — all auxiliary functions such as turning cutting fluids off and on. The CLU operates the drive mechanisms of the machine tool, receives feedback signals regarding the actual position and velocity of each of the axes and signals the completion of the operation. The DPU reads and processes the data sequentially. When a line has completed execution as signalled by CLU, the DPU reads and processes the next line of the programme and so on. To check whether the required lengths of travel have been obtained, a feedback transducer is provided. The feedback transducer sends the information of the actual position achieved to the control unit. If there is any difference between the input command signal and the actual position achieved, the drive unit is actuated by suitable amplifier from the error signal. Manual control or operator control helps the operator to perform some functions manually such as : motor start-stop, coolant supply control, axes movements, speed change, feed change etc. 16.3. CLASSIFICATION OF NC MACHINES NC machine tools can be classified in different ways. Based on the type of power drive or the actuation system used, these are three common NC systems : 1. Electro-mechanical 2. Hydraulic, and 3. Pneumatic The range of the machines performance capabilities and its applications will normally depend upon the type of the power source. Hydraulic power provides the largest power. The movement of the machine slides is more uniform in speed. However, the drawbacks are : Higher cost, noise, hydraulic contamination from leaking fluid and the need for additional equipment such as reservoir, intensifier, valves etc. Pneumatic power is least expensive, because the shop air can be tapped for this purpose. However, the motion of the worktable is not uniform. Electro-mechanical drives are the most precise and the most commonly used power sources. The group of electro-mechanical, hydraulic or pneumatic equipment used to control the motion of an NC machine is called the “Servo”. Based on control system features, NC machines are classified as : 1. Point-to-Point system 2. Straight line system 3. Contour system Numerically Controlled machine Tools 717 These three systems are illustrated in Fig. 16.3. Y Y B Y 15 A 5 X 5 X 10 0 X (a) Point-to-point (b) Straight Line (c) Contour Fig. 16.3. NC Control System. In point-to-point system, the machining is done at specific positions. The working-piece remains unaffected as the tool moves from one position to the next. This system is the simplest. In Fig. 16.3 (a), after drilling the hole at position A, the tool moves to position B, along the dotted line. A drilling machine is the best example of point-to-point system. Straight line system, Fig. 16.3 (b) is an extension of point-to-point system. Here the tool moves at a controlled feed rate in one axis direction at a time. The examples of straight line system are; stepped turning on lathe, pocket milling etc. In contour or continuous path system, Fig. 16.3 (c), there are continuous, simultaneous and co-ordinated motions of the tool and the workpiece along different co-ordinate axes. The various profiles, contours and curved surfaces are machined by this system. Depending on the feedback, NC machine systems are of two types : 1. Open loop system 2. Closed loop system As discussed above, a command signal is sent to the machine tool to carry out a certain operation. In the open loop system, there is no ‘feedback’ and no return signal to indicate whether the tool has reached the correct position at the end of the operation, or not. Hence, there are no means of knowing whether there is an error or not between the input command signal and the result achieved. An example of open loop system is a co-ordinate drilling machine. Fig. 16.4 (a) illustrates an “open loop system”. The command signal from the MCU is given to the servo-motor (here a stepper motor). The motor is driven a precise angular rotation for every pulse issued by CLU. So, the response of the motor is in incremental steps. An incremental step of is common. This will result in a corresponding linear movement of the lead screw (depending upon its lead) and hence of the machine slide. In closed loop system, a ‘feedback’ is built into the system, which automatically monitors the position of the tool. From the feedback signal to the control unit, it is known whether the tool has moved correctly to its new position or not. If not, its position is automatically controlled until it is in the right position. Such a device is known as a position feed back transducer. Common position feed back devices used on modern NC machines are : shaft encoders, linear scales or Inductosyns. In the contour or continuous path system, velocity control is also essential to ensure that the cutter path is as required by the profile. In this system, a velocity feed back is provided by a transducer known as tacho-generator attached to the feed drive system. A closed loop system is more expensive than an open loop system. A block diagram of a closed-loop system is shown in Fig. 16.4 (b). Since, there is a continuous monitoring of the positions, the drive is a continuous-position device. These servomotors provide a smoother and continuously controllable movement of the work-table. 