3 HISTORYUS Air Force commissioned MIT to develop the first "numerically controlled" machine in It was demonstrated in 1952.At first Computer Numeric Control machines were developed.Today, computer numerical control (CNC) machines are found almost everywhere, from small job shops in rural communities to companies in large urban areas.
4 DEFINITIONIn CNC (Computer Numerical Control), the instructions are stored as a program in a micro-computer attached to the machine. The computer will also handle much of the control logic of the machine, making it more adaptable than earlier hard-wired controllers.
5 CNC APPLICATIONS Machining 2.5D / 3D Turning ~ Lathes, Turning Centre Milling ~ Machining CentresForming2DPlasma and Laser CuttingBlanking, nibbling and punching3DRapid Prototyping
22 Utilization of computers in manufacturing applications has proved to be one of the most significant advantages & developments over the last couple of decades in helping to improve the productivity and efficiency of manufacturing systems.
23 ADVANTAGES of CNC Productivity Machine utilisation is increased because more time is spent cutting and less time is taken by positioning.Reduced setup time increases utilisation too.
24 PROFIT increases as COST decreases and as PRODUCTIVITY increases PROFIT increases as COST decreases and as PRODUCTIVITY increases. PRODUCTIVITY through AUTOMATION
25 any means of helping the workers to perform their tasks more efficiently AUTOMATIONtransfer of the skill of the operator to the machine
26 Transferred skill Results muscle powerengine driven machine toolsFirst industrial revolutionmanipulating skillmechanizationhard automationvision skilluse of position transducers, camerasincrease of accuracy, part recognitionbrain powercnc machines, industrial robots, soft automation, computer control of manufacturing systemssecond industrial revolution
27 EFFICIENCY OF MANUFACTURING COST = COST OF MANUFACTURING AND COST OF MATERIAL HANDLINGEFFICIENCY OF MANUFACTURINGPRODUCTIVITY = AVERAGE OUTPUT PER MAN-HOURPROFIT = INCOME - COST
28 ADVANTAGES of CNC Quality Parts are more accurate. Parts are more repeatable.Less waste due to scrap.
29 ADVANTAGES of CNC Reduced inventory Reduced setup time permits smaller economic batch quantities.Lower lead time allows lower stock levels.Lower stock levels reduce interest charges and working capital requirements.
30 ADVANTAGES of CNC Machining Complex shapes Slide movements under computer control.Computer controller can calculate steps.First NC machine built 1951 at MIT for aircraft skin milling.
31 ADVANTAGES of CNC Management Control CNC leads to CAD Process planning Production planning
32 DRAWBACKS of CNC High capital cost Machine tools cost $30,000 - $1,500,000Retraining and recruitment of staffNew support facilitiesHigh maintenance requirementsNot cost-effective for low-level production on simple partsAs geometric complexity or volume increases CNC becomes more economicalMaintenance personnel must have both mechanical and electronics expertise
34 The metal cutting operations (also called machining) is one of the most important manufacturing processes in industry today (as it was yesterday).
35 MACHINING IS THE REMOVAL OF MATERIALS IN FORMS OF CHIPS FROM THE WORKPIECE BY SHEARING WITH A SHARP TOOL.
36 The main function of a machine tool is to control the workpiece-cutting tool positional relationship in such a way as to achieve a desired geometric shape of the workpiece with sufficient dimensional accuracy.
37 Machine tool provides: work holdingtool holdingrelative motion between tool and workpieceprimary motionsecondary motion
38 between tool and workpiece Primary motionRelative motionbetween tool and workpieceSecondary motionCutting motionFeed motionCutting speedFeed rate
39 CLASSIFICATION OF THE CHIP REMOVING METHODS ACCORDING TO THE RELATIVE MOTION
40 CLASSIFICATION OF MACHINE TOOLS THOSE USING SINGLE POINT TOOLSTHOSE USING MULTIPOINT TOOLSTHOSE USING ABRASIVE TOOLSlathesshapersplanersboring m/c’setc.drilling m/c’smilling m/c’sbroaching m/c’shobbing m/c’sgrinding m/c’shoning m/c’s
44 ISO MACHINE TOOL AXES DEFINITIONS AXISMACHINE TOOL WITH SPINDLEMACHINE TOOL WITH NO SPINDLEZaxis of spindle,(+Z) as tool goes away from the work pieceperpendicular to work holding surface, (+Z) as tool goes away from the workpieceMACHINE TOOL WITH ROTATING WORKPIECEMACHINE TOOL WITH ROTATING TOOLHORIZONTAL AXISVERTICAL AXISXradial and parallel to cross slide, (+X) when tool goes away from the axis of spindlehorizontal and parallel to work holding surface, (+X) to the right when viewed from spindle towards work piecehorizontal and parallel to the work holding surface, (+X) to the right when viewed from spindle towards columnparallel to and positive in the principal direction of cutting (primary motion)Yapply right hand rules
45 RIGHT HAND RULE Vertical Machine Horizontal Machine
48 NUMERICALLY CONTROLLED MACHINE TOOLS: An NC machine tool is functionally the same as a conventional machine tool. The technological capabilities NC machine tools in terms of machining are no different from those of conventional ones. The difference is in the way in which the various machine functions and slide movements are controlled.
