CHAPTER 1 INTRODUCTION Introduction to NC Production of machined parts whose production is controlled by a computer Definition NC A method of accurately controlling the operation of a machine tool by a series of coded instructions consisting of numbers letters of the alphabet and sym ID: 701618
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Slide1
CNC
GOVT POLYTECHNIC
AMBALA CITYSlide2
CHAPTER 1
INTRODUCTIONSlide3
Introduction to NC
Production of machined parts whose production is controlled by a computer
.
Definition:
NC - A method of accurately controlling the operation of a machine tool by a series of coded instructions, consisting of numbers, letters of the alphabet, and symbols that the machine control unit can
understand
The concept of NC was proposed in 1940s by
JohnParsons
who recommended a method of automatic machine control that would guide a milling cutter to produce a curvilinear motion in order to generate smooth profiles on the work-pieces.Slide4
Components of NC machinesSlide5
Evolution of NC/CNC
Single
items produced by craftspeople
Interchangeable Parts
Eli Whitney (Cotton Gin)
Manual labor was still the most cost effective method.
WW II manufacturers could not maintain quantity & quality parts.Slide6
Conventional vs. CNC machine Slide7
Item
Conventional machine
CNC
machine
1. Movement
Acme screw
Ball screw
2. Feed
manual
motor
3.measurement
manual
Linear scaleSlide8
Three basic components of an NC system:
Input
medium:
- Part
program or instructions needed to drive the machine tool components - Instructions are prepared manually or by use of computer - Instructions include machining parameters (feed rate, cutting speed); sequence of actions (e.g., positioning & machine functions) - Instructions are stored in the form of tape (paper, magnetic); floppy diskettes; DNC download to CNC RAM
2
. Machine control unit (MCU): - Electronics & control hardware - Interpret instruction set - Execute instructions - Monitor results & correct where appropriate
3
. Machine tool: - Mechanical structure that performs the machining, including the components that drive each axis of motion (e.g., AC or DC motor; hydraulic actuator; stepper motor — choice affects speed of response, accuracy and power capacity).Slide9
INPUT UNIT
Input data includes information about the control medium, information about computer programs should also be given.
Control medium: perforated tape,
magnatic
tape, etc.
Type of dimensional programming: Absolute, Incremental or both etc.,
Number of digits in each dimensional word etc., Input resolution Information about programming methods and languages
Tape reader type - Mechanical or photo electric etc., Tape code - ISO, EIASlide10
MCU
MCU - Machine Control Unit - decodes NC codes to drive and monitor servo motor movements.Slide11
CNC Advantages vs. NC
Programs could be stored in computer memory.
Easier to edit.
More complex parts could be manufactured.
Use of 3d geometry.
Networking/
filesharing
w/ other computers.Slide12
Limitations of NC
Relatively high initial cost of equipment.
·
Need for part programming.
·
Special maintenance requirements.
·
More costly breakdowns.Slide13
Advantages Of CNC
Increased productivity after programming is completed
Reliability - reduces human error
Often eliminates need for special jigs and fixtures
Reduces location of part features
Makes possible the machining of complex shapes requiring simultaneous 3 axis
motion
Single part and production runs can be programmed and machined with minimum effort and cost.
Programs can readily be altered and re-run
Reduced inspection costs (more reliable)
Once programming, setup and verified the equipment can be operated by a less skilled operator.Slide14
Disadvantages
of CNC
Initial cost of CNC machine tools
Servicing of equipment
Larger machines require more space
Personnel must be trained in the programming and operation of this equipment.Slide15
Computer Numerical Control (CNC) Slide16
Direct Numerical Control (DNC)
Distributed (Direct) Numerical Control (DNC): ·
(
Direct) Central computer stores programs & directs NC operations; NC machines dependent on central computer.
(Distributed) Central computer stores programs and transfers programs to CNC machines.
Central computer provides management functions (e.g., part of MIS) Programs stored as cutter location (CL) files and post-processed for the machine assigned the job.
Major components: central computer; bulk memory; telecommunication (EDI); CNC machine tools.Slide17
DNCSlide18
Selection of components for NC/CNC
1. Parts from similar raw material, in variety of sizes, and/or complex geometries.
2
. Low-to-medium part quantity production.
