5 computer control

8/12/2019 5 Computer Control

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History

1955 - John Parsons and US Air Force define a need to

develop a machine tool capable of machining complex and

close tolerance aircraft parts with the same quality time

after time (repeatability). MIT is the subcontractor and

builds the machine for the project.


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History: Continued

1959 - MIT announces Automatic Programmed Tools

(APT) programming language

1960 - Direct Numerical Control (DNC). This eliminates

paper tape punch programs and allows programmers tosend files directly to machine tools


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History: Continued

1968 - Kearney & Trecker machine tool builders market

first machining center

1970s - CNC machine tools & Distributed Numerical

Control

1980s - Graphics based CAM systems introduced. Unixand PC based systems available


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History: Continued

1990s - Price drop in CNC technology

1997 - PC- Windows/NT based Open Modular

Architecture Control (OMAC) systems introduced to

replace firmware controllers.


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Numerical Control

Sections:

1. Fundamentals of NC Technology

2. Computer Numerical Control

3. Distributed Numerical Control4. Applications of NC

5. NC Part Programming


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Numerical Control (NC) Defined

Programmable automation in which the mechanical actions of a machine

tool are controlled by a program containing coded alphanumeric data

that represents relative positions between a work head (e.g., cutting

tool) and a work part

Machine

Control Unit

Power

Program

Instructions

Transformation

Process


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What is numerical control (NC)?

NC has been defined by the Electronic Industries Association (EIA)

asa system in which actions are controlled by the direct insertion of

numerical data at some points. The system must automatically

interpret at least some portion of this data

The term NCis used to describe the control of the various functions

of a machine using numeric data. In the early age of NC, machines

were fed with information by means of the punched tape. An Electro-

mechanical tape reader was used to load a machine tape into the

controller.


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NC Coordinate Systems

For flat and prismatic (block-like) parts:

Milling and drilling operations

Conventional Cartesian coordinate system

Rotational axes about each linear axis

For rotational parts:

Turning operations

Onlyx- and z-axes


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Motion Control Systems

Point-to-Point systems

Also called position systems

System 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 machining

System performs an operation during movement

(e.g., milling and turning)


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Interpolation Methods

1. Linear interpolation

Straight line between two points in

space

2. Circular interpolation

Circular arc defined by starting point,

end point, center or radius, anddirection

3. Helical interpolation

Circular plus linear motion

4. Parabolic and cubic interpolation

Free form curves using higher orderequations


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Absolute vs. Incremental Positioning

Absolute positioning

Move is: x= 40, y= 50

Incremental positioning

Move is: x= 20, y= 30.


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Computer Numerical Control (CNC)

Storage of more than one part program

Various forms of program input

Program editing at the machine tool

Fixed cycles and programming subroutines Interpolation

Acceleration and deceleration computations

Communications interface

Diagnostics


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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 technology Two way communication

Distributed numerical control (DNC)network

consisting of central computer connected to machine

tool MCUs, which are CNC Present technology

Two way communication


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Distributed Numerical Control

Machine

Control Unit

Transformation

Process

Machine

Control Unit

Machine

Control Unit

Central

Computer NC Pgms

BTR BTR BTR

Computer Network


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The EIA definition of DNC

A system connecting a set of numerically controlledmachines to a common memory for part program ormachine program storage with provision for on-demanddistribution of data to machines.

In DNC, several NC machines are directly controlled bya computer, eliminating substantial hardware from theindividual controller of each machine tool. The part-program is downloaded to the machines directly (thusomitting the tape reader) from the computer memory.


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DNC

DNC is a computer control system.

DNC is the method of controlling a CNC with a separate

computer.

This is used when the CNC's memory is not sufficient

enough to handle the machining program.

If a separate computer is used to control various CNC's

then the control system is called as Distributed Numeric

Control else it is called Direct Numeric Control.


