cnc knowhow summer project
TRANSCRIPT
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HOW CNC WORKSHOW CNC WORKS
Computer Numerical control is a computer-assisted process to control
general purpose Machines from instructions generated by processor and
stored in a memory system or storage media for present as well as future use.
Numerical Control means - Control by Numbers.
Controlling Machines by numerical command has brought about a
revolution in manufacturing .CNC can be adapted to any kind of machine
or process that requires direction by human intelligence. This text deals with
CNC milling centers, Turning centers, Boring Machines & Grinding
Machines.
CNC is a specific form of control system where position is the principal
control variables numerical values, representing desired position of tools &
symbolic information corresponding to tools & secondary function, are
recorded in some form where the information can be stored and revised
indefinitely. Hard drives tape readers & other converters transform this
information into signals that ultimately operate servomechanism on each
axis of machine whose motions are to be controlled.
Today 32 & 64 -bit-bus Microprocessors directly coupled with production
machines control systems are expanding both the Applications and the basic
definition of numerical control.
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OBJECTIVES OF CNC
Basic Objectives of CNC machines was to reduce cost for production of
machined parts. This objective has been achieved by CNC reducing
programming time, including operational capability of the NC systems in
place, & making the entire machining process more users friendly. CNC also
achieved this objective by reducing machining time, fixturing cost & tooling
storage, including cutter life & lowering the skills required to automatically
produce precision-machined components.
CNC has enabled accurate estimation of production process .CNC permitted
more efficient shop scheduling, exact cost prediction higher facilities
utilization, & more rapid return on equipment investment compared with
less sophisticated machine control techniques or manual operations.
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TYPES OF CNC SYSTEMSTYPES OF CNC SYSTEMS
CNC systems range from very simple to complex systems. Some of them
are:
1. Point to Point Control systems
2. Open loop control systems
3.Continous Path control systems
4.Closed Loop control systems
TYPES OF MONITORING CONTROL SYSTEMS
1.C-Numeric
2.Fanuc
CNC MACHINE MANUFACTURING COMPANIES
1.LAKSHMI MORI SEIKI
2.KIRLOSKAR WARNER SWASEY
3.H.M.T
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MOVEMENTS & MEASUREMENTS OF CNC AXIS
SYSTEM
A system of rectangular coordinates also the CARTESIAN COORDINATE
SYSTEM" is the basis for measuring CNC machine axis movement. All
point position are described in terms of distance ROM a common point
called as ORIGIN.(X0.000,0.000,Z0.000)& measured along certain mutually
perpendicular dimension line called AXIS. It is only necessary when
describing geometry of part to locate every point of part within a frame work
of 3 such MAJOR AXIS called X , Y , Z axis.
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PROGRAMMING OF CNC
CODED LANGUAGES:
CNC has clearly moved ahead to conversational programming formats that
have advanced entire programming process to instruction of machine in
everyday language, many part programs are stored in conventional program
system formats such as G & M codes.
These are:
N BLOCK Numbers (00-999)
G PREPATORY FUNCTIONS
G00 RAPID TRANSVERSE
G01 LINEAR INTERPOLATION
G02 CIRCULAR INTERPOLATION CLOCKWISE
G03 CIRCULAR INTERPOLATION ANTICLOCKWISE
G04 DWELL PROGRAMABLE
G10 OFFSET SETTING BY PROGRAM
G20 INCH DATA INPUT
G21 MM DATA INPUT
G25 SUBROUTINE CALL UP
G27 JUMP INSTRUCTIONS
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G28 REFRENCE POINT RETURN
G40 TOOL NOSE RADIUS COMPENSATION CANCEL
G41 TOOL NOSE RADIUS COMPENSATION LEFT
G42 TOOLNOSE RADIUS COMPENSATION RIGHT
G50 WORK CO-ORDINATE CHANGE
G64 FEED MOTORS
G65 DISC ACTIVATION
G70 FINISHING CYCLE
G71 MULTIPLE TURNING CYCLE
G72 MULTIPLE FACING CYCLE
G73 HIGH SPEED DRILLING CYCLE
G78 THREADING CYCLE
G81 DRILLING CYCLE
G82 DRILLING CYCLE WITH DWELL
G84 ROUGHING CYCLE LONGITUDNAL
G85 REAMING CYCLE
G88 ROUGHING CYCLE,FACING
G90 ABSOLUTE VALUE PROGRAMMING
G92 OFF SET OF REFERANCE POINT
G94 FEEDING PER MIN.
