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SCREW THREAD

General Objective: To understand the methods of testing and

measuring elements of ISO and BSW screw

threads.

Specific Objectives: At the end of the unit you will be able to :

Identify the methods of measuring major

diameter, minor diameter and mean diameter.

Measure and calculate major diameter, minor

diameter and mean diameter of a screw thread.

To check the thread form by using the optical

comparator.

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1.0 INTRODUCTION

All elements of the thread influence the strength and interchange ability of

screw thread, but the pitch, angle and effective diameter are much more

important than the other elements

1.1 ELEMENTS OF A THREAD

To understand and calculate the thread elements, the following

definition relating to screw threads should be known (Fig. 1.1).

root

pitch

1.1.1. Major Diameter

It is the largest diameter of the thread. This is the distance

between the crests of the thread measured perpendicular to the

thread axis.

Figure 1.1 Screw thread terminology

thread angle

maj

or

dia

met

er

min

or

dia

met

er

mea

n d

iam

eter

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1.1.2. Pitch/Mean Diameter

The diameter of the thread used to establish the relationship,

or fit, between an internal and external thread. The pitch diameter is

the distance between the pitch points measured perpendicular to the

thread axis. The pitch points are the points on the thread where the

thread ridge and the space between the threads are of the same width.

1.1.3. Minor Diameter

It is the smallest diameter of the thread. This is the distance

between the roots of the thread measured perpendicular to the thread

axis.

1.1.4. Thread Angle

This is the included angle of the thread form.

1.1.5. Pitch

It is the distance between the same points on adjacent threads.

This is also the linear distance the thread will travel in one

revolution.

1.1.6. Root

The surface of the thread that joins the flanks of adjacent

threads. The distance between the roots on opposite sides of the

thread is called the root, or minor diameter.

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1.2. MEASURING THE MAJOR DIAMETER

To measure major diameter of the screw, a micrometer, with anvils of

a diameter sufficient to span two threads, may be used,( Fig. 1.2). To

eliminate the effect of errors in the micrometer screw and measuring faces,

it is advisable first to check the instrument to a cylindrical standard of about

the same diameter as the screw. For such purposes a plug gauge or a set of

„Hoffman‟ rollers is useful.

anvil

1.3. MEASURING THE MINOR/CORE DIAMETER

The diameter over the roots of a thread may be checked by means of a

special micrometer adapted with a shaped anvils, (Fig. 1.3) or a micrometer

may be used in conjunction with a pair of vee pieces ( steel prisms ). The

second method is recommended ( Fig.1.5). The steel prisms on the

micrometer are pressed into the thread groove. The ends of the prisms are

slightly curved and parallel to the root thread. It is important , when

making the test, to ensure that the micrometer is positioned at right angles

to the axis of the screw being measured, and when a large amount of such

work is to be done, a special „floating bench micrometer‟ ( Fig. 1.4 ) is used.

It is because, it supports the screw and incorporates the micrometer

Figure 1.2 Checking the major diameter with a micrometer

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elements correctly located, as well as providing means for suspending the

vee prisms.

Fig. 1.4. A Floating Micrometer

Fig. 1.3 Checking the core diameter of a thread with an shaped anvil micrometer

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The prism values are stated as,

Dm = W – 2T

Note:

Dm - mean diameter

W - distance between two prism

T - prism height (known)

T

prism

W

1.4. MEASURING THE MEAN/PITCH/EFFECTIVE DIAMETER

The three-wire method is recognized as one of the best methods of

checking the pitch diameter because the results are least affected by any

error which may be present in the included thread angle. For threads which

require an accuracy of 0.001 in. or 0.02 mm, a micrometer can be used to

measure the distance over the wires. For threads requiring greater accuracy

an electronic comparator should be used to measure the distance over the

wires.

In the three-wire method, three wires of equal diameter are placed in

the thread; two on one side and one on the other side (Fig. 1.6). The wires

Figure 1.5 Checking minor diameter by using a micrometer and prisms

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used should be hardened and lapped to three times the accuracy of the

thread to be

inspected. A standard micrometer may then be used to measure the

distance over the wires. For greatest accuracy, the best size wire should be

used.

Figure 1.6 Three wire method

The hard round bars (wire) with the same size are positioned opposite

to the screw thread groove shown in the diagram above. The distance is

measured between the outside of the round bars. The most suitable wire

size is 0.57735p. In Fig. 1.7 P is the pitch of the screw thread. The suitable

wire size is quite hard to get, usually a size bigger than 0.57735p wire size

will be used.

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Fig. 1.7. Conditions when measuring with wires

1.4.1. Best Size Wires.

Wires which touch the thread at the pitch diameter are known

as "Best Size" Wires. Such wires are used because the measurements

of pitch diameter are least affected by errors that may be present in

the angle of the thread.

The above analysis for the distance over wires holds good

provided the wire touches the thread somewhere on its right side, and

provided the thread angle is correct. The extremes of wire sizes which

touch on the straight sides and which can be measured are shown at

(a) and (c), Fig.1.9. For ISO metric, unified and Whitworth threads

these limiting sizes are given in Table 1.1

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Thread

Form

Max.

Wire

Min.

Wire

„Best

Wire‟

Size range for

Best wire

ISO metric and

Unified

1.01p 0.505p 0.557p 0.534p

0.620p

Whitworth 0.853p 0.506p 0.564p 0.535p

0.593p

Note:

W = Distance over wires

DE = Pitch/ Effective Diameter

Dw = Wire diameter

= 600

From the Fig. 1.8, mean/pitch diameter can be calculated by applying

the following formula;

B

C

D

W

DE

P/2

r

A

h

E

60o

Pitch (P)

2

H

Figure 1.8. Three-wire measurement

Table 1.1. Wire sizes for thread measurement ( p = pitch of thread)

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AD = AB cosec 2

= r cosec 2

H = DE cot 2

= 2

P cot

2

CD = 0.5H = 4

Pcot

2

h = AD – CD = r cosec 2

–4

Pcot

2

and distance over wires (W)

= DE + 2h + 2r

= DE + 2 {r cosec 2

–4

Pcot

2} + 2r

= DE + 2r cosec 2

-2

Pcot

2 + 2r

= DE +2r ( 1 + cosec 2) –

2

Pcot

2

and, since 2r = d (the diameter of the wire),

W = DE + d ( 1 + cosec2

) –2

Pcot

2 (1)

From this general formula we may apply the special adaptation for

common threads.

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(a) ISO metric and unified Fig. 1.9 (a)

The effective diameter lies 0.3248p inside the crest of the thread,

Hence DE = D – 0.6496p

= 60 and cosec 2

= 2

cot 2

= 1.732

W (over wires) = DE + d (1 + cosec 2

) –2

Pcot

2

=D – 0.6496p + d(3) – 2

P (1.732)

= D +3d- 1.5156p (2)

Figure 1.9. a) ISO metric and unified b) Whitworth

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(b) Whitworth Fig. 1.9(b)

Depth of thread = 0.64p, so that DE = D – 0.64p

= 55 and cosec 2

= 2.1657 cot2

= 1.921

Hence W ( over wires) = DE + d { 1 + cosec 2

} - 2

P cot

2

= D -0.64p + d 3.1657) -2

P (1.921)

= D + 3.165d - 1.6 p (3)

1.5. OPTICAL COMPARATOR

An optical comparator or shadowgraph (Fig. 1.10a and 1.10b) projects

an enlarge shadow onto a screen where it may be compared to lines or to a

master from which indicates the limits of the dimensions or the contour of

the part being checked. The optical comparator is a fast, accurate means of

measuring or comparing the work piece with a master. It is often used when

the work piece is difficult to check by other method. Optical comparators are

particularly suited for checking extremely small or odd-shaped parts, which

would be difficult to inspect without the use of expensive gauges.

Optical comparators are available in bench and floor models, which

are identical in principle and operation. Light from a lamp passes through a

condenser lens and is projected against the work piece. The shadow caused

by the work piece is transmitted through a projecting lens system, which

magnifies the image and casts it onto a mirror. The image is then reflected

to the viewing screen and is further magnified in this process.

The extent of the image magnification depends on the lens used.

Interchangeable lenses for optical comparators are available in the following

magnifications: 5 x, 10 x, 31.25 x, 50 x, 62.5 x, 90 x, 100 x, and 125 x.

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A comparator chart or master form mounted on the viewing screen is

used to compare the accuracy of the enlarged image of the work piece being

inspected. Charts are usually made of translucent material, such as cellulose

acetate or frosted glass. Many different charts are available for special jobs,

but the most commonly used are linear-measuring, radius, and angular

charts. A vernier protractor screen is also available for checking angles.

Since charts are available in several magnifications, care must be taken to

use the chart of the same magnification as the lens mounted on the

comparator.

Many accessories are available for the comparator, increasing the

versatility of the machine. Some of the most common ones are tilting work

centres, which permit the work piece to be tilted to the required helix angle

for checking threads; a micrometer work stage, with permit quick and

accurate measuring of dimensions in both direction; and gauge blocks,

measuring rods, and dial indicators used on comparators for checking

measurement. The surface of the work piece may be checked by a surface

illuminator, which lights up the face of work piece adjacent to the projecting

lens system and permits this image to be projected onto the screen.

1.5.1. To check the angle of a 60o thread using an optical comparator

1. Mount the correct lens in the comparator.

2. Mount the tilting work centres on the micrometer cross-

slide stage.

3. Set the tilting work centres to the helix angle of the

thread.

4. Set the work piece between centres.

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5. Mount the vernier protractor chart and align it

horizontally on the screen.

6. Turn on the light switch.

7. Focus the lens so that a clear image appears on the

screen.

8. Move the micrometer cross-slide stage until the thread

image is centralized on the screen.

