Lab manual for manufacturing process

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LABORATORY MANUAL MANUFACTURING PROCESSES – 1 TA 201 LAB Department of Mechanical Engineering INDIAN INSTITUTE OF TECHNOLOGY KANPUR GENERAL INSTRUCTION 1. Every student should obtain a set of instruction sheets entitled manufacturing processes Laboratory. 2. For reasons of safety, every student must come to the laboratory in shoes. it is unsafe for the students to come to the laboratory wearing garments with parts that that hang about loosely. Students should preferably Use half-sleeve shirts. The Students should also ensure that floor around the machine is clear and dry (not oily) to avoid slipping. 3. An apron will be issued to each student. Students not wearing an apron will not be permitted to the work in the laboratory. 4. Instruments and tools will be issued from the tool room. Every student must produce his identity card for the purpose. Tools, etc. must be retuned to the tool room on the same day. 5. The student should take the permission of the Lab Staff / Tutor before handling any machine. 6. The student should not lean on the machine when it is working. 7. Power to the machines will be put off 10 minutes before the end of laboratory session to allow the students to return the tools. 8. Students are required to clear off the chips from the machine and lubricate the guides etc. at the end of the session. 9. Laboratory reports should be submitted on A4 size sheets. 10. Reports will not be returned to the students. Students may see the graded reports in the laboratory Manufacturing Processes I Page - 1 ME - Laboratory GENERAL INFORMATION This laboratory is aimed at providing an introduction to the Know-how of common processes used in industries for manufacturing parts by removal of material in a controlled manner. Auxiliary methods for machining to desired accuracy and quality will also be covered. The emphasis throughout the laboratory course will be on understanding the basic features of the processes rather than details of I constructions of machine, or common practices in manufacturing or acquiring skill in the operation of machines. Evidently, acquaintance with the machine is desirable and the laboratory sessions will provide adequate opportunity for this. LABORATORY EXERCISE I Turning (T) & NC Demonstration 6 hrs. II Milling 3 hrs. III Drilling and Fitting (D&F) 3 hrs. IV CNC Exercise 3 hrs. PROJECT The Project consists of conceptualization a device designing, planning of machining and other operations, fabrication of the components and assembly of the device. Before the final evaluation of the completed project a report has to be submitted. This report should contain general description of the completed project, design details, detailed drawings, and fabrication of the components, assembly I and testing, suggestions for improvements. I First turn : Project groups should be formed. II Second turn : Project groups name should be given to the tutor. III Third turn : Project discussion with Technical Staff / Guide with Material List. IV Fourth to sixth turn: Everything should be finalized during the 4th to 6th lab turn. So the work should start without any loss of time on the 7th lab turn. Number of components in the project should not be more than 25 (Excluding standard parts) LABORATORY EXERCISES SCHEDULE. EXERCISE POWER TRANSMITION Video Clip (10 MIN,) DRILLING CLASS LATHE MILLING & FITTING I A+B C D CNC DEMO. (60MIN.) 2 D C A+B 3 C+D A B CNC DEMO. (60 MIN.) 4 B A C+D 5 CNC EXERCISE NOTES: Each Section will be divided in to four groups (A, B, C & D). th th 6 to 12 Projects. Manufacturing Processes I Page - 2 ME - Laboratory MACHINING PROCESSES Machining is one of the processes of manufacturing in which the specified shape to the work piece is imparted by removing surplus material. Conventionally this surplus material from the workpiece is removed in the form of chips by interacting the workpiece with an appropriate tool. This mechanical generation of chips can be carried out by single point or multi point tools or by abrasive operations these are summarized below: Machining Processes I Single point tool operations Multi-point tool operations Abrasive operat:ons I I I 1. Turning 1. Milling 1. Grinding 2. Boring 2. Drilling 2. Lapping 3. Shaping 3. Tapping 3. Honing 4. Planing 4. Reaming 4. Super-finishing 5. Hobbing 6. Broaching 7. Sawing The process of chip formation in metal cutting is affected by relative motion between the tool and the workpiece achieved with the aid of a device called machine tool. This relative motion can be obtained by a combination of rotary and translatory movements of either the tool or the workpiece or both. The kind of surface that is produced by the operation depends on the shape of the tool and the path it traverses through the materials. When the workpiece is rotated about an axis and the tool is traversed in a definite path relative to the axis, a surface of revolution is generated. When the tool path is parallel to the axis, the surface generated is a cylinder as in straight turning or boring operations. Similarly, planes may be generated by a series of straight cuts without rotating the workpiece as in shaping and planning operations (Fig.3). In shaping the tool is reciprocating and the work piece is moved crosswise at the end of each stroke. Planning is done by reciprocating the workpiece and crosswise movement is provided to the tool. Surface may be machined by the tools having a number of cutting edges that can cut successively through the workpiece materials. In plane milling, the cutter revolves and moves over the work piece as shown Fig. 4. The axis of the cutter is parallel to the surface generated. Similarly in drilling, the drill may turn and be fed into the workpiece of the workpiece may revolve while the drill is fed into it (Fig.5). The machine tools, in general, provide two kinds of relative motions. The primary motion is responsible for the cutting action and absorbs most of the power required to perform the machining action. The secondary motion of the feed motion may proceed in steps or continuously and absorbs only a fraction of