Fundamentals and Applications: Computerized Numerically Controlled Machines - Essay Example

Computerized Numerically Controlled machines have been in use for almost 60 years for the manufacturing of simple and complex parts. Parsonsis considered to be one of the pioneering contributors for numerically controlled machines. Douglas Ross made pioneering efforts in developing automatic programming of NC machining. By developing APT, Ross removed the last impediment toward the broader use of NC for manufacturing industry as it resulted into the economic viability for myriad of industries from the view point of programming cost. The breakthrough with integrated circuits contributed enormously in the evolution of CNC machines. Nowadays in order to provide better online data transfer, CNCs can be connected to the internet. Companies having multi operations are benefitted a lot from this so that they can transfer their programmes to other locations. Today’s CNCs provide a very high level of automation. Any skillful operator can use all the features to increase his/her own as well as company’s productivity. Their capacity to deliver extreme precision has given a great boost to innumerable new state-of-the-art technologies as a multiplier effect to produce thousands of products in the last 30-40 years. The future of CNCs is extremely exciting and rewarding. It can be said that CNCs have been the back bone of engineering industries and will remain like that for future years to come. In the manufacturing and production sectors, automation using computers has become a common, if necessary trend. The application of Numerical Control (NC) and Computer Numerical Control (CNC) has seen production work become more effective in virtually all dimensions. With the three and five axis CNC machines, more complex shapes can be produced in real time, more accurately and without having to set and reset machines and workpieces over and over as I always the case with conventional machines. Having several structural components and coming in modular form, CNC machines are controlled using dedicated software and programs to automate and control its functions. Fundamentals and Applications of CNC Machines Computer Numerical Control (CNC) is the automated control of the machine tools by a computer program and a computer; the machine is controlled by a computer rather than a person (Mattson 2009, p.9). On the other hand, Numerical Control (NC) can be defined as a type of programmable automation that involves the use of a program of instructions to control the mechanical actions of machines or equipment. The program has set of instructions that include positioning of the workhead and workpiece and other instructions that are necessary to operate the machine. A workhead is the cutting tool and the workpiece is the object being processed. The application of numerical control is varied in many processes. Numerical control applications fall into two categories; (a) machine tool applications such as in milling, turning, drilling, and in other metal working, and (b) no machine tool applications such as in drafting, assembly and inspection (Groover 2008). Differences between CNC and NC Numerical control uses built in fixed logical functions and they are permanently wired into the control unit. The functions can not be changed by the machine operator or the programmer (Seames 2001). The system can interpret a program but it can not allow changes to be made. Numerical control also involves the compulsory application of punched tapes for program information input. Computer numerical control on the other hand involves the use of an internal micro processor, in this case a computer. The computer contains memory registers that store a variety of routines and are capable of controlling logical functions. As opposed to the numerical control, a machine operator or a programmer can change the program on the control and instant results achieved. The programs and logical functions used by CNC are usually stored on computer chips as software instructions. The logical functions are not controlled by hardware connections such as wires (Smid 2003, p1). Three-axis and Five-axis CNC Machines Three-axis CNC machines are quite similar to conventional machine tools in respect of the fact that all movements take place in the three axis only (Z, Y and X). They are always equipped with pallet and tool changers for high volume production. Fig 1: 3-axis CNC Machine Courtesy Parksmetal.com Five axis CNC machining defines the ability of the computer numerical control machine to perform simultaneous movement about five different axis according to Engineersedge (2010). In many cases, the five axis movement with the letters X, Y and Z representing the primary axis. The Z axis commonly refers to movement that is parallel to the tool spindle. The remaining two axes give the machine the ability to rotate about two primary axes; X and Y. The horizontal machine on the other hand has the x-y table, horizontal arbour, and rotary table. The five axis machine has greater advantage over its 3-axis counterpart given that it can make more complex shapes in one set-up. This in effect reduces machining time, errors, and setup time while increasing the rates of production (Engineersedge, 2010). Five axis machines are more accurate as machining is done based on a single datum point. Other advantages of such machines are related to their higher rigidity, higher cutting tool speeds, and reduced vibration. Some complex products that can be made include plastic mould tools turbine blades, and impellers (Engineersedge, 2010). Fig 2: 5-axis CNC Machine Courtesy Engineersedge.com Fig 3: Five axis Courtesy Engineersedge.com