3
. Similar processing operations & sequences among work pieces.
4
. Frequent changeover of machine for different part numbers.
5
. Meet tight tolerance requirements (compared to similar conventional machine tools).Slide19
Application of CNC Machine Tools
CNC mills and machining centers,
CNC lathes and turning centers
CNC electrical discharge machining (EDM)
CNC grinding machines
CNC cutting machines (laser, plasma, electron, or flame)
CNC fabrication machines (sheet metal punch press, bending machine, or press brake)
CNC welding machinesSlide20
AXIS IDENTIFICATION
2&3
axes CNC Machines
CNC lathes will be coming under 2 axes machines. There will be two axes along which motion takes place.
The saddle will be moving longitudinally on the bed (Z-axis) and the cross slide moves transversely on the saddle (along X-axis).
In 3-axes machines, there will be one more axis, perpendicular to the above two axes. Slide21
RIGHT HAND RULE FOR LINEAR AXIS IDENTIFICATIONSlide22
AXIS IDENTIFICATION(Lathe)Slide23
Axis identification (milling)Slide24
CHAPTER 2
CONSTRUCTION AND TOOLINGSlide25
MAIN ELEMENTS OF CNC MACHINES
To enable electronic automation with high rate of metal removal at optimum cutting conditions, maintaining high repetitive accuracies with utmost safety to the operator and the machine, CNC machines are specially designed.Slide26
MAIN ELEMENTS OF CNC MACHINES
i
) Machine structure
ii) Guide ways
iii) Spindle bearings & mounting
iv) Drive units
v) Mech. Power transmission
vi) Position feed back elements / systems
vii) Additional accessories / equipment
viii) Control software
ix) Chip removal system
x) Safety features Slide27
Machine structure
Structures are designed to withstand static, dynamic & thermal loads providing high stiffness, rigidity & damping properties. The material used is generally
mechanite
cast iron / special casting with nickel & copper elements. Welded structures also in wide usage.Slide28
Guide ways
TYPES OF GUIDE WAYS
1. Friction Guide ways: The relation between moving part (Guide) and stationary part (Guide way) contacting directly each other.
2. Anti Friction Linear Motion Guide ways: Here the contact between guide and guide way may be separated by the third element i.e. Ball, Roller or hydraulic oil film in case of Hydro dynamic or hydrostatic guide way systems. Slide29
1) HYDROSTATIC GUIDE WAY
2) AEROSTATIC GUIDE WAY
In Hydrostatic guide ways, the surface of slide is separated from the guide way by a very thin film of fluid supplied at pressure as high as 300 bar. Frictional wear and stick slip are entirely eliminated. A high degree of dynamic stiffness and damping are obtained with these guide ways, both characteristics contributing to good machining capabilities. Their application is limited due to high cost and difficulty in assembly.
In Aerostatic guide ways, the slide is raised on a cushion of compressed air which entirely separates the slide and the guide way surfaces. The major limitation of this type guide ways is a low stiffness, which limits its use for positioning application only. e.g. CMM and other measuring instruments. Slide30
RE CIRCULATING BALL BUSHINGS
The following are the advantages of the ball screws:
1) Low frictional resistance.
2) Low drive power requirement.
3) Lesser temperature rise.
4) Less wear hence longer life.
5) No stick slip effect.
6) High traverse speed.
7) High efficiency Slide31
There are two types of roller screws used planetary and re circulating. Both types provide backlash-free movement and their efficiency is of the same order (90 %) as ball screws. An advantage of roller screws is that because the pitch of the screw is smaller that the minimum pitch of the ball screw, the less complex electronic circuitry will provide more accurate positional control. Roller screws are much costlier than the ball screws. The rollers of both types of screw are positioned between the nut and the screw, and engage with the thread from inside the nut and on the outside of the screw. Slide32Slide33
Guideways
The
guideways
of CNC machines are made of steel. To reduce the frictional forces, they are coated with a strip of PTFE (Poly Tetra
Fluoro
Ethylene).Slide34
Ballscrews
On conventional lathes a
leadscrew
is used to converts the rotary motion of the hand wheel into linear motion of the slides.