the total power required for machining. When the secondary motion is added to the primary motion, machine surfaces of desired geometric characteristics are produced. . Consider a situation where both the cutting motions as well as the feed motion (provided at the end of each stroke) are rectilinear but perpendicular to each other. Here the machined surface produced is a plane. The line generated by the primary motion (cutting motion) is called the generatrix, while the line representing the secondary motion (feed motion) is called the directrix (Fig. 6a). Depending upon the shapes of the generatrix and the directrix and their relative orientations. Various geometries can be produced on the workpiece. Consider another case when the generatrix is a circle and the directrix is a line perpendicular to the plane of the generatrix. It is clear that in this situation the surface produced will be a cylinder (Fig. 6b). A tapered surface can be produced by merely changing the angle that the directrix makes with the plane of the generatrix. When the directrix is in the plane of the circular generatrix (Fig. 6c), lines are generated which results in a plain surface when a number of generatrices and directrices are placed side by side in the direction perpendicular to the plane of the generatrix. In actual practice, the cutting is performed by cutting edge and not a point. Thus,a series of generatrix directrix combinations are involved and the relative motion produces a surface rather than a line. Basically there are two methods of producing new surfaces, the tracing method and the generation method. In the tracing method the surface is obtained by direct tracing of the generatrices and when the surface produced is the envelope of the generatrics the process is known as generation. Figs. 6(a) & 6b), the plane and the cylindrical surfaces are obtained by direct tracing, while in Fig. 6(c) the final surface geometry is the envelope of the generatrices. Manufacturing Processes I Page - 3 ME - Laboratory Fig. 1 Straight turning Fig. 2 Straight boring Fig. 3 Shaping and planning Fig. 4 Plain milling Fig. 5 Drilling Fig.6 Concept of generatrix and directrix. (a) Rectilinear generatrix and directrix. (b) Directrix perpendicular to the plane of generatrix. (c) Directrix in the plane of generatrix. Manufacturing Processes I Page - 4 ME - Laboratory INTRODUCTION TO LATHE MACHINE AND EXERCISE ON TURNING PART (A) OBJECTIVE : To study the characteristic features of lathe. OUTLINE OF PROCEDURE i) Run the machine at low speed and observe the motions, which control the shapes of the surfaces produced. Note particularly the features, which control the geometrical form of the surface. ii) Learn the names of the major units and the components of each machine. Record these details (Table A). (Please ensure that the main isolator switch is off and check that the machine. cannot be inadvertently started. Do not remove guards). Use the manufacture's handbook for details that cannot be inspected. iii) Record the obtainable speed and feed values (Table B). iv) Note down the special features of the speed and feed control on each machine. v) Pay attention to the following : a. Size specification of various machine tools. b. Machine tool structures and guide ways I slide ways. c. Drive mechanism for primary (cutting) motion. d. Drive mechanism for secondary (feed) motion. OBSERVATIONS : (a) Record the following in a tabular form: Machine Tool Specifications (Table A) Type of Type & Speed given to Feed given to Machine Size Surface Make Produced Tool Work Tool Work lathe Speed and Feed Data (Table B) , ..,' , No. Lathe Speed Feed 1. 2. 3. 4. 5. 6. 7. 8. (b) Plot the lathe speeds against No. from Table B on a semi-log graph paper and show that the speed steps are in G.P. Manufacturing Processes I Page - 5 ME - Laboratory PART(B) OBJECTIVE : To make the part shown in the sketch from a mild steel rod on a Lathe. EQUIPMENT: List all tools and instruments used. OUTLINE OF PROCEDURE Hold the bar in a three jaw chuck and face the end with a right hand facing tool. Make central hole with a center drill. Repeat these' operations for the other end of the bar. Replace the chuck by a dog plate (Center plate) and hold the job in a carrier between centers. Turn the bar to the required diameter with rough cuts. Face the steps and finishes the diameters to the required sizes. Machine the roots and the groove with form tools. Machine the taper with the help of the cross-slide swiveling arrangement. Knurl the required surface. Cut the threads. OBSERVATIONS (a) Measure all dimensions (up to second decimal place) on the specimen turned by your group. Make a neat sketch and indicate all measured dimensions. (b) Discuss briefly how tapered portion was turned. (c) Show the calculation of the required gear ratio for thread cutting. (d) Sketch the main drive unit of 'the- lathe and show how the speed steps are obtained. Question Bank for laboratory quiz (ME Lab.) All answers must be brief and to the point. Sketches must be neat and self-explanatory. 1. What are the units of cutting speed and feed on machine tools? 2. What is the use of back gear arrangement in a lathe headstock? 3. How is the rotation imparted to a part, which is to be turned between centers? 4. What are the different ways of mounting the work on a lathe? 5. What is the use of Center drilling? 6. What is the function of chasing dial? 7. What is the difference between the lead and the pitch of a multistsrt thread? 8. Calculate the gear ratio between the spindle and the lead screw for cutting a screw with X threads per mm. The lead screw pitch is mm. 9. Why is the main spindle of a lathe hollow? 10. List the type of surfaces produced by turning. 11. Sketch the plan/top view of different types of cutting tools you have used during the lathe exercise and indicate their respective names. 12. What are the instruments for measuring the diameters of turned shafts? 13. What is the instrument normally used for-measuring lengths of various parts? 14. Explain how you can determine the taper angle of a taper pin. 15. Calculate the time required for single pass straight turning of a cylindrical bar (diameter D1 length L) at a spindle speed of N rpm and feed f in appropriate units. 