9. Remove the vernier protractor chart to show a reading of

30o.

10. Adjust the cross-slides until the image coincides with the

protector line.

11. Check the other side of the thread in the same manner.

Note: If the threaded angle is not correct or square with the centre

line, adjust the vernier protractor chart to measure the angle of the

thread image. Other dimensions of the threads, and width of flats,

may be measured with micrometer measuring stages or devices such

as rods, gauge blocks and indicators.

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helix angle

Figure 1.10 (a). Checking a thread form on an optical comparator

Figure 1.10 (b) Principle of the optical projector

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1.1. Draw and label a schematic drawing of how you would check

the core diameter of an external V-thread.

1.2. Using „best‟ wire sizes determine the distance of the wire for M

20 x 2.5 ISO metric thread.

1.3. Why is the three-wire method is one of the best method of

measuring the pitch diameter of a V thread?

1.4. With the aid of a labelled diagram, briefly explain how you

would use an optical comparator to check the thread angle of

60o

ACTIVITY

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GEAR

General Objective : To understand the concept of gears and gearing

Specific Objectives : At the end of the unit you will be able to:

Know the types and functions of gears in

engineering.

Know, sketch and label the parts of gears.

Understand the method of measuring spur gear.

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2.0 INTRODUCTION

Gears are used to transmit power positively from one shaft to another

by means of successively engaging teeth (in two gears). They are used in

place of belt drives and other forms of friction drive when exact speed ratios

and power transmission must be maintained. Gears may also be used to

increase or decrease the speed of the driven shaft, thus decreasing or

increasing the torque of the driven number.

2.1. TYPES OF GEARS

2.1.1. Spur gear

Spur gears, Fig. 2.1, are generally used to transmit power

between two parallel shafts. The teeth on these gears are straight

and parallel to the shafts to which they are attached. When two gears

of different sizes are in mesh, the larger is called the gear while the

smaller is called the pinion. Spur gears are used where slow to

moderate- speed drive are required.

.

Figure 2.1. Spur gears Figure 2.2. Internal gears

Gear

Pinion

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2.1.2. Internal gears

Internal gears, Fig. 2.2., are used where the shafts are parallel

and the centers must be closer together and that could be achieved

with spur or helical gearing. This arrangement, provides a stronger

drive since there is the greater area of contact than with the

conventional gear drive. It also provides speed reductions with a

minimum space requirement. Internal gears are used on heavy duty

tractors where much torque is required.

2.1.3. Helical gears

Helical gears, Fig.2.3, may be used to connect parallel shafts or

shafts which are at an angle. Because of the progressive rather than

intermittent action of the teeth, helical gears run more smoothly and

quietly than spur gears. Since there is more than one tooth in

engagement at any one time, helical gears are stronger than spur

gears of the same size and pitch. However, special bearing (thrust

bearings) are often required on shafts to overcome the end thrust

produced by these gears as they turn.

2.1.4. Herringbone gears

Figure 2.3. Helical gears

Figure 2.4. Herringbone gears

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Herringbone gears, Fig. 2.4., are resembles of two helical

gears placed side by side, with one half having a left-hand helix and

the other half a right-hand helix. These gears have a smooth

continuous action and eliminate the need for thrust bearings.

2.1.5. Bevel gears

When two shafts are located at an angle with their axial lines

intersecting at 90o, power is generally transmitted by means of bevel

gears, Fig. 2.5.

2.1.6. Miter gears

When the shafts are at right angles and the gears are of the

same size, they are called miter gears, Fig. 2.6..

Figure 2.5. Bevel gears

Figure 2.6. Miter gears

Figure 2.7. Angular bevel gears

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2.1.7. Angular bevel gears

However, it is not necessary that the shafts be only at right

angles in order to transmit power. If the axes of the shafts intersect

at any angle other 90o, the gears are known as angular bevel gears,

Fig. 2.7.

2.1.8. Hypoid gears

Bevel gears have straight teeth very similar to spur gears.

Modified bevel gears having helical teeth are known as hypoid gears.

The shafts of these gears, although at right angles, are not in the

same plane and, therefore, do not intersect. Hypoid gears are used in

automobile drives, Fig. 2.8.

2.1.9. Worm and worm gear

When shafts are at right angles and considerable reduction in

speed is required, a worm and worm gear may be used, Fig. 2.9. The

worm, which meshes with the worm gear, may be single or multiple

start thread. A worm with a double-start thread will revolve the

Figure 2.8. Hypoid gears

Figure 2.9. Worm and worm gears

Worm

Worm gear

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worm gear twice as fast as a worm with a single-start thread and the

same pitch.

2.1.10. Rack and pinion

When it is necessary to convert rotary motion to linear motion,

a rack and pinion may be used, Fig. 2.10. The rack, which is actually

a straight or flat gear, may have straight teeth to mesh with a spur

gear, or angular teeth to mesh with a helical gear.

2.2. GEAR TERMINOLOGY

Fig. 2

Figure 2.10. Rack and pinion

circular pitch

clea

ran

ce

addendum

dedendum

face width

addendum circle

face

flank

thooth

thickness

top land/peak

pitch circle

dedendum

circle

root

pitch

diamete

r outside

diamete

r

base

diamete

r

pitch

liner

Fig. 2.11 Parts of a spur gear

Pinion

Rack

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2.2.1. Addendum

Addendum is the radial distance between the pitch circle and

the outside diameter or the height of the tooth above the pitch.

2.2.1. Dedendum

Dedendum is the radial distance from the pitch circle to the

bottom of the tooth space.

2.2.3. Pitch diameter

Pitch diameter is the diameter of the pitch circle which is equal

to the outside diameter minus two addendums.

2.2.4. Base diameter

The diameter of the circle from which the involute is

generated; which is equals to pitch diameter times the cosine of the

pressure angle.

2.2.5. Pitch circle

Pitch circle is the circle through the pitch point having its

centre at the axis of the gear.

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2.2.6. Pitch line

The line formed by the intersection of the pitch surface and the

tooth surface.

2.2.7. Face width - The width of the pitch surface.

2.2.8. Tooth thickness

The thickness of the tooth measured on the pitch circle.

2.2.9. Top land - The surface of the pitch cylinder.

2.2.10. Base diameter - The diameter of the root circle.

2.2.11. Root - The bottoms of the tooth surface.

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2.3. MEASUREMENT AND TESTING OF GEARS

2.3.1. Gear-tooth vernier caliper

The gear-tooth vernier, Fig.2.12, is an instrument for

measuring the pitch-line thickness of a tooth. It has two scales and

must be set for the width (w) of the tooth, and the depth (h) from the

top, at which the width occurs.

AO = R

Figure 2.12. The gear-tooth vernier caliper

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NOTE: The following considerations of gear elements, the

symbols below will be used for the quantities.

T/t = No. of teeth

P = Diametral pitch ( inch gear )

P = Circular pitch

D/d = Diameter of pitch circle

R/r = Radius of pitch circle

= pressure angle

M = Modul

Add/A = Addendum

Ded/D = Dedendum

Circular pitch = x Modul M

The angle subtended by a half tooth at the centre of the gear

( AOB), Fig. 2.12, is given by,

= 4

1x

T

360 =

T

90; T = no. of teeth

AB = 2

w = AO sin

T

90 = R sin

T

90

D = Modul x No. of Teeth, and

R = R 2

MT

i.e. D = 2R =MT and R = 2

MT

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Hence 2

w = R sin

T

90 =

2

MT sin

T

90

and w = MT sin T

90 (1)

To find h we have that h = CB = OC – OB

But OC = R + Add = 2

MT +M

And OB = R cos T

90 =

2

MT cos

T

90

Hence h = 2

MT +M -

2

MT cos

T

90

= 2

MT +M -

2

MT cos

T

90] (2)

= M + 2

MT [ 1- cos

T

90 ]

For diametral-pitch gears, (1) becomes w = P

Tsin

T

90

And (2) becomes h = P

1[ 1 +

2

T ( 1 – cos

T

90)

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Example:

To calculate the gear tooth vernier setting to measure a gear of

33T, 6 modul.

w = MT sin T

90 = 6 x 33 sin

33

90

= 198 sin 2o 43.5‟ = 198 x 0.0476

= 9.42 mm.

h = M [ 1 + 2

T ( 1 – cos

2

T)]

= 6 [ 1 + 2

33 ( 1 – cos

33

90) ]

= 6 [ 1 + 2

33 (0.0011) ]

= 6.11 mm

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2.4. PLUG METHOD OF CHECKING FOR PITCH DIAMETER AND

DIVIDE OF TEETH

The tooth vernier gives us a check on the size of the individual tooth,

but does not give a measure of either the pitch diameter or the accuracy of

the division of the teeth.

Fig. 2.13 shows a rack tooth symmetrically in mesh with a gear tooth

space, the curved sides of the gear teeth touching the straight rack tooth at

the points A and B on the lines of action. O is the pitch. If now we consider

the rack

tooth as an empty space bounded by its outline, a circle with centre at O and

radius OB would fit in the rack tooth and touch it at A and B (since OA and

OB are perpendicular to the side of the rack tooth). Since the rack touches

Figure 2.13

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the gear at these points, the above circle (shown dotted) will rest against the

gear teeth at points A and B and will have its centre on the pitch circle.

In triangle OBD: OB = radius of plug required.

OD = 4

1 circular pitch

= 4

m

< B = 90o, < O =

OB = OD cos

= 4

m cos

Dia of plug = 2OD

= 2

m cos

This is the diameter of a plug which will rest in the tooth space and

have its centre on the pitch circle. Notice that the plug size remains the

same for all gears having the same pitch and pressure angle.

With such plugs placed in diametrically opposite tooth spaces, it is a

simple matter to verify the gear pitch diameter. The accuracy of the spacing

over any number of teeth may be found as shown in chordal calculations.

