DILS.pdf

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TERM PAPER REPORT

ON

CNC MACHINES

SUBMITTED BY:- SUBMITTED TO:-

Dileep Yadav Mr. Laxman Kumar Pandey

B.Tech+M.Tech (MAE) Co-ordinator (MAE)

Enroll. No.:- A20422411006 ASET

3rd Semester

AMITY SCHOOL OF ENGINEERING AND TECHNOLOGY

AMITY UNIVERSITY RAJASTHAN

(November,2012)

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Declaration

Certified that the Report entitled CNC MACHINEING submitted by DILEEP YADAV

with Enrollment No.:- A20422411006 on November, 2012 is his own work and has

been carried out under my supervision.

It is recommended that the candidate may now be evaluated for his work by the

University.

Dileep Yadav Mr. Laxman Kumar Pandey

Signature: Signature:

Date: 6th November, 2012 Designation: Co-ordinator(MAE)

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Acknowledgement

I, Dileep Yadav Of B.Tech+M.Tech(M&A), Amity School of Engineering and

Technology, Amity University Rajasthan, gratefully acknowledge the guidance, support

and cooperation of all the faculties members of ASET for completing my term paper

report on “CNC Machining”.

I would like to heartedly thanks Mr. Laxman Kumar Pandey (Co-ordinator MAE)

whose encouragement, guidance, and support from the start till the end of my report

enabled me to develop an understanding about this topic.

I have gained a lot of knowledge about this topic and think that it is a good way of

improving our knowledge on new technologies.

Dileep Yadav

B.Tech+M.Tech

MAE(3rd Semester)

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Abstract

CNC stands for Computer Numerical Control and has been around since the early

1970's. Prior to this, it was called NC, for Numerical Control.

In Industry it is not efficient or profitable to make everyday products by hand. On

a CNC machine it is possible to make hundreds or even thousands of the same item in

a day.

First a design is drawn using design software, then it is processed by the

computer and manufactured using the CNC machine. CNC machine can be used to

machine design woods, plastics and aluminium. In industry, CNC machines can be

extremely large. The Duet is one of the smaller CNCs and is ideal for use in schools.

The primary objective of this report is the formulation and analysis of the

machining process plan to complete the fabrication of the premounting base with CNC

machinery.

This Machine has not only brought a wonderful processing and working in the

field of engineering but also helped in the saving of time and energy of any company or

organization in manufacturing field by providing a complete ease of handling it.

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Contents

Topic Page No.

CHAPTER 1: INTRODUCTION 6

1.A) About CNC 6

1.B) A Brief History Of CNC 8

CHAPTER 2: ABSENCE AND PRESENCE OF CNC 10

CHAPTER 3: PARTS AND FUNCTIONING OF THEM IN CNC MACHINE 11

3.A) Central Processing Unit 12

3.B) Speed Control unit 12

3.C) Servo- Control Unit 12

3.D) Operator Control Unit 14

3.E) Machine Control Unit 15

CHAPTER 4: MODE OF OPERATING A CNC MACHINE 16

4.A) Manual Mode 16

4.B) Manual Data Input Mode 17

4.C) Automatic Mode 17

4.D) Reference Mode 18

4.E) Input Output mode 18

CHAPTER 5: TYPE OF CNC MACHINE 19

a) CNC Mills 19

b) CNC Lsthes 19

c) CNC Grinders 19

CHAPTER 6: GENERAL WORKING OF A CNC AMCHINE 20

CHAPTER 7: VARIOUS FUNCTIONS OF CNC 21

CHAPTER 8: TYPES OF CNC PROGRAMMINGS USED 22

CHAPTER 9: ADVANTAGE OF CNC MACHINES 22

CHAPTER 10: BIBLIOGRAPHY 25

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1. INTRODUCTION

1.A) ABOUT CNC

CNC, commonly known as computer numerical control which is governed by the

computer programming instead of manually controlling the machine.

CNC Machining is a process used in the manufacturing sector that involves the

use of computers to control machine tools. Tools that can be controlled in this manner

include lathes, mills, routers and grinders. The CNC in CNC Machining stands for

Computer Numerical Control.