49 The functions and motions such as; turning the spindle on and off setting cutting speeds setting feed rate turning coolant on and off moving tool with respect to workpiece are performed by Machine Control Unit (MCU) in NC machine tools.
54 PART PROGRAMA 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 exampleN10 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
55 PROGRAM INPUT DEVICEThe program input device is the mechanism for part programs to be entered into the CNC control. The most commonly used program input devices are keyboards, punched tape reader, diskette drivers, throgh RS 232 serial ports and networks.
56 MACHINE CONTROL UNITThe machine control unit (MCU) is the heart of a CNC system. It is used to perform the following functions:Read coded instructionsDecode coded instructionsImplement interpolations (linear, circular, and helical) to generate axis motion commandsFeed axis motion commands to the amplifier circuits for driving the axis mechanismsReceive the feedback signals of position and speed for each drive axisImplement auxiliary control functions such as coolant or spindle on/off, and tool change
57 TYPES of CNC CONTROL SYSTEMS Open-loop controlClosed-loop control
58 OPEN-LOOP CONTROL SYSTEM In open-loop control system step motors are usedStep motors are driven by electric pulsesEvery pulse rotates the motor spindle through a certain amountBy counting the pulses, the amount of motion can be controlledNo feedback signal for error correctionLower positioning accuracy
59 CLOSED-LOOP CONTROL SYSTEMS In closed-loop control systems DC or AC motors are usedPosition transducers are used to generate position feedback signals for error correctionBetter accuracy can be achievedMore expensiveSuitable for large size machine tools
60 CONTROL Desired path (p, v, a) Path generator 3-axis position control (encoder feedback)Velocity control (tachometer feedback)Torque control (current feedback)Path generatorLinear interpolationCircular interpolationComplex path interpolation (contouring)
61 DRIVE SYSTEMA drive system consists of amplifier circuits, stepping motors or servomotors 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 stepping motors which in turn rotate the ball lead-screws to position the machine table.
62 STEPPING MOTORSA stepping motor provides open-loop, digital control of the position of a workpiece in a numerical control machine. The drive unit receives a direction input (cw or ccw) and pulse inputs. For each pulse it receives, the drive unit manipulates the motor voltage and current, causing the motor shaft to rotate bya fixed angle (one step). The lead screw converts the rotary motion of the motor shaft into linear motion of the workpiece .
64 RECIRCULATING BALL SCREWS Transform rotational motion of the motor into translational motion of the nut attached to the machine table.
65 RECIRCULATING BALL SCREWS Accuracy of CNC machines depends on their rigid construction, care in manufacturing, and the use of ball screws to almost eliminate slop in the screws used to move portions of the machine.
66 COMPONENTS OF RECIRCULATING BALL SCREWS Ball nut (anti-backlash)WaysLinear bearings
68 POSITIONINGThe positioning resolution of a ball screw drive mechanism is directly proportional to the smallest angle that the motor can turn.The smallest angle is controlled by the motor step size.Microsteps can be used to decrease the motor step size.CNC machines typically have resolutions of mm or better.
69 MACHINE TOOLCNC 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.
70 FEEDBACK SYSTEMThe 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.
81 CNC Programming Manual Computer-assisted CAD/CAM Write code directly Draw cutter pathCAD/CAMDraw the partCutter path is generated
82 VELOCITY FEEDBACK Tachometers: Electrical output is proportional to rate of angular rotation.Encoders, Resolvers, Potentiometers:Number of pulses per time is proportional to rate change of position.
