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Edit Comment Close Premium member Presentation Transcript CAD/CAM OVERVIEW: CAD/CAM OVERVIEW CNC Machine Tools, CNC Machining and Introductory NC programming FMS(TGS) 1 Dr. Tafesse GebresenbetCNC Machine tools and machining: CNC Machine tools and machining Lecture outline Basic concepts of Machining & historical overview of CNC machines CNC control of manufacturing systems Coordinate systems and machine tools Types of NC machines The MCU and other components of the NC system Position and motion control in NC machines Interpolation schemes Machine Tool Technology for NC machines NC part programming 2 FMS(TGS)Historical overview: Historical overview Historical Development 15 th century – Machining Metal 18 th century - Industrialization, production type machine tools 20 th century - F.W. Taylor tool metal, HSS Automated production equipment Screw machines Transfer lines Assembly lines ------ using cams and preset shops Programmable automation NC PLC Robots 3 FMS(TGS)PowerPoint Presentation: 4 FMS(TGS)Introductory concepts: Introductory concepts 5 FMS(TGS)Introductory concepts: Introductory concepts 6 FMS(TGS) THOSE USING SINGLE POINT TOOLS THOSE USING MULTIPOINT TOOLS THOSE USING ABRASIVE TOOLS lathes shapers planers boring machines etc. drilling machines milling machines broaching machines hobbing machines etc. grinding machines honing machines etc.Introductory concepts: Introductory concepts 7 FMS(TGS)Introductory concepts: Introductory concepts Numerical control (NC) is a technique for controlling machine tools or processes by using coded command instructions. The NC controller interprets these instructions and then converts them into two types of control signals: motion control signals and miscellaneous control signals Motion control signals are a series of electric pulse trains that are used to control the position and the speed of the machine table and spindle . Each pulse activates a motion of one basic length-unit (BLU) which is the minimum increment size of the NC control system . 8 FMS(TGS)Introductory concepts: Introductory concepts The number of pulses transmitted to each axis determines the incremental axis position, and the frequency of these pulses regulates the axis speed. Miscellaneous control functions are a set of on/off signals to implement the control of the speed and direction of the spindle rotation, control of coolant supply, selection of cutting tool, automatic clamping and unclamping , etc. NC is often referred to as the older generation of numerical control technology. NC systems are hard-wired controls in which most functions are implemented by electronic hardware based upon digital circuit technology . 9 FMS(TGS)Introductory concepts: Introductory concepts Computer Numerical Control (CNC) Computer numerical control (CNC) is the numerical control system in which a dedicated, stored program computer is built into the control to perform basic and advanced NC functions. CNC controls are also referred to as soft-wired NC systems because most of their control functions are implemented by the control software programs . The control signals in CNC systems are in the form of binary words . Each word contains a fixed number of bits, 32 bits or 64 bits are commonly used. Each bit of data produces one BLU motion in the controlled axis. Theoretically a 32-bit word could represent one of up to 2 32 = 4,294,967,296 different axial positions . If the system resolution is, for example, BLU = 0.0001 in., this number can represent up to 429,969 in. possible motions, which is more than enough for all types of applications 10 FMS(TGS)Introductory concepts: Introductory concepts Direct Numerical Control (DNC) Direct numerical control uses a shared computer to simultaneously control the operation of a group of NC machine tools . The main tasks performed by the computer are to program and edit part programs as well as download part programs to NC machines . The idea of direct numerical control began in the mid-1960s in Cincinnati Milacron and General Electric. 11 FMS(TGS)Introductory concepts: Introductory concepts Distributive Numerical Control (DNC) The main concept of distributive numerical control is to use a network of computers to coordinate the operation of a group of CNC machine tools . This new form of DNC systems began to evolve in the early 1980s and grew with the development of computer and communication technologies. Today many CNC machines, together with robots, programmable logic controllers and other computer-based controllers, have been integrated into DNC systems to make automated manufacturing systems possible. 12 FMS(TGS)Advantages of NC with Small Lot Sizes: Advantages of NC with Small Lot Sizes 1. Reduced non-production time 2. Reduced fixturing 3. Reduced lead time 4. Greater manufacturing flexibility 5. Easier to accommodate engineering design changes on the work piece 6. Improved accuracy and reduced human error. 13 FMS(TGS) NC is most appropriate for the following conditions 1. Frequently processed parts with small to medium lot sizes 2. Complex part geometry 3. Close tolerances 4. Need of much metal removal 5. 