ISAT 331- Numerical Control - CISAT Sharepoint

ugliestmysticAI and Robotics

Nov 14, 2013 (3 years and 10 months ago)

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egekwu_nc

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ISAT 331

Manufacturing Control
-

Numerical
Control

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Numerical Control (NC)


is the use of coded numerical data (information) in
the automatic
control
of equipment positioning.


NC controls Applications:


motion of cutting tool


moving part being machined or assembled


fiber placement in filament winding composites


chip placement and assembly in electronic component
manufacturing.

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NC system

An NC system is made of
(Fig 8.1)

{Rabie}

4

NC system

An NC system is made of
(Fig previous slide)



Program of instructions
(part program)



Machine Control Unit
(MCU)

Hardware (and software) that converts program
instruction into electrical signals that in turn control
the mechanical actions of the machine



Machine
(Processing equipment)




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NC/CNC/DNC


NC


numerical control


Control functions controlled by “inflexible physical
electronic components e.g. punched paper tape readers;
no memory MCU and program cannot be stored.


CNC


computer numerical control


Microprocessor is an integral component of MCU


Motion interpolation handled by ‘soft
-
wired” features


Program editing and use of
canned cycles

(preprogrammed functions e.g. tapping of holes)
possible

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NC/CNC/DNC continued


DNC


distributed numerical control


Facilitated by existence of LANs (Ethernet, etc.)


Jobs are centrally processed (tool motion commands for
all parts in facility developed) post
-
processed and
stored in host computer; then sent to specific machines
for machining


Facilitates efficient assignment of jobs to machines, and
improves other performance metrics ??
E.G.

{Rabie}

7


The purpose is to provide
a means of locating the
tool in relation to the
work piece


NC Coordinate System


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NC process steps


fig 9.1

1.
Receive part definition from design engineering

2.
Generate a process plan


specify machines (processes)


define operational sequence


specify standard time estimates
-

setup & others


tooling required


raw materials

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NC Process steps contd.

3.
Generate NC program or code


geometry and motion statements.

4.
Process NC code


develop cutter location data set (CLDATA)


APT language is one example of a processor


generic processing of NC code is produced

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NC process steps contd.

5.
Post processing


CLDATA is further processed to conform to
specific machine tools


machine tools differ in the way they respond
to the dynamic behavior of a particular
machine ( e.g. coolant flow rate)

6.
Input numerical information to machine.

{ Zarrugh }

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NC Programming Methods

NC
-

words

Language
-
based

Graphics
-
based

Programmer

Language

Processor

machine

code


High
-
level

language


APT

Drawing

geometry

CL Data

CL Data

Programmer


machine

code

Drawing

geometry

machine

code


CAD

System

NC Graphic

Processor

geometry

IGES

DXF

Language

Processor

CL Data

Programmer

{ Zarrugh}

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NC
in CAD/CAM Environment

Design

(CAD)

CAD

file

Develop full product

definition including
geometry, tolerance,
surface finish and
material callout.

Process

Planning

Process & routing

instructions

Develop mfg
steps and
sequence

NC

Processor

Part program

& CL DATA

Generate generic
instructions for
running the NC

cutter along the
path prescribed
by the geometry.

Usually language
-

based statements.

Post
-

processing

Convert generic
instructions to

specific machine
commands in low
level machine
language.

Execute machine
code on the NC
machine tool to
produce the part.

machine

code

Process is fully automated
for simple applications

[Zarrugh]

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NC Programming Methods


The part program may be developed using



low
-
level language
(NC

words)

>>
Manual



Language based
(high
-
level language e.g APT
-

Automatically
Programmed Tools) >> partially develops geometry



Graphics
-
based >>
Fully develops geometry

(Auto CAD)



CAD/CAM system >>
Fully automated? (MasterCAM)


[Zarrugh]

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Manual NC Programming

Blocks & Words


The part programmer prepares a list of machine coded
statements arranged in lines
.