718 A Textbook of Production Technology Machine table Command signal (pulse train from MCU) Stepper Gear Amplifier Lead screw motor box (a) Open loop Machine table Comparator Command signal from MCU position sensor Amplifier Servo Gear motor box Lead screw Feed back signal (b) Closed loop Fig. 16.4. Control Systems. A.C. servo-motors have better reliability, better performance-to-weight ratio and lower power consumption. So, these are rapidly replacing the DC servomotor on new NC machine tools. 16.4. PROGRAMMING FOR NC MACHINES As discussed above, the complete information for producing a component on an NC machine, is punched on a paper tape. The information punched on the paper tape includes : 1. the co-ordinate values of the entire tool path. 2. the co-ordinate values are prefixed with certain codes indicating the type of movement of the tool (point-to-point, straight line, contour) from one co-ordinate to the next. 3. the co-ordinate values are also suffixed with certain codes indicating the various machine functions, such as, start/stop, spindle coolant etc. The co-ordinate values are also supplemented with other functions such as feed rates, spindle speeds, etc. Programming to obtain the punched paper tape can either be done manually or with the help of a computer. Simple point-to-point programs can be easily developed manually, but more complex ones, as well as almost all contouring programs are developed with the help of computers. Before making the part programme, the programmer first studies the part drawing and decides upon : the proper sequence of operations, the cutting tools, the path of the cutter/tool, speeds and feeds at various points and the other related information, such as starting and stopping of the machine etc. 16.4.1. Manual Programming. The first step is to establish the zero reference axes on the part drawing and determine the co-ordinate dimensions for each operation. Also, the set up instruments to establish the workpiece properly on the machine table with respect to the tool, are established. Then, all the data and the instructions are entered in a ‘program sheet’ in a particular format acceptable to the machine tool-control unit combination. This sheet includes the following information : the co-ordinate dimensions for each operation, the spindle traverse that determines the depth of cut, the spindle speed and feed, tool change. After preparing the ‘programme sheet’, the programmer uses it to prepare the punched paper tape on a typewriter-like tape punching machine. Numerically Controlled machine Tools 719 16.4.2. Computer Programming. To develop the program with the help of a computer, specialized programming languages, such as APT (Automatically Programmed Tool) have been developed to facilitate the production of punched paper tapes with relatively little knowledge of computers. The various steps in computer programming are : 1. Writing the ‘part programme manuscript’ using the simplified programming language. 2. Copying the part programme manuscript onto a deck of punched cards with the help of a keypunch operator. This is known as the ‘part programme’. On each card, a specific machine-tool instruction is punched. Punched Program tape manuscript Part program Magnetic tape Computer Conversion program equipment Computer Machine tool Postprocessor control program Fig. 16.5. Computer Programming. 3. The ‘part programme’ alongwith ‘computer programme’ and ‘post-processor programme’ are then fed to a general-purpose computer. The ‘computer programme’ is a previously prepared deck of punched cards, or tape, that contain instructions for the computer for executing the part- programme instructions, and also those from `post-processor programme'. The computer translates the ‘part programme’ into numbers and computing instructions and performs the calculations. The output of the computer, which is translated with the aid of ‘post processor programme’ into a form acceptable to the particular machine tool, may be in the form of a magnetic tape or a punched paper tape. If the output is in the form of a magnetic tape, it is converted into punched tape with the help of conversion equipment. The punched tape is now ready to be fed into machine tool control. The complete scheme of computer programming is shown in Fig. 16.5. APT the first programming language used for NC machine tools was developed in 1950. It used English type statements. After that, many subsets of APT and new languages have been developed. In APT-II, the complete job of part program preparation from the part drawing was undertaken by the computer. Then came the APT Long Range Program (ALRP). In 1965, ALRP was changed to Computer- Aided Manufacturing-I (CAM-I). The subsets of APT and its expanded forms and other new languages are given below : ADAPT (Adaptation of APT) :for smaller components. UNIAPT :handles full power of APT on a smaller computer. It differs from APT only in its internal design of processor. 