On the surface, It may look like a normal PC controls the machines, but the computer

unique software and control console are what really sets the system apart for in use of

CNC.

Under CNC Machining, machine tools function through numerical control. A

computer program is customized for an object and the machines are programmed with

CNC machining language (called G-code) that essentially controls all features like feed

rate, coordination, location and speeds. With CNC machining, the computer can control

exact positioning and velocity. CNC machining is used in manufacturing both metal and

plastic parts.

First a CAD drawing is created (either 2D or 3D), and then a code is created that

the CNC machine will understand. The program is loaded and finally an operator runs a

test of the program to ensure there are no problems. This trial run is referred to as

"cutting air" and it is an important step because any mistake with speed and tool

position could result in a scraped part or a damaged machine.

There are many advantages to using CNC Machining. The process is more precise than

manual machining, and can be repeated in exactly the same manner over and over

again. Because of the precision possible with CNC Machining, this process can produce

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complex shapes that would be almost impossible to achieve with manual machining.

CNC Machining is used in the production of many complex three-dimensional shapes. It

is because of these qualities that CNC Machining is used in jobs that need a high level

of precision or very repetitive tasks.

A CNC Machine.

For better output and handling of CNC machine one should have a good background in

mathematics, industrial arts and mechanical drafting, as well as computer usage.

While people in most walks of life have never heard of this term, CNC has touched

almost every form of manufacturing process in one way or another. If you'll be working

in manufacturing, it's likely that you'll be dealing with CNC on a regular basis.

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1.B) A Brief History of CNC

The first commercial NC machines were built in the 1950's, and ran from punched tape.

While the concept immediately proved it could save costs, it was so different that it was

very slow to catch on with manufacturers. In order to promote more rapid adoption, the

US Army bought 120 NC machines and loaned them to various manufacturers so they

could become more familiar with the idea. By the end of the 50's, NC was starting to

catch on, though there were still a number of issues.

For example, G-code, the nearly universal language of CNC we have today, did not

exist. Each manufacturer was pushing its own language for defining part programs (the

programs the machine tools would execute to create a part).

1959 CNC Machine: Milwaukee-Matic-II was first machine with a tool changer.

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A number of key developments brought CNC rapidly along during the 1960's:

Standard G-Code Language for Part Programs: The origin of g-code dates

back to MIT, around 1958, where it was a language used in the MIT

Servomechanisms Laboratory. The Electronic Industry Alliance

standardized g-code in the early 1960's.

CAD came into its own and started rapidly replacing paper drawings and

draftsmen during the 60's. By 1970, CAD was a decent sized industry with

players like Intergraph and Computer vision, both of whom I consulted for

back in my college days.

Minicomputers like the DEC PDP-8's and Data General Nova's became

available in the 60's and made CNC machines both cheaper and more

powerful.

By 1970, the economies of most Western countries had slowed and employment

costs were rising. With the 60's, having provided the firm technology foundation that

was needed, CNC took off and began steadily displacing older technologies such as

hydraulic tracers and manual machining.

US companies had largely launched the CNC revolution, but they had been

overly focused on the high end. The Germans were the first to see the opportunity to

reduce prices of CNC, and by 1979 the Germans were selling more CNC than the US

companies. The Japanese repeated the same formula to an even more successful

degree and had taken the leadership away from the Germans just one year later, by

1980. In 1971, the 10 largest CNC companies were all US companies, but by 1987, only

Cincinnati Milacron was left and they were in 8th place.

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2. ABSENCE AND PRESENCE OF CNC

MACHINES

While there are exceptions to this statement, CNC machines typically replace (or work

in conjunction with) some existing manufacturing processes. Take one of the simplest

manufacturing processes, drilling holes, for example:

A drill press can of course be used to machine holes. (It's likely that almost everyone

has seen some form of drill press, even if you don't work in manufacturing.) A person

can place a drill in the drill chuck that is secured in the spindle of the drill press. They

can then (manually) select the desired speed for rotation (commonly by switching belt

pulleys), and activate the spindle. Then they manually pull on the quill lever to drive

the drill into the work piece being machined.