16. Main scale of a Vernier has 10 divisions/cm and the least count of the instrument is 0.01mm. What should be the length of each division on the Vernier scale? 17. How are the spindle speeds changed? 18. What is the relation between spindle speed and cutting speed? 19. What is the relation between feed rate (mm/rev.) and spindle speed? 20. Are the back gears used to get lower or higher spindle speeds? 21. Draw figures of left hand and right hand turning tools. 22. How is the size of lathe specified? 23. Why are follow-rest and steady-test used? 24. What is Live Center and Dead center of the lathe? Manufacturing Processes I Page - 6 ME - Laboratory MILLING: INTRODUCTION AND PRACTICE PART (A) OBJECTIVE: To study the characteristic features of Milling machine. OUTLINE OF PROCEDURE i) Run the machine at low speed and observe the motions, which control the shapes of the surfaces produced. Note particularly the features, which control the geometrical form of the surface. ji) Learn the names of the major units and the components of each machine. Record these details (Table A). (Please ensure that the main isolator switch is off and check that the machine cannot be inadvertently started. Do not remove guards). Use the manufacture's handbook for details that cannot be inspected. jii) Record the obtainable speed and feed values (Table B). iv) Note down the special features of the speed and feed control on each machine. v) Pay attention to the following: a. Size specification of various machine tools. b. Machine tool structures and guide ways I slide ways. c. Drive mechanism for primary (cutting) motion. d. Drive mechanism for secondary (feed) motion. OBSERVATION : Record the following in a tabular form: Machine Tool Specifications (Table A) Type of Size Speed given to Feed given to Machine Type & Make Surface Produced Tool Work Tool Work Milling m/c Speed and Feed Data (Table B) No. Milling m/c. Speed Feed 1. 2. 3. 4. 5. Manufacturing Processes I Page - 7 ME - Laboratory PART (B) OBJECTIVE: To machine the hexagonal head and the slot shown in the sketch on the specimen, EQUIPMENT: List all tools / cutters and instruments used. OUTLINE OF PROCEDURE Fit the helical cutter on the arbor and the specimen between the centers of the dividing head and the tail center. Carefully adjust the work piece so that the cutter just touches the top surface of the specimen. Calculate the necessary depth of cut and then mill the six faces of the hexagonal head in succession. Change the cutter and mill the rectangular slot. OBSERVATIONS (a) Measure all dimensions (up to second decimal place) on the specimen milled by your group. Make a neat sketch and indicate all measured dimensions. (b) Explain in brief how the required indexing was obtained with the dividing head. (c) Explain up-milling and down-milling operations. Which one did you use for slot milling and why? (d) Explain the advantages of using a helical milling cutter. Question Bank for laboratory quiz (ME Lab.) All answers must be brief and to the point. Sketches must be neat and self-expanatory. 1. How is a milling cutter mounted? 2. What is the main difference between a horizontal and a vertical milling machine? 3. Explain what is meant by a universal milling machine. 4. Why are helical tooth milling cutters usually preferred over straight tooth cutters for slab milling? 5. Why is down milling generally avoided? 6. What are the advantages of up milling? 7. What special - attachment is needed in the milling machine to perform down milling? 8. In what respect does a slitting saw differ from a narrow milling cutter? 9. How are milling cutters generally classified? 10.What is the difference between a plain milling cutter and a side-milling cutter? 11.How does a universal dividing head differ from a plain dividing head? 12.When does a universal indexing become essential? Manufacturing Processes I Page - 8 ME - Laboratory SHAPING: INTRODUCTION AND PRACTICE PART (A) OBJECTIVE: To study the characteristic features of Shaper. OUTLINE OF PROCEDURE i) Run the machine at low speed and observe the motions, which control the shapes of the surfaces produced. Note particularly the features, which control the geometrical form of the surface. ii) Learn the names of the major units and the components of each machine. Record these details (Table A). (Please ensure that the main isolator switch is off and check that the machine cannot be inadvertently started. Do not remove guards). Use the manufacture's handbook for details that cannot be inspected. iii Record the obtainable speed and feed values (Table B). iv) Note down the special features of the speed and feed control on each machine. v) Pay attention to the following: a. Size specification of various machine tools. b. Machine tool structures and guide ways I slide ways. c. Drive mechanism for primary (cutting) motion. d. Drive mechanism for secondary (feed) motion. OBSERVATION Record the following in a tabular form: Machine Tool Specifications (Table A) Type of Speed given to Feed given to Type & Make Machine Size Surface Tool Work Tool Work Produced Shaper M/c. Speed and Feed Data (Table 2) No. Shaper M/c. Speed Feed 1. 2. 3. 4. 5. Manufacturing Processes I Page - 9 ME - Laboratory PART (B) OBJECTIVE: To machine a V-block as shown in the sketch out of the workpiece provided. EQUIPMENT List all tools and instruments used. OUTLINE OF PROCEDURE Hold the work piece in a vice and machine the bottom surface shown in the sketch. Invert the casting in the vice and machine the top surface till the desired height is obtained. Machine the inclined faces using right and left hand tools. Finally machine the groove. OBSERVATIONS (a) Measure all dimensions (up to second decimal place) on he specimen machined by your group. Make a neat sketch and indicate all measured dimensions. (b) Calculate the machining time for the bottom surface of the specimen. (c) Explain -the quick return mechanism. (d) Explain the use of clapper box on the machine. Question Bank for laboratory quiz (ME Lab.) All answers must be brief and to the point. Sketches must be neat and self-explanatory. 