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Abstract:

Computer Numerical Control (CNC)

machines have become a major part in

almost all types of manufacturing processes

and is used in a wide range of applications.

From aerospace applications to the

manufacturing of energy systems and

medical robots, we see a major reliance on

CNC machines. New machine tools in CNC

have enabled industry to consistently

produce parts to accuracies undreamed. The

same part can be reproduced to the same

degree of accuracy any number of times if the

CNC program has been properly prepared

and the computer properly programmed. The

operating commands which control the

machine tool are executed automatically with

amazing speed, accuracy, efficiency, and

repeatability.

Introduction:

Computer Numerical Control involves the

replacement of the conventional hard wired

NC controller unit by a small computer (mini

or microcomputer).CNC may be considered

to be a means of operating a machine through

the use of discrete numerical values fed into

the machine, where the required input

technical information is stored on a kind of

input media such as floppy disk, hard disk,

CD ROM, DVD, USB Flash Drive, or RAM

card. The machine follows a predetermined

sequence of machining operations at the

predetermined speeds necessary to produce a

work piece of the right shape and size and

thus according to completely predictable

results. A different product can be produced

through reprogramming and a low-quantity

production run of different products is

justified.

History:

In 1947, the U.S. Air Force found that the

complex designs and shapes of aircraft parts

such as helicopter rotor blades and missile

components were causing problems for

manufacturers, who could not keep up to

projected production schedules. At this time,

John Parsons, of the Parsons Corporation, of

Traverse City, Michigan, began

experimenting with the idea of making a

machine tool generate a thru-axis curve" by

using numerical data to control the machine

tool motions. In 1949, the U.S. Air Material

Command awarded Parsons a contract to

develop NC and in turn speed up production

methods. Parsons subcontracted this study to

the Servomechanism Laboratory of the

Massachusetts Institute of Technology

(MIT), which in 1952 successfully

demonstrated a vertical spindle Cincinnati

Hydrotel, which made parts through

simultaneous three-axis cutting tool

movements. In a very short period of time,

almost all machine tool manufacturers were

producing machines with NC. At the 1960

Machine Tool Show in Chicago, over a

INTRODUCTION TO COMPUTER NUMERICALLY

CONTROLLED (CNC) MACHINES AND

ITS ADVANTAGES

Bulon Ch. Hazarika, 15-22-308, CAD CAM & Automation, NIT Silchar, Assam

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hundred NC machines were displayed. Most

of these machines had relatively simple

point-to-point positioning, but the principle

of NC was now firmly established. From this

point, NC improved rapidly as the electronics

industry developed new products. At first,

miniature electronic tubes were developed,

but the controls were big, bulky, and not very

reliable. Then solid-state circuitry and,

eventually, modular, or integrated circuits

were developed. The control unit became

smaller, more reliable, and less expensive.

The development of even better machine

tools and control units helped spread the use

of NC from the machine tool industry to all

facets of manufacturing.

Data Processing: NC data processing (with

numbers, letters, and symbols) is done in a

computer or machine control unit (MCU) by

adding, subtracting, multiplying, dividing,

and comparing. The computer can be

programmed to recognize an A command

before a B command, an item 1 before an

item 2, or any other elements in their

sequential order. It is capable of handling

numbers very quickly; the addition of two

simple numbers may take only one billionth

of a second (a nanosecond).

NC Evolves into CNC: The introduction of

software-based controls in the early 1970s

replaced the NC hardware design with

complete computer logic that had more

capacity and could be programmed for a

variety of functions at any time. This made it

possible to revise, modify, or update CNC

programs or parts of programs at any time on

a computer. In turn, CNC machines became

easier to use with their menu-selected

displays, advanced graphics, and ease of

programming.

Types of CNC Machines:

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. Used

to make 3D prototypes, moulds, cutting dies,

printing plates and sights.

CNC Lathes:

They cut metal that is often turning at fast

speeds. CNC lathes are able to make fast,

precision cuts using index able tools and

drills with complicated programs. Normally,

they cannot be cut on manual lathes. They

often include 12 tool holders and coolant

pumps to cut down on tool wear.

CNC Grinders:

Grinding metal process 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.

CNC Drilling:

Drilling is commonly used for mass

production. The drilling machine (drilling

press) is used to create or enlarge holes.

Drilling machine for different jobs;

The bench drill: For drilling holes through

raw materials such as wood, plastic and metal

The pillar drill: A larger version that stands

upright on the floor. As the bench drill, it can

be used to drill larger pieces of materials and

produce bigger holes.

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CNC Boring:

Process of enlarging an existing hole or

internal cylindrical surface. This can be

accomplished on a lathe or a machine tool

specifically designed for the process, such as

a horizontal boring machine.

CNC Grinding:

CNC grinding machine is to use abrasives on

the workpiece surface grinding machine.

Most of the grinder is to use high-speed

rotation of the grinding wheel and grinding,

the minority is to use the oil stone, belt and

other abrasive and free abrasive processing,

such as honing machines, super finishing

machines, abrasive belt grinding machines,

grinding machines and polishing machines.