As you can easily see, there is a lot of manual intervention required to use a drill

press to drill holes. A person is required to do something almost every step along the

way! While this manual intervention may be acceptable for manufacturing companies

if but a small number of holes or work pieces must be machined, as quantities grow,

so does the likelihood for fatigue due to the tediousness of the operation. And do note

that we've used one of the simplest machining operations (drilling) for our example.

There are more complicated machining operations that would require a much higher

skill level (and increase the potential for mistakes resulting in scrap work pieces) of

the person running the conventional machine tool. (We commonly refer to the style of

machine that CNC is replacing as the conventional machine.)

By comparison, the CNC equivalent for a drill press (possibly a CNC machining

center or CNC drilling & tapping center) can be programmed to perform this operation

in a much more automatic fashion. Everything that the drill press operator was doing

manually will now be done by the CNC machine, including: placing the drill in the

spindle, activating the spindle, positioning the work piece under the drill, machining

the hole, and turning off the spindle.

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3. PARTS AND FUNCTIONING OF THEM IN

CNC MACHINES

A CNC system basically consists of the following :-

Central processing unit (CPU)

Speed Control Unit

Servo-control unit

Operator control panel

Machine control panel

Other peripheral device

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3.A) Central Processing Unit (CPU)

The CPU is the heart and brain of a CNC system. It accepts the information

stored in the memory as part program. This data is decoded and transformed into

specific position control and velocity control signals. It also oversees the movement of

the control axis or spindle whenever this does not match the programmed values, a

corrective action is taken.

All the compensations required for machine accuracy (like lead screw pitch error,

tool wear out, backlash, etc.) are calculated by the CPU depending upon the

corresponding inputs made available to the system. The same will be taken care of

during the generation of control signals for the axis movement. Also, some safety

checks are built into the system through this unit and the CPU unit will provide

continuous necessary corrective actions. Whenever the situation goes beyond control of

the CPU, it takes the final action of shutting down the system in turn the machine.

3.B) Speed Control Unit

This unit acts in unison with the CPU for the movement of the machine axes. The

CPU sends the control signals generated for the movement of the axis to the servo

control unit and the servo control unit convert these signals into the suitable digital or

analog signal to be fed to the machine tool axis movement. This also checks whether

machine tool axis movement is at the same speed as directed by the CPU. In case any

safety conditions related to the axis are overruled during movement or otherwise they

are reported to the CPU for corrective action.

3.C) Servo-Control Unit

The decoded position and velocity control signals, generated by the CPU for the

axis movement forms the input to the servo-control unit. This unit in turn generates

suitable signals as command values. The servo-drive unit converts the command

values, which are interfaced with the axis and the spindle motors .The servo-control unit

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receives the position feedback signals for actual movement of the machine tool axes

from the feedback devices (like linear scales, rotary encoders, resolves, etc.).The

velocity feedback is generally obtained through tacho generators. The feedback signals

are passed on to the CPU for further processing. Thus the servo-control unit performs

the data communication between the machine tool and the CPU.As explained earlier,

the actual movements of the slides on the machine tool is achieved through servo

drives. The amount of movement and the rate of movement are controlled by the CNC

system depending upon the type of feedback system used, i.e. closed-loop or open-

loop system .

Closed-loop System

The closed-loop system is characterized by the presence of feedback. In this system,

the CNC system send out commands for movement and the result is continuously

monitored by the system through various feedback devices. There are generally two

types of feedback to a CNC system -- position feedback and velocity feedback.

Position Feedback

A closed-loop system, regardless of the type of feedback device, will constantly try to

achieve and maintain a given position by self-correcting. As the slide of the machine

tool moves, its movement is fed back to the CNC system for determining the position of

the slide to decide how much is yet to be traveled and also to decide whether the

movement is as per the commanded rate. If the actual rate is not as per the required

rate, the system tries to correct it. In case this is not possible, the system declares fault

and initiates action for disabling the drives and if necessary, switches off the machine.