1. What is the driving mechanism on the shaping machine? 2. Why is quick return effect important? 3. What happens to the quick return ratio when the stroke length is reduced? 4. How is feeding done on a shaping machine? 5. Why is clapper box provided on a shaper? Manufacturing Processes I Page - 10 ME - Laboratory DRILLING AND FITTING PART (A) OBJECTIVE: To study the characteristic features of Drilling machine. OUTLINE OF PROCEDURE i) Run the machine at low speed and observe the motions, which control the shapes of the surfaces produced. Note particularly the features, which control the geometrical form of the surface. ji) Learn the names of the major units and the components of each machine. Record these details (Table A). (Please ensure that the main isolator switch is off and check that the machine cannot be inadvertently started. Do not remove guards). Use the manufacture's handbook for details that cannot be inspected. jii) Record the obtainable speed and feed values (Table B). iv) Note down the special features of the speed and feed control on each machine, v) Pay attention to the following: a, Size specification of various machine tools, b, Machine tool structures and guide ways I slide ways. c. Drive mechanism for primary (cutting) motion, d. Drive mechanism for secondary (feed) motion. OBSERVATION Record the following in a tabular form: Machine Tool Specifications (Table A) Speed given to Feed given to Type & Type of Surface Machine Size Make Produced Tool Work Tool Work Drilling m/c Speed and Feed Data (Table B) Drilling M/c. No. Speed Feed 1 2 3 4 Manufacturing Processes I Page - 11 ME - Laboratory PART (B) OBJECTIVE: To drill, file, as shown in the sketch, ream and tap holes on the mild steel plate. EQUIPMENT : List all tools and instruments used. OUTLINE OF PROCEDURE File all the sides of the mild steel work piece ensuring with a trisquare that all the angle are 90°. Mark the centers of the holes. Mark the circle for the diameters to be drilled. Punch at the center and at two points on the periphery of the required hole. Drill and ream the holes as required. Tap the hole using a set of three taps. OBSERVATIONS (a) Measure all dimensions (up to second decimal place) on the specimen made by your group. Make a neat sketch and indicate all measured dimensions. (b) Explain how power is transmitted from drill spindle to drill shank.' (c) Sketch a reamer and show its main features. (d) Explain why a set of three taps was used. Question Bank for laboratory quiz (ME Lab.) All answers must be brief and to the point. Sketches must be neat and self-explanatory. 1. To which elements (tool and work) the speeds and feeds are provided on 2. Lathe (ii) Milling machine (m) Shaper and (iv) Drilling machine. 3. What type of speed variation mechanism is provided in the drilling machines you have studies? 4. What material is generally selected for the machine tool structure? 5. What types of guides are used for the main sideways of the basic machine tools? 6. How are the sizes of various basic machine tools specified? 7. Why are square threads used on driving screws of machine tools? 9. Which of the following process are intermittent? (a) Milling (b) Drilling (c) Shaping (d) Turning 10. What makes the simultaneous rotation of the spindle and the feed motion possible on drilling machines? 11. What are the functions of flutes on a twist drill? 12. Explain how power is transmitted from the drill spindle the drill shank? 13. Why are drilled holes generally slightly larger than drill diameter? 14. What is the primary purpose of reaming? 15. Sketch a twist drill and name its principle pats. 16. Name the principal kinds of reamer. 17. How is the diameter of a drilled hole measured? 18. What is the approximate order of magnitude by which hole diameter increases after reaming? 19. What will happen when the drilling is done with dull drill? 20. What are the types tapes in a hand operated tap set? 21. What is a course file used for soft work materials? 22. Describe very briefly the important features of files? 23. Why the number of flutes on a reamer always even? 24. How is the drill held in spindle? 25. Why is the cutting fluid generally not used during drilling cast iron? 26. How are the files classified? 27. Distinguish between second cut and double cut files. 28. What is a scraper? 29. Does a saw blade cut on a return stroke? 30. What operation other than hole drilling can be performed on drilling machine? Manufacturing Processes I Page - 12 ME - Laboratory Overview of Numerical Control A new technique for controlling the machine / production tools, the Numerical Control (NC) was developed in mid 50's. Prior to this, all the machine / production tools were manually operated and controlled. Quality of the products produced by manually operated machines is totally dependent on the skills and mind status of the human operator. Numerical control machines are more accurate than manually operated machines, can produce components more uniformly, faster and in the long-run tooling costs are smaller but the initial investment is higher. Numerical Control (NC) has been defined by the Electronic industries Association (EIA) as 'a system in which actions are controlled by direct insertion of numerical data at some point'. This s -stern automatically interprets symbolic instructions (numerical dai3) to control machine tools and other manufacturing systems. Symbolic instructions or the numerical data required to produce a part is called a 'part program. Traditionally, in the NC machining, part drawing of the component to be machined is studied by the NC programmer who translates the information on the drawing to the necessary programme which issues operational instructions to the machine tools. The programme represents the path or action at every moment that the machine tool must take to properly machine the part as described by the engineering drawing. In the initial stages of NC development the programmed instructions stored on punched tapes where interpreted by electromechanical tape readers connected to the machine tool. The main problem with tapes was that it was very difficult to