Each line is called a

block


and contains a string of

words


having coded numerical data to
move

the tool
from one location to another.



A block also contains
machining instructions

(speed, feed
rate, tools, coolant, etc) to

be executed. The order of
execution depends on the type of word.

[zarrugh]

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Types of Words

N_ G _ X_ Y_ F_ S_ T_ M_

N = statement sequence number

G = preparatory functions dealing with motion control parameters

X, Y, Z = dimensional words followed by absolute or relative



positions


F = feed rate code used for contouring and straight cut followed


by feed rate in inch/min

S = spindle speed code followed by speed in rev/min (rpm)

T = tool word to specify a particular tool by number

M = miscellaneous functions (e.g control of coolant, clamp, etc)

Cod
e

Number

Word

Block

N110 G04 X
-
5.200 Y0.901 F60 S710 T650 M03

[Zarrugh}

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General Guidelines 1


The sequence of words (address character plus parameters)
must

appear in the following order:


N, G, X, Y, Z, F, S, T, M.



Only one of each address character can be used per block, except for
G
-
codes, which can occur more than once,
if the words are of
different G
-
word types



Example



N1 G01 G90 X.20 Y.25 is
valid
.



N1 G01 G00 X.20 Y.25 is
not valid
.



[Zarrugh]

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General Guidelines (cont.)


Some words
(usually dimensional words)

do not have to be
repeated in every block. The controller will use the latest
value for each word type until it changes.



Example





N3G1X.5Y2.25F30


N4Y3.5



Spaces and insignificant zeroes are optional

[zarrugh]

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General Guidelines (cont.)



The
first block

in any program should move the tool to a safe location HOME. The
location should allow the removal of the finished part and/or put another part:



N0G0Z.1

;move rapidly to z=0.1 inches (away from part)



N1M3



;turn spindle on



The
second block

will prompt the operator to start the spindle ON.



The
third block

will move the tool to a START location to be ready for machining.



N2G00G90X.5Y.5 ;move rapidly to absolute coordinates (.5,.5)



The
end

of a part program should move the tool back to the HOME location



N92G0Z.1


;move tool rapidly to z=0.1, away from part



N93X.5Y.5M5 ;move tool rapidly to coordinates (.5,.5)




and turn spindle off.



N94M2


;end of program

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Motion Control (machining plan)


Point

To
-
Point (PTP)


Contouring


Straight
-
Cut: motion along one major axis e.g.
sawing operation



[ Zarrugh}

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Point
-
To
-
Point (PTP) Control

P
1

P
2

Many paths

are possible

P
1

P
2

Stationary

tool

workpiece
motion


Relative motion between the tool and
the work piece is controlled


In PTP control, only the
final
destination

is controlled, but
not the
actual path
(no contact between tool
and work piece)


Typically a
work piece is moved

with
respect to the tool
(at full speed)

until
the desired location is reached.


The tool performs the required action,
such as drill a hole, spot weld or punch
a shape with the
work piece stationary
.


[Zarrugh]

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Contouring Control


In contouring systems, such as NC mills and
NC lathes, all axes can be moved
simultaneously to
generate a continuous
smooth path in space
.



In general, the
axes do not move at constant
velocity

so that curvilinear (not a straight line)
motion, such as circular motion, can be
generated.

Zarrugh

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Interpolation Scheme


An
interpolator

is necessary to generate
intermediate points

on the
path between the points.




Most common is linear interpolation. The tool moves on
series of
straight lines

between these two points
(linear interpolation?)