Velocity feedback

In case no time constraint is put on the system to reach the final programmed position,

then the system may not produce the required path or the surface finish accuracy.

Hence, velocity feedback must be present along with the position feedback whenever

CNC system are used for contouring, in order to produce correct interpolation and also

specified acceleration and deceleration velocities. The tacho generator used for velocity

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feedback is normally connected to the motor and it rotates whenever the motor rotates,

thus giving an analog output proportional to the speed of motor. The analog voltage is

taken as speed feedback by the servo-controller and swift action is taken by the

controller to maintain the speed of the motor within the required limits.

Open-loop system

The open loop system lacks feedback. In this system, the CNC system send out signals

for movement but does not check whether actual movement is taking place or not.

Stepper motors are used for actual movement and the electronics of these stepper

motors is run on digital pulses from the CNC system. Since system controllers have no

access to any real time information about the system performance, they cannot

counteract disturbances appearing during the operation. They can be utilized in point to

point system, where loading torque on the axial motor is low and almost constant.

Servo-drives

The servo-drive receives signals from the CNC system and transforms it into actual

movement on the machine. The actual rate of movement and direction depend upon the

command signal from CNC system. There are various types of servo-drives, viz., dc

drives, ac drives and stepper motor drives. A servo-drive consists of two parts, namely,

the motor and the electronics for driving the motor.

3.D) Operator Control Panel

The operator control panel provides the user interface to facilitate a two-way

communication between the user, CNC system and the machine tool. This consists of

two parts:

Video Display Unit (VDU)

Keyboard

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Video Display Unit (VDU)

The VDU displays the status of the various parameters of the CNC system and the

machine tool. It displays all current information such as:

Complete information of the block currently being executed.

Actual position value, set or actual difference, current feed rate, spindle speed.

Active G functions.

Main program number, subroutine number .

Display of all entered data, user programs, user data, machine data, etc..

Alarm messages in plain text.

Soft key designations.

In addition to a CRT, a few LEDs are generally provided to indicate important operating

modes and status. Video display units may be of two types:

1. Monochrome or black and white displays

2. Color displays.

3.E) Machine Control Panel (MCP)

It is the direct interface between operator and the NC system, enabling the

operation of the machine through the CNC system.

During program execution, the CNC controls the axis motion, spindle function or

tool function on a machine tool, depending upon the part program stored in the memory.

Prior to the starting of the machine process, machine should first be prepared with some

specific tasks like:-

Establishing a correct reference point

Loading the system memory with the required part program

Loading and checking of tool offsets, zero offsets, etc.

For these tasks, the system must be operated in specific operating mode so that these

preparatory functions can be established.

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4. MODES OF OPERATING A CNC MACHINE

Generally, the CNC system can be operated in the following modes:-

Manual mode

Manual data input (MDI) mode

Automatic mode

Reference mode

Input mode

Output mode, etc.

4.A) Manual mode

In this mode, movement of a machine slide can carried out manually by pressing the

particular jog button (+ or -). The slide (axis) is selected through an axis selector switch

or through individual switches (e.g., X+, X-, Y+, Y-, Z+, Z-, etc.). The feed rate of the

slide movement is prefixed. CNC system allows the axis to be jogged at high feed rate

also. The axis movement can also be achieved manually using a hand wheel interface

instead of jog buttons. In this mode slides can be moved in two ways:

Continuous moving,

Incremental moving.

Continuous mode

In This mode, the slide will move as long as the jog button is pressed.

Incremental mode

Hence the slide will move through a fixed distance, which is selectable. Normally,

system allows jogging of axes in 1, 10, 100, 1000, 10000, increments. Axis movement is

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at a prefixed feed rate. It is initiated by pressing the proper jog+ or jog- key and will be

limited to the no of increments selected even if the jog button is continuously pressed.

For subsequent movement the jog button has to be released and once again pressed.

4.B) Manual Data Input (MDI) Mode

In this mode the following operation can be performed:

Building a new part program

Editing or deleting of part program stored in the system memory

Entering or editing or deleting of:

Tool offsets (TO)

Zero offsets (ZO)

Test data, etc.