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Example:

Calculate for a 36Tgear of 5 mm module and 20o pressure angle, (a) plug size

(b) distance over two plugs placed in opposite spaces, (c) distance over two

plugs spaced 10 teeth apart.

Solutions:

(a) Dia of plug = 2

m cos

= 2

5 cos 20o

= 7.854 x 0.9397

= 7.38 mm

Pitch dia of gear = mT

= 5 x 36

= 180 mm

(b) Distance across plugs in opposite spaces = 180 + 7.38

= 187.38 mm

(c) Distance across plugs spaced 10 teeth apart (Fig.2.14)

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Angle subtended by 10 teeth = 10 x 36

360

= 100o.

In triangle OAB:

AB = OA sin 50o

= 90 x 0.766

= 68.94

Centre distance of plugs = 2 x AB

= 2 x 68.94

= 137.88 mm.

Distance over plugs = 137.88 + 7.38

= 145.26 mm.

Figure 2.14

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2.5 MEASURE AND INSPECT OF SPUR GEAR

Mengukur tebal perentas dengan angkup vernier gigi gear

Rajah di atas menunjukkan sebuah angkup vernier gigi gear.

Angkup tersebut dilengkapkan dengan plat penahan yang boleh dilaraskan

mengikut ukuran adendum gear yang hendak diukur.

Kemudian hujung plat yang terletak di antara rahang angkup itu dikenakan pada

puncak gigi gear.

Rahang angkup vernier dilaraskan untuk mendapat ukuran tebal perentas gigi.

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2.6 KAEDAH PERENTAS MALAR

Perentas malar ialah satu garis rentas yang panjangnya sentiasa sama bagi semua

gigi gear yang mempunyai pic di garis pusat dan sudut tekan sama , walaupun

bilangan gigi bagi gear mungkin berbeza.

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2.7 KAEDAH TANGEN TAPAK

Alat pengukur seperti angkup vernier yang besar, tolok tinggi vernier,

pembandingan tangen tapak atau mikrometer tebal gigi iaitu sejenis mikrometer

yang dipasang dengan andas yang besar berbentuk plat bulat,boleh lah digunakan

untuk mengukur jarak rentang itu.

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2.8 MEMERIKSA GARIS PUSAT PIC BAGI GEAR TAJI

Dalam kaedah ini sepasang guling (rola) atau palam piawai digunakan bersama

mikrometer luar.

Garis pusat guling hendaklah bersesuaian dengan pic dan sudut tekanan bagi gear

hendak diuji.

Jika gear bergigi genap, guling guling itu di letakkan dalam lurah yang

bersetentangan.

Jika gear bergigi ganjil, kedudukan guling mestilah pada lurah-lurah yang paling

hampir bersetentangan.

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2.9. THE INDEXING OR DIVIDING HEAD

The indexing or dividing head is one of the most important

attachments for the milling machine. It is used to divide the circumference

of a work piece into equally spaced divisions when milling gears, splines,

squares and hexagons. It may also be used to rotate the work piece at a

predetermined ratio to the table feed rate to produce cams and helical

grooves on gears, drills, reamers, and other parts.

2.10. INDEX HEAD PARTS

The universal dividing head set consists of the headstock with index

plates, headstock change and quadrant, universal chuck, footstock, and the

centre rest ( Fig 3.9 ). A swiveling block mounted in the base enables the

headstock to be tilted from 5o below horizontal position to 10o beyond the

vertical position. The side of the base and the blocks are graduated to

indicate the angle of the setting. Mounted in the swiveling block is a

spindle, with 40-tooth worm wheel attached, which meshes with a worm (

Fig. 3.10 ). The worm , at right angles to the spindle, is connected to the

index crank, the pin of which engages in the index plate. A direct indexing

plate is attached to the front of the spindle.

A 60o centre may be inserted into the front of the spindle, and a

universal chuck may be threaded onto the end of the spindle.

The footstock is used in conjunction with the headstock to support

work held between centers or the end of work held in a chuck. The footstock

centre may be adjusted longitudinally to accommodate various lengths of

work and may be raised or lowered off centre. It may also be tilted out of

parallel with the base when cuts are being made on tapered work.

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Long, slender work held between centers is prevented from bending

by the adjustable centre rest.

Figure 3.10 Section through a dividing head, showing the worm wheel and worm shaft

Figure 3.9. A universal dividing head set

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2.11 METHODS OF INDEXING

The main purpose of the indexing or dividing head is to divide the

work piece circumference accurately into any number of divisions. This may

be accomplished by the following indexing methods: direct, simple, angular,

and differential. However, this modul will only cover direct and simple

indexing.

Direct indexing

Direct indexing is the simplest form of indexing. It is

performed by disengaging the worm shaft from the worm wheel by

means of an eccentric device in the dividing head. Some direct

dividing heads do not have a worm and worm wheel but rotate on

bearings. The index plates contain slots, which are numbered , and a

spring-loaded tongue lock is used to engage in the proper slot. Direct

indexing is used for quick indexing of the work piece when cutting

flutes, hexagons, squares, and other shapes.

The work is rotated the required amount and held in place by a

pin which engages in to a hole or slot in the direct indexing plate

mounted on the end of the dividing head spindle. The direct indexing

plate usually contains three sets of hole circles or slots: 24, 30, and 36.

The number of divisions it is possible to index is limited to numbers

which are factors of either 24, 30, or 36. The common divisions that

can be obtain by direct indexing are listed in Table 3.3

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Plate

Hole

Number

24 2, 3, 4, -, 6, 8, - ----- 12 …………24

30 2, 3, -, 5, 6, -, -, -, 10, -, -, 15, ……….30

36 3, 4, -, 6, -, 9, -, 12, -, 18,…………… 36

Example:

What direct indexing is necessary to mill eight flutes on a reamer blank?

As the 24 hole circle is the only one divisible by eight (the required of

divisions), it is the only circle which can be used in this case.

Indexing = 8

24 = 3 holes on a 24-hole circle.

Note: Never count the hole or slot in which the index pin is engaged.

Simple Indexing

In simple indexing, the work is positioned by means of the

crank, index plate, and sector arms. The worm attached to the crank

must be engaged with the worm wheel on the dividing head spindle.

Since there are 40 teeth on the worm wheel, one complete turn of the

index crank will cause the spindle and the work to rotate one-fortieth

of a turn. Similarly, 40 turns of the crank will revolve the spindle and

Table 3.3. Direct Indexing Divisions

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work one turn. Thus there is a ratio of 40:1 between the turns of the

index crank and the dividing head spindle.

To calculate the indexing or the number of turns of the crank

for most divisions, it is necessary only to divide 40 by the number of

division (N) to be cut, or

Indexing = N

40

Example:

The indexing required to cut eight flutes would be:

8

40= 5 full turns of the index crank

If, however, it was necessary to cut seven flutes, the indexing would be

7

40= 5

7

5 turns

Five complete turns are easily made; however, the five seventh of a turn

involves the use of the index plate and sector arms.

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1. State three (3) characteristics of the following gears

i. helical gear

ii. spur gear

2. Sketch and name six (6) parts of a spur gear

3. Calculate the diameter of plug which will lie in the tooth space of a 5

mm module gear with its centre on the pitch circle. If the gear has

50T, find (a) distance over two such plugs spaced in opposite spaces,

(b) distance over two plugs spaced 12 spaces apart ( = 20o)

(J: 1. 7.38 mm (a) 257.38 mm (b) 178.52 mm)

4. Determine the diameter of a plug which will rest in the tooth space of

a 4 mm module 20o rack, and touch the teeth at the pitch line.

Calculate (a) the distance over two such plugs spaced 5 teeth apart.

(b) The depth from the top of the plug to the top of the teeth.

(J: 5.9 mm (a) 59 mm (b) 10.664 mm)

ACTIVITY

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SURFACE TEXTURE

General Objectives: To understand the importance of surface texture

in engineering.

To understand the methods of calculating the

surface roughness.

Specific Objectives : At the end of this unit you will be able to:

Identify the surface finish symbols that appear on

a drawing.

Identify the surface texture terms/ definitions.

Calculate the arithmetic mean value, Ra.

Calculate the root-mean-square average, Rq.

Calculate the maximum roughness height, Rt.

Compare Ra and Rq.

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4.0 DEFINITION

Surface Texture is defined as a degree of finish conveyed to the

machinist by a system of symbols devised by a Standards Association, eg.

ASA – American Standards Association, BS – British Standards

Modern technology has demanded improved surface finishes to ensure

proper functioning and long life of machine parts. Pistons, bearings, and

gears depend to a great extent on a good surface finish for proper functioning

and therefore, require little or no break-in period. Finer finishes often

require additional operation, such as lapping or honing. The higher finishes

are not always required on parts and only result in higher production costs.

To prevent overfinishing a part, the desired finish is indicated on the shop

drawing. Information specifying the degree of finish is conveyed to the

machinist by a system of symbols devised by Standards Associations, eg.

ASA American Standards Association and BS British Standards. These

symbols provide a standard system of determining and indicating surface

finish. The inch unit for surface finish measurement is microinch (µin), while

the metric unit is micrometer (µm)

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4.1. SURFACE TEXTURE TERMS AND DEFINITIONS

Surface profile Error of form Waviness

Roughness

Regardless of the method of production, all surfaces have their own

characteristics, which are collectively referred to as surface texture, Fig. 4.1.