Figures 4 and 5: Complex machining and components Courtesy Engineersedge.com Basic Component and Features of CNC Machines A numerical system consists of the following basic components; (a) a program of instructions, (b) a machine control unit (MCU), and (c) processing equipment. The program of instructions directs the action of the processing equipment through detailed step-by-step commands. The program of instructions is referred to as part program and the individual who prepares the part program is referred to as a part programmer. Common methods used fro programming are manual and computer-aided part programming. The program of instructions is implemented by the MCU (Alavala, 2008). The machine control unit comprises of a microcomputer and related control hardware such as interface components and feedback control elements. The microcomputer stores the program of instructions and converts each command into mechanical action of the processing equipment thus executing an action. The processing equipment performs the actual productive work like machining and accomplishes the task through processing steps that transform the object being processed into a completed part (Gizelbach, 2009). The object being processed in this case is the starting workpiece. The CNC is usually built based on modular principles and the machine tool is formed by bolting together several units (Seames 2001). The base of the machine supports a two axes slide way system; the work table is located here. The third axis is created relative to the two axes. The machine headstock contains the motor drive system and the spindle assembly. The machining horizontal spindle configuration has more machining capabilities than the vertical spindle configuration. The features of CNC machines include: (a) the capability of the CNC controllers to store multiple programs, (b) their capability to create multiple data entries, (c) the program can be edited while still in the MCU computer memory thus a program tested and corrected at the machine site, (d) frequently used machining cycles can be stored and be executed by a part program, (e) the alignment of the machine tool on the machine tool table by use of machine tool axes is made possible by the software options in the CNC system, (f) cutter length and size compensation are made automatically in the computed tool path, (g) acceleration and deceleration calculations are achieved when the cutter path is abruptly changed; feed rate is decelerated in regard to the tool path change and accelerated after direction change, (Groover 2008), (h) communication interface that links the machine to computer driven devices and other computers, (i) diagnostic capabilities to detect any malfunction and system breakdown diagnosis, and (j) the control functions can be modified to meet the desired requirements thus allowing greater flexibility of adapting new options without adding them to the controller components (Nayaran 2008, p.271) CNC Machine Tool Axes Drives Ball Screws: The CNC has a ground precision lead screw with a recirculating ball bearing nut that converts rotating motion into linear motion. The recirculating ball reduces friction thus virtually eliminating backlash. Ball screws last longer and they provide high-speed capability (Gizelbach 2009). Re-circulating ball screws and nuts are used rather than lead screws and nuts. Lead screws and nuts exhibit backlash properties and high frequency and it leads to erratic motion, heavy wear and inaccurate positioning. Backlash is reduced by preloading assembly and future wear is reduced by backlash compensation techniques. Position Feedback Device Selection of a position feedback device is crucial in determining the accuracy of the CNC machine tool. Rotary and linear transducers are common in many modern CNC machine tools (Seames 2001). Rotary Transducer and Linear Transducer They are used on machine tools in which precision ball screws move the slides. Transducers monitor the ball screw angular position. There are inaccuracies due to translation errors between the linear movement and screw ball rotation. The precision is good for majority of applications. Rotary transducer operation is based on either optical diffraction (digital encoders) or electrical coupling (synchro-resolver). Synchro-resolver is a transformer. The range of electrical coupling is achieved by rotating one set of rotor relative to the stator. When AC voltage is applied and the two coils are parallel, the stator coil induces a maximum voltage in rotor cell. Inductive coupling varies with the stator and rotor coil angular displacement. Digital encoder produces a digital indication of the angular displacement. Rotary encoders are of two types; incremental and absolute encoders.Linear transducers measure the slide’s linear position directly and are fixed adjacent to the axis slide path (Kesavan and Ramanath 2006, p32). The Coordinate System Programming NC equipment requires definition of the standard axis system and through this the position of the workhead relative to the workpart is defined. Two axis systems that are based on Cartesian coordinate system are used in NC systems and they include those; (a) for flat and prismatic parts consisting of three linear axes, and (b) for rotational parts. All machinery movements are based on the Cartesian coordinate system and the system gives a better understanding of numerical control (Seames 2001, p26). The Cartesian coordinate system is made up of lines and/or planes that run vertically, horizontally and intersect with each other. The planes or lines are perpendicular to each other and they intersect at an angle of 90°. The system can be added with a third line or a plane