G95 FEED PER REVOLUTION G50 MAXIMUM SPINDLE SPEED
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M MISCELLANEOUS FUNCTIONS
* M00 PROGRAMME STOP
* M01 OPTIONAL STOP
* M02 END OF PROGRAMME
* M03 SPINDLE ANTICLOCKWISE
* M04 SPINDLE CLOCKWISE
* M05 SPINDLE STOP
* M08 COOLENT OFF
* M09 COOLING ON
* M10 CHUCK CLOSE
* M11 CHUCK OPEN
* M12 LOW CHUCKING PR.* M13 REGULAR CHUCKING PR.
* M18 TAILSTOCK IN
* M19 TAILSTOCK OUT
* M20 QUILL LEFT
* M21 QUILL RIGHT
* M22 REVERSE INDEX
* M23 FORWARD INDEX
* M28 PARTS CATCHER OPEN
* M29 PARTS CATCHER CLOSE
* M30 PROGRAM REWIND
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* M36 CANCEL M37
* M37 IGNORE UPTO SPEED FEED
* M40 DRY RUN WITHOUT SPINDLE
* M41 DRY RUN WITH SPINDLE
* M42 NORMAL MODE
* M48 CANCEL M39
* M49 FEED RATE & SPINDLE SPEED
* M90 IN POSITION CHECK ON
* M91 IN POSITION OFF
* M92 SOFT JAW BORING
* M97 DISABLED
* M98 SUB PROG. CALL
* M99 SUB PROG. END
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INTRODUCTION
A computer numerical control has a microprocessor based computer in the
system to store and process the data for the numerical control of motions of
the machine tools. The NC system is the heart and brain of the CNC
machine which enable the operation of the various machine members such
as slides, spindles etc & also optimizes the matching with proper parameters
involved in a matching operation.
CNC systems are constructed with an NC unit integrated with a PLC.The
NC controls the spindle movement and the machine such as solenoids, relay
coils etc.working together the NC & PLC enable the machine tool to operate
automatically.
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BACKGROUND
Development of CNC machines is an outstanding contribution to
manufacturing industries. This has made possible to automate the machining
process with flexibility to handle small to medium batch quantities in part
production.
CNC technology was applied on basic metal cutting machines. Later CNC
machines capable of performing multiple operations were developed.
Further, the concept of multi operations on the same machine was extended
for cylindrical component machining and led to the development of turning
centers and grinding centers.
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ADVANTAGES OF CNC MACHINES
HIGHER FLEXIBILITY
INCREASE IN PRODUCTIVITY
CONSISTENCY IN QUALITY
REDUCTION IN SCRAP RATE
RELIABILITY IN OPERATION
REDUCTION IN MANPOWER
REDUCTION IN NON-PRODUCTIVE TIME
SHORTER CYCLE TIME
HIGHER ACCURACY
REDUCE LEAD TIME
LESSER FLOOR SPACE
OPERATIONAL SAFETY
MACHINING OF ADVANCED MATERIALS
REDUCTION IN MATERIAL HANDLING
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CONFIGURATION OF CNC SYSTEM
A CNC system basically consists of the following:
CPU
SERVO CONTROL
OPERATOR CONTROL PANEL
MACHINE CONTROL PANEL
OTHER PERIPHERAL DEVICES
PLC
CNC SYSTEM CONSISTS OF FOLLOWING PCBS
POWER SUPPLY
NC CPU
MEMORY CARD
COMMAND CARD
FEEDBACK CARD
OPERATOR PANEL
NC COUPLING CARD
PC COUPLING CARD
INPUT/OUTPUT CARD
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PC CPU
CENTRAL PROCESSING UNIT
This is the heart and brain of a CNC system. This translates the part program
to position control signals. It also oversees the movement of control axis of
spindle and the program signals corrective action is taken.
It also checks whether all the required conditions are within the control of
the CPU if not then action of shutting down the machine is taken.
A. POWER SUPPLY
It is switching mode power supply it supplies all received dc
voltage(12v,15v,5v) to other PCBs.
B. NC CPU
The whole CNC circuit is divided into two portions:
NC PART
PC PART
NC (numerical controller part) -all numerical process/calculation take place.