100% inspection required parts 6. Expensive parts where processing mistakes are costly 7. Need of many operations on the part 8. Likely engineering design changesIntroductory concepts: Introductory concepts FMS(TGS) 14 Increased productivity Reduced production costs Facilitation of complex machining operations Improved production planning and control Facilitation of flexible automation High accuracy and repeatability Reduced indirect operating costs Greater flexibility Lower operator skill requirement high initial investment high maintenance requirement, and not cost-effective for low productions runs Advantages Limitations Advantages and Limitations of CNC The main advantages of using CNC technology are to reduce product cost, improve product quality, and facilitate production planning and control . These benefits can be realized through these nine causes:Computer Control of Manufacturing Systems: Computer Control of Manufacturing Systems NUMERICAL CONTROL (NC) A form of programmable automation Numbers, letters, and symbols are coded to define a program of instructions for a particular work part or job. Two categories of numerical control applications : Machine tool applications (drilling, milling, etc.) Non-machine tool applications (assembly, drafting, etc.) Basic Components of NC 1. Program of instructions 2. Machine control unit 3. Processing equipment Drive system Machine tool Feedback system 15 FMS(TGS)Computer Control of Manufacturing Systems: Computer Control of Manufacturing Systems 16 FMS(TGS)PowerPoint Presentation: 17 FMS(TGS) CNC Dual turret centerPowerPoint Presentation: 18 FMS(TGS)PowerPoint Presentation: 19 FMS(TGS) CNC vertical Milling MachinePowerPoint Presentation: 20 FMS(TGS) Double axis machining centerPowerPoint Presentation: 1. Program of instructions: Detailed step-by-step commands that direct the processing equipment . The most common medium on which the program is submitted to the machine control unit has been 1-in.-wide punched tape. More recently, magnetic tape cassettes and floppy diskettes and computer via RS-232-C communication are used. 2. Machine Control Unit (MCU): Electronics and control hardware that read and interpret the program of instruction and convert it into mechanical actions of the mach. Tool or other processing equipment . Implement interpolations (linear, circular, and helical ) to generate axis motion commands Feed axis motion commands to the amplifier circuits for driving the axis mechanisms Receive the feedback signals of position and speed for each drive axis Implement auxiliary control functions such as coolant or spindle on/off, and tool change 3. Processing Equipment: Component that performs useful work. Ex. Work table, spindle, motors. 21 FMS(TGS)The MCU and other Components of the NC System: The MCU and other Components of the NC System Drive System A drive system consists of amplifier circuits, drive motors, and ball lead-screws . The MCU feeds control signals (position and speed) of each axis to the amplifier circuits. The control signals are augmented to actuate drive motors which in turn rotate the ball lead-screws to position the machine table. Machine Tool CNC controls are used to control various types of machine tools. Regardless of which type of machine tool is controlled, it always has a slide table and a spindle to control of position and speed. The machine table is controlled in the X and Y axes, while the spindle runs along the Z axis. Feedback System The feedback system is also referred to as the measuring system . It uses position and speed transducers to continuously monitor the position at which the cutting tool is located at any particular time . The MCU uses the difference between reference signals and feedback signals to generate the control signals for correcting position and speed errors. 22 FMS(TGS)Coordinate System and Machine Motions: Coordinate System and Machine Motions Coordinate System -I The purpose is to provide a means of locating the tool in relation to the work piece. Numerical control coordinate system is defined with respect to the machine tool table . Depending on the type of NC machine, the part programmer may have several options for specifying the location . One of these options: 1. * Fixed zero : the origin is always located at the same position on the machine table . All locations must be defined by x and y coordinates relative to that fixed origin. * Floating zero: Modern NC machines allow the machine operator to set the zero point at any position on the machine table . 