change the instructions on the tape. Even to make very minor instructions in the programme, new tape had to be made in addition to interrupting machining operations. Another draw-back of using the tapes was that they had to be run as many times as the number of components required, which decreased the life of tape. With rapid developments in computer technology and its capabilities, the problems associated with punched paper/plastic tape were solved. Rapid development in computer technology extended numerical control (NC) to direct numerical control (dNC), computer numerical control (CN C) and distributed numerical control (DNC). The problems faced by NC lead to the development of a concept known as direct numerical control (dNC) which eliminates the use of tape as a medium of carrying the programmed instructions. In dNC many machine tools are connected to a host computer through a data transmission link as shown in Fig. 15.13. Here, the NC programmes required to operate the machines are stored in the host computer and are fed to the machine tool connected through the data transmission lines. Even though dNC eliminates the use of tape, it will suffer whenever the host computer goes down. This shortfall of dNC led to the development of computer numerical control (CNC) which allowed NC machines at remote locations to be connected to the host computer. The development: of programmable logic controllers (PLC) and micro-computers lead to the development of computer numerical control (CNC). In CNC technology, each machine tool has a PLC or a micro-computer which allows the programmes to be input and stored at ea.ch and every machine. In addition, programmes can be developed off-line and down-loaded to the micro-processors/PLCs present at individual machine tools. Even though it rectified the problems associated with the down-time of host computer as in dNC, but a new problem known as data management came into picture. For example the same prorarnmme may get loaded on many different microcomputers Witch control the machine tools as there is no communication present among them. This problem of data management was solved by the development of distributed numerical control (DNC). Distributed numerical control (Fig. 15.14) \Vas developed by combining the positive point of both direct numerical control (dNC) and computer numerical control (CNC). Hence, in distributed numerical control (DNC) both host and local computers are present at individual machine tool. Here host computers are used as main storage devices and programmes are down-loaded to the mini-computers present at various machines where they are stored or transmitted to the NC machines. Mini-computer controllers also serve as back-up memory whenever Manufacturing Processes I Page - 13 ME - Laboratory the host computer is down. Therefore, the NC machines do not have to be down when the host computer is down. An effective data transmission network from the host computer to the micro-computer controls the NC machines and is key to the success of distributed numerical control systems. In rest of this book, NC is used as synonym for CNC. Various components present in NC machine tool are: Machine tool Machine Control Unit (MCU) Communication interface and accessories The machine tool may be any type of machine tool used in the manufacturing industry. Machine control unit (MCV) is the control unit that reads and interprets the numerical data/part programme from the tap or any other media and passes on this information in the form of electrical signals to various activators / drive mechanisms of the machine to operate the machine tool in the desired way. Numerical control Machines are classified based on the type of motion control, the presence of feed-back loops, the power drives the positioning system used and the number of axes of motion which can be controlled. Two types of motion controls are used on NC machines. They are point-to-point or continuous-path controlled. As the name implies, the point-to-point (PTP) controlled machines move in a series of steps from one point to the other point. A machine with PTP control can perform very limited number of machining opera ions. In almost all the cases where PTP control is used, its function is to move the machine table or the spindle to a desired position to perform the machining operation at that position/point. In general, no machining is performed when the movement from one point to other is taking place, hence movement is made as rapidly as possible. When there is no need of machining between two points, path to be followed between them is not specified. Continuous path machines move uniformly and evenly by controlling the motion of two are more axes simultaneously. Here, the paths to be followed along with the destinations have to be mentioned as the machining is invariably done along that path. Figure 15.15 shows the cutting paths followed by PTP and continuous path machines. Depending on the presence of the feedback, they are classified as open-loop system or closed-loop system (Fig. 15.16 and 15.17). In closed-loop systems, a transducer sends the current table position to the control unit and the driving unit (typically motor) stops running as the table reaches its desired position, hence accurate positioning can be achieved with closed-loop control systems. Closed-loop NC systems use servomotors where as open-loop systems use stepper motors. NC machines are classified into three types: hydraulic, pneumatic or electric based on the type of power used to drive the axes and spindle. Most of the modern NC machines use electric drives (AC or DC servo motors) to drive its axes and spindle because of its compact size, ease of control and low cost. Manufacturing Processes I Page - 14 ME - Laboratory Depending on the positioning system used on the machine, they are classified as incremental or absolute positioning machines. Modern NC machine tools allow the programmer to choose either of the above mentioned positioning systems or both of them together through part programme. In absolute positioning system, the end point of the tool for a particular move has to be mentioned in the programme