84 CNC CUTTERSTurning center cuttersMachining center cutters
85 TURNING CENTER CUTTERS Types of cutters used on CNC turning centersCarbides (and other hard materials) insert turning and boring toolsCeramicsHigh Speed Steel (HSS) drills and taps
86 STANDART INSERT SHAPES V – used for profiling, weakest insert, 2 edges per side.D – somewhat stronger, used for profiling when the angle allows it, 2 edges per side.T – commonly used for turning because it has 3 edges per side.C – popular insert because the same holder can be used for turning and facing. 2 edges per side.W – newest shape. Can turn and face like the C, but 3 edges per side.S – Very strong, but mostly used for chamfering because it won’t cut a square shoulder. 4 edges per side.R – strongest insert but least commonly used.
88 MACHINING CENTER CUTTING TOOLS Most machining centers use some form of HSS or carbide insert endmill as the basic cutting tool.Insert endmills cut many times faster than HSS, but theHSS endmills leave a better finish when side cutting.
89 MACHINING CENTER CUTTING TOOLS (cont’d) Facemills flatten large surfaces quickly and with an excellent finish. Notice the engine block being finished in one pass with a large cutter.
90 MACHINING CENTER CUTTING TOOLS (cont’d) Ball endmills (both HSS and insert) are used for a variety of profiling operations such as the mold shown in the picture.Slitting and side cutters are used when deep, narrow slots must be cut.
91 MACHINING CENTER CUTTING TOOLS (cont’d) Drills, Taps, and ReamersCommon HSS tools such as drills, taps, and reamers are commonly used on CNC machining centers. Note that a spot drill is used instead of a centerdrill. Also, spiral point or gun taps are used for through holes and spiral flute for blind holes. Rarely are hand taps used on a machining center.
92 TOOL HOLDERSAll cutting tools must be held in a holder that fits in the spindle. These include end mill holders (shown), collet holders, face mill adapters, etc. Most machines in the USA use a CAT taper which is a modified NST 30, 40, or 50 taper that uses a pull stud and a groove in the flange. The machine pulls on the pull stud to hold the holder in the spindle, and the groove in the flange gives the automatic tool changer something to hold onto. HSK tool holders were designed a number of years ago as an improvement to CAT tapers, but they are gaining acceptance slowly.
94 CNC PROGRAMMING Offline programming linked to CAD programs. Conversational programming by the operator.MDI ~ Manual Data Input.Manual Control using jog buttons or `electronic handwheel'.Word-Address Coding using standard G-codes and M-codes.
95 Basics of NC Part Programming: During secondary motion, either the tool moves relative to the workpiece or the workpiece moves relative to the tool. In NC programming, it is always assumed that the tool moves relative to the workpiece no matter what the real situation is.
96 The position of the tool is described by using a Cartesian coordinate system. If (0,0,0) position can be described by the operator, then it is called floating zero.
97 In defining the motion of the tool from one point to another, either absolute positioning mode or incremental positioning mode can be used.
98 1. Absolute positioning. In this mode, the desired target position of the tool for a particular move is given relative to the origin point of the program. 2. Incremental positioning. In this mode, the next target position for the tool is given relative to the current tool position.
99 Structure of an NC Part Program: Commands are input into the controller in units called blocks or statements.Block Format:1. Fixed sequential format2. Tab sequential format3. Word address format
100 EXAMPLE: Assume that a drilling operation is to be programmed as: 1. The tool is positioned at (25.4,12.5,0) by a rapid movement.2. The tool is then advanced -10 mm in the z direction at a feed rate of 500 mm/min., with the flood coolant on.3.The is then retracted back 10 mm at the rapid feed rate, and the coolant is turned off.
102 Modal commands: Commands issued in the NC program that will stay in effect until it is changed by some other command, like, feed rate selection, coolant selection, etc. Nonmodal commands: Commands that are effective only when issued and whose effects are lost for subsequent commands, like, a dwell command which instructs the tool to remain in a given configuration for a given amount of time.
104 INFORMATION NEEDED by a CNC 1. Preparatory Information: units, incremental or absolute positioning2. Coordinates: X,Y,Z, RX,RY,RZ3. Machining Parameters: Feed rate and spindle speed4. Coolant Control: On/Off, Flood, Mist5. Tool Control: Tool and tool parameters6. Cycle Functions: Type of action required7. Miscellaneous Control: Spindle on/off, direction of rotation, stops for part movementThis information is conveyed to the machine through a setof instructions arranged in a desired sequence – Program.