23 FMS(TGS)PowerPoint Presentation: Coordinate System -II 2. * Absolute Positioning : The tool locations are always defined in relation to the zero point. * Incremental Positioning: The next tool location must be defined with reference to the previous tool location . Machine Axes Designation & Direction The EIA-267-B standard specifies fourteen axes for describing the linear and rotary motions of CNC machines . This includes nine linear axes and five rotary axes . The nine linear axes are further divided into the following three groups: (see figure below) Primary linear axes (X,Y, and Z) Secondary linear axes (U, V, and W) Tertiary linear axes (P, Q, and R) 24 FMS(TGS)PowerPoint Presentation: The primary axes (X, Y, and Z) are assigned to the primary slide table . The secondary linear axes (U, V, and W) are added to the primary axes for defining the movement of the second moving slide or spindle. Similarly, the tertiary linear axes (P, Q, and R) are used to represent the linear motion of the third slide or spindle. The five rotary axes consist of three primary rotary axes (A, B, and C) and two special axes (D and E) (right figure). Their definitions are: 25 FMS(TGS)PowerPoint Presentation: Machine Axes Designation Machine axes are designated according to the "right-hand rule". When the thumb of right hand points in the direction of the positive X axis , the index finger points toward the positive Y axis , and the middle finger toward the positive Z aixs . The left figure shows the right-hand rule applied to vertical machines, while the right figure applies to horizontal machines. 26 FMS(TGS)Direction of Machine Axes: Direction of Machine Axes Direction of Machine Axes CNC controls use the positive (+) and negative (-) sign to indicate the motion direction of the machine axes. This is how we define the directions. +Z direction: is the direction which increases the distance between the workpiece and the cutting tool. -Z direction: is in the opposite direction of +Z. +X direction: (a) In vertical machines, it is the direction to the right when observed from the spindle toward its supporting column. (b) In horizontal machines, it is pointed to the right when viewed from the spindle axis toward the workpiece . -X direction: is in the opposite direction of +X. +Y direction: follows the right-hand rule: when the thumb points toward the +X axis and the middle finger is directed toward the +Z axis, the index finger points in the direction of the +Y axis. -Y direction: is in the opposite direction of +Y. 27 FMS(TGS)PowerPoint Presentation: Reference Zero Points Reference zero points are the base or starting points that are chosen as the reference for calculating the coordinates of the other points . Also, reference zero points are called the zero points . CNC controls use the following four types of reference zero points to facilitate the programming of tool paths: Machine zero point Reference return point Work zero point Program zero point Machine Zero Point The machine zero point is the origin of the machine coordinate system. It is set by the machine tool manufacturer and can not be changed. The machine zero is labeled with an M and represented by this symbol: The location of the machine zero point varies from manufacturer to manufacturer. 28 FMS(TGS)PowerPoint Presentation: For turning machines, the machine zero is normally located at the center of the spindle end face (left figure). In milling machines, the machine zero is usually at the extreme limit of each axis travel (right figure). Normally the machine zero is not directly used as the reference point for writing part programs. It may be used in one of the following three a pplications: 1. Initial setup of the machine 2. As the reference point for other reference points such as reference return points, work zeros, and program zeros 3. As the tool change position 29 FMS(TGS)PowerPoint Presentation: Reference Return Point Reference return points are the locations to which the machine table or the spindle is returned. They are identified by the letter R and are represented by the symbol Some CNC controls allow defining up to four reference return points. Normally, the machine zero is set to be the first reference return point in milling machines [(Figure below (left)]. The second, third, and fourth reference return points are specified by setting their parameter values. They can be set at any convenient location within the work envelope. In turning machines, the reference return point is located in the extreme end of the work envelope [Figure below (right)] The location of the first reference return point is precisely predetermined in each moving axis in relation to the machine zero point. Because of this, it can be used for calibrating and regulating the measuring system of the slide table and spindle. 30 FMS(TGS)PowerPoint Presentation: Specifically, the reference point is used in four situations 1 . When the control is powered up, all axes always must be positioned at the reference return point to calibrate the measuring system. 2. The machine must be re-positioned to the reference return point for reestablishing the proper coordinate values in situations such as losing the current position data due to electrical failure or improper operation. 3. All axes must be retracted to the reference point before the tool change can take place . 4. At the end of the part program , all axes must be retracted to the reference return point to reset the control system for re-running the part program or running a new part program. 