that is perpendicular to the other two lines or planes. Any axis system for flat and prismatic parts comprises of three linear axes in the Cartesian coordinate system. The three linear axes are x, y and z. Majority of the machine tools use the x- and y-axes to move and put into position the worktable that the part is attached. The other axis (x-axis) vertically positions the cutting tool. The position scheme described is used in most simple NC applications such as punching of flat metal sheets and drilling. The machine tools are programmed by defining a sequence of x coordinates. Right-hand rule is used to distinguish negative from positive angles. The rule involves the use of the right hand with the thumb pointing in the direction of the positive linear axis (+x, +y, or +z) and the fingers curled in the positive rotational direction. The rotational axes serve the following functions; orientation of the workhead or the tool, or the orientation of the workpart to show different surfaces for machining (Groover 2008). Fig 6: The coordinate system Courtesy Engineersedge.com Motion Control System Machine tools perform their processes in either of the following ways; at discrete positions on the workpiece (processes performed include spot welding and drilling) or while the workhead is moving (processes performed include continuous arc welding and turning). These movements are accomplished by the following two motion control systems, point-to-point and continuous system. Point-To-Point System It is also referred to as positioning system. In this type of system, the tool is moved to a programmed location, the path taken to arrive at the location does not matter. When the move is completed, a processing action is done such as punching a hole or drilling. The program contains a series of points at which the operation can be performed. Continuous Path System It is also referred to as contouring system. The system has the capability of controlling two or more axes continuously and simultaneously. The tool trajectory is controlled relative to the workpart. The machine tool is capable of generating angular surfaces, 2D curves and 3D contours. Interpolation is very important in contouring. The path in a contouring system requires circular arcs and smooth nonlinear shapes. A problem arises when generating these shapes because NC system is continuous while NC is digital, for example, to generate a circular path, the circle has to be divided into a series of straight lines sections that are approximate to the curve. Thus the tool has to be commanded to machine each line section in succession. Decimal and Binary System The decimal system involves the use of the digits 0 to 9. It is comprised in the powers of base 10. The binary system makes use of the digits zero and one. The only difference with decimal system is that they are constructed on the powers of base 2. MCU can recognize values in decimal system and convert them into binary data. A circuit can be open (that is off and it is represented by 0) or closed (that is on and it is represented by 1). Tapes are being out phased by computers. A tape has eight channels that can be punched by holes, a punched hole represents digit 1 (Gizelbach, 2009). CNC Software The CNC software are of three types and they are; (a) operating system software, (b) machine interface software, and (c) application software. The operating system interprets the NC part programs and generates the required response to drive the axes of the machine tool (Smid, 2003). It also performs diagnostic routines in the CNC system. The operating system is installed by controller manufacturer and is stored in the ROM of the MCU. It consists of: (a) an editor – allows the machine operator to perform file management functions such as inputting and editing NC part programs, (b) control program – translates the part program instructions, carries out interpolation and acceleration/deceleration calculations, and finishes related functions to generate coordinate signals per axis, and (c) an executive program – manages the execution of the MCU 110 operations and CNC software. Machine interface software links the CPU and the machine tool in order to accomplish CNC auxiliary functions according to Smid (2003). The machine interface software is usually written in form of ladder logic diagrams (the 110 signal are implemented through programmable logic controller that is interfaced to the MCU). Application software comprises of NC part programs written for machining applications in user’s plant. Fig 7: CNC Machine components Courtesy Macpowercnc.com The Drive Motors: Majority of the drive motors are electrical and they control machine slide movement on CNC machines. Servomotors: Direct current servomotors consist of speed motors that rotate when voltage is applied (Seames, 2001). They are used to drive gear mechanisms and ball screws generating a high torque output that remains constant throughout the motion. Alternating current servomotors are also controlled by regulating voltage and they require less maintenance than DC servomotors. Hydraulic servomotors are more powerful than electric servomotors and thus used in large CNC machines. Fig 9: Servo drives Courtesy Drive Loxin2002.com Stepper Motor: Stepper motors convert (Machine Control Unit) MCU electrical pulse into fixed rotational steps. They are used in low-torque situations using open-loop control systems. Stepper motors are being replaced by servomotors which utilize closed-loop systems. Machine Spindles: The spindle is a device that holds the cutting tool on a CNC mill or workpieces on CNC turning centre. The machine spindle can rotate clockwise or anticlockwise and it has the ability