4.C) Automatic Mode (Auto and Single Block)

In this mode the system allows the execution of a part program continuously. The part

program is executed block by block. While one block is being executed, the next block

is read by the system, analyzed and kept ready for execution. Execution of the program

can be one block after another automatically or the system will execute a block, stop the

execution of the next block till it is initiated to do so (by pressing the start button).

Selection of part program execution continuously (Auto) or one block at a time (Single

Block) is done through the machine control panel. Many systems allow blocks (single or

multiple) to be retraced in the opposite direction. Block retrace is allowed only when a

cycle stop state is established. Part program execution can resume and its execution

begins with the retraced block. This is useful for tool inspection or in case of tool

breakage. Program start can be effected at any block in the program, through the

BLOCK SEARCH facility.

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4.D) Reference Mode

Under this mode the machine can be referenced to its home position so that all the

compensations (e.g., pitch error compensation) can be properly applied. Part programs

are generally prepared in absolute mode with respect to machine zero. Many CNC

systems make it compulsory to reference the slides of the machine to their home

positions before a program is executed while others make it optional.

4.E) Input Mode and Output Mode (I/O Mode)

In this mode, the part programs, machine setup data, tool offsets, etc. can be

loaded/unloaded into/from the memory of the system from external devices like

programming units, magnetic cassettes or floppy discs, etc. During data input, some

systems check for simple errors (like parity, tape format, block length, unknown

characters, program already present in the memory, etc.). Transfer of data is done

through a RS232C or RS485C port.

Fig. CNC machine showing other peripherals

PROGRAMMING

UNIT

TAPE

READER

TAPE

PUNCHER

PRINTERS

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5. TYPES OF CNC MACHINES

a) CNC Mills

These machining centers use computer controls to cut different materials. They are able

to translate programs consisting of specific number and letters to move the spindle to

various locations and depths. Many use G code, which is a standardized programming

language that all CNC machines understand, while others use proprietary languages

created by their manufacturers. These proprietary languages are often simpler than G

code, but not transferable to other machines.

b) CNC Lathes

Lathes are machines that cut metal that is often turning at fast speeds. CNC lathes are

able to make fast, precision cuts using indexable tools and drills with complicated

programs for parts that normally cannot be cut on manual lathes. These machines often

include 12 tool holders and coolant pumps to cut down on tool wear. CNC lathes have

similar control specifications and can read G code as well as the manufacturer's

proprietary programming language.

c) CNC Grinders

Grinding metal is a process that uses a coated wheel that slowly removes metal to

create a part. Through the years, grinding was done on a manual machine, but with the

advent of CNC technology, the grinding process has advanced dramatically. Enter the

program, which includes the various passes that the wheel will make, and hit start. It is

one of the easiest types of CNC machines to work with as they often only contain a

single grinding wheel.

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6. GENERAL WORKING OF CNC MACHINE

Electronic industries association defines numerical control as “A system in which action

is controlled by direct insertion of numerical data. The system must automatically

interpret at least some potion of data.”

In simple word numerical control means control by numbers. In NC machine tools the

main function is to control the displacement and positioning of slides, spindle, speed,

feed rate, selection of tool and many other auxiliary functions. NC directs the machine

tool to achieve all these function in a very controlled and systematic manner .The major

elements that comprises NC machine tools are :-

Control system – CNC

The machine tool

Servo drive units

Feedback devices

Operator control

Electrical cabinet

CAD/Drawing software(Creates the parts to be machined)

CAM Software(Convert the part file to G-Code)

CNC Control Software(Turns the G-code file into motion- usually by

controlling the parallel port.)

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7. VARIOUS FUNCTIOS OF CNC

The main functional controls that a CNC performs are:-

A. Axes position and velocity control

B. Spindle speed

C. Miscellaneous functions.

Various functional codes

1. Preparatory function: these are commands which prepare the machine for

different modes of movements like position countering, thread cutting etc.

2. Dimensional Data: Movement of machine tool slides in one or more axes is

determined by dimensional data entered in the program.