Leadscrews
have a high coefficient of friction and backlash. On CNC machines a
ballscrew
is used instead of a
leadscrew
. Friction and backlash are much less than on
leadscrews
.Slide35
Swarf Removal
Efficient chip removal system eliminates thermal effects & thus improves the quality of cutting and the job being machined.
Centrifuging of chips at source integrated into a
swarf
blowing system
Airflow isolation slide damper
Rotary valve
Blowline
inlet connectionSlide36
Safety
Suitable covers for guide-ways etc., and electronic interlocks for the safety of the operating personnel and machine are provided.Slide37
AUTOMATIC TOOL CHANGER
HYDROMOTOR IS USED FOR THE ROTATION OF THE MAGAZINE.
MAGAZINE
MOVES IN RAPID ONE TOOL AHEAD (SENSED BY PROXIMITY SWITCH)
MAGAZINE CREEPS TO THE POSITION IN A VERY LOW
SPEED
(SENSED BY PROXIMITY SWITCH).
WHEN THE MAGAZINE IS IN POSITION, LOCATING PIN ENTERS THE HOLE IN THE MAGAZINE AND POSITIONS THE MAGAZINE ACCURATELY. Slide38
ATC
Simple CNC machines work with a single tool. Turrets can work with a large number of tools. But if even more tools are required, then ATC is provided. The tools are stored on a magazine. It allows the machine to work with a large number of tools without an operator. The main parts of an automatic tool changer are the base, the gripper arm, the tool holder, the support arm and tool magazines.Slide39
Types of ATC
Depending on the shape of the magazine, ATC can be of two types: Drum Type changers are used when the number of tools is lower than 30. The tools are stored on the periphery of the drum. Chain type changers are used when the number of tools is higher than 30(The number is different depending on the design and manufacturer. It is important to note that the number of tools for the drum type is fewer than the chain type). But the tool search speed will be lower in this caseSlide40
SPINDLE DIRECT ATCSlide41
ATC TYPESlide42
WORKING OF ATCSlide43Slide44
Programming Methods-APT
Part definition
P1=Point/12,20,0
C1=Circle/Center,P1,Radius,3
LN1=Line/C1. ATANGL,90
Cutter Commands
TLRT,GORT/LN1.TANTO,C1
GOFWD/C1,TANTO,L5Slide45
Programming Methods-CAM
Computer Aided Machining (CAM) Systems
Graphic representation of the part
PC based
Integrated CAD/CAM functionality
“Some” built-in expertise
Speed & feed data based on material and tool specifications
Slide46
Programming Methods-CAM
Tool & material libraries
Tool path simulation
Tool path editing
Tool path optimization
Cut time calculations for cost estimatingSlide47
Programming Methods-CAM
Import / export capabilities to other systems
Examples:
Drawing Exchange Format (DXF)
Initial Graphics Exchange Standard (IGES)Slide48
The Process CAD to NC File
Start with graphic representation of part
Direct input
Import from external system
Example DXF / IGES
2D or 3D scan
Model or Blueprint
(At this point you have a graphics file of your geometry)Slide49
The Process CAD to NC File
Define cutter path by selecting geometry
Contours
Pockets
Hole patterns
Surfaces
Volume to be removed
(At this point the system knows what you want to cut)Slide50
The Process CAD to NC File
Define cut parameters
Tool information
Type, Rpm, Feed
Cut method
Example - Pocket mill zig-zag, spiral, inside-out
Rough and finish parameters
(At this point the system knows how you want to cut the part)Slide51
The Process CAD to NC File
Execute cutter simulation
Visual representation of cutter motion
Modify / delete cutter sequences
(At this point the system has a “generic” cutter location (CL) file of the cut paths)Slide52
The Process CAD to NC File
Post Processing
CL file to machine specific NC code
Filters CL information and formats it into NC code based on machine specific parameters
Work envelope
Limits - feed rates, tool changer, rpm’s, etc.
