ch 7 numerical control - fkm.utm.myjinhoe/notes/skmp4722 modern... · system moves to a location...
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
Ch 7 Numerical Control
Sections:1. Fundamentals of NC Technology2. Computer Numerical Control3. DNC4. Applications of NC5. Engineering Analysis of NC Positioning Systems6. NC Part Programming
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
Numerical Control (NC) Defined
Form of programmable automation in which the mechanical actions of a machine tool or other equipment are controlled by a program containing coded alphanumeric dataThe alphanumeric data represent relative positions between a workhead (e.g., cutting tool) and a workpartWhen the current job is completed, a new program can be entered for the next job
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
Basic Components of an NC System
1. Program of instructionsPart program in machining
2. Machine control unitControls the process
3. Processing equipmentPerforms the process
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
Basic Components of an NC System
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
NC Coordinate Systems
For flat and prismatic (block-like) partsMilling and drilling operationsConventional Cartesian coordinate systemRotational axes about each linear axisRight hand rule
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
Coordinate Axis System for Flat and Prismatic Parts
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
NC Coordinate Systems
For rotational parts:Turning operationsConventional Cartesian coordinate system, but only x- and z-axesy-axis not needed in turning
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Coordinate Axis System for Rotational Parts
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
Motion Control Systems
Point-to-Point systemsAlso called position systemsSystem moves to a location and performs an operation at that location (e.g., drilling)Also applicable in robotics
Continuous path systems Also called contouring systems in machiningSystem performs an operation during movement (e.g., milling and turning)
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
Point-To-Point Control in NCDrilling of Three Holes in Flat Plate
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Continuous Path Control in NCProfile Milling of Part Outline
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Interpolation Methods
1. Linear interpolationStraight line between two points in space
2. Circular interpolationCircular arc defined by starting point, end point, center or radius, and direction
3. Helical interpolationCircular plus linear motion
4. Parabolic and cubic interpolationFree form curves using higher order equations
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Circular Interpolation
Approximation of a curved path in NC by a series of straight line segments, where tolerance is defined on only the inside of the nominal curve
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Circular Interpolation
Approximation of a curved path in NC by a series of straight line segments, where tolerance is defined on only the outside of the nominal curve
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Circular Interpolation
Approximation of a curved path in NC by a series of straight line segments, where tolerance is defined on both the inside and outside of the nominal curve
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Absolute and Incremental Positioning
Absolute positioningLocations defined relative to origin of axis system
Incremental positioningLocations defined relative to previous positionExample: drilling
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Absolute vs. Incremental Positioning
The workhead is presently at point (20, 20) and is to be moved to point (40, 50)
In absolute positioning, the move is specified by x = 40, y = 50
In incremental positioning, the move is specified by x = 20, y = 30.
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Computer Numerical Control (CNC) –Additional Features
Storage of more than one part programVarious forms of program inputProgram editing at the machine toolFixed cycles and programming subroutinesInterpolationAcceleration and deceleration computationsCommunications interfaceDiagnostics
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Configuration of CNC Machine Control Unit
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DNC
Direct numerical control (DNC) – control of multiple machine tools by a single (mainframe) computer through direct connection and in real time
1960s technologyTwo way communication
Distributed numerical control (DNC) – network consisting of central computer connected to machine tool MCUs, which are CNC
Present technologyTwo way communication
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General Configuration of a Direct Numerical Control System
Connection to MCU is behind the tape reader (BTR). In distributed NC, entire programs are downloaded to each MCU, which is CNC rather than conventional NC
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Distributed Numerical Control Configurations
Switching network
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Distributed Numerical Control Configurations
Local area network (LAN)
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Applications of NC
Machine tool applications:Milling, drilling, turning, boring, grindingMachining centers, turning centers, mill-turn centersPunch presses, thermal cutting machines, etc.