31 FMS(TGS)Work Zero Point : Work Zero Point Work Zero Point A work zero point is the origin of the workpiece's coordinate system. It is used to determine the work's coordinate system in relation to the machine zero point. The work's zero points are often referred to as setup points because they are the locations for setting up the workpiece on the machine table. Some CNC controls allow the use of multiple work zero points in one machine setup or operation. The work zero point is labeled by W and represented by the symbol The work zero point can be chosen by the programmer at any convenient location within the working envelope of the machine. It is recommended that you place the work zero point in a way that it can be easily located and measured on the workpiece. 32 FMS(TGS)Work zero point: Work zero point two common methods of choosing the work zero point for turnings (top figure) and the bottom figure shows milling examples. 33 FMS(TGS)PowerPoint Presentation: TYPES OF NC SYSTEMS 1. Point-to-point: The objective is to move the cutting tool to a predefined location. Once the tool reaches the desired location, the machining operation is performed at that location. 2. Straight cut: Capable of moving the cutting tool parallel to one of the major axes. Workpieces of rectangular configurations may be fabricated. Angular cuts are not possible. 3. Contouring: The most complex, the most flexible, and the most expensive type of machine tool control. Capable of performing both PTP and straight-cut operations. In addition, simultaneous control of more than one axis movement of the machine tool. Straight or plane surfaces at any orientation, circular paths, conical shapes, most any other mathematically definable form are possible. 34 FMS(TGS) Types of NC SystemsPosition and Motion Control in an NC System: Position and Motion Control in an NC System The data read into the MCU define machine table positions. Each axis is equipped with a drive unit which is connected to the table by means of a lead screw. The axis positioning system may be designed as either an open-loop or a closed-loop system. 35 FMS(TGS)CNC Drive Systems: CNC Drive Systems CNC Drive Systems The CNC drive systems can be either open loop or closed loop type. The main difference between the two systems depends on whether the system has a feedback loop to insure the accuracy of system performance. Open-loop NC systems make use of stepping motors. Each pulse generated my MCU drives the stepping motor by a fraction of one revolution, step angle. = Step angle (integer) = number of step angles 36 FMS(TGS)PowerPoint Presentation: P= number of pulses received by the motor f p = pulse rate (frequency of the pulse train) t= duration of the pulse train S= rotational speed of the stepping motor angle of rotation=P angle of rotation=f p t By controlling the number of pulses to the motor the position of the table is controlled without feedback sensors. Stepping motors are used on NC systems where the load is relatively small. 37 FMS(TGS)CNC Drive Systems: CNC Drive Systems Open Loop System No feedback loop is used in open loop drive systems. The drive motor acts upon the control commands from the machine control unit (MCU). The system simply assumes the machine table will reach the target position. There is no way for the MCU to know the actual performance of the system. An open loop system is very sensitive to the load resistance. Position and velocity error may occur when a heavy cutting resistance is encountered. 38 FMS(TGS)CNC Drive Systems: CNC Drive Systems Open loop drives are typically used in PTP systems in which the cutting tool does not engage with the work piece during positioning. They can also be used in light-loaded cutting machines. Open loop systems are less expensive, but they are prone to load resistances during machining. 39 FMS(TGS)PowerPoint Presentation: A closed-loop system uses position sensors to measure the table position relative to the input value for the axis. Generally uses dc servomotor or hydraulic actuator. Various feedback sensor devices are used. Ex. Optical encoder. Optical encoder consists of a light source, a photodetector,, and a disk which is connected to the rotating shaft whose angular position is to be measured. As the disk rotates, the openings on the disk cause the light source to be seen as a series of flashes. The photodetector emits an electrical signal equal to the number flashes which are counted by the MCU. = angle between the openings in the disk N d = number of openings in the encoder disk angle of rotation= P 40 FMS(TGS)PowerPoint Presentation: Pulses generated by the optical encoder is compared with the input position command The error is used to control a dc servo motor, which in turn drives the machine table. Closed-loop NC systems are more appropriate for processes that generate a significant load during operation. Ex: milling, turning. Accuracy: A measure of the control system’s capacity to position the machine table at a desired location . Related to the control resolution of an NC system. Control resolution is the capability of the MCU to divide the range of a given axis movement in to closely spaced points. It is the distance between adjacent control points. 