720 A Textbook of Production Technology NELAPT :developed by National Engineering Laboratory in U.K. This language contains advanced features not available in APT. EXAPT (Extended APT) : was developed in Germany. It is both geometrically and technologically oriented. Its various versions are available : (i) EXAPT 1 : for positioning machine tools, e.g., drilling machines, (ii) EXAPT 2 : for turning centres. (iii) EXAPT 3 : for milling operations. AUTOSPOT (Automatic System for Positioning Tools) : was developed by IBM in 1962 for three axes, point-to-point motion control. COMPACT :This language was developed for simultaneous servising of multiple users from a remote computer over telephone lines. COMPACT - II :This is the latest version of COMPACT. This is available to the users on a time sharing basis. SPLIT (Sunstrand Processing Language Internally Translated) : This language can handle upto 5-axis positioning and possesses contouring capability. 16.5. METHODS OF LISTING THE CO-ORDINATES OF POINTS IN NC SYSTEM There are two methods of mentioning the co-ordinates of points while programming for an NC system : 1. Absolute System. 2. Incremental System. In the absolute system, the co-ordinates of points are mentioned with respect to one reference point, that is, datum. In the incremental system, the co-ordinates are written in the programme with respect to the previous point. Referring to Fig. 16.3 (a), the co-ordinates of points A and B, in the two system, are given as below : Absolute system X-Co-ordinate Y-Co-ordinate Point A 5 5 Point B 10 15 Incremental System Point A 5 5 Point B 5 10 16.6. APPLICATIONS OF NC MACHINES The major applications of NC machines are : 1. For parts, which are complex and it will not be possible to manufacture them very accurately on conventional machines, due to human error involved. 2. For parts which are frequently subjected to design changes. 3. Repetitive and precision quality parts which are to be produced in low to medium batch quantity. 4. In situations where the investment on tooling and fixture inventory will be high if the parts are made on conventional machine tools. 5. To cut down ‘lead time’ in manufacture. Numerically Controlled machine Tools 721 16.7. ADVANTAGES AND DISADVANTAGES OF NC MACHINES Advantages 1. Greater accuracy. 2. Lesser production cost per piece due to reduction in lead time and also set up time. 3. Improved product quality and provision of high order of repeatability. 4. High production rates as the machining conditions (feeds and speeds) are optimised and the non-machining times are reduced to a minimum. Blue print Part programming Programming Computer aided Manual programming programming post-processor Tape or disk Tape reader CNC Mini computer Memory NC machine tool Fig. 16.6. 5. Less scrap due to consistent accuracy and absence of operator errors 6. Reduced inventory of `parts in process' because parts can be made economically in smaller quantities. 7. Less operator skill is required to run NC machine. 8. Due to reduced idle time, the machine utilization is better. 9. Changes in part design can be incorporated very easily and at a low cost by simply changing parts of tape program. 10. Excellent reliability as the control equipment now is virtually all made of solid state modulus. 11. Lower tooling costs as expensive jigs and fixtures are not required. 12. Reduced cycle time and increased tool life. 722 A Textbook of Production Technology The major disadvantage of NC machines are their costs. Therefore, the machines must have sufficient use to justify the investment. Programming of NC machines has been greatly simplified and machine availability above 95% is common. 16.8. COMPUTER NUMERICAL CONTROL (CNC) AND DIRECT NUMERICAL CONTROL (DNC) Computer has been associated with NC right from the begining, and over the years this association has resulted in many new conceptions like : Computer Numerical Control (CNC). Direct Numerical Control (DNC), Computer Aided Design (CAD), Computer Aided Manufacture (CAM), adaptive Control (AC), and Integrated Manufacturing System. It is clear from the above discussion that in conventional NC systems, the instructions (program) stored on a paper tape (in a coded language) are read by tape reader. The instructions are processed by DPU and the command signals are controlled by CLU. There is no provision for storage or memory in MCU. For every new component, the tape needs to be loaded again, since the program is not stored in MCU and so can not be called back. The entire data input and data handling sequence including control functions are determined only