with respect to the origin. But in incremental system, the end point of the tool for a particular move has to be mentioned with respect to the current tool position. Origin is a reference point used within the NC part program as the basis of defining tool location and other geometric entities. While developing an NC programme the programmer assumes that the tool is located at some specific point relative to the workpiece to be machined, and this point is designated as origin. In practice, during set-up and before starting the programme the operator has to move tile tool to a position which is designated as origin in that particular programme and then presses a button on control panel to select that as the origin. It means that the origin could be any where within the machine work space, hence it is called a floating zero. In the older generation NC machine tools, this facility was not available. The origin was a fixed point and the programmer had to write the programme with respect to that particular point. The purpose of any production system is to produce a part in the most economic way without compromising on the desired qualities. Many large and small industries have undertaken the conversion from manual machines and processes to NC to get benefits. Some of these are listed below: Flexibiiity is high Scheduling is easier Set-up lead and processing times arc less Greater uniformity and accuracy in cutting Lover overall tooling costs Low inventory Inspection cost is less Higher degree of interchangeability of parts and tools In spite of the above mentioned benefits, there are some negative points as well. The most prominent ones are: Large initial investments Programming costs Training and retraining costs for the existing work force Numerical-Control Programming 10.1 NC PART PROGRAMMING 10.1.1 Coordinate Systems In an NC system, each axis of motion is equipped with a separate driving source that replaces the hand wheel of the conventional machine. The driving source can be a DC motor, a stepping motor, or a hydraulic actuator. The source selected is determined mainly based on the precision requirements of the machine, as described in Chapter 9. The relative movement between tools and workpieces is achieved by the motion of the machine tool slides. The three main axes of motion are referred to as the X, Y. and Z axes. The Z axis is perpendicular to both the X and Y axes in order to create a right-hand coordinate system, as shown in Figure 10.1.A positive Motion in the Z direction moves the cutting tool away from the workpiece. This is detailed as follows: Z AXIS 1. On a workpiece -rotating machine, such as a lathe, the Z axis is parallel In the spindle, and the positive Motion moves the tool away from the workpiece (figure 10.2). 2. On a tool-rotating machine, such as a milling or boring machine, the Z axis is parallel to the tool axis, and the positive motion moves the tool away from the workpiece (Figures 10.3 and 10.4). Manufacturing Processes I Page - 15 ME - Laboratory 3. On other machines, such as a press, a planing machine, or shearing machine, the Z axis is perpendicular to the tool set, and the positive motion increases the distance between the tool and the workpiece. X AXIS 1. On a lathe, the X axis is the direction of tool movement, and the positive motion moves the tool away from the workpiece 2. On a horizontal milling machine, the X axis is parallel to the table. 3. On a vertical milling machine, the positive X axis points to the right when the programmer is facing the machine. The Y axis is the axis left in a standard Cartesian coordinate system. 10.1.2 NC Program Storage Media Modern CNC controllers provide several ways of transferring data. Perhaps the most typical data-communication methods used to transfer part program files is an RS-232C interface (see Chapter 8). An NC part program is stored in a file on a computer or a CNC controller. The file download (or upload) can be initiated by setting up a transfer mode on the CNC controller. On the other side of the communication cable is a computer that sends or receives data byte by byte. The operator must start and end the data-transfer process on both the CNC controller and the computer. Some machines use higher-level protocols to ensure an error-free data transfer. Two of the higher-level protocols used are Kermit and Xmodem. Kermit and Xmodem arc widely accepted in the computer-to-computer telecommunication file- transfer process. These protocols allow the file transfer to be controlled by either the computer or the controller. The computer can send and retrieve data directly. Some machines also provide local-area network (LAN) instead of serial communication. Ethernet and MAP are two technologies used. Some CNC controllers allow the entire controller function to be initiated from a remote computer through the data-communication network. 10.1.3 A BCD (binary-coded decimal) or ASCII (American Standard Code for Information interchange) code is frequently used in NC applications. BCD: An eight-track punched tape is one of the more common input media for NC systems. Hence, all data in the form of symbols, letters, and numbers must be: represent able by eight binary fields. The BCD code has been devised to satisfy this requirement. In a BCD code, the numerals 0 through 9 are specified using only the first four tracks, quantities 1, 2. 4, and 8. Note that the four numbers 1.2,4. and 8. added together as needed make all numbers from 1 to 15. Letters, symbols and special instructions are indicated by using tracks 5 through 8 in Manufacturing Processes I Page - 16 ME – Laboratory combination with the numeral tracks. A complete BCD character set, based on EIA Standard RS244A, is illustrated in Figure 10.6. Each BCD character must have all odd number of holes. By punching a parity bit along with all even bit strings, all characters have an odd number of holes. If an even number of holes is detected, it is by definition an error, and a parity check occurs. This simple method provides some: protection from input errors resulting in part damage. ASCII: ASCII was formulated to standardize punched-tape codes regardless of applications (Pressman and Williams, 1979). Hence, ASCII is used in computer and telecommunications as well as in NC applications. ASCII code was devised to support a large character set that includes uppercase and lowercase letters and additional special symbols not used in NC application Figure 10.6 illustrates the ASCII subset applicable to NC. Many new control systems now accept both BCD and ASCII codes. It is likely that the move toward ASCII standardization will progress as older NC equipment is replaced. 