Certain guidelines have been established to identify surface texture in terms

of well-defined and measurable quantities (Figure 4.2)

Figure 4.1. Standard terminology and symbols to describe surface

finish

Lay

direction

Waviness

width

Roughness

spacing

Roughness

Height, Rt

Waviness

height

Flaw

Roughness

width cutoff

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4.1.1. Flaws

Flaws or defects, are random irregularities, such as scratches,

cracks, holes, depression, seams, tears or inclusions. These defects can

be caused during the machining or production process such as

molding, drawing, forging, machining, eg, holes cause by air bubbles

during casting, crack and tears by forging and drawing process.

4.1.2. Lay

Lay or directionality, is the direction of the predominant

surface pattern caused by the machining process and it is usually

visible to the naked eye.

4.1.3. Roughness

Roughness is defined as closely spaced, irregular deviation on a

scale smaller than that of waviness. It is caused by the cutting tool or

the abrasive grain action and the machine feed. Roughness may be

superimposed on waviness.

4.1.3.1. Roughness height

Roughness height, Ra is the deviation to the centre line

in micro inches or micrometers.

4.1.3.2. Roughness width

Roughness Width is the distance between successive

roughness peaks parallel to the nominal surface in inches or

millimeters.

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4.1.4. Waviness

Waviness is a recurrent deviation from a flat surface, much like

waves on the surface of water. It is measured and described in terms

of the surface between adjacent crests of the waves (waviness width)

and height between the crests and valleys of the waves (waviness

height). Waviness can be caused by:

a) deflection of tools, dies or the work piece

b) force or temperature sufficient to cause warping

c) uneven lubrication

d) vibration

e) any periodic mechanical or thermal variations on the system

during manufacturing operations.

4.1.5. Profile

The contour of a specified section through a surface.

4.1.6. Microinch and micrometer

The unit of measurement used to measure surface finish.

The microinch is equal to 0.000 001 inch and the micrometer

equals to 0.000 001 meter.

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4.2. STANDARD SYMBOLS TO DESCRIBE SURFACE TEXTURE/FINISH

0.02 – 2

6.3

1.6 0.01

Figure 4.2 A sample of a surface texture/finish designation

Symbols‟ definition:

0.02 – Maximum waviness height (mm)

2 - Maximum waviness width (mm)

6.3 - Maximum roughness height ( m)

1.6 - Minimum roughness height ( m)

0.01 - Maximum roughness width (mm)

- Lay symbol (Lay perpendicular to the line

representing the surface to which the

symbol is applied)

Sometimes, the roughness number is used as a substitute for the

roughness value eg. N7 is equals to 1.6 µm, (Table. 4.1). Table 4.2 shows an

average surface roughness produced by standard machining processes.

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Table 4.1. Roughness number and value

µm 50 25 12.5 6.3 3.2 1.6 0.8 0.4 0.2 0.1 0.05 0.025

Roughness

number N12 N11 N10 N9 N8 N7 N6 N5 N4 N3 N2 N1

Table 4.2 Average surface roughness produced by standard machining

processes

PROCESS MICROINCHES MICROMETERS

Turning 100 - 250 2.5 - 6.3

Drilling 100 - 200 2.5 - 5.1

Reaming 50 - 150 1.3 - 3.8

Grinding 20 - 100 0.5 - 2.5

Honing 5 - 20 0.13 - 0.5

Lapping 1 - 10 0.025 - 0.254

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4.3. SYMBOLS FOR SURFACE ROUGHNESS

The following symbols indicate the direction of the lay (Table

4.3)

Lay

Symbol

Interpretation Examples

= Lay parallel to the line representing the

surface to which the symbol is applied

Lay perpendicular to the line representing the

surface to which the symbol is applied.

Lay angular and both direction to line

representing the surface to which symbol is

applied

Lay multidirectional

Lay approximately circular relative to the

centre of the surface to which the symbol is

applied

Lay approximately radial relative to the

centre of the surface to which the symbol is

applied

X

M

C

R

C

R

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Pitted, protuberant, porous, or particulate

non-directional lay

4.4 SURFACE ROUGHNESS

Surface roughness is generally described in two methods: arithmetic

mean value and root-mean-square average.

4.4.1 The Arithmetic Mean Value, Ra.

Ra, formerly identified as AA for arithmetic average or CLA for

centre-line average is based on the schematic illustration of a rough

surface, which is shown in (Figure 4.4). The arithmetic mean value,

Ra, is defined as

Ra = n

fedcba ... (4.4.1)

Where, all ordinates, a, b, c, …, are absolute values, and n is the

number of readings

Figure 4.3. Standard lay symbols for engineering surfaces

P P

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4.4.2. The Root-Mean-Square Average, Rq.

Rq, formerly identified as RMS is defined as

Rq = n

dcba ...2222

(4.4.2)

The datum line AB in figure 4.4 is located so that the sum of

the area above the line is equal to the sum of the areas below the line.

The units generally used for surface roughness are µm (micrometer, or

micron) or µin (microinch). ( Note, 1µm = 40 µin and 1µin = 0.025 µm

).

3.4.3. Maximum Roughness Height, Rt

Maximum roughness height, Rt, is defined as the height from

the deepest trough to the highest peak. It indicates how much

material has to be removed in order to obtain a smooth surface by

polishing or other means

A f g h i j k B

a b c d e

Centre line (datum line)

Figure 4.3. Coordinates used for surface – roughness using equations 4.4.1 &

4.4.2

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Rt = 5

)()( 10864297531 hhhhhhhhhh

h1 h3 h6 h5 h7

h9

h2 h4 h8 h10

Where,

h1, h2…......hn - height of ordinates in mm

M - magnification

4.5. COMPARISON OF Ra AND Rq

The arithmetic mean value, Ra was adopted internationally in the

mid-1950s and is used widely in engineering practice. Equations 4.4.1 and

4.4.2 show that there is a relationship between Rq and Ra, as shown by the

ratio Ra

Rq. The table 4.4 below gives this ratio for various surfaces:

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Table 4.4 Ratio for various surfaces

Surface Ra

Rq

Sine Curve 1.1

Machining by cutting 1.1

Grinding 1.2

Lapping and honing 1.4

In general, a surface cannot be describe by its Ra and Rq value alone,

since these values are averages. Two surfaces may have the same roughness

value but have actual topography which is very different. A few deep

troughs on an otherwise smooth surface, for example, do not affect the

roughness values significantly. However, the type of surface profile can be

significant in terms of friction, wear and fatigue characteristics of a

manufactured product.

It is therefore, important to analyze the surface in great detail,

particularly for parts used in critical applications. Some 130 parameters

have been identified thus far for measuring surface roughness.

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4.1. Explain why present-day standards relating to surface texture

are very important to industry .

4.2. List and explain the types of defects found on surfaces.

4.3. Explain the following terms:

a) roughness

b) waviness

c) lay

4.4. What do Ra, Rq and Rt stand for?

4.5. Describe how you would use the surface roughness comparator

gauge.

4.6 Define the symbol on figure below.

0.03 – 1.5

3.2

1.6

0.01

ACTIVITY 4A

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COMPUTER NUMERICAL CONTROL

General Objective :To understand the concept and principles of

computer numerical control (CNC) system.

Specific Objectives : At the end of the unit you will be able to :

Understand the main components of the CNC

system,

Understand the point-to-point system

(positioning),

Understand the contouring system (continuous

system), and

Write a simple CNC milling program.

.

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6.0 INTRODUCTION

Computer numerical control is a system in which a control

microcomputer is an integral part of a machine or a piece of equipment

(onboard computer). The part programmes can be prepared at a remote site

by programmer, and it may incorporate information obtained from drafting

software packages and from machining simulations, in order to ensure that

the part programme is bug free. The machine operator can, however, easily

and manually programme onboard computers. The operator can be modify

the programs directly, prepare programme for different parts, and store the

programmes.

Because of the availability of small computers having a large memory,

microprocessor(s), and programme-editing capabilities, CNC systems are

widely used today. The availability of low-cost programmable controllers

also played a major role in the successful implementation of CNC in

manufacturing plants.

Numerical Control is a system where machine action is created from

the insertion of Numeric Data. The Numeric Data is, in the beginning,

written words in an easily understood code of letters and numbers

(alphanumeric characters) known as a programme, which in turn is

converted by the machine control unit (MCU) into the electrical signals used

to control the machine movements.

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The relationship between the words "Numerical" and "Control" is

shown below.

NUMERICAL CONTROL

Two important points should be made about N.C. First, the actual

N.C. machine tool can do nothing more than it was capable of doing before a

control unit was joined to it. There are now new metal removing principles

involved. N.C. machines position and drive the cutting tools, but the same

milling cutters, drills, taps, feeds, and other tools still perform the cutting

operations. Cutting speeds, feeds, and tooling principles must still be

adhered to. Given this knowledge, what is the real advantage of numerical

control?

Primarily, the idle time or time to move into position for new cuts is

limited only by the machine's capacity to respond. Because the machine

receives commands from the machine control unit (MCU), it responds

without hesitation. The actual utilisation rate or chip making rate is

therefore much higher than on a manually operated machine.

The second point is that numerical control machines can initiate

nothing on their own. The machine accepts and responds to commands from

the control unit. Even the control unit cannot think, judge, or reason.

Without some input medium, e.g., punched tape or direct computer link, the

An instructional expression,

in a language of numbers,

which represents a series of

commands for specific

machine tool movements

To control such machine

actions as:

Directing Altering

Commanding Timing

Prescribing Ceasing

Sequencing Guiding

Initiating

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machine and control unit will do nothing. The N.C. Machine will perform

only when the N.C. tape is prepared and loaded and cycle start is initiated.

6.1. NC OPERATION

CNC stands for Computer Numerical Control. An N.C. system in

which a dedicated stored program computer is used to perform basic control

functions.

The functions of a CNC Controller are:

1. To read and store programme information.

2. To interpret the information in a logical command sequence.

3. To control the motion of the machines mechanical members.

4. To monitor the status of the machine.

The interpretation of programme commands by a machine control unit

and its conversion of those commands into machine motion is complex. The

basic elements and operation of a typical NC machine are shown in Fig. 6.1.