Other NC applications:Component insertion machines in electronicsDrafting machines (x-y plotters)Coordinate measuring machinesTape laying machines for polymer compositesFilament winding machines for polymer composites
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Common NC Machining Operations
Turning
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Common NC Machining Operations
MillingDrilling
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CNC Horizontal Milling Machine
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NC Application Characteristics (Machining)
Where NC is most appropriate:1. Batch production2. Repeat orders3. Complex part geometries4. Much metal needs to be removed from the starting
workpart5. Many separate machining operations on the part6. The part is expensive
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Advantages of NC
Nonproductive time is reducedGreater accuracy and repeatabilityLower scrap ratesInspection requirements are reducedMore complex part geometries are possibleEngineering changes are easier to makeSimpler fixturesShorter lead timesReduce parts inventory and less floor spaceOperator skill-level requirements are reduced
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Disadvantages of NC
Higher investment costCNC machines are more expensive
Higher maintenance effortCNC machines are more technologically sophisticated
Part programming issuesNeed for skilled programmersTime investment for each new partRepeat orders are easy because part program is already available
Higher utilization is required
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NC Positioning System
Typical motor and leadscrew arrangement in an NC positioning system for one linear axisFor x-y capability, the apparatus would be piggybacked on top of a second perpendicular axis
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Analysis of Positioning NC Systems
Two types of NC positioning systems:1. Open-loop - no feedback to verify that the actual
position achieved is the desired position2. Closed-loop - uses feedback measurements to
confirm that the final position is the specified positionPrecision in NC positioning - three measures:1. Control resolution2. Accuracy3. Repeatability
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Open-Loop Motion Control System
Operates without verifying that the actual position achieved in the move is the desired position
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Closed-Loop Motion Control System
Uses feedback measurements to confirm that the final position of the worktable is the location specified in the program
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Optical Encoder
Device for measuring rotational position and speedCommon feedback sensor for closed-loop NC control
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Precision in NC Positioning
Three measures of precision:1. Control resolution - distance separating two adjacent
addressable points in the axis movement2. Accuracy - maximum possible error that can occur
between the desired target point and the actual position taken by the system
3. Repeatability - defined as ±3σ of the mechanical error distribution associated with the axis
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Definitions of Control Resolution, Accuracy, and Repeatability
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NC Part Programming
1. Manual part programming2. Computer-assisted part programming3. Part programming using CAD/CAM4. Manual data input
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Binary Coded Decimal System
Each of the ten digits in decimal system is coded with four-digit binary numberThe binary numbers are added to give the valueBCD is compatible with 8 bits across tape format, the original storage medium for NC part programsEight bits can also be used for letters and symbols
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Creating Instructions for NC
Bit - 0 or 1 = absence or presence of hole in the tapeCharacter - row of bits across the tapeWord - sequence of characters (e.g., y-axis position)Block - collection of words to form one complete instructionPart program - sequence of instructions (blocks)
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Block Format
Organization of words within a block in NC part programAlso known as tape format because the original formats were designed for punched tapeWord address format - used on all modern CNC controllers
Uses a letter prefix to identify each type of wordSpaces to separate words within the blockAllows any order of words in a blockWords can be omitted if their values do not change from the previous block
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Types of Words
N - sequence number prefixG - preparatory words
Example: G00 = PTP rapid traverse moveX, Y, Z - prefixes for x, y, and z-axesF - feed rate prefixS - spindle speedT - tool selectionM - miscellaneous command
Example: M07 = turn cutting fluid on
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Example: Word Address Format
N001 G00 X07000 Y03000 M03N002 Y06000
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Issues in Manual Part Programming
Adequate for simple jobs, e.g., PTP drillingLinear interpolation
G01 G94 X050.0 Y086.5 Z100.0 F40 S800Circular interpolation
G02 G17 X088.0 Y040.0 R028.0 F30Cutter offset
G42 G01 X100.0 Y040.0 D05
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Computer-Assisted Part Programming
Manual part programming is time-consuming, tedious, and subject to human errors for complex jobsMachining instructions are written in English-like statements that are translated by the computer into the low-level machine code of the MCUAPT (Automatically Programmed Tool)The various tasks in computer-assisted part programming are divided between
The human part programmerThe computer
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Computer-Assisted Part Programming
Sequence of activities in computer-assisted part programming
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Part Programmer's Job
Two main tasks of the programmer:1.Define the part geometry2.Specify the tool path
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Defining Part Geometry
Underlying assumption: no matter how complex the part geometry, it is composed of basic geometric elements and mathematically defined surfacesGeometry elements are sometimes defined only for use in specifying tool pathExamples of part geometry definitions:
P4 = POINT/35,90,0L1 = LINE/P1,P2C1 = CIRCLE/CENTER,P8,RADIUS,30
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Specifying Tool Path and Operation Sequence
Tool path consists of a sequence of points or connected line and arc segments, using previously defined geometry elementsPoint-to-Point command:
GOTO/P0Continuous path command
GOLFT/L2,TANTO,C1
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Other Functions in Computer-Assisted Part Programming
Specifying cutting speeds and feed ratesDesignating cutter size (for tool offset calculations)Specifying tolerances in circular interpolationNaming the programIdentifying the machine tool
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Cutter Offset
Cutter path must be offset from actual part outline by a distance equal to the cutter radius
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Computer Tasks in Computer-Assisted Part Programming
1. Input translation – converts the coded instructions in the part program into computer-usable form
2. Arithmetic and cutter offset computations – performs the mathematical computations to define the part surface and generate the tool path, including cutter offset compensation (CLFILE)
3. Editing – provides readable data on cutter locations and machine tool operating commands (CLDATA)
4. Postprocessing – converts CLDATA into low-level code that can be interpreted by the MCU
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NC Part Programming Using CAD/CAM
Geometry definitionIf the CAD/CAM system was used to define the original part geometry, no need to recreate that geometry as in APT
Automatic labeling of geometry elementsIf the CAD part data are not available, geometry must be created, as in APT, but user gets immediate visual feedback about the created geometry
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Tool Path Generation Using CAD/CAM
Basic approach: enter the commands one by one (similar to APT)
CAD/CAM system provides immediate graphical verification of the command
Automatic software modules for common machining cycles
Profile millingPocket millingDrilling bolt circles
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Examples of Machining Cycles in Automated NC Programming Modules
Pocket milling
Contour turning
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Examples of Machining Cycles in Automated NC Programming Modules
Facing and shoulder facing
Threading (external)
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Manual Data Input
Machine operator does part programming at machineOperator enters program by responding to prompts and questions by systemMonitor with graphics verifies tool pathUsually for relatively simple parts
Ideal for small shop that cannot afford a part programming staffTo minimize changeover time, system should allow programming of next job while current job is running