Cnc Knowhow Summer Project

8/4/2019 Cnc Knowhow Summer Project

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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.

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