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Ch 7 Numerical Control

Sections:

1. Fundamentals of NC Technology

2. Computer Numerical Control

3. DNC

4. Applications of NC

5. Engineering Analysis of NC Positioning Systems

6. NC Part Programming

Numerical Control (NC) Defined

Form of programmable automation in which the mechanicalactions of a machine tool or other equipment arecontrolled by a program containing coded alphanumericdata

The alphanumeric data represent relative positionsbetween a workhead (e.g., cutting tool) and a workpart

When the current job is completed, a new program can beentered for the next job

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Basic Components of an NC System

1. Program of instructions

Part program in machining

2. Machine control unit

Controls the process

3. Processing equipment

Performs the process

Program of Instructions

Step-by-step commands

Part programmer

The program is coded on a suitable medium forsubmission to the machine control unit

Three class of language to be coded:

- Machine language

- Assembly language- High level language C++, Pascal, Fortran

- Object Oriented Language

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Machine Control Unit

Microcomputer and related control hardware

Stores and executes the program

Hardware includes interface component and readingdevices

Software system software, calculation algorithms, andtranslation software

Computer numerical control(CNC)

Processing Equipment

Performs the actual productive work

Driven by instructions

Common examples the worktable and spindle, motorsand controls

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Basic Components of an NC System

NC Coordinate Systems

Two axis systems

Flat and prismatic workparts

Rotational parts

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NC Coordinate SystemsFor flat and prismatic (block-like) parts

Milling and drilling operations

Conventional Cartesian coordinate system

Rotational axes about each linear axis

Right hand rule

Coordinate Axis System for

Flat and Prismatic Parts

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Flat and Prismatic Parts The x- and y-axes are used to move and position the

worktable to which part is attached

The z-axis is used to control the vertical position of thecutting tool

NC Coordinate Systems

For rotational parts:

Turning operations

Conventional Cartesian coordinate system, but only x- andz-axes

y-axis not needed in turning

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Rotational Parts

Motion Control Systems

Point-to-Point systems

Continuous path systems

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Point-to-Point Systems Also called position systems

System moves to a location and performs an operation atthat location (e.g., drilling)

The programs consists of a series of points locations atwhich operations are performed

Also applicable in robotics

Point-To-Point Control in NC

Drilling of Three Holes in Flat Plate

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Continuous Path Systems Capable of continuous simultaneous control of two or

more axes

Also called contouring systems in machining

System performs an operation during movement (e.g.,milling and turning)

Continuous Path Control in NC

Profile Milling of Part Outline

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Interpolation Methods

Important aspect of contouring

Why it is needed?

Continuous (Equipment) vs Digital (NC)

Interpolation Methods

1. Linear interpolation

Straight line between two points in space

2. Circular interpolation

Circular arc defined by starting point, end point, centeror radius, and direction

3. Helical interpolation

Circular plus linear motion

4. Parabolic and cubic interpolation Free 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 onlythe insideof the nominal curve


Approximation of a curved path in NC by a series ofstraight line segments, where tolerance is defined on onlythe outsideof the nominal curve

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Approximation of a curved path in NC by a series of

straight line segments, where tolerance is defined on boththe inside and outsideof the nominal curve

Absolute and Incremental Positioning

Absolute positioning

Locations defined relative to origin of axis system

Incremental positioning

Locations defined relative to previous position

Example: drilling

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Absolute vs. Incremental Positioning

The workhead is presentlyat point (20, 20) and is to bemoved 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.

Computer Numerical Control (CNC)

Definition

An NC system whose MCU is based on a dedicatedmicrocomputer rather than on a hard-wired controller.

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Additional FeaturesPlease discuss!!!