Applications:

CNC machines are widely used in the metal

cutting industry and are best used to produce

the following types of product:

Parts with complicated contours

Parts requiring close tolerance and/or good

repeatability

Parts requiring expensive jigs and fixtures

if produced on conventional machines

Parts that may have several engineering

changes, such as during the development

stage of a prototype

In cases where human errors could be

extremely costly

Parts that are needed in a hurry

Small batch lots or short production runs.

The applications of CNC include both for

machine tool as well as non-machine tool

areas. In the machine tool category, CNC is

widely used for lathe, drill press, milling

machine, grinding unit, laser, sheet-metal

press working machine, tube bending

machine etc. Highly automated machine

tools such as turning center and machining

center which change the cutting tools

automatically under CNC control have been

developed. In the non-machine tool category,

CNC applications include welding machines

(arc and resistance), coordinate measuring

machine, electronic assembly, tape laying

and filament winding machines for

composites etc.

Classification of CNC Systems:

CNC machine tool systems can be

classified in various ways such as:

1. Point-to-point or contouring : depending on whether the machine

cuts metal while the work piece

moves relative to the tool

2. Incremental or absolute : depending on the type of

coordinate system adopted to

parameterize the motion

commands

3. Open-loop or closed-loop : depending on the control system

adopted for axis motion control

Point-to-point systems:

Point-to-point (PTP) systems are the ones

where, either the work piece or the cutting

tool is moved with respect to the other as

stationary until it arrives at the desired

position and then the cutting tool performs

the required task with the motion axes

stationary. Such systems are used, typically,

to perform hole operations such as drilling,

boring, reaming, tapping and punching. In a

PTP system, the path of the cutting tool and

its feed rate while traveling from one point to

the next are not significant, since, the tool is

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not cutting while there is motion. Therefore,

such systems require only control of only the

final position of the tool. The path from the

starting point to the final position need not be

controlled.

Fig 1.1: Point to point systems

Contouring systems:

In contouring systems, the tool is cutting

while the axes of motion are moving, such as

in a milling machine. All axes of motion

might move simultaneously, each at a

different velocity. When a nonlinear path is

required, the axial velocity changes, even

within the segment. For example, cutting a

circular

Fig 1.2: Contouring Systems

Elements of a CNC System

A CNC system consists of the following 6

major elements:

a. Input Device

b. Machine Control Unit

c. Machine Tool

d. Driving System

e. Feedback Devices

f. Display Unit

Input Devices

a. Floppy Disk Drive:

Floppy disk is a small magnetic storage

device for CNC data input. It has been the

most common storage media up to the 1970s,

in terms of data transfer speed, reliability,

storage size, data handling and the ability to

read and write. Furthermore, the data within

a floppy could be easily edited at any point as

long as you have the proper program to read

it. However, this method has proven to be

quite problematic in the long run as floppies

have a tendency to degrade alarmingly fast

and are sensitive to large magnetic fields and

as well as the dust and scratches that usually

existed on the shop floor.

b. USB Flash Drive:

A USB flash drive is a removable and

rewritable portable hard drive with compact

size and bigger storage size than a floppy

disk. Data stored inside the flash drive are

impervious to dust and scratches that enable

flash drives to transfer data from place to

place. In recent years, all computers support

USB flash drives to read and write data that

make it become more and more popular in

CNC machine control unit.

c. Serial communication

The data transfer between a computer and a

CNC machine tool is often accomplished

through a serial communication port.

International standards for serial

communications are established so that

information can be exchanged in an orderly

way. The most common interface between

computers and CNC machine tools is referred

to the EIA Standard RS-232. Most of the

personal computers and CNC machine tools

have built in RS232 port and a standard RS-

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232 cable is used to connect a CNC machine

to a computer which enables the data transfer

in reliable way. Part programs can be

downloaded into the memory of a machine

tool or uploaded to the computer for

temporary storage by running a

communication program on the computer and

setting up the machine control to interact with

the communication software.

d. Ethernet communication:

Due to the advancement of the computer

technology and the drastic reduction of the

cost of the computer, it is becoming more

practical and economic to transfer part

programs between computers and CNC

machines via an Ethernet communication

cable. This media provides a more efficient

and reliable means in part program

transmission and storage. Most companies

now built a Local Area Network (LAN) as

their infrastructure. More and more CNC

machine tools provide an option of the

Ethernet Card for direct communication

within the LAN.

e. Conversational Programming:

Part programs can be input to the controller

via the keyboard. Built-in intelligent software

inside the controller enables the operator to

enter the required data step by step.

Machine Control Unit (MCU)

The machine control unit is the heart of the

CNC system. There are two sub-units in the

machine control unit: the Data Processing

Unit (DPU) and the Control Loop

Unit (CLU).

a. Data Processing Unit

On receiving a part program, the DPU firstly

interprets and encodes the part program into

internal machine codes. The interpolator of

the DPU then calculate the intermediate

positions of the motion in terms of BLU

(basic length unit) which is the smallest unit

length that can be handled by the controller.