to move up and down or up and down. They have a programmable speed range and are driven by alternating current (ac) or direct current (dc). DC motors have fixed variable speeds while AC motors utilize step drive and switch to move from high to low speed range. Machine Body: The machine body consists of a saddle, table, column and bed. Bed is the base for the components and it is hardened to support the saddle which provides the Y axis movement. The table provides X axis movement and it is mounted on the saddle. The column gives support to the spindle head unit which provides Z axis movement. The Tool Changer Manufacturing a part requires the following tools Manual Manual tool change is performed when the spindle is off. It involves loosening of the collet and changing the bit. Sometimes a program is needed for each tool change. Multiple Head Multiple heads can accommodate different tools and they are common in work enters machines. The multiple heads are mounted on one axis and are activated by a program. Separate moving parts can lower the accuracy of the machine. Automatic Tool Changers Automatic tool changers are specially designed spindles that use drawbar to draw a tapered tool holder into the spindle bottom tapered receptacle. The drawbar has fingers that grasp a knob that is screwed into the tool holder end. Retraction of the drawbar drags the tool holder firmly into the spindle taper (Albert 2009, p.47). Fig 8: Automatic tool changer Courtesy Kutenich.com Machine Control Unit (MCU) The MCU consists of the following; computing capability, ROM (read only memory) and the circuit switching ability that controls the functioning of the CNC machines. The machine functions are encoded in the ROM. The following are general features of an MCU; emergency stop button, load meters, keyboard pad, cycle start button, power on/off buttons, feed and speed override knobs, manual pulse generator, feed hold button, axis select knob, tool select/clamp switch, miscellaneous function switches and a CRT screen. CNC Programming There are four methods of CNC programming; manual programming, computer-assisted manual programming (part programming), parametric programming and geometric programming (computer programming). a) Geometric/Computer Programming Geometric programming involves the definition of the workpiece geometry and the computer calculates the coordinate and prepares instructions as a manual program. b) Computer-Assisted Manual Programming/Part Family Programming Before a CNC programmer begins writing the part program, it is important for the programmer to evaluate the drawing to get an idea about the part. Evaluation involves looking at the dimensioning method, tolerance, drawing units and scales, surface finish requirements, drawing revisions, material type, size, shape and condition, title block formation, omissions and other errors and a bill of materials if it is available (Smid 2006, p.2) The program is usually written in English-like statements and converted into low level machine language. This type of programming saves time and it is more accurate and efficient than manual programming. The programming tasks involve the computer and human part programmer. The programmer writes the machining instructions in English-like statements. The statements are translated by the computer into low level machine language that the machine tool controller can interpret and execute. The programmer defines the part geometry and specifies the operation sequence and tool path. Apart from defining the part geometry and specifying the path, the programmer has to; name the program, identify the machine tool for the job to be done, specify the feed rates and cutting speeds, designate cutter size, and specify circular interpolation tolerance. The computer does the following tasks; translating the input, computation of the arithmetic and cutter offset, editing, and post-processing. The first three tasks are performed under language processing program supervision. The last task, post-processing is performed by a separate computer program. The program is entered using high-level part programming language such as APT. c) Parametric Programming Parametric programming is a form of manual programming that makes use of parameters instead of the known coordinates. d) Manual Programming This is the basic technique of programming NC machine tools. The programmer has to do the following; plan the machine tool component layout, specify the required cutting tools, set the component datum and machine positions, select the speeds and cutting feeds, and do conversion of geometrical dimension information of engineering drawing into control system standard language (Groover 2008). Numerical Control (NC) Programming Procedure The NC programmer transfers the drawing details into the manufacturing plan. ON machine programming is usually used when the part geometry is simple. OFF machine programming is usually used when the part geometry is complicated. On Machine Programming New NC control systems have the capability of representing the cutter path and at times full graphic image of completed part. The programmer can create a new program and verify it and at the same time running the machine (Smid, 2006). On machine programming is common in two dimensional machining like sheet metal nibbling, turning, electrical discharge machining, and laser cutting. Off Machine Programming It is the standard method of NC programming. In this type of programming, the programmer has access to material/cutting tools references and graphic facilities (Smid, 2009). The following simultaneous steps are considered: Component Drawing Details The programmer has to be familiar with the part drawing