G & M function capabilitiesSlide53
Output: NC Code
Numerical Control (NC) Language
A series of commands which “direct” the cutter motion and support systems of the machine tool.Slide54
Output: NC Code
G-Codes (G00, G1, G02, G81)
Coordinate data (X,Y,Z)
Feed Function (F)
Miscellaneous functions (M13)
N - Program sequence number
T - Tool call
S - Spindle commandSlide55
Output: NC Code
NC Program Example
N01G90 G80
N03 GOO T12 M06
N05 GOO X0 Y0 Z.1 F10 S2500 M13
N07 G1Z-.5
N09 G02 X-10. I0J0F20
N13 X0Y10
N17 X10Y0
N19 X0Y-10
N21 X-10Y0
N23 M2Slide56
Example of CNC Programming
What What Must Be Done To Drill A Hole On A CNC Vertical Milling MachineSlide57
Top
View
Front
View
Tool Home
1.) X & Y Rapid To Hole PositionSlide58
Top
View
Front
View
2.) Z Axis Rapid Move
Just Above Hole
3.) Turn On Coolant
4.) Turn On Spindle
.100”Slide59
Top
View
Front
View
5.) Z Axis Feed Move to
Drill HoleSlide60
Top
View
Front
View
6.) Rapid Z Axis Move
Out Of HoleSlide61
Top
View
Front
View
9.) X&Y Axis Rapid
Move Home
7.) Turn Off Spindle
8.) Turn Off CoolantSlide62
Top
View
Front
View
Tool At Home
O0001
N005 G54 G90 S600 M03
N010 G00 X1.0 Y1.0
N015 G43 H01 Z.1 M08
N020 G01 Z-.75 F3.5
N030 G91 G28 X0 Y0 Z0
N035 M30
N025 G00 Z.1 M09
Here’s The CNC Program!Slide63
Top
View
Front
View
Tool At Home
O0001
O0001
Number Assigned to this programSlide64
Top
View
Front
View
Tool At Home
O0001
N005 G54 G90 S600 M03
N005 Sequence Number
G54 Fixture Offset
G90 Absolute Programming Mode
S600 Spindle Speed set to 600 RPM
M03 Spindle on in a Clockwise DirectionSlide65
Top
View
Front
View
O0001
N005 G54 G90 S600 M03
N010 G00 X1.0 Y1.0
G00 Rapid Motion
X1.0 X Coordinate 1.0 in. from Zero
Y1.0 Y Coordinate 1.0 in. from ZeroSlide66
Top
View
Front
View
O0001
N005 G54 G90 S600 M03
N010 G00 X1.0 Y1.0
N015 G43 H01 Z.1 M08
G43 Tool Length Compensation
H01 Specifies Tool length compensation
Z.1 Z Coordinate .1 in. from Zero
M08 Flood Coolant OnSlide67
Top
View
Front
View
O0001
N005 G54 G90 S600 M03
N010 G00 X1.0 Y1.0
N015 G43 H01 Z.1 M08
N020 G01 Z-.75 F3.5
G01 Straight Line Cutting Motion
Z-.75 Z Coordinate -.75 in. from Zero
F3.5 Feed Rate set to 3.5 in./min.Slide68
Top
View
Front
View
O0001
N005 G54 G90 S600 M03
N010 G00 X1.0 Y1.0
N015 G43 H01 Z.1 M08
N020 G01 Z-.75 F3.5
G00 Rapid Motion
Z.1 Z Coordinate .1 in. from Zero
M09 Coolant Off
N025 G00 Z.1 M09Slide69
Top
View
Front
View
O0001
N005 G54 G90 S600 M03
N010 G00 X1.0 Y1.0
N015 G43 H01 Z.1 M08
N020 G01 Z-.75 F3.5
N030 G91 G28 X0 Y0 Z0
G91 Incremental Programming Mode
G28 Zero Return Command
X0, Y0, Z0
X,Y,& Z Coordinates at Zero
N025 G00 Z.1 M09Slide70
Top
View
Front
View
O0001
N005 G54 G90 S600 M03
N010 G00 X1.0 Y1.0
N015 G43 H01 Z.1 M08
N020 G01 Z-.75 F3.5
N035 M30
N030 G91 G28 X0 Y0 Z0
N025 G00 Z.1 M09
M30 End of ProgramSlide71
Output: NC Code - Canned CyclesSlide72
CAD to NC CodeSlide73
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