# Unit 5 Numerical Control

Electronique - Appareils

8 nov. 2013 (il y a 7 années et 11 mois)

361 vue(s)

Unit 5 Numerical Control

Sections:

1.
Fundamentals of NC Technology

2.
Computer Numerical Control

3.
Distributed Numerical Control

4.
Applications of NC

5.
NC Part Programming

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

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

Only
x
-

and
z
-
axes

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)

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, and
direction

3.
Helical interpolation

Circular plus linear motion

4.
Parabolic and cubic interpolation

Free form curves using higher order
equations

Absolute vs. Incremental Positioning

Absolute positioning

Move is: x

= 40,
y

= 50

Incremental positioning

Move is: x

= 20,
y

= 30.

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

Machine Control Unit

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

Distributed Numerical Control

Machine

Control Unit

Transformation

Process

Machine

Control Unit

Machine

Control Unit

Central

Computer

NC Pgms

BTR

BTR

BTR

Computer Network

Applications of NC

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

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

Precision

NC Part Programming

1.
Manual part programming

2.
Manual data input

3.
Computer
-
assisted part programming

4.

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

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 complete
instruction

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 original
formats were designed for punched tape

-

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

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

N001 G00 X07000 Y03000 M03

N002 Y06000

Cutter Offset

Cutter path must be
offset from actual
part outline by a
distance equal to

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

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;

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 v
ertical edge at left of part.

Rapid move to target point, cancel offset, spindle stop.

End of program, stop machine.

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

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

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

Tool path is sequence of points or connected line and arc
segments

Point
-
to
-
Point command: GOTO/P4

Continuous path command: GOLFT/L1,TANTO,C1

NC Part Programming Using

CNC Milling

CNC Punching