The functional elements in numerical control and the components involved

follow:

a. Data input: The numerical information is read and stored

in the tape reader or in computer memory

b. Data processing: The programmes are read into the

machine control unit for processing.

c. Data output: This is information is translated into

commands (typically pulsed commands) to the servomotor

(Fig. 6.2 and 6.3). The servomotor then moves the table (on

which the work piece is mounted) to specific positions,

through linear or rotary movements, by means of stepping

motors, leadscrews, and other similar devices.

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Computer:

Input command,

Processing,

Output command

Dri

ve

sig

nal

Po

siti

on

fee

db

ack

Lim

it s

wit

ches

Figure 6.1. A schematic illustration of the major component of a computer numerical control machine tool

Comparator DAC

Lead screw Position sensor

Dc

servomotor Gear

Work table

Feedback signal

Input

Stepping

motor Gear

Work table

Lead screw

Pulse train

Figure 6.3. A closed-loop control system for a numerical-control machine

Figure 6.2. An open-loop control system for a numerical-control machine

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6.2. INDUSTRIAL APPLICATION

6.2.1. Metal Machining

Lathes of all types

Milling Machines of all types

Drilling Machines

Jig borers

Electric Discharge Machining (including wire cut machines)

Laser cutting machines

Machining centres

Turning centres

All types of grinding machines

Gear cutting machines

6.2.2. Metal Forming

Punching and nibbling

Guillotines

Flame cut and profiling

Folding

Pipe bending

Metal spinning

6.2.3. Finishing

Plating

Painting

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6.2.4. Assembly Joining -

Pick and place robots, spot and seam welding machines and robots,

riveting, looming of wires and assembly of components into printed

circuit boards.

6.3. CNC AXIS CONVENTIONS

CNC axis classification follows the three-dimensional Cartesian

coordinate system and is established in BS 3635: 1972: Part 1. Fig. 5.3 shows

the tree primary axes and the associated rotational axes.

Most machines have two or three slide ways placed at right angles to

one another. On CNC machines each slide is fitted with a control system,

and is identified with either the letter X, Y or Z.

Conventions have been adopted as to the naming of each axis. The

axis of the main spindle, whether it is the axis of the tool spindle or the axis

about which the work piece rotates is called the Z axis.

The X axis is the motion of the largest travel of the primary movement

(in case there is more than one).

The Y axis then makes the third motion and is the shorter primary

movement.

In addition to these primary linear axes, there is provision for Rotary

axes. They are designated A, B and C, with A rotary about the X axis, B

rotary about the Y axis, and C rotary about the Z axis.

It is often required to command a motion parallel to X, Y or Z axes

within the realm of a secondary motion, or a tertiary motion within special

automatic cycles such as describing the amount of finish allowance on a

turned part, or to describe the distance of advancement of a drill during a

drilling cycle etc. etc.

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Linear Axes X Y Z

Rotary Axes A B C

Secondary Linear U V W

Interpolation I J K

Tertiary motion codes differ considerably, but the address characters

variously used are P, Q, R, D, L, E, and H.

The z-axis is parallel to the main spindle of the machine. It will be

horizontal on a lathe or horizontal machining centre and vertical on a

vertical machining centre.

The x-axis is always horizontal and at 90o to z.

The y-axis is at right angles to both the x and z axes.

z

y

x

table rotation

spindle

Figure 6.3. CNC axes

Table 6.1. NC axes

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6.4. NC MACHINE SUB-UNIT

We have already seen the many and varied applications of numerical

control to the manufacturing and other industries, now we will look at the

methods of controlling machines. There are three sub units to study:

The machine tool itself.

The control unit.

The control system.

6.4.1. The Machine Tool

A machine tool is a device designed to cut away surplus

material and leave a component of the required shape and size. To do

this a machine tool must be capable of:

- Holding the work piece securely

- Holding the cutting tool securely and driving it with suitable power.

- Moving the tool and work piece relative to one another precisely

enough to achieve accuracy of size and surface finish.

In addition, provision must be made for altering the spindle

speed and feed rates, tool changing, supply of coolant etc. On a

conventional machine an operator controls these functions and sets or

alters them when he considers it necessary, the decision resulting

from his training, skill and experience. Obviously, the machine

settings may differ between operators as will the time taken to read

scales, set positions, change tools, alter speeds and feeds, engage

drives and set up the work piece etc. CNC Automatic Control can be

applied to these functions and so result in consistent and reduced

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machining times through optimised cutting data, fast accurate

positioning between cuts and fast automatic tool changing.

6.4.2. The Control Unit

The CNC Machine Control Unit (MCU) has to read and decode

the part programme, and to provide the decoded instructions to the

control loops of the machine axes of motion, and to control the

machine tool operations.

The main grouping of parts of a control could be considered to be:

The Control Panel.

The Tape Reader,

The Processors

The first part of the control panel is the human interface that

allows various modes of machine or control operation to be initiated,

from switching on and homing, to programme loading and editing, to

setting work positions and tool offsets, manually controlled

movements and commencing the automatic cycling of a programme.

Information about machine status and condition is available to the

operator via VDU screens, gauges, meters, indicator lights and

readouts.

The tape reader is the device used to transfer the programme

information contained on a programme tape into the control unit.

Most tape readers are of the photo-electric type which offers high

speed reading with reliability and accuracy providing the tape is in

good condition and the reader is kept clean and free of paper dust

particles.

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The processors within a control are the electronic circuits that

permit conversion of part programme data into machine motions and

they may be classified into two main sections. The data processing

unit and the axis control processor. The function of the data processor

is to decode the commands of the part program, process it and provide

data to the axis control processor which then operates the slide drives

and receives feedback signals on the actual position and velocity of

each axis.

The Data Processing Unit includes the following functions:

i. The input device, such a tape reader.

ii. Reading circuits and parity checking logic.

iii. Decoding circuits for distributing data to the controlled axes

iv. An interpolator to supply velocity commands to the axes,

either singly or in combination.

The axis control processor consists of the following circuits:

i. Position control loops for each and all axes.

ii. Velocity control loops.

iii. Deceleration and backlash take up circuits.

An MCU is adaptable to virtually any machine, the differing

control motions and codes being a result of the way the control has

been programmed. This permanent resident program is known as an

executive programme and resides in the read only memory (ROM) of

the control, whereas the N.C. programme resides in the Random

Access Memory (RAM). RAM allows external access and alteration if

necessary, while ROM is programmed by the manufacturer and

cannot be accessed through the control keyboard.

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6.4.3. Control System

There are two types of control systems used on NC machines.

The point-to-point system and the continuous-path system.

Point-to-point systems are not so common these days, but they operate

only in straight lines, which are suitable for positioning moves on a

drilling machine or limited use on a lathe or milling machine, where

at best 45% cuts are possible with two axes running continuous path

controls allow angular path and radius motion because the control

interpolator has the ability to move the axis drive motors at varying

velocities.

The point-to-point controls were NC controls, while the

continuous path controls could be NC or CNC controls.

NOTE: NC is a general term used for Numerical Control and is also a

term used to describe controls that run directly off tape. CNC is a specific

term for Computer Numerical Control. CNC Machines are all NC machines,

but NC controlled machines are not CNC machines.

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6.6. NC PROGRAMMING

6.6.1. Job Planning

1. Sketch the part. Add incremental or absolute dimensions.

2. Ascertain fixturing. Select fixtures which have minimal

projections

above the part.

3. Identify a set-up point. Locate the set-up point near:

1. A corner of the part

2. A spot above the fixture

Consider space requirements for:

1. Part loading and unloading

2. Tool change.

4. Plan operation sequence Mark sequence pattern of sketch.

Test program data for accuracy.

5. Record necessary data for

each movement of the table

and tool on the program

sheet.

6. Record instructions for Identify, specific:

the machine operator. 1. Tools needed.

2. Speed and feed data

3. Tool change points

4. Console switch setting

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6.6.2. Incremental

The word "incremental" may be defined as a dimension or a

movement with respect to the preceding point in a prescribed

sequence of points. Each positioning move is described quantitatively

in distance and in direction from a previous point rather than from a

fixed zero reference point.

In incremental mode all moves are with respect to the last

position reached.

N10 G91

N15 G01 X10.Y10.F300.

N20 Y10.

N25 X20.

N30 X10.Y20.

N35 X20-Y-30.

N40 X-10.Y-10.

N45 X-50.

N50 M02

40

30

20

10 20 30 40 50 60

10

Y 4

0 4

0

X

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6.6.3. Absolute

The data in the absolute system describes the next location

always in terms of its relationship to the fixed zero point. The zero

point when used as a programme datum is known as the programme

origin.

The G90 code sets the control up in absolute mode. All moves are

performed with respect to the axes zero.

N10 G90

N15 G01X10.Y10.F300.

N20 Y20.

N25 X30.

N30 X40.Y40.

N35 X60.Y10.

N40 X50.Y0.

N45 X0.

N50 M02

40

30

20

10 20 30 40 50 60

10

Y 4

0 4

0

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6.6.4. Linear Interpolation

Under this command the machine tool will move in a straight

line at a defined feed rate. Combined axis motions (angled moves) will

be executed at the programming feed rate as the control will reduce

the velocity of both axes accordingly.

E.g. G01 X200. F250.

G01 Move in a straight line

X200. A distance of 20O.mm

F250. At a feed rate of 250.mm/min.

NOTE: If a new line with G01 is listed again somewhere below, the F250

does not have to be written again. This is called modal.

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Example:

A block as shown below is to be machined, write a program in absolute

mode.

%

G90 -

G01 X100 F300

Y70

X90. Y80

X20.