and undefined details are corrected. There is no room for assumptions about the geometry or its accuracy. If need be, a designer is consulted. Machine Tool Selection The appropriate machine tool for the work is selected and selection is based on range of the NC machines in a company and limitations of the existing machine tools. Layout Planning Poor layout planning is not allowed as it can bring problems. The NC process plan assists the programmer to locate and clamp the workpiece. Clamps, vices, fixtures, and chucks are used to secure the workpiece in a way it is capable of withstanding the cutting forces, need for a coolant, and removal of swarf and evasion of cutter/fixture collisions. Geometry/Cutting Tool Relationship The work is clearly defined in datum points by the designer and these are the points to be used by the designer. Cartesian and Polar coordinates are used in general machining of rotational and prismatic parts. Sometimes the cutting tool motion relative to the component is considered; either moving tool and stationary workpiece, or stationary workpiece or moving tool or a combination of both. One dimension can be referenced by absolute or incremental values. Absolute positioning involves measuring the dimension from a fixed datum or a point of origin whereas incremental dimension is measured from its predecessor. Cutting Tool Specification It is influenced by the company standards, and programmer’s machining practice experience and knowledge. Cutting tool suppliers recommend selection of appropriate feed rate and cutting speed to suit the operation. The operator can modify the programmed values (the spindle speed rate and feed rate) through over-ride control switch, usually 0 to 120 percent of programmed values. Specifications are also included in the cutting tool holder; cutting tool holder connects the tool cutting tip and the machine spindle. CNC machine manufacturers identify three cutting tool specifications and they are maximum tool length, diameter and the weight (Smid 2009, p.58). Work Place Definition The programmer establishes the workplanes. Safe plane (programmed Z 0 position) is the plane near or on the part. X axis controls the depth of the cut and it also defines the safe zone. The safe zone controls the occurrence of rapid motion. Numerical Control (NC) Manuscript The NC program is usually written in word variable block format. The program is set in a consistent and logical manner by a programming sheet. The program is accompanied by a process planning sheet that registers cutting tool data, datum points and the machining details. The Basics of Part Programming The tool can move either relative to the workpiece or the workpiece relative to the tool during secondary motion. In NC programming, the tool moves relative to the workpiece and it is not dependent on the real situation (Smid, 2006). Modal commands are issued in NC programming and they remain in effect until changed by other commands such as coolant selection and feed rate selection. Non-modal commands are only effective when issued and they are lost in subsequent commands. Dwell command instructs the tool to stay in a certain configuration for a given time. The machine instructions are composed of a letter followed by a number. The letters are associated with a specific type of information that is needed by the machine. Letters that are used in codes include N, G, X, Y, Z, A, B, C, I, J, K, F, S, T, R, M. G-codes serve preparatory functions and involve actual tool moves (Smid, 2009). M-codes serve miscellaneous functions and involve actions that are necessary for machining such as coolant on/off and spindle on/off. CNC Positioning System Open-loop System (Stepper Motor) and Closed-loop System (Servo Motor) The positioning control systems that are used on CNC machines are open-loop and closed-loop systems (Seames, 2001). The open loop system is usually used with stepper motor and it operates without verification of actual position achieved by the move if it is similar to the desired position. It is a cheap system because it does not require other hardware and electronics to provide positional feedback. The close-loop system on the other hand has resolvers that monitor the spindle and table movement and transfers this data to the MCU and comparison is done between the current and programmed position. Adjustments are done until the transmitted signals equal the feedback signals. Servomotors with closed-loop systems are accurate in monitoring velocity and slide position. Machine Tool Positioning Accuracy During processing, there is conversion of coordinate axis values that are found in the NC part program into relative tool and workpart positions. The rotating leadscrew driven by a servomotor or a steeping motor moves the worktable linearly. For each revolution, the table moves a distance similar to the pitch of the screw. Velocity of the worktable is determined by leadscrew rotational speed. Positioning accuracy of the cutting tool affects the precision of the machine tool. The positioning accuracy is in respect to the workpiece and structural deformations between them (Altintas 2000, p.65). Precision in Positioning For any processing done by a CNC system, the positioning system must have a high degree of precision. Precision in CNC positioning can be defined by control resolution, accuracy and repeatability. Control resolution is the ability of the control system to split axis movement total range into closely spaced points (addressable points) that are distinguishable by MCU. Addressable points are locations on the axis that the worktable can be directed to go. Control resolution should be as small as possible. Control resolution is affected by electromechanical factors such as lead screw pitch and drive system gear ratio. Accuracy of an axis in a positioning system can be defined as the occurrence of a maximum possible error between the wanted target point and the actual position occupied by the system (Seames, 2001). Repeatability is defined as the ability of the positioning to go back to a previously programmed addressable point. Advantages and Disadvantages of CNC Machining CNC and NC machines have numerous benefits over conventional or traditional machine tools. On the other hand, they have a number of limitations. The advantages and disadvantages of CNC as stated by Gizelbach (2009) are highlighted below. Advantages 1. Reduction of non-productive time because there is less interaction between the machine and the operator. 