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Central processing unit of NC part is called NC-CNC.
C. MEMORY CARD
It is having three portion/area in one area. It store job machining program
and stored on ram module.m/c operator requires to change it.
In second portion it store m/c parameter. This portion also contains ram.
In third portion it contain CNC system program and it is EPROM which is
non-volatile.
D. COMMAND CARD
This card executes axis command voltage (analog) and spindle command
voltage (analog) to axis drive & spindle drive respectively.
E. FEEDBACK CARD
This card receive feedback signal (digital) from feedback device for
positional or rotational(rpm) calculation.
F. OPERATOR PANEL
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It connects CNC system to monitor (display unit) for displaying messages.
G. NC COUPLING CARD
It interfaces NC part & PC part while compile data to assembling language.
H. PC COUPLING CARD
It connects peripheral units (tape reader, printer, floppy drive etc.) with CNC
system. For downloading & uploading of m/c data, program.
I. INPUT/OUTPUT CARD
It interface I/P devices (limit switch, proximity switches, overload etc.) &
O/P devices (solenoids, counter, relay etc) of m/c side CNC system.
J. PC CPU
It contain central processor unit of programmable control processor.
Program of m/c logic is stored in EPROM module on this card.
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MODES OF OPERATION
CNC system can be operated in the following modes:
MANUAL MODE
MANUAL DATA INPUT(MDI) MADE
AUTOMATIC MODE
SET UP MODE
INPUT/OUTPUT MODE
MANUAL MODE
Here, movement of the machine slide can be done manually by pressing
particular buttons. Selection of slide is done through an axis selector switch
feedrate of slide movement is prefixed.
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MANUAL DATA INPUT MODE
Following operations can be performed: -
1. Editing of the part program stored in system memory.
2. Building a new part program.
3. Entering tool offsets in the system memory.
AUTOMATIC MODE
System allows execution of part program continuously. Part program is
executed block by block while one block is read by the system, analyzed &
kept ready for execution. Execution of the block can be continuos or system
will execute a block, stop execution of next block till it is initiated to do so.
Selection of part program block by block (auto) or one at a time (single) is
done through machine control panel.
Block retrace is allowed when cycle stop state is established. Part program
execution can be resumed and its execution begins with the retraced block.
This is important for tool inspection.
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SET UP MODE
The m/c can be referenced to its home position so that all the compensation
can be applied. Part programs are generally prepared in absolute mode with
respect to the m/c zero.
I/O MODE
The part program, m/c setup data, tool offsets etc can be loaded into memory
of the system from external sources like programming units or floppy drives.
COMPENSATION FOR M/C ACCURACIES
Machine accuracy is the accuracy of the movement of the carriages and
tables and is influenced by: -
1.The geometric accuracy of the alignment of the sideways.
2. The deflection of the bed due to load
3.Any temperature gradients existing through the machines.
4. The accuracy of the screw thread of any drive screws and the amount of
backlash.
5. The amount of twist of the shaft, which will influence the measurements
of rotary transducers.
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TOOL MOVEMENT MODES
In a CNC/NC machines, usually the tool moves with respect to the
workpiece which remains at the same place. In some cases ex. In a milling
machine, the workpiece moves with respect to the tool. However, from
programmer point of view this aspect hardly makes any difference as only
relative motion between the tool and workpiece is important. While writing
programs it is always assumed that tool moves with respect to the
workpiece. For the second type of the machine, the workpiece automatically
moves in the opposite direction so as to produce the desired relative motion.
Three types of motion control are used in a CNC machine:
Point to point placement
Axial cut
Contour cutting
POINT TO POINT PLACEMENT
Such a control simply places the tool over desired location in desired
sequence. There is no control over the speed of tool movement between the
selected points which always a fast traverse. This type of control can be used
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in drilling, punching or similar operations where the location of tool at the
timing of operation is important.
AXIAL CUT
This control allows the tool to move along any major axes with desired
speed. Therefore cutting along x,y,z axes is possible. Limitation being
simultaneous motion along two axes is not possible so it cant make angular
cut.
CONTOUR CUTTING
This is the most flexible but the most expensive type of control .It permits
simultaneous control of more than one axes movement of the tool.