10.1.4 Tape Input Formats The organization of words within blocks is called the tape format (EIA Standard RS274) (Groover, 1980). Four tape formats are used for NC input (Pressman and Williams, 1979): 1. The fixed sequential format requires that each NC block be the same length and contain the same number of characters. This restriction enables the block to be divided into substrings corresponding to each of the NC data types. Because block length is invariant, all values must appear even if some types are not required. 2. The block-address format eliminates the need for specifying redundant information in subsequent NC blocks through the specification of a change code. The change code follows the block sequence number and indicates which values arc to be changed relative to the preceding blocks. All data must contain a predefined number of digits in this form t. 3. The tab sequential format derives its name because words are listed in a fixed sequence and separated by depressing the tab key (TAB) when typing the manuscript on a Flexowriter. Two or more tabs immediately following one another indicate that the data that -would normally occupy the null locations are redundant and have been omitted. An example of tab sequential NC code is T001 T01 T07500 T06250 T10000 T612 T718 T T EOB T002 T T08725 T06750 T T T T EOB T003 T T T T05000 T520 T620 T01 T EOB (T represents a tab character.) 4. The word-address format places a letter preceding each word and is used to identify the word type and to address the data to a particular location in thecontroller. The X prefix identifies an X-coordinate word, 30 S prefix identifies spindle speed, and so on. The standard sequence of words in 2t block for a three axis NC machine is. N word, G word, X word, Y word, Z word, F word, S word, T word, M word and EOB A word-address NC code is N001 G01 X07500 Y06250 Z10000 F612 5718 EOB N002 X08752 Y06750 EOB N003 Z05000F520 S620 M01 EOB A block of NC part program consists of several words. A part program written in this data format is called a G-code program. A G-code program contains the following words: N. G, X, Y, Z, A, B. C. I, J. K, F, S, T, R. M Through these words, all NC control functions can be programmed. An EIA standard. RS-273, defines a set of standard codes. However, it also allows for the customizing of certain codes. Even with this standard, there is still a wide variation of cod format. A program written for one controller often does not run on another. It is, therefore, essential to refer to the programming manual for the target machine before a program is written. In this section, before the meaning of each word is explained, we will first analyze the requirements of an NC control. 10.1.5.1 Basic requirement of NC machine control. To control a machine, it is necessary to begin by defining the coordinates of the tool motion. It is necessary to Manufacturing Processes I Page - 17 ME - Laboratory specify whether the motion is a positioning motion (rapid traverse) or a feed motion (cutting). 'The feed motion includes linear motion and circular motion. Linear motion requires the destination coordinates. When circular interpolation is used, the center of the circle must be given in addition to the destination. Before a cutting motion is called out, the spindle must be turned to the desired rpm and the feed speed must be specified. The spindle can rotate either clockwise or counterclockwise. Sometimes coolant is required in machining, and the coolant may be applied in flood or mist form. If an automatic tool changer is present, the next tool number has to be known to the controller before a tool can be changed to the machine spindle. The sequence to change the tool also needs to be specified. It is often desirable to aggregate a fixed sequence of operations such as drilling holes into a cycle. Using cycle codes can drastically reduce programming effort. Additional information is needed for specific cycle operations. Finally, there are other programming functions, such as units-inch or metric-positioning system-absolute or incremental, and so on. All of these activities can (and in some cases must) be controlled through the NC controller and related part program. These control functions and data requirements are summarized in what follows: (a) Preparatory functions: the words specify which units, which interpolator, Absolute or incremental Programming, which circular interpolation plane, cutter compensation, and so on. (b) Coordinates: define three translational (and three rotational) axes. (c) Machining parameters: specify feed and speed. (d) Tool control: specifics tool diameter, next tool number, tool change, and so on. (e) Cycle functions: specify drill cycle, ream cycle, bore cycle, mill cycle, and clearance plane. (d) Coolant control: specify the coolant condition, that is, coolant on/off, flood. And mist. (g) Miscellaneous control: specifies all other control specifics, that is, spindle on/of, tape rewind, spindle rotation direction, pallet change, clamps control, and so on. (h) Interpolators: linear, circular interpolation, circle center, and so on. These control functions are programmed through program words (codes). 