by the fixed circuit interconnections of DPU and CLU. Any modifications will need a corresponding change in the circuitry of the system. Thus, the conventional NC system, which is also known as “Hard-wired system” is a rigid one. Reloading of tape each time a component is to be manufactured results in loss of time and sometimes errors in reading. Also, modifications and editing in the program is not possible. To overcome the above mentioned drawbacks of conventional NC systems, Computer Numerical Control (CNC) has been developed. Here, a bare minimum of electronic hardware is used and the functions of MCU of the conventional NC system are partly or wholly taken over by a dedicated micro-computer of the machine tool. The control program (called Executive programme) is stored which performs all the basic NC functions. Thus, in this system, the machine control data comes directly from a micro-mini- computer, instead of from a continuously read tape (as in conventional NC system). By changing or modifying the executive programme (through paper tape), the system can be used for another machine tool. Thus, this system is flexible or ‘soft wired system’. For repetitive production, the memory of the dedicated mini-computer can be utilized for temporary storage of the part programme being run. However, tape or disc is still the medium of transmitting the part programme from the programming computer to the dedicated micro-mini-computer of the machine tool, Fig. 16.6. However, the instructions (program) can also be entered in the memory of the micro-computer with the help of a key-board (known as direct programming). As computer-Aided Design (CAD) systems become more common generating data bases, the use of tape for storing program will decrease with the corresponding increase in the use of floppy disk, hard disk and other computer storage devices for this purpose. Again, computer-aided programming is being replaced by computer- aided manufacturing (CAM), where the programs for the NC machine tools can be generated on the graphic terminal directly from the data base created by CAD (CAD file), with the help of CAD/ CAM software. Advantages of CNC Systems over Conventional NC Systems 1. Because the computer can be readily and easily reprogrammed, therefore, the system is very flexible. The machine can manufacture a part followed by other parts of different designs. 2. More versatility. Editing and debugging programs, reprogramming; and plotting and printing part shapes are simpler. 3. Program to manufacture a component can be easily called. This saves time and eliminates errors due to tape reading. 4. Greater accuracy. Numerically Controlled machine Tools 723 5. Ease of operation. 6. The micro-processors used in place of hard-wired NC circuits are very reliable and have self diagonistic features. This makes trouble shooting extremely easy. 7. The dedicated computers need less memory storage and even small computers possess large memory. In DNC system, several NC machines can be controlled by a large central computer. Here, direct link may be established between the programming computer and the large central control computer. But, this system is very expensive and a highly skilled software knowledge is needed and can usually be justified only in large corporations or if the system is expanded to include process management and control (CAM and CAD) Fig. 16.7. The NC units used in the conventional DNC system may be : 1. of the conventional hard-wired type, with the tape reader being replaced by a direct communicating link to the Central Computer, or Blue print Part programming CAD Print display Stock Plant control, etc. Computer Management information Work monitoring Part program reporting schedule Mini computer NC machine tool Fig. 16.7. 2. Speicialized units which like CNC system, use a micro/mini computer as the MCU. DNC system has the main drawback that if the Central mainframe computer goes down, all the machine tools become inoperative. This drawback has been overcome by a more recent definition of DNC as “Distributed Numerical Control” in place of “Direct Numerical Control”, still abbreviated as DNC. In this system, a Central large computer is still used as in conventional DNC. But, the individual NC machines are not directly controlled by this central computer. Each NC machine has its own dedicated on-board micro-computer just like a CNC system. All these invidual micro- computers are linked to the central large computer, which serves as the Central control system. 724 A Textbook of Production Technology This arrangement overcomes the main drawback of the conventional DNC system, in that if the central computer goes down, the NC machines will not become inoperative. Moreover, this system provides a larger memory and computational capabilities and increased flexibility. DNC, system appeared in the market earlier than CNC system. but with the development of dedicated mini-computers, the benefits of DNC system can be realised in CNC system. Moreover, with the availability of small computers with large memory, micro-processors, and program editing capabilities, CNC machines are widely used at present. Also, the availability of low cost-programmable logic controllers (PLCs) has helped in the successful implementation of CNC systems. 