X10. Y90.

XO. Y80

Y0.

M02

6.6.5. Circular Interpolation

In circular interpolation mode G02 will cause the path to be

transcribed in a clockwise direction and G03 will cause

counter-clockwise motion.

G02 - Clockwise

G03 - Counterclockwise

In circular interpolation there are a number of points to be

remembered:

The end point of the arc is defined as X and Y coordinates

exactly the same as if commanding linear motion.

The centre of the arc is defined with respect to the start point in

the I and J words as an "increment" from this point.

100

70

70

80

90

10

0 10

0

10

0

80

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For G02 and G03 to function the feed rate "F." must be specified.

Example:

N5%

N10 G90

N15 G01 Y110. F200.

N20 G02 X20. I10.

N25 G03 X30. Y100. I10.

N30 G01 X90.

N35 G02 X100. Y90. J-10.

N40 G01 Y10,

N45 G02 X90. Y0. I-10.

N50 G01 X0.

N55 M02

6.7. PROGRAM DEFINITION

To enable the machine to operate automatically it is necessary to put

into its memory a programme or set of instructions to carry out the required

operation.

a) Programme.

A programme is a series of instructions to the machine, set out

in sequence to -produce a complete machining operation. A

programme is made up of a series of blocks.

b) Block.-

A block or programme line is a set of instructions to the

machine that are carried out simultaneously. A block is made up of

100 1

10

All radius – R10

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one or more Words and is terminated by an End of Block which is the

Line Feed Character.

c) Word.

A word is a specific instruction to the machine that will affect a

particular machine function. Every word consists of a Letter Code and

a Numerical value.

Examples of Dimensional Words: X100. Y2.345 F0.25

Examples of Non-Dimensional Words: N25 G90 M03 S1200

Dimension words can be written in various ways, depending on the

control. Let's take the examples X100. Y2.345 some older controls cannot

accept decimal points, so both dimensional words would be written X100000

Y2345, with Y showing all decimal places. With these controls, if the X word

was written as X100, it would be interpreted as one-tenth of a millimeter,

not one hundred millimeters.

If a control accepts decimal points, then ALL dimensional words

should have a decimal point. On any control, non-dimensional must NOT

have a decimal point. The method of writing words beginning with a letter

is known as word address format and is now almost universally used.

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6.7.1. Program

(Start of Program)

(Material 25.mm. dia.)

(Grip 120.m.m. from Front of Jaws)

N01 G71G90G95

N02 G50X100.Z130.

N03 S2000M03

(Select Turning & Facing Tool)

N05 GO1X2.F.O4

N06 GOOZ120.

N07 X24.

N08 G01Z20.

N09 X26.

N10 G00X100.Z130.T0100

N11 M02

WORD ADDRESS The letter at the beginning of each word is called

the address character.

e.g. X Y Z for Axis designating word

F for Feed rates

G for Preparatory functions

M for miscellaneous function

N for Sequence numbers

N04 G00X26.Z119.T0101 BLOCK

WORDS

N10 GOOX100. Z130. T0100

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CNC mills, drills and machining centers are all equipped with cycles

to perform drilling, reaming, counter boring, boring and tapping operations.

Some others have pocketing cycles, slot cutting cycles, hole pattern cycles

etc, all of which are designed to save programming time and effort.

CNC lathes usually have cycles to cover drilling, grooving/parting,

screw cutting, repetitive cut (automatic roughing) operations and others.

Each cycle has its own G code to control the sequence of motions and an

accompanying set of words to define the parameters of those motions. These

words have addresses such as: R,P,Q,D,E,I,K,H,B etc.

6.7.2. Program Preparation

CNC programmes can be prepared manually, where the programmer

usually roughs the programme out on paper, then produces it via a keyboard

device of the type detailed below, or by assisted preparation in which a

computer plays a predominant role -such as when CAD/CAM packages have

been installed for design and programming.

The programmer must posses knowledge and skills in planning

machining sequences, fixturing, cutting data, cutting tools, calculations, as

well as being familiar with the machines he is programming. To implement

these skills to best effect a programmer should be prepared to observe

critically his programs in use and modify them as necessary in order to gain

maximum machine utilization.

6.7.3. Operation of program

Before a machine can set into automatic motion a program must be

checked for errors. A simple typing mistake - an incorrect code, a minus sign

instead of a zero, the exclusion of a decimal etc, could cause and expensive

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machine crash. Anyone who considers their programmes to be without error

and not in need of careful and conscientious trialing has an attitude problem

and is placing expensive machinery and operators safety at risk.

There may be many ways in which a programme can be checked for

errors, but a programme can only be proved 100% by running the machine

and producing a part.

Error checking can be performed in a variety of ways:

Verification: Read through the print-out (NOT the handwritten

manuscript) carefully - sometimes mistakes can be seen

easily.

Trialing: This involves the execution of the programme without

actually cutting the part and may be carried out in

several ways depending on the type of machine, or

control, or even the philosophy of the person in charge.

Adhere to the later unless

you can put up good reasons for alteration.

Trialing usually consists of running the machinewith the single

block switch active, that is, each block will only be executed by

pressing cycle start, in conjunction with the programme

being displayed on the screen.

Quite often the dry run mode is switched on to hasten

Proceedings. 'Dry Run' results in all machine motion

being executed at a preset rate, usually in the region of

50% to 80% of the rapid traverse capability of the

machine. The actual axisvelocity can be overridden from

0% to 100%. The disadvantage of dry running a

programme is that feed rates will be masked, and

attention must be paid to determining theactual

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programmed feed rate for each block. This may be

displayed on the screen.

Every movement the machine makes during programme

trialing should be expected and accountable to the

programmer, if not, those motions should be checked for

viability, and if necessary, a more thorough

understanding of the machine operation should be

sought.

Editing: Wherever errors are found, they should be corrected and

rechecked, be it on the machine or at the programming

station. Whenever a programme is edited on the

machine, a note should be made on the print-out so the

master or original programme can also be corrected. A

better method is to punch out a programme from the

control after successfully producing a component.

6.8. TYPES OF CONTROL SYSTEM

There are two basic types of control systems in numerical control:

point-to-point and contouring.

a. In a point-to-point system, also called positioning, each axis of the

machine is driven separately by lead screws and, depending on the

type of operation, at different velocities. The machine moves

initially at maximum velocity in order to reduce non-productive

time, but decelerates as the tool approaches its numerically

defined position. Thus, in an operation such as drilling (or

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punching a hole), the positioning and cutting take place

sequentially (Fig. 5.4).

After the hole is drilled or punched, the tool retracts upward

and moves rapidly to another position, and the operation is

repeated. The path followed from one position to another is

important in only one respect. It must be chosen to minimize the

time of travel, for better

efficiency. Point-to-point systems are used mainly in drilling,

punching, and straight milling operations.

Position Coordinate

(X)

Coordinate

(Y)

C.P. -15 15

Point 1 10 -10

Point 2 55 -10

Point 3 55 -55

Point 4 10 -55

Position Coordinate

(X)

Coordinate

(Y)

C.P. -15 15

Point 1 25 -25

Point 2 45 0

Point 3 0 -45

Point 4 -45 0

Incremental (G90) Absolute (G91)

Figure 5.4. Point-to point system

C.P

15

15

(0,0)

10

10 45

45

1 2

3 4

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b. In a contouring system (also known as a continuous path system),

the positioning and the operations are both performed along

controlled paths but at different velocities. Because the tool acts as it

travels along a prescribed path (Fig. 5.5), accurate control and

synchronization of velocities and movements are important. The

contouring system is typically used on lathes, milling machines,

grinders, welding machinery, and machining centres.

6.9. PROGRAMMING CODES

A number of standard codes are used to reduce the amount of

programming effort needed to command commonly used machining

operations, instructions and conditions. These are commonly known as:

G codes – call up machining commands

M codes – call up machine control activities

T codes – call up tool selection

F codes – call up feed rates

S codes – call

Figure 5.5. Continuous path by a milling cutter

Cutter

radius

Cutter

path

Machined

surface

Work piece

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- modal codes remain active after being entered, unless they are

cancelled by another G code; and

- non-modal codes are only active in the programme block in which they

appear.

6.9.1. G codes (preparatory codes)

The majority of manufacturers follow the same practice in designation

of codes, but their detailed implementation mav differ.

Sample G codes

GOO Rapid movement for position

GOI Linear interpolation used for straight-line feed

G02 Circular interpolation, clockwise

G03 Circular interpolartion, counterclockwise

G04 Dwell, a programmed stop to the tool movement

G17 Circular interpolation xy plane

G18 Circular interpolation xz plane

G19 Circular interpolation yz plane

G20 Inch units

G21 Millimetre units

G28 Return to home position

G29 Return from home position

G31 Reverses programmed direction of x axis

G32 Reverses programmed direction of y axis

G41 Tool radius compensation left

G42 Tool radius compensation right

G43 Tool length compensation-positive direction

G44 Tool length compensation-negative direction

G70 Imperial unit

G71 Metric units

G80 Cancel canned cycle

G81 Drilling cycle

G82 Drilling cycle with dwell

G83 Deep hole drilling

G84 Tapping cycle

G85 89-boring cycles

G90 Absolute mode

G91 Incremental mode

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6.9.2. M codes

These control the auxiliary functions of the machine.