Storage of more than one part program

- More memory expansions

Various forms of program input

- multiple data entry capabilities

- RS-232 (Serial), Parallel, USB

Program editing at the machine tool

- can be tested and corrected at the machine site

- can also be optimized


Additional Features

Fixed cycles and programming subroutines

- full instructions vs call statement

Interpolation

Positioning features for setup

- By software options

Cutter length and size compensation

- Manual entering may differ from program, thus

computed tool path will do the compensation- Use of tool length sensor

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Input/Output Interface

Provides communication between components

Transmit and receives data and signals to and fromexternal devices

Eg., Operator control panel, display

Machine Tool Controls

Hardware component control position and velocity, androtational speed of the machine tool spindle

Consist of a drive control circuit and a feedback sensorinterface

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Sequence Controls

Additional functions

Eg., on/off actuations, interlocks, and discrete numericaldata

CNC Software

Operating system software

- editor

- control program

- executive program

Machine interface software

- communication link

Application software

- for machining applications

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DNC

Direct numerical control (DNC) control of multiplemachine tools by a single (mainframe) computerthrough direct connection and in real time

1960s technology

Two way communication

Distributed numerical control (DNC) networkconsisting of central computer connected to machinetool MCUs, which are CNC

Present technology Two way communication

Components of Direct NC

Central Computer

Bulk memory at the central computer site

Set of controlled machines

Telecommunications line to connect the machines tothe central computer

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General Configuration of a

Direct Numerical Control System

Connection to MCU is behind the tape reader (BTR). Indistributed NC, entire programs are downloaded to eachMCU, which is CNC rather than conventional NC

Distributed NC (DNC)

The central computer is connected to MCUs, which arethemselves computers.

Advantages

- easier and less cost

- possible of expansion

- flexibility and reliability

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Distributed Numerical Control

Configurations

Switching network

Distributed Numerical Control

Configurations

Local area network (LAN)

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Applications of NC

Machine tool applications:

Milling, drilling, turning, boring, grinding

Machining centers, turning centers, mill-turn centers

Punch presses, thermal cutting machines, etc.

Other NC applications:

Component insertion machines in electronics

Drafting machines (x-y plotters)

Coordinate measuring machines

Tape laying machines for polymer composites

Filament winding machines for polymer composites

Common NC Machining Operations

Turning

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Common NC Machining Operations

Milling

Drilling

CNC Horizontal Milling Machine

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NC Application Characteristics

(Machining)Where NC is most appropriate:

1. Batch production

2. Repeat orders

3. Complex part geometries

4. Much metal needs to be removed from the startingworkpart

5. Many separate machining operations on the part

6. The part is expensive

Advantages of NC Nonproductive time is reduced

Greater accuracy and repeatability

Lower scrap rates

Inspection requirements are reduced

More complex part geometries are possible

Engineering changes are easier to make

Simpler fixtures

Shorter lead times Reduce parts inventory and less floor space

Operator skill-level requirements are reduced

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Disadvantages of NC

Higher investment cost

CNC machines are more expensive

Higher maintenance effort

CNC machines are more technologically sophisticated

Part programming issues

Need for skilled programmers

Time investment for each new part

Repeat orders are easy because part program is

already available

Higher utilization is required

NC Positioning System

Typical motor and leadscrew arrangement in an NC

positioning system for one linear axis

For x-ycapability, the apparatus would be piggybacked ontop 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 actualposition achieved is the desired position

2. Closed-loop - uses feedback measurements toconfirm that the final position is the specified position

Precision in NC positioning - three measures:

1. Control resolution

2. Accuracy

3. Repeatability

Open-Loop Motion Control System

Operates without verifying that the actual positionachieved 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 theprogram

Optical Encoder

Device for measuring rotational position and speed

Common feedback sensor for closed-loop NC control

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Precision in NC PositioningThree measures of precision:

1. Control resolution - distance separating two adjacentaddressable points in the axis movement

2. Accuracy - maximum possible error that can occurbetween the desired target point and the actual positiontaken by the system

3. Repeatability - defined as 3of the mechanical errordistribution associated with the axis

Definitions of Control Resolution,

Accuracy, and Repeatability

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NC Part Programming

1. Manual part programming

2. Computer-assisted part programming

3. Part programming using CAD/CAM

4. Manual data input

Binary Coded Decimal System

Each of the ten digits in decimal system is coded withfour-digit binary number

The binary numbers are added to give the value

BCD is compatible with 8 bits across tape format, theoriginal storage medium for NC part programs