41 FMS(TGS)CNC Drive Systems: CNC Drive Systems Closed Loop System With closed loop drive systems, feedback sub-systems are used to monitor the actual output and correct any discrepancy between desired and actual system performance. Feedback sub-systems may be either an analog or digital type. Analog systems measure the variation of physical systems such as position and velocity in voltage level. Tachometers are typically used to measure the velocity, while resolvers for position. There are two feedback loops in CNC drive systems: position loop and velocity loop. The position loop is the outer loop that consists of a comparator, an amplifier circuit, a velocity loop, a resolver, and a resolver interface. 42 FMS(TGS)PowerPoint Presentation: n= number of bits for an axis number of control points= 2 n Repeatability: Ability of the control system to return to a given location that was previously programmed into the controller. repeatability errors are caused by mechanical errors. 43 FMS(TGS)CNC Drive Systems: CNC Drive Systems In operation, the comparator receives the reference signal from CNC control and the position feedback signal from the resolver. The difference of these two signals represents the existing positional error. The comparator produces the positional error signal and feeds this signal to the amplifier to drive the servo motor for correcting the error. The velocity loop is the sub-loop within the position loop. It consists of a comparator, an amplifier circuit, a tachometer attached to either the lead screw or the servo motor, and a tachometer interface. 44 FMS(TGS)CNC Drive Systems: CNC Drive Systems Digital feedback systems ( Fig below) use a digital position transducer to measure the position. Encoders are popular digital position transducers. An up-down counter and a digital-to-analog converter are used in place of the comparator and amplifier. The velocity loop in a digital feedback system is the same as that found in the analog feedback system. 45 FMS(TGS)Interpolation Schemes: Interpolation Schemes Interpolation - One of the important aspects of contouring. All circular arcs and nonlinear shapes cannot be defined mathematically except by approximation. To cut along a circular path, the circle must be divided into a series of straight line segments. Developed to prevent the punch tape from being too long for having end points of each line segment. Permit the programmer to generate instructions with a few input parameters. Calculate the intermediate points the cutter must follow. The int. rout. types: 1. Linear int. 4. Parabolic int. 2.Circular int. 5. Cubic int. 3. Helical int. 46 FMS(TGS)Interpolation in CNC : Interpolation in CNC CNC Interpolations CNC controls use various forms of interpolation to execute contouring tool paths. Interpolation is the process of producing a series of intermediate data points between given coordinate positions . The interpolation points are implemented by various types of CNC control interpolators. The two main functions provided by an interpolator are: compute intermediate coordinate positions along the programmed path compute the axial velocity of an individual axis along the contour path The already discussed five possible the following five possible types of interpolations: • Linear interpolation • Circular interpolation • Helical interpolation • Parabolic interpolation • Cubic interpolation All modern CNC controls furnish linear, circular, and helical interpolation; few controls support parabolic and cubic interpolations. 47 FMS(TGS)Interpolation in CNC : Interpolation in CNC Linear interpolation moves the tool from the starting point to the target point along a straight line. Linear interpolation can be implemented in a 2-D plane or 3-D space. It is used in two categories of applications: machine lines and on the following page, approximate irregular curves and surfaces . 48 FMS(TGS)Interpolation in CNC : Interpolation in CNC Circular Interpolation Circular interpolation is programmed to cut circular arcs in one of three principal planes (XY-plane, ZX-plane, or YZ-plane) 49 FMS(TGS)Interpolation in CNC : Interpolation in CNC Helical Interpolation Helical interpolation combines the two-axis circular interpolation and a linear interpolation in the third axis. It can be used in the following three configurations: Circular arc in XY-plane and linear interpolation in Z axis Circular arc in ZX-plane and linear interpolation in Y axis Circular arc in YZ-plane and linear interpolation in X axis 50 FMS(TGS)Interpolation in CNC : Interpolation in CNC Parabolic Interpolation Parabolic interpolation uses three non-collinear points to approximate curves that are of free form. It is mainly used in mold and die making where free form designed, rather than precisely defined shapes, are preferred. The use of two or more successive parabolic interpolations can approximate higher-order curves. The advantage of using parabolic interpolation is the reduction of programmed points by as much as 50 times that required by the linear interpolation mode. Parabolic interpolation and parabolic interpolation of a higher order curves 51 FMS(TGS)Interpolation in CNC : Interpolation