MOO Program stop

M02 End of program

M03 Spindle on, clockwise

M04 Spindle on, counter clockwise

M05 Spindle off

M06 Tool change

M07 Oil mist coolant on

M08 Flood coolant on

M09 Coolant off

M30 End of tape

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6.10. WRITING A PROGRAM

Position Coordinate (X) Coordinate (Y)

C.P. 0 0

P. 1 45.0 -25.0

P. 2 70.0 -25.0

P. 3 60.0 -65.0

P. 4 45.0 -50.0

P. 5 60.0 -50.0

P. 6 49.393 -75.607

P. 7 38.787 -65.0

P. 8 15.0 -65.0

Figure 5.6. To cut a „S‟-slot/groove with a point-to-point method and a continuous path/contouring system

Table 5. Reference points and X and Y coordinates to cut a „S‟-slot/groove with a point-to-point method and a continuous path/contouring system

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To machine the above component (as in Fig.5.6), below is the

programme that can be followed;

N10 G71 G90 S1500 T1

N20 G00 X0 Y0

N30 G00 X70.0 Y-25.0 Z10.0

N40 G01 Z-5.0 F250

N50 G03 I-25.0 J0

N60 X45.0 Y-50.0

N70 G01 X60.0 Y -50.0

N80 G02 I0 J-15.0

N90 X49.393 Y-75.607

N100 G01 X38.787 Y-65.0

N110 X15.0 Y-65.0

N120 Z10.0

N130 G00 M00

N140 G00 X0 Y0

Description of The Above Programme

NXX – block number

Block No. 10 – set machine to use metric unit, incremental coordinate,

spindle speed 1500 rpm, choose tool no. 1.

Block No. 20 – rapid movement to centre point (C.P).

Block No. 30 - rapid movement to point 1 (P. 1), cutting tool distance

is

5.0 mm from the surface of the work piece.

Block No. 40 – cutting tool cuts 10.00 mm deep, feed 250 mm/min

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Block No. 50 – circular interpolation, counter clockwise, radius 25.0

mm

Block No. 60 – tool ends interpolation cutting at P. 4

Block No. 70 – linear interpolation until P. 5

Block No. 80 - circular interpolation, clockwise, radius 15.0 mm

Block No. 90 - tool ends interpolation cutting at P. 6

Block No. 100 - linear interpolation until P. 7

Block No. 110 - linear interpolation until P. 8

Block No. 120 – tool rises up 10.0 mm

Block No. 130 – program stops

Block No. 140 - rapid return to centre point (C.P).

6.11. ADVANTAGES OF COMPUTER NUMERICAL CONTROL

i. The component programming tape and the tape reader are used

once only when the programme is copied into the computer

memory, not only this practice wills same time but it will also

reduce errors.

ii. The programming tape can be edited on the shop floor, when

the machine is placed/located. Editing, correction and

optimising; such

as machine tool operations, spindle speeds and speeds; are

usually done in the test run of the tape.

iii. Computer numerical control can easily changes into metric

system if the programme is in the imperial units.

iv. It is widely used in industry. It is easily adaptable in a

computerised industry system.

v. Increased flexibility – the machine can produce a specific part,

followed by other parts with different shapes, and at reduces

cost.

vi. Greater accuracy – computers have a higher sampling rate and

faster operation.

vii. More versatility – editing and debugging programmes,

reprogramming, and plotting and printing part shape are

simpler.

viii. Programmes are stored on the machine ready for use.

ix. Programmes and data can be modified on the machine.

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6.1. Briefly state four (4) advantages of numerical control system.

6.2. You are given a drawing of a component. List down the steps you would take to

operate a NC machine in order produce the component.

6.3. Write a short paragraph on three (3) basic components of a numerical control

system.

6.4 Numerical control machine can be done in absolute coordinate (G90) and

incremental coordinates (G91). What is the difference between the two coordinates.

6.5 By using G90 and G 91 coordinates write a program to cut a component in is the

.below figure.

ACTIVITY 6

30

70

J20

20

60

35

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Answer 6.5

Point G 90 G 91

X Y X Y

Origin Point 0 0 0 0

1 30 -70 30 -70

2 30 -40 0 30

3 70 -15 40 25

4 90 -35 20 -20

5 90 -70 0 -35

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SHIELDED GAS ARC WELDING

General Objective: To understand the principles of shielded gas arc

welding i.e. TIG and MIG welding.

Specific Objectives : At the end of the unit you will be able to :

Identify the principles of shielded gas arc

welding i.e. TIG and MIG welding.

Elaborate on the TIG and MIG welding

principles, welding procedures, welding

machines, gas, etc.

State the advantages and disadvantages of

TIG and MIG compared to manual arc

welding.

State the weaknesses of TIG and MIG welding

and how to prevent them.

.

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7.0. INTRODUCTION

The objective of welding is to produce a welding joint that contains the

same mechanical properties as the base metal. The objective can be

achieved if the molten metal is free from atmospheric air. If not, nitrogen

and oxygen gases in the atmosphere will be absorbed by the melting pool.

The welding produced will have small pore that will weaken the weld.

To prevent the welding, molten metal and the end of the filler rode

and electrodes from atmospheric air pollution before the molten metal

become solid inert gas is blown out from the welding point. These gases will

cover the welding pools, the filler rod points and electrode tips to avoid

oxidation.

7.1. TUNGSTEN INERT GAS (TIG)

The welding of aluminium and magnesium alloys by the oxy-acetylene

and manual metal arc processes is limited by the necessity to use a corrosive

flux. The gas shielded, tungsten arc process enables these metals and a wide

range of ferrous alloys to be welded without the use of a flux. The choice of

the either a.c. or d.c. depends upon the metal to be welded. For metals

having refractory surface oxides such as aluminium and its alloys,

magnesium alloys and aluminium bronze, a.c. is used whilst d.c. is used for

carbon and alloy steels, heat-resistant and stainless steels, cooper and its

alloys, nickel and its alloys, titanium, zirconium and silver.

The arc burns between a tungsten electrode and the work piece within

a shield of the inert gas argon, which excludes the atmosphere and prevents

contamination of electrode and molten metal. The hot tungsten arc ionizes

argon atoms within the shield to form a gas plasma consisting of almost

equal numbers of free electrons and positive ions. Unlike the electrode in

the manual metal arc process, the tungsten is not transferred to the work

and evaporates very slowly, being classed as „non-consumable‟. Small

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amount of other elements are added to the tungsten to improve electron

emission.

Figure 7.1. TIG welding equipment

Gas flow

Water inlet

Water outlet Welding

machine

Torch

Work piece

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7.1.1. Preparation of Metal.

Gas tungsten-arc processes must start with clean metal which

has the proper joint design i.e., V, U, or J. Mechanical and chemical

cleaning are often necessary to prepare the base metal. The edges of

the joint should be shaped to permit adequate fusion and penetration.

It is common practice to reduce or bevel the adjoining edges to 1.6 mm

thickness.

A strip (backup bar) to support the back side of the base metal

should be used when needed. This is especially helpful on aluminium

since it aids in shielding. The backup bar may be removed after

welding.

Figure 7.2. TIG in progress. The tungsten does not melt into the puddle for filler. This is a nonconsumable electrode.

Shielded gas

Electrode

(tungsten)

Filler rode

arc

Melting pool

Inert/noble

gas

Work piece

20 – 30o

80 – 90o

Direction of travel

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7.1.2. Joint Fit.

Good joints make it easier to obtain a good weld. In production

work, carefully fitted joints can help save money and can help the

welding operator develop standardized welding techniques. Root

opening (distance apart) and angle of bevel are two major factors

requiring close tolerance when fitting joints.

7.1.3. Welding Machine.

Gas tungsten-arc welding requires a conventional welding

machine, with the following accessories:

1. Torch, lead cable, and hoses.

2. Inert gas supply and flow meter for measuring

amount of shielding gas.

3. Water cooling system for water-cooled torches.

Air-cooled torches are limited to 150 ampere capacity.

4. High-frequency spark unit attached to the output

leads of the power supply (to start and stabilize arc).

The finished weld will be greatly affected by type of current and

polarity. For example, aluminium is welded with alternating current

plus superimposed high-frequency current (ACHF). Stainless steel is

welded with direct current straight polarity (DCSP). Improper

electrical connections will cause (a) the electrode to overheat, (b) poor

penetration, or (c) insufficient cleaning effect upon the base metal.

Current selection must be made with care. When an electrode

is connected to the negative terminal (DCSP), electrons pass through

the arc to bombard the base plate (Fig. 7.3).

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This causes nearly 70% of the arc heat to accumulate in the

base metal to assist fusion and penetration. When the electrode is

made positive (DCRP), a cleaning effect is created on the surface of

the base plate (Fig. 7.4).

Deep penetration

Work piece

Figure 7.3 Power supply with direct current straight polarity

Direction of electron

travel

Welding

machine

Positive surface

particles travel

Electrode

Positive surface

particles travel

Direction of electron

travel

Electrode Welding

machine

Work piece

Shallow penetration

Figure 7.4 Power supply with direct current reverse polarity

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96

In welding aluminium this method is used to remove surface

oxidation. While an electrode positive connection furnishes a cleaning

effect, it also heats the tungsten electrode. The electrode may get hot

enough to melt, transfer to the weld pool, and contaminate the base

metal. When this happens, the electrode must be removed, its end

broken off, and it must be ground to shape.

Alternating current offers the advantages of both direct current

straight polarity (DCSP) and direct current reverse polarity (DCRP).

Gas tungsten-arc welding of aluminium and magnesium requires an

AC power supply (Fig. 7.5).

Gas tungsten-arc welding is not recommended for metal more

than 20 mm thick. Welds have been completed on 25 mm thick plate

but require a great deal of time and, consequently, are expensive.

Most applications are less than 12 mm thick, and require less than

500 amperes of current.

Electrode

Surface

particles lifted Electron flow

Welding

machine

Work piece

Medium penetration

Figure 7.5 Alternating current power supply

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7.1.4. Welding Torch.

The welding torch has a round collet which compresses to hold

the electrode and a nozzle to control the gas (Fig. 7.2). Water-cooled

torches are used when current values exceed 150 amperes.