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TOOL CHANGING DEVICES
A number of tools may be required for making a complex part. In CNC
machine, tools are changed through program instructions. Tools are fitted in
tool magazine or drum. When a tool needs to be changed the drum rotates to
an empty position, approaches the old tool and pulls it , then it again rotates
to position the new tool, fits it and then retracts . This is typical tool
changing sequence of an automatic tool changer.
Tool changing time is of the order of a few seconds this saves time and thus
increases productivity.
WORK HANDLING DEVICES
Some of the machining centers provide more than one separate pallets which
can be of linear or rotary types. These pallets simply move or rotate or
interchanging there positions on the machining table.While machining is
being done on job kept on one pallet, the other pallets are accessible to the
operator for clamping /unclamping raw material/ finished products. This
saves a lot of material handling and set up time, resulting in higher
productivity.
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SPINDLE SPEED AND FEEDRATE SELECTION
Let V = Cutting speed
= Angular velocity of spindle
N = RPM
D = Diameter in mm
Then cutting speed V = D/2
= DN/60 in mm/sec.
A very high cutting speed reduces tool life due heat generation, whereas
very low speed waste machining time and may break the tool due to large
cutting forces. So spindle speed should be properly chosen. Feed also should
have proper value .a high value may break the tool due to large cutting
forces, and surface finish also will be poor, where low wastes machining
time. Usually for heavier cuts, slow RPM is recommended.
The largest depth of cut with highest feedrate safely possible is selected.
This reduces cycle time and has a negligible effect on the tool life. In any
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case costwise reducing the machining time is usually more important than
increasing the tool life.
PROGRAMMING PROCEDURE
The basic steps involved in an NC/CNC programming are:
(a) Process planning.
(b) Part programming.
(c) Program entry.
(d) Program verification.
(e) Production.
PROCESS PLANNING
The design /development department of manufacturing industry design a
path and prepare its detailed drawing. For a simple part, three or even two
orthographic views are sufficient to describe it fully.
After the drawing is finalized the programmer carries out the task of process
planning. Process planning is the procedure of deciding what operations are
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to be done on the component, in what order and with what tooling and work
holding facilities.
PART PROGRAMMING
The next step after finalizing the machining scheme is part programming the
first step in part programming is to define a component zero point at a
convenient location on the part this become the origin of the co-ordinate
system for programming. The axis remains parallel to the machine axis. In
the program, all the co-ordinates in the part drawing are specified with
respect to this co-ordinate system.
The program must follow the standard being used by the MCU (called
simply controller also). FANUC controller has now become industry
standard. Program written for a specific controller cant be used on a
different controller. Where some of the commands (G and M codes) may
have the different meanings. This text follows FANUC standard. So the
given program can only be used on a FANUC machine.
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PROGRAM ENTRY
After the program is ready it needs to be transmitted to the MCU. The most
common input medium has been one-inch wide tape over the past 40 years.
Even today it is being used in some machines, though they have now being
more or less completely replaced by the other media such as floppy disk.
Most of the CNC machines provide both on line and offline software for
program coding and simulation. Since programming takes substantial
amount of time, one cant afford to engage a production machine for this
purpose. Hence offline software are invariably used for programming using
a standalone computer. These software provide necessary editing features to
make correction in the typed program Once the program is ready and
verified, it may be transferred to the machine through floppy disk.
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PROGRAM VERIFICATION
Once the part program is entered on the machine and execution command is
given for producing the part, the operator has no control over machine
movements except minor modifications such as over riding spindle RPM
and feed. In case of any undesired movement of tool, the machine can only
be stopped through the emergency stop switch or the reset button .Any
carelessness may result in serious damage to the machine so the program
must be thoroughly verified before executing it on the machine for
production.
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PRODUCTION
Production is the simplest part of a CNC procedure. The person responsible
for this need not be a skilled worker. In fact one semi skilled worker may
handle several CNC machines on the shop floor.
CNC operator has to first check the tool position on the tool turret.If there is
any discrepancy from the part program , he may either change tool positions
on the turret or modify the program to correct the tool index no.
The next thing to be done by the operator is tool offsetting. Infact as already
mentioned the details of tool offset can be stored on a floppy disk, which can
be reloaded in case of power failure or for a subsequent use on the machine
Finally the operator has to check coolant level, air pressure extra.If every
thing is okay he give the execution command for production.