3. Miscellaneous Function: Some of the important miscellaneous function which

worth to be considered here are coolant on or off, Spindle CW or CCW, program

stop etc.

4. Speed Function(S): this function pertains to speed of spindle.

5. Feed Function (F): It pertains to feed rates of the slides.

6. Tool Function (T): this function pertains to the selection of required tool for the

particular operation.

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8. TYPES OF CNC PROGRAMMING USED

Here are four of the most common types of CNC software:

- CNC Calculators to help calculate Feeds and Speeds ( G-Wizard CNC Calculator for

example).

- CNC Program Editors to help us manage g-code(eg. G-Wizard Editor).

- CAD programs used to create drawings and 3D models of the parts we want to

machine.

- CAM programs that start from a CAD drawing and produce the g-code our CNC

machines execute. There are many more types of CNC software out there.

9. ADVANTAGES OF CNC MACHINES

a) Productivity

Since cutting tool is brought to its machining position much more efficiently than it was

done manually by the machine operator, NC machine is spending much more time per

shift cutting than in past. Conventional machines very seldom remove metal for more

than 15% of total available time under normal batch production conditions. Whereas

CNC machine tools should be capable of removing metal for between 50% and 75% of

available time. When working on medium batch production, CNC machining has around

4 to 1 productivity advantage over conventional machine. The actual advantage may

vary from batch to batch depending upon the complexity of components to be produced

and is normally proportional to the number of conventional operation required to

produce the components.

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b) Flexibility in design and production

Machine can switch over to different job as set up times are low and sudden changes in

sales requirement are much more easily catered for. This enables the formulation of

more aggressive marketing plans. The use of CNC machines also give designers

freedom to design components which, by conventional means, are often impossible to

produce. Change of design can also be easily incorporated as it means change of tape.

c) Inspection

High position accuracies and repeatability are inherent features of CNC machines and

reduce inspection time considerably. Normally a 100% inspection of the first component

produced by a new tape is all that is necessary to prove the tape and tooling.

Subsequently it is required to have only sample inspection. In process gauging and

inspection is also provided on modern CNC machines.

d) Floor space

One CNC machine can replace five to six conventional machines. Thus manufacturing

activities of a company can be expanded without increasing the floor area

proportionately.

e) Inventory

By using CNC machine, procurement sizes and batch sizes can be reduced because of

shorter lead time’s .This results in substantial saving. Lead time is time taken to

progress a batch of component through a batch of production shop and is proportional

to number of operation required by conventional methods. For example a component

which requires 112 set ups by conventional methods may requires only 1 or 2 set ups in

CNC machining center reducing total product flow times.

f) Material Handling

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Handling of component from machine to machine which is necessary on conventional

machine is significantly reduced on CNC machine, as all the operations are performed

on one machine. This obviously reduces labor cost.

g) Tooling

This ability to complete machine part in a single setup means that fewer and simpler

fixtures are required, which in turns requires less storage space and maintenance. The

simpler a fixture is, the less expensive is to manufacture it.

h) Operator’s Skill

Dependence on skilled labor can be dispensed with. The accuracy of part produced with

CNC machines machine depend upon accuracy and ability of machine and tape – and

not on individual operator.

i) Scrap and Rework

Drastic reduction in scrap is achieved because of the inherent accuracy and

repeatability of CNC machine.

j) Costing

Time required to produce a component is a function of machining cycle of CNC

machines and is not influenced by operator’s efficiency or variation in labor’s rate, a

great stability of prices can often be achieved throughout the life cycle of the respective

product. Also cost accounting becomes very precise.

k) Better Management Information and Control

With various advantages of CNC machines, decisions effecting unit cost, delivery and

quality are firmly placed in the hands of management and not of the machine operator.

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BIBLIOGRAPHY

CNC Programming handbook by Mr. Peter Smid.

Managing CNC operations by Mike Lynch

Wikipedia.www.wikipedia.org/wiki/Main

B.H.E.L. senior engineer Mr. B.K. Garg

CNC information and easy CNC by Mr. David Benson.

http://www.cnccncmachines.com

http://www.cnccookbook.com