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1.0 TITLE: CNC Milling (Non Traditional Machining) 2.0 OBJECTIVE: 1. To develop more understanding about development and principle of CNC machine. 2. To learn various type of modern CNC machines and CNC modes of operations. 3. To understand the fundamentals of part programming in terms of the various steps needed to be taken for completing a successful CNC program. 4. To understand the different preparatory (G codes) and miscellaneous functions (M codes) as used in CNC part program. 5. To understand the advantages and disadvantages of CNC machines and their applications. 1
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Page 1: CNC Milling Machine

1.0 TITLE: CNC Milling (Non Traditional Machining)

2.0 OBJECTIVE:

1. To develop more understanding about development and principle of CNC machine.

2. To learn various type of modern CNC machines and CNC modes of operations.

3. To understand the fundamentals of part programming in terms of the various steps

needed to be taken for completing a successful CNC program.

4. To understand the different preparatory (G codes) and miscellaneous functions (M

codes) as used in CNC part program.

5. To understand the advantages and disadvantages of CNC machines and their

applications.

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3.0 INTRODUCTION

Numerical control (NC) refers to the automation of machine tools that are operated by

abstractly programmed commands encoded on a storage medium, as opposed to manually

controlled or mechanically automated via cams alone. The first NC machines were built in the

1940s and '50s, based on existing tools that were modified with motors that moved the

controls to follow points fed into the system on paper tape. These early servomechanisms

were rapidly augmented with analog and digital computers, creating the modern computer

numerical controlled (CNC) machine tools that have revolutionized the design process. In

modern CNC systems, end-to-end component design is highly automated using CAD/CAM

programs. The programs produce a computer file that is interpreted to extract the commands

needed to operate a particular machine, and then loaded into the CNC machines for

production. Since any particular component might require the use of a number of different

tools, modern machines often combine multiple tools into a single "cell".

Diagram 1.1: CNC milling machine

Modern CNC mills differ little in concept from the original model built at MIT in

1952. Mills typically consist of a table that moves in the Y axis, and a tool chuck that moves

in X and Z (depth). The position of the tool is driven by motors through a series of step-down

gears in order to provide highly accurate movements, or in modern designs, direct-drive

stepper motors. As the controller hardware evolved, the mills themselves also evolved. One

change has been to enclose the entire mechanism in a large box as a safety measure, often

with additional safety interlocks to ensure the operator is far enough from the working piece

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for safe operation. Mechanical manual controls disappeared long ago. CNC-like systems are

now used for any process that can be described as a series of movements and operations.

These include laser cutting, welding, friction stir welding, ultrasonic welding, flame and

plasma cutting, bending, spinning, pinning, gluing, fabric cutting, sewing, tape and fiber

placement, routing, picking and placing (PnP), and sawing.

Diagram 1.2: the Data Processing in a CNC Machine Tool

Features of CNC

CNC systems include additional features beyond what is feasible with conventional hard-

wired NC. These features, many of which are standard of most CNC machine control units

(MCU) where others are optional, include the following:

1. Storages of more than one part. With improvements in computer storage technology,

newer CNC controllers have sufficient capacity to store multiple programs.

2. Various forms of program input. Hard-wired MCUs are limited to punched tape as the

input medium for entering part programs, whereas CNC controllers possess multiple data

entry capabilities.

3. Program editing at the machine tool. CNC permits a part program to be edited while it

resides in the MCU computer memory. Hence, the process of testing and correcting a

program can be done entirely at the machine site rather than returning to the programming

office to edit the tape.

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4. Fixed cycles and programming subroutines. The increased memory capacity and the

ability to program the control computer provide the opportunity to store frequently used

machining cycles as macros that can be called by the part program. Instead of writing the

full instructions for the particular cycle into every program, a call statement is included in

the part program to indicate that the macro cycle should be executed.

5. Interpolation. Linear and circular interpolation is sometimes hard-wired into the control

unit, but helical, parabolic and cubic interpolations are usually executed in a stored

program algorithm.

6. Positioning features for set up. Setting up the machine tool for a given workpart involves

installing and aligning a fixture on the machine tool table. The alignment task can be

facilitated using certain features made possible by software option in CNC system.

Position set is one of these features. With position set, the operator is not required to

locate the fixture on the machine table with extreme accuracy.

7. Cutter length and size compensation. In older style controls, cutter dimensions had to be

set very precisely to agree with the tool path defined in the part program.

8. Acceleration and deceleration calculation. This feature is applicable when the cutter

moves at high feed rates. It is designed to avoid tool marks on the work surface that would

be generated due to machine tool dynamics when the cutter path changes abruptly.

9. Communication interface. Most modern CNC controllers are equipped with RS-232 or

other communication interface to allow machine to be linked to other computers and

computer driven devices.

10. Diagnostic. Many CNC systems possess an online diagnostics capability that monitors

certain aspects of the machine tool to detect malfunctions or sign of impending

malfunctions or to diagnose system breakdowns.