The interpolation scheme influence tolerances
(accuracy)

on tool path
(Figure 8.8
-
next slide)



Others
-

Circular, Helical, etc

Zarrugh

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Interpolation Scheme


Others
-

Circular, Helical, etc

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APT Organization Structure


APT organization structure contain following
sub
-
divisions:

1.
Part Definition
: contain elements of geometry that
physically describe the part

2.
Machining Specification
: defines characteristics
specific to a machine e.g. coolant flow rate

3.
Machining plan
: specifies tool motions that control
the removal of material in relation to defined part
geometry in


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Language
-
Based Programming: APT
Part Definition




Two major types of motion statements


Point to Point


Contouring



Point
-
To
-
Point


FROM/<point location> Defines initial cutter center; may be x, y,
z coordinates


PTA = POINT/4,3,6


FROM/PA


GOTO/<pt location> Rapid move of cutter to a point


GOTO/P1 OR GOTO/0.3,2.0,3.6


GODTA/dx,dy,dz Incremental move of cutter from current
position


GODLTA/0,0,+0.9

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APT (Automatically
Programmed Tool) Programming


Typical APT geometry
statements:


Geometric type

APT Word

Point

POINT

Line

LINE

Plane

PLANE

Circle

CIRCLE

Pattern (of parts)

PATERN

Cylinder

CYLDR

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APT Prog.
-

Machining Plan contd.

Contouring


For contouring, three surfaces have to be defined
to guide the tool (see figs. 9.34 and 9.35)


Part surface
: On top of which the tool rides


Drive surface:

Against which the tool moves


Check surface:
At which motion must stop


Tool must be brought in contact with the
control surfaces using
Initial Motion
Statements:

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Contouring Motion Statements


Initial Motion Statement:


GO/ <c.p.>, drive surface, <c.p.>, part surface,
<c.p.>, check surface


<c.p.> is a
cutter specifier

parameter


E.G: TO, ON, PAST, OR TANTO


GO/TO,CIR1,ON,PLN1,TO,LN1


Initial motion statements are not repeated in the
program, but regardless defines the part surface
for the entire APT program.

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Contouring Motion Statements


Intermediate Motion Statements
-

control the
actual part cutting


Motion Word/drive surface,<c.p.>,check
surface


c.p. are as defined for initial motion statements


Motion Words are: GOLFT, GOFWD,
GORGT, GOBACK, GOUP, GODOWN

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APT Programming
-

Machining
Specification


Part Definition (Geometric) and Motion statements
constitute approx. 70% of an APT program, while
Marching statements the remaining 30%. Machining
Specification statements are grouped into:


Postprocessor statements

-

to control specific
auxiliary functions e.g. spindle speed, feed rate, etc


Tolerance and cutter specifications

-

specify cutter
size and permissible deviations from of cuts from ideal


Initial and Termination
-

these specify the beginning
and end of an APT program.

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Postprocessor statement example


MACHIN/<postprocessor>,<unit> Defines post processor
and specific machine to be used

COOLNT/ON
-

Turns coolant ON or OFF

SPINDLE/ON
-

Turns spindle ON or OFF

SPINDLE/3000,CCLW
-

set spindle at 3000 RPM CCW

FEEDRAT/4
-

feed rate set at 4 in/min

TOOLNO/381,4
-

use tool #381having 4 units of length

TURRET/6
-

selects tool in the #6 position of the automatic
tool changer

END
-

the end statement of the CNC program

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Machining Spec.
-

Tolerance and Cutter
Specifications

OUTTOL/.0005
-

outer tol. or maximum over
cutting error.

INTOL/.0002
-

inner tolerance or maximum
undercutting error

TOLER/.0003
-

maximum outer and inner
tolerance

CUTTER/.250
-

specifies size of cutter
-

diameter = 0.125 units.

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Machining Specs.
-

Initial and
Termination statements


PARTNO DF197654
-

first statement
identifying part no.


FINI
-

stops compilation of APT part
program; is always last statement of an APT
program.

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APT Machining Plan Example

GO/TO,LI.ON,PLN1,ON,L7

GORGT/L1,PAST,L2

GOLFT/L2,TO,L3

GORGT/L3,TANTO,C3

GOFWD/C1,TANTO,L4

GOFWD/L4,PAST,L5

GOLFT/L5,PAST,L6

GOLFT/L6,PAST,L7

GOLFT/L7,PAST,L1

GOTO/SP


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APT program for sample part


Bedworth Fig 9.44