1 0.1.5.2 NC words A specific NC function may be programmed using an NC word or a combination of NC words. All functions can be programmed in one block of a program. Many CNC controllers allow several of the same “word" be present in the same block. Thus, several functions can be included in one block, this is normally done by using a word- address format, which is the most popular format used in modern CNC controllers. The sequence of the words within one block is usually not important, except for the sequence number that must be the first word in the block. In order to make a program more readable, it is a good practice to follow a fixed sequence. Each word consists of a symbol and a numeral. The symbol is either N, G, X, V, and so on. Numerals follow as data in a prespecified format. For example, the format for an X word might he ".3.4:' which means three, digits before the decimal point and four digits after the decimal are used. The function of each NC word (code) and their applications are discussed in what follows: N-code. A part program block usually begins with an “N” word. The N word specifies the sequence number. It is used to identify the block within the program. It is especially useful for program editing. For example when the format is “4” a proper sequence number would be N00I0 It is a good practice to program N values in increments of 10 or greater. This allows additional blocks to be inserted between two existing blocks. G-code. The G-code is also called preparatory code or word. It is used to prepare the MCU for control functions. It indicates that a given control function is requested or that a certain unit or default be taken. There are modal functions and non modal functions. Modal functions are those that do not change after they have been specified once, such as unit selection. Non modal functions are active in the block where they are specified. For example, circular interpolation is a non modal function. Some commonly used G-codes arc listed in the Table 10.1. Some of these functions are explained in what follows. G00 is the rapid traverse code that makes the machine move at maximum speed. It is used for positioning motion. When G01. G02, or G03 are specified, the machine moves at the feed speed. G01 is linear interpolation; G02 and G03 are for circular interpolation. For circular interpolation, the tool destination and the circle center are Manufacturing Processes I Page - 18 ME – Laboratory Programmed in one block (explained later). G04 (dwell) is used to stop the motion for a time specified in the block. G08 and G09 codes specify acceleration and deceleration, respectively. They are used to increase (decrease) the speed of motion (feed speed) exponentially to the desired speed. Before an abrupt turn, decelerate the tool. Rapid acceleration in the new direction may cause a tool to break. The best accuracy can be obtained with acceleration and deceleration codes on and set to lower values. Most NC controllers interpolate circles on only TABLE 10.1 G CODES G00 Rapid traverse G40 Cutter compensation cancel G01 Linear interpolation G41 Cutter compensation left G02 Circular interpolation CW G42 Cutter compensation right G03 Circular interpolation CCW G70 Inch format G04 Dwell G71 Metric format G08 Acceleration G74 Full-circle programming off G09 Deceleration G75 Full-circle programming on G17 X-Y plane G80 Fixed-cycle cancel G18 Z-X Plane G81-89 Fixed cycles G19 Y-Z Plane G90 Absolute dimension program G91 Incremental dimension XY, YZ, and XZ planes. The interpolation plane can be selected using G 17, G18 or G 19. When a machine is equipped with thread-cutting capability, (G33-G35), the part program must specify the proper way to cut the thread. Codes G4O- G43 deal with cutter compensation. They simplify the cutter-center offset calculation. More details of cutter compensation are discussed later in Section 10.2.2. Most canned cycles are manufacturer-defined. They include drilling, peck drilling, spot drilling, milling, and profile turning cycles. The machine-tool manufacturer may assign them to one of the nine G codes reserved for machine manufacturers (GS)-G89). A user also can program the machine using either absolute (G90) or incremental (G91) coordinates. In the same program, the coordinate system can be changed. In order to simplify the presentation, most of the examples given in this chapter use absolute coordinate. Many controllers also allow the user to use either inch units (G70) or metric units (G71). Because hardwired NC circular in- terpolators work only in one quadrant and many CNC systems allow full-circle interpolation, a (G74) code emulates NC circular interpolation for CNC controllers. G75 returns the CNC back to the full-circle circular interpolation mode. X,Y,Z ,A, B and C-Codes. These words provide the coordinate positions of the tool. X, Y and Z define the three translational (Cartesian) axes of a machine. A. B. C are used for the three rotational axes about the X, Y, and Z axes. For a three axis there can be only three translational axes. Most applications only require X. Y. and Z codes in part programs. However, for four, five, or six-axis machine tools. A, B, and C are also used. The coordinates may be specified in decimal number (decimal programming) or integer number (BLU programming). For a controller with a data format of "3.4", to move the cutter to (1.12,2.275, 1.0), the codes are : X1.1200 Y2.2750 Z1.OOO In BLU programming, the programmer also may need to specify leading zero(s), or trailing-zero formats. A leading- zero format means that zeros must be entered in the space proceeding the numeric value. In this format, the controller locates the decimal point by counting the digits from the beginning of a number. In trailing-zero format, it is reversed. The number specified is in the BLU unit. The data format "3.4" implies that a BLU equals 0.0001 in. (fourth decimal place). By using the data from preceding example, the leading-zero program would be X.0112 Y002275 Z00l In the trailing-zero format, the program looks like X.11200 Y22750 Z10000 For circular motion, more information is needed. A circular are is defined by the start and end points, the center, and the direction. Because the start point is always the current tool position, only the end point, the circle center. And the direction needs to be specified. I, J. and K words are used to specify the center. Usually, circular interpolation works only on either X- Y, Y-Z. or X-Z planes. When interpolating a circular are on the X- Y plane, Manufacturing Processes I Page - 19 ME - Laboratory

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