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General Configuration of aDirect 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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Configurations

Switching network


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Configurations


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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


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Common NC Machining Operations

TURNING


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Common NC Machining Operations


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NC Application Characteristics

(Machining)

Batch and High Volume production

Repeat and/or Repetitive orders

Complex part geometries

Mundane operations Many separate operations on one part


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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 isalready available

Higher utilization is required


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Cost-Benefits of NC

Costs High investment cost

High maintenance effort

Need for skilled programmers

High utilization required

Benefits

Cycle time reduction

Nonproductive time reduction

Greater accuracy and repeatability

Lower scrap rates

Reduced parts inventory and floor space

Operator skill-level reduced


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Precision in NC Positioning

Three measures of precision:

1. Control resolution - distance separating two adjacent

addressable points in the axis movement

2. Accuracy - maximum possible error that can occur

between the desired target point and the actual positiontaken by the system

3. Repeatability - defined as 3of the mechanical error

distribution associated with the axis


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Precision


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NC Part Programming

1. Manual part programming

2. Manual data input

3. Computer-assisted part programming

4. Part programming using CAD/CAM


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Manual Part Programming

Binary Coded Decimal System

Each of the ten digits in decimal system (0-9) is

coded with four-digit binary number

The binary numbers are added to give the value BCD is compatible with 8 bits across tape format, the

original 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)


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Block Format

Organization of words within a block in NC part program

Also known as tape format because the original

formats were designed for punched tape

Word address format - used on all modern CNC

controllers

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 forx, y, and z-axes

F - feed rate prefix

S - spindle speed

T - tool selection

M - miscellaneous command Example: M07 = turn cutting fluid on


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Example: Word Address Format

N001 G00 X07000 Y03000 M03

N002 Y06000


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Cutter Offset

Cutter path must be

offset from actual

part outline by a

distance equal tothe cutter radius


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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


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Example

NC part program code

N001 G21 G90 G92 X-050.0 Y-050.0 Z010.0;N002 G00 Z-020.0 S1989 M03;

N003 G01 G94 G42 Y0 D05 F398;N004 G01 X075.0;N005 G01 X150.0 Y043.02;

N006 G01 Y070.0;

N007 G01 X080.0;N008 G17 G02 X050.0 Y100.0 R030.0;N009 G01 Y125.0;

N010 G01 X0;

N011 G01 Y0

N012 G40 G00 X-050.0 Y-050.0 Z010.0 M05;N013 M30;

Comments

Define origin of axes.Rapid to cutter depth, turn spindle on.

Bring tool to starting y-value, start cutter offset.Mill lower horizontal edge of part.Mill angled edge at 35 degrees.

Mill vertical edge at right of part.

Mill horizontal edge leading to arc.Circular interpolation around arc.Mill vertical step above arc.

Mill top part edge.

Mill vertical edge at left of part.

Rapid move to target point, cancel offset, spindle stopEnd of program, stop machine.


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Manual Data Input

Machine operator does part programming at

machine

Operator enters program by responding to

prompts and questions by system

Monitor with graphics verifies tool path

Usually for relatively simple parts

Ideal for small shop that cannot afford a part

programming staff

To minimize changeover time, system should

allow programming of next job while current job

is running


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Computer-Assisted Part Programming

Write machine instructions using natural language

type statements

Statements translated into machine code of the MCU

APT (Automatically Programmed Tool) Language


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Computer-Assisted Part Programming

Sequence of activities in computer-assisted partprogramming


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Sample Statements

Part is composed of basic geometric elements and

mathematically defined surfaces

Examples of statements:

P4 = POINT/35,90,0

L1 = LINE/P1,P2

C1 = CIRCLE/CENTER,P8,RADIUS,30

Tool path is sequence of points or connected line and arcsegments

Point-to-Point command: GOTO/P4

Continuous path command: GOLFT/L1,TANTO,C1

C


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NC Part Programming Using

CAD/CAM


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YouTube

CNC Milling

CNC Punching

CNC Adhesive Bonding

CNC Drug Insertion

CNC Bioprocessing

CAD/CAM

Etc.

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