Advantages of CNC machine

CNC machine has several advantages with emphasis on machine tool applications.

When the production application satisfies the characteristics needed, CNC yields many

benefits over manual production methods. The benefits translate into economic saving for the

user company. Some of the advantages are:

i. Nonproductive time is reduced through fewer setup, less setup time, less workpiece

handling time, and automatic tool changes.

ii. Greater accurancy and repeatability.

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iii. Lower scrap rates.

iv. Inspections requirements are reduced.

v. More complex part geometries are possible

vi. Enginnering changes can be accommodated more gracefully.

vii. Simple fixture are needed.

viii. Shorter manufacturung lead times.

ix. Reduced parts inventory.

x. Less floorspace required

xi. Operator skill level requirements are reduced.

Machine Control Unit (MCU)

CNC machine is fitted with MCU which is performs the various controlling functions

under the program control. The MCU may be generally housed in a seperated cabinet like

cabinet body or may be mounted on the machine itself. Apperance wise it looks like a

computer with a display panel generally of small size and a number of button to control the

machine tool along with a keyboard. This control unit control the motion of cutting tool,

spindle speeds, feed rate, tool changes, cutting fluids application and several other functions

of the mahine tool. The MCU consists of the following components and subsystems: (1)

central processing unit, (2) memory, (3) input and output interface, (4) control for machine

tool axes and spindle speed, and (5) sequence control for other machine tools. This subsystem

interconneted by means of a system bus.

Numerical Control Mode

The controller have number of modes in which the can operate. There could be four possible

modes in which the controller can function in relation to a machine centre.

1. Termed as point to point mode. In this, the control has the capability to operate all the

three axes, but not necessarily simultaneously. It would be possible to move the tool to

any point (in X and Y axes) and carry out the machining operation in one axis (Z axis) at

that point.

2. Improvement over point to point mode. The machine tool has the capability to carry out a

continuous motion in each of the axis direction.

3. A control system, which has simultaneous motion capability in any two axes.

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4. The highest form of control that gives the capability of simultaneous three or more axes

motion.

Part Program

Part program is a very important software element in the NC manufacturing system. It

is a detailed plan of manufacturing instructions required for machining the part as per

drawing. The format standardized by ISO. For example,

N30 G00 X120 Y45 Z 85N40 G90N50 G03 X200 I-100 J0 F200

ISO Standards for coding

CHARACTER ADDRESS FOR CHARACTER ADDRESS FOR

AAngular dimension around X

axisN Sequence number

BAngular dimension around Y

axisO References rewind stop

CAngular dimension around Z

axisP Third rapid traverse dimension

DAngular dimension around 3rd

feed functionQ Second rapid traverse dimension

EAngular dimension around 2nd

feed functionR First rapid traverse dimension

F Feed function S Spindle speed functionG Preparatory function T Tool function

H Unassigned USecondary motion dimension

parallel to X*

I Distance to arc centre to X VSecondary motion dimension

parallel to Y*

J Distance to arc centre to Y WSecondary motion dimension

parallel to Z*K Distance to arc centre to Z X Primary X motion dimensionL Do not use Y Primary Y motion dimensionM Miscellaneous function Z Primary Z motion dimension

Preparatory functions

This is denoted by ‘G’. It is a pre-set function associated with the movement of

machine axes and the associated geometry. It has two digits as per ISO specifications. Some

examples are shown:

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CODE FUNCTION

G00 rapid traverse

G01 line interpolation

G02 ciccular interpolation, clockwise

G03 circular interpolation, anti-clockwise

G04 dwell

G05 hold/delay

G06 parabolic interpolation

G33 thread cutting, constant lead

G53 deletion of zero offset

G64 change in feed rate or speed

G80 canned cycle cancelled

Miscellaneous functions

These functions operate some controls on the machine tool and affect the running of

the machine. Generally, only one M-code is supposed to be given in a single block. The ISO

standard examples are:

CODE FUNCTION

M00 program stop, spindle and coolant off

M01 optional programmable stop

M02 end of program

M03 spindle on, clockwise

M04 spindle on, anticlockwise

M09 coolant off

M10 clamp

M31 interlock by-pass

M68 unclamp piece art

4.0 APPARATUS

1. CNC centroid milling machine

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2. Machine control unit (MCU)

3. Plastic workpiece (100x100x15)

5.0 EXPERIMENTAL PROCEDURE

A demonstration was carried on by our lecturer on:

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1. At first, we have to check the programs to ensure that the machinery will function properly and that the output will meet specifications. We downloaded the NC program to the controller by key in the data, cabling RS232 or APT after the programming work is completed.

2. Tool layout. Before the machining was started, the tool layout must be check so that we got the desired shape of workpiece.

3. Workpiece setting. In this step, operator loaded the proper cutting tools into the tool holder and positions the workpiece that is piece of metal or plastic that is being shaped on the CNC machine tool.