The calculated data are passed to CLU for

further action.

b. Control Loop Unit

The data from the DPU are converted into

electrical signals in the CLU to control the

driving system to perform the required

motions. Other functions such as machine

spindle ON/OFF, coolant ON/OFF, tool

clamp ON/OFF are also controlled by this

unit according to the internal machine codes.

Fig 1.3: display and MCU

Machine Tool

This can be any type of machine tool or

equipment. In order to obtain high accuracy

and repeatability, the design and make of the

machine slide and the driving leadscrew of a

CNC machine is of vital importance. The

slides are usually machined to high accuracy

and coated with anti-friction material such as

PTFE and Turcite in order to reduce the stick

and slip phenomenon. Large diameter

recirculating ball screws are employed to

eliminate the backlash and lost motion.

Other design features such as rigid and heavy

machine structure; short machine table

overhang, quick change tooling system, etc.

also contribute to the high accuracy and high

repeatability of CNC machines.

Driving System

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The driving system is an important

component of a CNC machine as the

accuracy and repeatability depend very much

on the characteristics and performance of the

driving system. The requirement is that the

driving system has to response accurately

according to the programmed instructions.

This system usually uses electric motors

although hydraulic motors are sometimes

used for large machine tools. The motor is

coupled either directly or through a gear box

to the machine leadscrew to moves the

machine slide or the spindle. Three types of

electrical motors are commonly used.

a: DC Servo Motor

b: AC Servo Motor

c: Stepping Motor

e: Linear Motor

Feedback Device

In order to have a CNC machine operating

accurately, the positional values and speed of

the axes need to be constantly updated. Two

types of feedback devices are normally used,

positional feedback device and velocity

feedback device

a: Positional Feedback device

b: Velocity Feedback device

Display Unit

The Display Unit serves as an interactive

device between the machine and the operator.

When the machine is running, the Display

Unit displays the present status such as the

position of the machine slide, the spindle

RPM, the feed rate, the part programs, etc. In

an advanced CNC machine, the Display Unit

can show the graphics simulation of the tool

path so that part programs can be verified

before the actually machining. Much other

important information about the CNC system

can also displayed for maintenance and

installation work such as machine

parameters, logic diagram of the programmer

controller, error messages and diagnostic

data.

Fig 1.4: Block diagram of elements of CNC

Axis of motions in CNC machine:

In generally, all motions have 6 degrees of

freedom. In other words, motion can be

resolved into 6 axes, namely, 3 linear axes

(X, Y and Z axis) and 3 rotational axes (A, B,

and C axis).

A CNC turning Centre has two axis along

which the table can slide to perform the

operations while CNC milling has three axis

of motions.

Fig 1.5: Coordinate system of standard lathe

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Fig 1.6: Standard CNC milling axis of motions

Part Program:

A part program is a set of instructions often

referred to as blocks, each of which refers to

a segment of the machining operation

performed by the machine tool. Each block

may contain several code words in sequence.

These provide:

1. Coordinate values (X, Y, Z, etc.) to specify

the desired motion of a tool relative to a work

piece. The coordinate values are specified

within motion code word and related

interpolation parameters to indicate the type

of motion required (e.g. point-to-point, or

continuous straight or continuous circular)

between the start and end coordinates. The

CNC system computes the instantaneous

motion command signals from these code

words and applies them to drive units of the

machine.

2. Machining parameters such as, feed rate,

spindle speed, tool number, tool offset

compensation parameters etc.

3. Codes for initiating machine tool functions

like starting and stopping of the spindle,

on/off control of coolant flow and optional

stop. In addition to these coded functions,

spindle speeds, feeds and the required tool

numbers to perform machining in a desired

sequence are also given.

4. Program execution control codes, such as

block skip or end of block codes, block

number etc.

5. Statements for configuring the subsystems

on the machine tool such as programming the

axes, configuring the data acquisition system

etc.

Basic Codes in CNC Machines:

Preparatory Codes (G ) for initial machining setup and operating

conditions.

Miscellaneous Codes ( M) for on/off of coolant flow, spindle rotation etc.

Spindle Speed (S)

Feed Rate (F)

Tool number (T)

Fig 1.7: Example of a typical block

Advantages of CNC machines:

-Flexibility in operations is improved, as it

has the ability to produce complex shapes

with good dimensional accuracy;

-Repeatability;

-Reduced scrap loss:

-Easier to program;

-Easy store, edit and change existing

programs;

-Stabilize manufacturing costs;

-Avoids human errors;

-Safer to operate;

-Usually generates closer tolerances than

manual machines

-High production rates

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REFERENCES:

Mickell P. Groover & Emory W. Zimmers, Jr Computer Aided Design and Manufacturing

P.N Rao CAD/CAM Principles and Applications

INDUSTRIAL CENTRE, THE HONGKONG POLYTECHNIC UNIVERSITY Computer

Numerical Control (CNC)

P.Radhakrishnan , S Subramanyan & V.Raju CAD/CAM/CIM