16.9. MACHINING CENTRE A machining centre or work centre consists of a single, but sometimes, two machine tools with the specific feature of an automatic tool changer and capable of performing a number of operations (drilling, tapping, milling, boring and turning etc.) on a workpiece. Most machines are numerically controlled, but other types of controls will work as well. The major advantage is that the job needs clamping on the workholding surface only once; the machine then performs a variety of machining operations on all the job's faces except the base. Work handling time is thus decreased because there is no movement of the workpiece from one machine to another. Some machines are equipped with two work-tables that can be moved into or out of the machine. While the work is going on one works table, the next component can be set up on the other work-table. When the job is complete on the first work-table, it is moved out of the way and the second work-table is moved into position. For storing the preset tools, various magazine systems are used. These preset tools are removed from their slots by a hand-arm mechanism’. When a particular machining operation is completed, the tool is removed from the spindle and returned to its storage slot, then another tool is picked up and mounted in the tool spindle. A machining centre is mainly used for batch production of main components of a product. The main components ofa product are usually small (about 10%) but are expensive (they represent about 50% of the product value) because they have considerable material value and usually require a large amount of machining. For such components, the machining centre is generally most economical. Tool magazine Table Base CNC System Fig. 16.8 Numerically Controlled machine Tools 725 Machining centres have high metal removal rate capabilities. The high degree of accuracy and multi-operation in the same set up, make the machining centres highly versatile and increase productivity. Some models are made with manual tool change facility. Fig. 16.8 shows a typical machining centre. 16.10. ADAPTIVE CONTROL Adaptive control (AC) is another step towards true automation, wherein the vital element of decision making is added. As the component is being manufactured, the important variables are measured and then if need be, certain variables are altered within programmed limits, to get as accurate the finished part as possible. For example, in drilling, the torque on the drill is measured and speed and feed or both are adjusted within programmed limits. Similarly, in milling process, feed and/or speed and depth of cut are controlled in response to signals from torque, force, vibration and dimensional transducers. The limits set by the process, e.g., surface finish, maximum feeds, speeds and depth of cut, cutting force and torque; and those set by the machine tool, e.g., vibration and elastic deformation, are programmed. Adaptive control is still in its infancy, since, the effect of the various process variables on the finished part, is still relatively unknown. For the adaptive control to grow, process reliability has to be improved and also a more quantitative understanding of the process has to be gained. Another major problem is the on line measurement of the dimensional, without which the tool wear can not be compensated for with any great accuracy. PROBLEMS 1. Justify the need for NC machines. 2. Define NC 3. With the help of a diagram, explain the working of a NC machine tool. 4. List the main component of a NC machine tool and explain their functions. 5. Compare a closed loop system with an open loop system. 6. Classify the NC machines. 7. Discuss the programming of NC machines. 8. Give the applications of NC machines. 9. List the advantages and disadvantages of NC machines. 10. Define CNC and DNC. 11. List the advantages of CNC systems over the conventional NC systems. 12. List the main advantages of DNC. 13. What is “Distributed Numerical Control” ? 14. What is a machining centre ? 15. List the main advantages of a machining centre. 16. What is “Adaptive Control” ? 17. Write briefly, how numerical control works ? 18. Explain the main difference between point to point and continuous path type of numerically controlled machine tools. 19. Under what conditions of production the numerically controlled machine tools are employed ? 726 A Textbook of Production Technology 20. What is point-to-point type of path motion of NC ? For which machines this system is employed ? 21. What is straight line NC ? 22. What is continuous path NC ? 23. How manual programming of a NC machine is done ? 