in CNC Cubic Interpolation Cubic interpolation approximates the surfaces defined by third-order geometry. It involves the motion of three axes to machine complex shapes such as automobile sheet metal dies. 52 FMS(TGS)The Machining Process: The Machining Process Machining is a manufacturing process in which the geometry of the work is changed by removing excess material by means of the relative motion between a cutting tool and the work piece. Five basic types of machining processes: 1. Turning 4. Planning 2. Drilling 5. Grinding 3. Milling Parameters -called cutting conditions- used to control the processes are speed, feed, and depth of cut. 53 FMS(TGS)PowerPoint Presentation: (Cutting) Speed (V): relative velocity of the tool with respect to the work surface. (surface feet per minute ( sfpm ), m/min) Rotational (Spindle) Speed (S): Speed of cutting tools that rotate (rev/min). Feed (f): Lateral displacement of the cutting tool relative to the work on each pass or revolution of the tool (in./rev, in./pass). Feed rate ( f r ): Lateral travel rate of the tool (in./min). Depth of cut: The distance the tool penetrates below the original surface of the work (in., mm). For milling and drilling we need to convert from surface speed to spindle rotation speed and vice versa. D= diameter of the cutter (in.) Spindle speed in a turning operation may be found if we take D as the diameter of the cylindrical work piece. 54 FMS(TGS)PowerPoint Presentation: For rotational machining operations feed might be converted to feed rate: f r =Sf In a milling operation, the feed may be determined from the chip load on one tooth of the cutter. Chip load= chip width taken by the milling cutter f= (number of teeth on the cutter)(chip load) Metal Removal Rate (MRR)= the rate at which metal is removed during machining (in. 3 /min). MRR (for turning op.)= 12Vfd MRR (for drilling op.)= (area of the drill)f r MRR (for milling op.)= (area of the cut)f r Tm= time required to accomplish the cutting op. L= length of the workpiece in the direction of feed travel 55 FMS(TGS)Machine Tool Tech. for NC: Machine Tool Tech. for NC Numerical control machine tools have been designed for many different processes: Drill presses Milling machines Turning machines Boring mills Profiling and contouring mills Surface grinders and cylindrical grinders Punch presses for sheet metal hole punching Presses for sheet metal bending. 56 FMS(TGS)PowerPoint Presentation: Machining center is a machine tool capable of performing several different machining operations on a work part in one setup under program control The mach. center is capable of milling, drilling, reaming, tapping, boring, facing, and similar operations. Characterizations of an NC machining center : Automatic tool-changing capability Automatic work part positioning Pallet shuttle Vertical mach centers for flat work, and horizontal mach. centers for cube-shaped parts. 57 FMS(TGS)PowerPoint Presentation: Other machines and equipment utilizing NC-type controls: Electrical wire-wrap machines Component insertion machines Drafting machines Coordinate measuring machines Flame cutting, plasma arc cutting, laser cutting, and similar machines Tube bending Cloth cutting Knitting Riveting Filament winding 58 FMS(TGS)NC Part Programming: NC Part Programming Tape Reader Electrical-mechanical device for winding and reading the punched tape containing the program of instructions The punched tape consists of eight parallel tracks of holes The presence or absence of a hole in a certain position represents bit information Techniques used in NC tape readers to sense the hole pattern in the tape: Photoelectric cells Electrical contact fingers Vacuum method The use of photoelectric cells is faster and more reliable. 59 FMS(TGS)NC Part Programming: NC Part Programming PUNCHED TAPE 1 in. wide Can be made out of several materials (paper, Mylar-reinforced paper, Mylar-coated aluminium, certain plastics) Holes punched by manual or computer controlled punch machines Eight columns of holes for programming, ninth column of holes between the third and the fourth columns for winding Binary system used to represent single-digit numbers Numbers, alphabetical letters, plus and minus signs, and other symbols coded on the tape 60 FMS(TGS)NC Part Programming: NC Part Programming 61 FMS(TGS)PowerPoint Presentation: 62 FMS(TGS) Fifth column is used as a check -parity- for odd number of holes. A bit is a binary digit. 