105 BLOCK FORMAT Sample Block N135 G01 X1.0 Y1.0 Z0.125 F5Restrictions on CNC blocksEach may contain only one tool moveEach may contain any number of non-tool move G-codesEach may contain only one feedrateEach may contain only one specified tool or spindle speedThe block numbers should be sequentialBoth the program start flag and the program number must be independent of all other commands (on separate lines)The data within a block should follow the sequence shown in the above sample block
106 WORD-ADDRESS CODINGExample CNC ProgramEach instruction to the machine consists of a letter followed by a number.Each letter is associated with a specific type of action or piece of information needed by the machine.N5 G90 G20N10 M06 T3N15 M03 S1250N20 G00 X1 Y1N25 Z0.1N30 G01 Z F5N35 X3 Y2 F10N40 G00 Z1N45 X0 Y0N50 M05N55 M30Letters used in CodesN,G,X,Y,Z,A,B,C,I,J,K,F,S,T,R,M
107 G & M Codes G-codes: Preparatory Functions involve actual tool moves. Example CNC ProgramG-codes: Preparatory Functions involve actual tool moves.M-codes: MiscellaneousFunctions – involve actionsnecessary for machining (i.e.spindle on/off, coolant on/off).N5 G90 G20N10 M06 T3N15 M03 S1250N20 G00 X1 Y1N25 Z0.1N30 G01 Z F5N35 X3 Y2 F10N40 G00 Z1N45 X0 Y0N50 M05N55 M30
111 M Codes M00 Program stop M01 Optional program stop M02 Program end M03 Spindle on clockwiseM04 Spindle on counterclockwiseM05 Spindle stopM06 Tool changeM08 Coolant onM09 Coolant offM10 Clamps onM11 Clamps offM30 Program stop, reset to start
112 N Codes Gives an identifying number for each block of information. It is generally good practice to increment each block number by 5 or 10 to allow additional blocks to be inserted if future changes are required.
113 X,Y, and Z CodesX, Y, and Z codes are used to specify the coordinate axis.Number following the code defines the coordinate at the end of the move relative to an incremental or absolute reference point.
114 I,J, and K CodesI, J, and K codes are used to specify the coordinate axis when defining the center of a circle.Number following the code defines the respective coordinate for the center of the circle.
115 F,S, and T Codes F-code: used to specify the feed rate S-code: used to specify the spindle speedT-code: used to specify the tool identification number associated with the tool to be used in subsequent operations.
116 Application of Some Codes G01 Linear Interpolation Format: N_ G01 X_ Y_ Z_ F_Linear Interpolation results in a straight line feed move.Unless tool compensation is used, the coordinates are associated with the centerline of the tool.
117 Application of Some Codes G01 Linear Interpolation . As an example, for the motion that occurs in x-y plane with the same maximum speed for the x- and y-axis, initial motion is at an angle of 45o to the axes until motion in one ofthe axes is completed and then the balance of the motion occurs in the other axis. This is called point-to-point motion.
118 Application of Some Codes G01 Linear Interpolation
119 Application of Some Codes G01 Linear Interpolation G01 is another preparatory function to specify that the tool should be moved to a specified location along a straight line path. It is referred to as linear interpolation.This function is typically used to specify machining of straight features such as turning a cylindrical surface in turning, cutting a slot in milling, etc.
120 Application of Some Codes G01 Linear Interpolation
121 G01 Linear Interpolation N10 G00 X1 Z1 N15 Z0.1 N20 G01 Z F5 N25 X2 Z2 F10XZ
122 G02 Circular Interpolation G02 is also a preparatory function to specify that the tool should be moved to a specified location along a circular path in a clockwise direction. In order to specify the path to the MCU, the end point of the arc and the location of the center of the arc should be specified. Within the block in which the G02 code is programmed, the center of the arc is given by specifying its location relative to the start of the arc.
