Unit 5 Numerical Control

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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.
Part programming using CAD/CAM

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


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

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


Cutter Offset

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

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;

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 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

C1 = CIRCLE/CENTER,P8,RADIUS,30



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
CAD/CAM

YouTube


CNC Milling


CNC Punching


CNC Adhesive Bonding


CNC Drug Insertion


CNC Bioprocessing


CAD/CAM


Etc.