4. In machining process, after operator load workpieces and cutting tools into a machine, after that the start button was pressed. Operator monitored the machine for problems, and measure the parts produced to check that they match specifications.

6.0 DATA AND RESULT

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ADDRESS

X YA 14 14B 14 86C 30 86D 70 86E 86 86F 86 14G 70 14H 30 14

X Y Z Fix Rate(F) Radius(R)

N01 G00 X14 Y14 Z5 N02 G01 X14 Y14 Z2 F0.5 N03 G01 X14 Y86 Z2 N04 G01 X30 Y86 Z2 N05 G03 X70 Y86 Z2 F0.2 R27N06 G01 X86 Y86 Z2 F0.5 N07 G01 X86 Y14 Z2 N08 G01 X30 Y14 Z2 N09 G03 X30 Y14 Z2 R27N10 G01 X14 Y14 Z2

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7.0 DISCUSSSION

From this experiment we can see the result of the work piece when using non-traditional

machine CNC milling machine. This CNC milling are control by Centroid control, is a

powerful PC based CNC that eliminates many typical limitations of other controls. This

Centroid control is so easy to use. These controls are perfect for proto-typing, mold working,

short or long run production. The Centroid control can be ordered in 3, and 4 axis

configurations. The package provides automatic control of the spindle motor, the flood pump;

the mister and lube pump (with auto low-lube detection with most models).

Below is the result that we get with using this machine:-

• For the above product, we use the polymer material called ‘Perspex’.

• As we got the result, we could determine that the final shape of the product is very

fine cutting; no burrs and the dimensional is precise and accuracy.

Before we can get this result we need to program the code using NC programming. In order to

ensure CNC machine do all the required operation, there are commands that must be included

in the CNC programs in proper order. The command that being used in CNC programs are in

form of G and M codes, which each codes have their own function. So the codes that have

been use to make this product we can see below:-

G00-rapid traverse

G01-linear

G02-circular counter clockwise

G03- circular counter clockwise

CD-G03 X70 Y86 R27

DE-G01 X86 F0.5

EF-G01 X86 Y14

FG-X30

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GH-G03 X70 Y14 R27

HA-G01 X14

Before we do this operation zero set-up for the work piece and machine tool must carried out.

Generally, there are two types of method that can be used in zero set-ups for work piece and

machine tool. The first method is to assign program zero in the program. This is the old

method. By using this method, the programmer will tells the control how far the distance of

the starting position of the machine from the program zero point. Usually this method is done

by using G50 command. The second method to assign program zero is through some form of

offset. This is the latest method. In this method, the cutting tool is pointed to the work piece

and zero position is set at the computer control or input data. Between these two methods, the

second one is the better way to assign program zero. The important of these zero set up before

running the operation is in order to get a positive value for the coordinates. It also allowed for

a fixing a number of positions on the machine table whose coordinates can be entered into the

controller as a permanent memory. This zero set-ups helps to avoid errors from occur. On the

run-test graphic screen, the present status such as the position of the machine slide, the spindle

RPM, the feed rate and the part programs will be displayed when the machine is running. We

can also see the graphic simulation of the tool path. Much other important information about

CNC system can also be seen on the screen. It is important for maintenance and installation

work such as machine parameter, logic diagram of the programmer controller, error message

and diagnostic data.

So these machine much better than conventional machine because the flexibility of operation

is improved, as well as the ability to produce complex shapes with good dimensional

accuracy, good repeatability, reduced scrap loss, high production rates and high product

quality. We also can reduce tooling cost, because templates and other fixtures are not

required. With CNC machine more operation can be performed with each setup, and the lead

time for setup and machining required is less. Furthermore, design changes are facilitated, and

inventory is reduced. Programs also can be prepared rapidly, and they can be recalled at any

time, by utilizing microprocessors. Less paperwork is involved.

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8.0 CONCLUSION

1. The objective to acquaint students with NC programming and CNC Milling Machine

has been achieved.

2. From this experiment, we can see that NC programming and CNC Milling Machine

help a lot especially in milling process.

3. Milling machines are being replaced rapidly by this CNC Machines (computer

numerical-control machines), which are more versatile and have capable of milling,

drilling, boring and also tapping with repetitive accuracy.

4. Milling process will be more easily and less time has been taken to complete the

process.

5. Other than that, this machine has the ability to produce complex shapes with high

dimensional accuracy besides it can reduce the scrap loss.

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9.0 REFERENCES

i. Mikell P. Groover, Automation, Production Systems and Computer Integrated

Manufacturing, 2nd edition, Prentice Hall, 2001

ii. P. N. Rao, CAD/CAM Principles and Applications, 2nd edition, Mc Graw Hill,

2004,

iii. Serope Kalpakjian, W.R. Schmid, Manufacturing Technology and Fundamental,

5th edition, Prentice Hall, 2004

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