123 G02 Circular Interpolation (CW) The G02 command requires an endpoint and a radius in order to cut the arc.I,J, and K are relative to the start point.N_ G02 X2 Y1 I0 J-1 F10orN_ G02 X2 Y1 R1
125 Canned CyclesThe sequence of some machining operations is may be the same for any part and for any machine. For example, drilling a hole involves the following steps:Position the tool above the point where the hole will be drilledSet the correct spindle speedFeed the tool into the workpiece at a controlled feed rate to a predetermined depthRetract the tool at a rapid rate to just above the point where the hole started
128 Three Main parts of a CNC program Part 1- Program PetupN5 G90 G21 (Absolute units, metric)N10 M06 T2 (Stop for tool change, use tool # 2)N15 M03 S1200 (Turn the spindle on CW to rpm)
129 Three Main parts of a CNC program Part 2- Chip RemovalN20 G00 X1 Y1 (Rapid to X1,Y1 from origin point)N25 Z (Rapid down to Z0.125)N30 G01 Z F100 (Feed down to Z at mm/min)N35 G01 X2 Y2 (Feed diagonally to X2,Y2)N40 G00 Z1 (Rapid up to Z1)N45 X0 Y0 (Rapid to X0,Y0)
130 Three Main parts of a CNC program Part 3- System ShutdownN50 M05 (Turn the spindle off)N55 M00 (Program stop)
132 G-CODE PROGRAMFirst pass : conventional mill to a depth of around edge profile. Tool 1 is a ½ inch dia. end mill.%:1002N5 G90 G20N10 M06 T1N15 M03 S1200N20 G00 X0.125 Y0.125N30 Z0.125N35 G01 Z F5N40 X3.875N45 Y4.125N50 X0.125N55 Y0.125
133 Second pass: conventional mill to a depth of 0.25 around edge profile. N35 Z-0.250N40 X3.875N45 Y4.125N50 X0.125N55 Y0.125N60 Z0.125
134 Third pass: conventional mill to a depth of 0 Third pass: conventional mill to a depth of around pocket profile.N65 G00 X1.25 Y1.0N70 G01 Z F5N75 X1.75N80 Y2.5N85 X1.25N90 Y1.0N95 Z0.125
135 Fourth pass: climb mill to a depth of 0.125 across remaining material. N100 Y2.125N105 X2.625N110 Z0.125N115 G00 X-5 Y-5 Z5N120 M05N125 M30
136 Advanced features:Execution of the part of the program in a rotated or mirrored position.Ability to scale the program and produce larger or smaller programs.Three dimensional circular interpolation which produces a helical shape.Parabolic and cubic interpolation.
137 Program Loading:Through keyboardThrough punched tape readerThrough diskette driveThrough RS 232 serial portThrough network interface card
138 Direct Numerical Control (DNC): A system in which a central computer downloads the NC programs block by block to many NC machine tools simultaneously is called Direct Numerical Control (DNC) system.
139 Direct Numerical Control (DNC): This system used to work with the early NC machine tools which can not read more than a block of information at a time. The central computer feed the program information one block at a time. When the machine execute the information, the next block of information would be fed.
140 Distributed Numerical Control (DNC): Distributed NC is known by the same acronym as Direct Numerical Control (DNC). After the introduction of CNC, the machine tools have had the capability of storing large amount of information. Therefore, there have been no need to have drip feed information system, like, Direct Numerical Control. Instead, Distributed Numerical Control is introduced. In such a system, a host computer communicate with many CNC machine tools via networks and download or upload programs.
141 Distributed Numerical Control (DNC): With Distributed Numerical Control systems, it is possible to monitor the activities in individual CNC machine tools on host computer.Therefore, better shop floor control can be achieved.
142 Computer Aided Part Programming: NC program preparation may be tedious and difficult if the part to be machined has a complex geometry. The main difficulty is to find out the cutter locations during the machining. Computers may be used to assist the programmers in preparing the NC codes.
143 Advantages of applying computer-aided part programming include the following: 1. It reduces the manual calculations involves in determining the geometric characteristics of the part.It provides the cutter path simulation.It provides tool collision checking.It shortens the program preparation time.It makes the program preparation easier.
144 The Aerospace Industries Association sponsored the work that led to the first part programming language, developed in MIT in 1955.This was called: Automatically Programmed Tools (APT).APT is an English like simple programming language which basically produce the Cutter Location (CL) data.Using the cutter location data, the program can generate the actual NC codes by using a postprocessor .
145 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, CADAM, ProEngineer, MechanicalDesktop (Auto Desk).