Eight 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 tape

Character - row of bits across the tape

Word - sequence of characters (e.g., y-axis position)

Block - collection of words to form one completeinstruction

Part program - sequence of instructions (blocks)

Block Format

Organization of words within a block in NC part program

Also known as tape format because the originalformats were designed for punched tape

Word address format - used on all modern CNCcontrollers

Uses a letter prefix to identify each type of word

Spaces to separate words within the block

Allows any order of words in a block Words can be omitted if their values do not

change from the previous block

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Types of Words

N - sequence number prefix

G - preparatory words

Example: G00 = PTP rapid traverse move

X, Y, Z - prefixes for x, y, and z-axes

F - feed rate prefix

S - spindle speed

T - tool selection

M - miscellaneous command

Example: M07 = turn cutting fluid on

Example: Word Address Format

N001 G00 X07000 Y03000 M03

N002 Y06000

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Issues in Manual Part Programming Adequate for simple jobs, e.g., PTP drilling

Linear interpolation

G01 G94 X050.0 Y086.5 Z100.0 F40 S800

Circular interpolation

G02 G17 X088.0 Y040.0 R028.0 F30

Cutter offset

G42 G01 X100.0 Y040.0 D05

Computer-Assisted Part Programming

Manual part programming is time-consuming, tedious,and subject to human errors for complex jobs

Machining instructions are written in English-likestatements that are translated by the computer intothe low-level machine code of the MCU

APT (Automatically Programmed Tool)

The various tasks in computer-assisted partprogramming are divided between

The human part programmer

The computer

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Computer-Assisted Part Programming

Sequence of activities in computer-assisted partprogramming

Part Programmer's Job

Two main tasks of the programmer:

1.Define the part geometry

2.Specify the tool path

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Defining Part Geometry

Underlying assumption: no matter how complex the partgeometry, it is composed of basic geometric elements andmathematically defined surfaces

Geometry elements are sometimes defined only for use inspecifying tool path

Examples of part geometry definitions:

P4 = POINT/35,90,0

L1 = LINE/P1,P2

C1 = CIRCLE/CENTER,P8,RADIUS,30

Specifying Tool Path and

Operation Sequence

Tool path consists of a sequence of points or connectedline and arc segments, using previously defined geometryelements

Point-to-Point command:

GOTO/P0

Continuous path command

GOLFT/L2,TANTO,C1

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2008 Pearson Education, Inc ., Upper Saddle River, NJ. A ll rights reserved. This material is protected under all copyright laws as they currently exist.No portion of this material may be reproduced, in any form or by any means, without permissi on in writing from the publisher. For the exclusive use of adopters of the book

Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

Other Functions in Computer-Assisted

Part Programming Specifying cutting speeds and feed rates

Designating cutter size (for tool offset calculations)

Specifying tolerances in circular interpolation

Naming the program

Identifying the machine tool

Cutter Offset

Cutter path must beoffset from actualpart outline by adistance equal tothe cutter radius

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Computer Tasks in Computer-Assisted

Part Programming1. Input translation converts the coded instructions in the

part program into computer-usable form

2. Arithmetic and cutter offset computations performs themathematical computations to define the part surface andgenerate the tool path, including cutter offsetcompensation (CLFILE)

3. Editing provides readable data on cutter locations andmachine tool operating commands (CLDATA)

4. Postprocessing converts CLDATA into low-level codethat can be interpreted by the MCU

NC Part Programming Using

CAD/CAM

Geometry definition

If the CAD/CAM system was used to define the originalpart geometry, no need to recreate that geometry as inAPT

Automatic labeling of geometry elements

If the CAD part data are not available, geometry mustbe created, as in APT, but user gets immediate visualfeedback 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 graphicalverification of the command

Automatic software modules for common machiningcycles

Profile milling

Pocket milling

Drilling bolt circles

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)

Manual Data Input

Machine operator does part programming at machine

Operator enters program by responding to prompts andquestions by system

Monitor with graphics verifies tool path

Usually for relatively simple parts

Ideal for small shop that cannot afford a part programmingstaff

To minimize changeover time, system should allowprogramming of next job while current job is running

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