2. There is greater repeatability (a variety of machining operations can be performed) and accuracy (there is no need for an operator to control machine movements). 3. Scrap rates are lower because of the ability of CNC machines to perform various operations and high accuracy coupled with less human errors. 4. Inspection requirements are reduced because of constituent quality and greater accuracy. 5. There is possibility of complex part geometries. 6. Engineering changes are easy to do. 7. Manufacturing lead times are shorter thus jobs can be performed quickly. 8. Floor spaced required is less and thus creating more storage space. 9. Part inventories are reduced thus leading to increased production speed and set up time. 10. Reduction of operator skill level requirements. 11. Improved safety because the operator is protected from cutting tool and moving machine parts. 12. Lower cutting tool costs because the machine is capable of generating a tool path that conforms to the tool shape. 13. Lower fixture costs because a part can be produced by a few fixtures. 14. The cutting tool life is increased because CNC machining optimizes the speeds and feeds. 15. Machining time is reduced due to optimized cutting feeds and high rapid-positioning speeds. 16. There is better production scheduling and costs are saved due to the accuracy and high machining time control (Gizelbach 2009). Disadvantages 1. The initial investment required is high because of the work required to be done. 2. Programming cost and computer time are needed. 3. High NC equipment utilization. 4. Trained personnel are required for maintenance (Gizelbach 2009). Conclusions As science will make inroads in artificial intelligence, it is quite likely that future CNC machines will turn more and more user friendly but that will come with a cost attached to it. The sophistication will bring many new features but affordability will be a big question for many small and medium sized companies. However, in all likelihood basic CNC machines with 3-axis movements will be a preferred choice for common applications, which do not come under purview of high-tech areas. The future of CNCs is extremely exciting and rewarding. It can be said that CNCs have always been the back bone of engineering industries and will remain like that for future years to come. CNC machines have numerous benefits over their traditional counterparts, but not without a few limitations. With powerful programs and software, the machines can produce several pieces in short durations of time with high accuracy. One of the greatest beauties of the CNC is their capability to continue working accurately even after having to stop due to inherent problems such as blunting of tools. Their level of automation allows them to fast and accurately replace faulty tools, thanks to their advanced control systems. Safety being at the forefront in their design, the machines are rarely associated with injuries. This is because they are normally well guarded, the operator only having to feed them with part programs which may be developed on site or in a different computer room. In all respect, the futures of organizations that rely on the current and hopefully future CNCs remain bright. The application of more elements of artificial intelligence and new technologies in the design of future CNCs will definitely guarantee higher efficiency and effectiveness in production. References Alavala, C. R. (2008) CAD/CAM: Concepts and applications. India: PHI Learning Pvt. Ltd. Albert, A. (2009) Understanding CNC routers. Vancouver, BC: FPInnovations FORINTEK. Altintas, Y. (2000) Manufacturing automation: Metal cutting mechanics, machine tool vibrations, and CNC design. Cambridge, UK: Cambridge University Press. Engineersedge (2010) Five axis CNC Machining, retrieved 26th November, 2010 http://www.engineersedge.com/manufacturing/five-axis-cnc-machining.htm Gizelbach, R. (2009) CNC machining. Tinley Park: The Goodheart-Willcox Company. Groover, M. P. (2008) Automation, production system, and computer-integrated manufacturing. 3rd edn. New Jersey: Pearson Education Inc. Kesavan, R. & Ramanath, B. V. (2006) Manufacturing Technology-II. Firewall Media. Mattson, M. (2009) CNC programming: Principles and applications. Clifton Park, NY: Cengage Learning. Nayaran, L. (2008) Computer aided design and manufacturing. India: PHI Learning Pvt. Ltd. Seames, W. S. (2001) Computer numerical control: Concepts and programming. 4th edn. Albany, NY: Cengage Learning. Smid, P. (2003) CNC programming handbook: A comprehensive guide to practical CNC programming. 2nd edn. New York, NY: Industrial Press Inc. Smid, P. (2006) CNC programming techniques: An insider’s guide to effective methods and applications. New York, NY: Industrial Press Inc. Smid, P. (2009) CNC control setup for milling and turning: Mastering CNC control systems. New York, NY: Industrial Press Inc. Read More