0 or 1 depending on the absence or presence of a hole Out of a row of bits, a character is made A word is a collection of characters used to form part of an instruction Out of a collection of words, a block is formed A block is a complete NC instruction End-of-block (EOB) symbol is used to separate blocksNC Part Programming: NC Part Programming Part Program A part program is a series of coded instructions required to produce a part. It controls the movement of the machine tool and the on/off control of auxiliary functions such as spindle rotation and coolant. The coded instructions are composed of letters, numbers and symbols and are arranged in a format of functional blocks as in the following example. N10 G01 X5.0 Y2.5 F15.0 | | | | | | | | | Feed rate (15 in/min) | | | Y-coordinate (2.5") | | X-coordinate (5.0") | Linear interpolation mode Sequence number 63 FMS(TGS)NC Words: NC Words S EQUENCE N UMBER (N- Words ): Identify the block P REPARATORY W ORK (G- Words ): Prepare the controller for instructions that are to follow C OORDINATES (X- , Y- , and Z- Words ): Give coordinate positions of the tool F EED R ATE (F- Word ): Specifies the feed rate in a machining operation (in./min) C UTTING S PEED (S- Word ): Specifies the cutting speed of the process (rev/min) T OOL S ELECTION (T- Word ): Specifies the tool to be used in the operation. Only be needed for machines with a tool turret or automatic tool changer M ISCELLANEOUS F UNCTION (M- Word ): Specifies certain miscellaneous or auxiliary functions which may be available on the machine tool. The last word in the block. 64 FMS(TGS)Tape Formats: Tape Formats 1. Word address format: A letter precedes each word and is used to identify the word type Words may be in any order 2. Tab sequential format: Words listed in a fixed sequence Each word separated by tab key The repeating words in are not retyped 3. Fixed block format Least flexible format Fixed sequence of words Same length and format characters in each word 65 FMS(TGS)Methods of NC Part Programming: Methods of NC Part Programming 1. Manual part programming: Machining instructions specified on a manuscript Manuscript: a listing of the relative tool and workpiece locations, preparatory commands, speed/feed specifications, etc. Different manuscripts for different machine tool and tape format NC tape typed with the help of data on manuscript More appropriate for point-to-point operations 66 FMS(TGS)PowerPoint Presentation: 2. Computer-assisted part programming: Part programmer defines the geometry of the work part specifies the tool path and/or operation sequence The computer makes input translation arithmetic calculations cutter offset computation post processing 67 FMS(TGS)PowerPoint Presentation: 3. Manual data input: Require much documentation and procedure Operator manually enters the programming data The system stays idle while the programming instructions are being transferred 4. NC programming using CAD/CAM Integrates design engineering and manufacturing engineering Easier to prepare models of parts Design file of a part stored in database, used for similar parts in the future Helps to generate toolpaths 68 FMS(TGS)PowerPoint Presentation: 5. Computer-automated part programming Automation of the complete NC part programming procedure NC Part Programming Languages Over 100 NC part programming languages Some of the languages which are still in use: APT (Automatically Programmed Tools) AUTOSPOT (Automatic System for Positioning Tools) SPLIT (Sundstrand Processing Language Internally Translated) COMPACT II ADAPT (Adaptation of APT) EXAPT (Extended Subset of APT) 69 FMS(TGS)PowerPoint Presentation: 70 FMS(TGS)PowerPoint Presentation: 71 FMS(TGS)PowerPoint Presentation: 72 FMS(TGS)PowerPoint Presentation: NC Functional Blocks NC controls simultaneously execute all NC words contained in a block . Therefore, NC blocks are normally organized according to their tasks. Table 1.7 gives some common functional blocks used in part programming CNC functional blocks 73 FMS(TGS) Function Block Explanation Typical Examples 1. Start feature Set the control to proper operating modes at the beginning of a part program. This block is also used after a tool change. G90 G80 G40 G17 2. Coordinate system setting Define the work zero point. G92 Xx Yy Zz G54 3. Tool length offset Offset the difference between the programmed tool length and the actual tool length G43 Hh 4. Tool motion Generate tool paths to machine the workpiece G0 Xx Yy Zz G1 Xx Yy Zz G2/G3 Xx Yy ZzPowerPoint Presentation: Function Block Explanation Typical Examples 5. Cutter diameter compensation Offset the cutter in a specified direction by a given amount od distance. G41/G42 Xx Yy Hh/Dd G40 Xx Yy 6. Fixed cycle Generate a series of tool paths to perform hole operations. G8_ Xx Yy Zz Rr Ff 7. Tool change Select a tool and cause a tool change Tt M6 8. Spindle control Command spindle rotation speed and direction. Ss M3/M4 M5 9. Reference point return Return the tool to the machine home position G91 G28 Z0 G91 G28 X0 Y0 10. Program end Specify the end of part program M2 M30 74 FMS(TGS)PowerPoint Presentation: 75 FMS(TGS)PowerPoint Presentation: 76 FMS(TGS)PowerPoint Presentation: 77 FMS(TGS)CAD/CAM Based Part Programming: CAD/CAM Based Part Programming CAD/CAM Based Part Programming: The output of any CAD package include the geometric data of the part to be machined. Therefore, many CAD/CAM package can produce cutter location (CL) data to be used for NC code generation. There is still to be a process planning module for a workable NC code generation. Some of the CAD/CAM packages that have the NC code generation capabilities are Computervision , CATIA, Unigraphics , CADAM, ProEngineer , MechanicalDesktop (Auto Desk). 78 FMS(TGS) You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.