Ch 7 Numerical Control - Networ-based Systems Lab

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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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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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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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Basic Components of an NC System

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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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Coordinate Axis System for Flat and Prismatic Parts

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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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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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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