24. How computer programming of a NC machine is done ? 25. Discuss the control system of a NC machine. 26. Write the functions of MCU, DPU and CLU, in a NC machine. 27. Write on some of the programming languages used for NC machine tools. 28. Write on the methods of listing the co-ordinates of points in NC system. 29. What are "information sources" is an NC machine tool? 30. Write the information carriers in an NC systems ? 31. Write the aim of the manufacturing activity in the modern age of business competition? 32. How are the NC Machines classified? 33. On the basis of power drive, what are the various types of NC systems? 34. What is pneumatic control? 35. What is its drawback? 36. Write the advantages and drawbacks of hydraulic control. 37. What is electro-mechanical drive? 38. What is "Servo"? 39. Write on different types of feedback systems in NC machines. 40. What is the difference between NC and CNC machines? 41. When should an NC machines be used from the point of view of economy? 42. Write on : CNC scene in India. 43. What are PLCs? 44. What is Retro-fitting ? The medium and small scale industries (which play a significant role in the economy of developing countries) can ill afford NC machine tools, because of their very high initial investment. Their problem can be overcome by providing assessories to their conventional/ general purpose equipment so as to get the advantages of advanced CNC technology. Their performance is not equal to that of CNC machines, but is much higher than the conventional machine tools. This is know as "Retro-Fitting.  Chapter 17 Surface Finishing Processes 17.1 INTRODUCTION Whatever may be the manufacturing process, an absolutely smooth and flat surface can not be obtained. The machine elements or parts retain the surface irregularities left after manufacturing. The surface of a part is its exterior boundary and the surface irregularities consist of numerous small wedges and valleys that deviate from a hypothetical nominal surface (Fig. 17.1, which shows a surface on a highly magnified scale). These irregularities are responsible to a great extent for the appearance of a surface and its suitability for an intended application of the component. These surface irregularities are usually understood in terms of surface finish, surface roughness, surface texture or surface quality. Heat exchanger tubes transfer heat better when their surfaces are slightly rough rather than highly finished. Brake drums and clutch plates etc. work best with some degree of surface roughness. However, if a film of lubrication must be maintained between two moving parts, the surface irregularities must be small enough (smooth surface) so that they do not penetrate the oil film under the most severe operating conditions. The examples are : Bearings, journals, cylinder bores, piston pins, bushing, helical and worm gears, seal surfaces and machine ways etc. In gears, smooth surfaces are also necessary to ensure quiet operations. For components which are subjected to load reversals, sharp irregularities act as stress raisers constituting the greatest potential source of fatigue cracks. Therefore, the surfaces of components which are subjected to high stresses and load reversals are finished highly smooth. Fig. 17.1. A Profile of Surface Irregularities. Evaluation of Surface Roughness : As per B.I.S (Bureau of Indian Standards), it is assessed in terms of Centre Line Average (CLA) or Arithmatic Average (AA) and is denoted as Ra. Referring to Fig. 17.2, Arithmatic average or Centre line average is difined as : the average value of the ordinates from the center line (the centre line AB is located such that the sum of areas above the line is equal to sum of areas below the line), the algebra sign of the ordinates is not considered that is, 727 728 A Textbook of Production Technology z L 1 AA = CLA = Ra = y( x ) dx L 0 Where L = Roughness width cut-off or the sampling length. This is the maximum width of surface irregularities that is included in the measurement of rougness height. n ∑ |Y | 1 i Approximately, Ra = n Where n = number of vertical ordinates y1  y 2  y 3 ...... y n i.e. Ra = n A1  A2  A3 ..... An = L A = L Units of Surface Roughness: Surface rougness or surface finish is measured in microns or micrometres, 1 micron = 10–3 mm = 10–6 m = 1  m. The secondary manufacturing processes (the conventional metal machining methods, Chapter 8) are all finishing processes. The range of surface finish obtained with these methods for average applications is given in Table 17.1. To achieve better surface finish, other surface finishing processes such as lapping, b c A1 A5 a d Centre Line A3 Y1 Y2 Y3 Y4 A3 5 6 7 A Y5 Y6 Y7 B A2 1 2 3 4 e A4 g f L Fig. 17.2. Surface Roughness. Honing, Buffing and super finishing etc. are used. These processes will be discussed in this chapter. The typical ranges of surface finish obtained by these methods is given in Table 17.1.