Maintenance of either torch is more time consuming than with the

metal-arc process. Careful selection of nozzle size, proper shaping of

the working end of the electrode and correct extension of electrode

beyond nozzle are important. Nozzle size influences the flow of gas.

End shape of electrode and extension of electrode beyond nozzle

control the stability of the arc. Further, it is important that electrode

diameter match current value (Table 7.1). If the current is too high for

the diameter of an electrode, the life of the electrode will be reduced.

When the current is too low for a given electrode diameter, the arc will

not be stable.

Electrode

Size

(Diameter,

Inches)

Nozzle or

Cup Sizes

WELDING CURRENT IN AMPERES

ACHF DCSP DCRP

Pure

Tungsten

Thoriated

Tungsten

Pure or

Thoriated

Pure or

Thoriated

0.020 4,5 5-15 5-20 5-20 *

0.040 4,5 10-60 15-80 15-80 *

1/16 4-6 50-100 70-150 70-150 10-20

3/32 5-7 100-160 140-235 150-250 15-30

1/8 6-8 150-210 225-325 250-400 25-40

*Not applicable.

Table 7.1. Selection of nozzle size and electrode size for gas tungsten-arc

welding

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The end of the electrode should remain bright, as if it was

polished. On some metals, such as aluminium and magnesium, the

end is contaminated when starting or by touching the base plate.

Contamination can be burned off by welding on a scrap plate of metal,

or it can be removed by grinding (Fig. 7.6). The electrode should be

adjusted to extend beyond the nozzle a distance equal to the electrode

diameter (Fig. 7.7)

Figure 7.6 Electrode shapes for gas shielded tungsten-arc welding

3/8” max

Electrode diameter

Figure 7.7. Adjustment of electrode from nozzle

Grind here

AC

30o

45o

15o

DCSP DCRP

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7.1.5. Shielding Gas.

Gas used with this process produces an atmosphere free from

contamination and also provides a path for arc transfer. The path

creates an environment that helps stabilize the arc. The gas and arc

activity also perform a cleansing action on the base metal. Both argon

and helium are generally used for this process but argon is preferred

because it is cheaper and provides a smoother arc. Helium, however,

helps produce deeper penetration (Table 7-2).

7.1.6. Filler Metal.

Filler metals are selected to meet or exceed the tensile strength,

ductility, and corrosion resistance of the base metal. The usual

practice is to select a filler metal having a composition similar to that

of the base metal. For most efficient application, select clean filler

metals of proper diameter; the larger the diameter of the filler metal,

the more heat is lost from the weld pool.

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Metal Shielding Gas Remarks

Aluminium Argon Easy starting

Good cleaning action.

Helium Faster and more penetration.

Argon-10% helium Increase in penetration over pure argon.

Stainless steel Argon Better control of penetration (16 gauge

and thinner).

Argon-helium

mixtures

Higher welding speeds.

Copper and

nickel

Argon Easy to control penetration and weld

contour on sheet metal.

Argon-helium Increases heat into base metal.

Helium Highest welding speed.

7.2. TIG WELDING TECHNIQUES

After the base metal has been properly cleaned and clamped or tacked

together, welding can be started. On aluminium, the arc is usually started

by bringing the electrode near the base metal at a distance of about one

electrode diameter so that a high-frequency spark jumps across the gap and

starts the flow of welding current. Steel, copper alloys, nickel alloys, and

stainless steel may be touched with the electrode without contamination to

start the arc. Once started, the arc is held stationary until a liquid pool

appears. Filler rod can be added to the weld pool as required (Fig. 7.8).

Highest current values and minimum gas flow should be used to produce

clean, sound welds of desired penetration (Table 7-3).

Table 7.2 Selection of gases for manual application of tungsten-arc welding.

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Material Aluminium Stainless Steel Magnesium Deoxidized

Copper

Type of Current ACHF DCSP ACHF DCSP

1.6mm electrode

Current:

Argon:

Passes:

60-80

15 cfh

1

80-100

11 cfh

1

60

13 cfh

1

110-140

15 cfh

1

3.2mm electrode

Current:

Argon:

Passes:

125-145

17 cfh

1

120-140

11 cfh

1

115

19 cfh

1

175-225

15 cfh

1

4.7mm electrode

Current:

Argon:

Passes:

190-220

21 cfh

1

200-250

13 cfh

1

120-175

19 cfh

1,2

250-300

15 cfh

1 at 257.4*

*Preheat to temperature indicated.

The shielded gas is pure argon and pre-heating is required for drying

only to produce welds of the highest quality. All surfaces and welding wire

should be degreased and the area near the joint and the welding wire should

be stainless steel wire brushed or scrape to remove oxide and each run

brushed before the next is laid.

The angles of torch and filler rod are shown in Fig. 7.8. After

switching on the gas, water, welding current and HF unit, the arc is struck

by bringing the tungsten electrode near the work (without touching down).

The HF sparks jump the gap and the welding current flows. Arc length

should be about 3 mm. Practice starting by laying the holder on its side and

bringing it to the vertical position, but using the ceramic shield as a fulcrum

can lead to damage to the holder and ceramic shield. The arc is held in one

Table 7.3 Operating data for TIG

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position on the plate until a molten pool is obtained and welding is

commenced, proceeding from right to left, the rod being fed into the forward

edge of the molten pool and always kept within the gas shield. It must not

be allowed to touch the electrode or contamination occurs. A black

appearance on the weld metal indicates insufficient argon supply.

The flow rate should be checked and the line inspected for leaks. A

brown film on the weld metal indicates presence of oxygen in the argon while

a chalky white appearance of the weld metal accompanied by difficulty in

controlling the weld indicates excessive current and overheating. The weld

continues with the edge of the portion sinking through, clearly visible, and

the amount of the sinking which determines the size of the penetration bead

is controlled by the welding rate.

30o

15o

Direction of

travel

Figure 7.8. Example of TIG

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7.3. METAL INERT GAS (MIG)

It is convenient to consider, under this heading, those applications

which involve shielding the arc with argon, carbon dioxide (CO2) and

mixtures of argon with oxygen and/or CO2, since the power source and

equipment is essentially similar except for gas supply. With the tungsten

inert gas shielded arc welding process, inclusions of tungsten become

troublesome with currents above 300 A. The MIG process does not suffer

from these advantages and larger welding current giving greater deposition

rates can be achieved. The process is suitable for welding aluminium,

magnesium alloys, plain and low-alloy steels, stainless and heat-resistant

steel, copper and bronze, the variation being filler wire type of gas shielding

the arc.

The consumable electrode of bare wire is carried on the spool and is

fed to a maually operated or fully automatic gun through an outer flexible

cable by motor-driven rollers of adjustable speed, and rate of burn-off of the

electrode wire must be balance by rate of wire feed. Wire feed rate

determines the current used.

In addition, a shielding gas or gas mixture is fed to the gun together

with welding current supply, cooling water flow and return (if the gun is

water cooled) and a control cable from gun switch to control contractors.

A d.c. power supply is required with the wire electrode connected to the

positive pole ( Fig. 7.9).

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During this process an electric arc is used to heat the weld zone. The

electrode is fed into the weld pool at a controlled rate and the arc is shielded

by a protective gas such as argon, helium, or carbon dioxide (Fig. 7.9). Gas

metal-arc welding can be either the short-circuiting process or the spray-arc

process (Fig. 7.10).

Inert/noble gas

Melting pool

Arc Shielded gas

Work piece

Figure 7.10. MIG in progress

Figure 7.9 . MIG welding equipment

Spool of electrode

wire

Control head

forelectrode feed

and gas supply

Inert gas

cylinder

Electrode feed

rools

Welding power

cable

Arc welding

power supply

Gas flow

meter

Contactor lead,welding

current,electrode, and inert gasto welding

gun

Contacto

r cable Ground

cable

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The short-circuiting arc process (short arc) operates at low currents

and voltages. For example, 18-gauge sheet metal can be welded at 45 amps

and 12 volts.

In contrast, the spray-arc process uses high currents and voltages,

e.g., Arc action is illustrated in Fig. 7.12. This results in high heat input to

the weld area, making possible deposition rates of more than 0.4 lb per

minute. (The deposition rate is the weight of filler metal melted into the

weld zone

per unit of time.) Most applications of the spray-arc process are in thick

metal fabrications, e.g., in heavy road-building machinery, ship construction,

and beams for bridges.

Work piece

Work piece

Figure 7.11. Mechanics of the short circuiting transfer process as shown between the electrode and work piece. Electrode dips into pool an average of 90 times a second

Electrode maintains steady arc length

Figure 7.12. Mechanics of the spray-arc transfer process as shown between the electrode and work

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All metal inert-gas (MIG) welding is classified as semi-automatic,

since the electrode feeds into the weld according to a preset adjustment.

After making an initial adjustment, the welding operator merely moves the

gun along the joint. For effective applications, the welding operator needs

information concerning power requirements, welding gun, selection of

shielding gas, type of filler metal, and job procedures.

7.3.1. Power Requirements.

Conventional power supplies used for shielded metal-arc

welding are not satisfactory. A welding machine designed for the MIG

process is called a constant potential power source; it produces a

constant voltage and also permits the operator to adjust electrode feed

rates. The adjustments on the power supply are voltage, slope (limits

current), and wire feed rate. Welding current is established by

selecting a wire feed rate. Slope adjustment to limit current is not a

problem with spray-arc type transfer. However, in short-circuiting arc

processes, limitations on short-circuit current are essential to prevent

excessive spatter.

The electrode feed mechanism, an important part of the

welding machine, consists of a storage reel for electrode wire and a

power drive which feeds the electrode into the weld at a controlled

rate.

Metal Shielding Gas Remarks

Aluminium and copper Argon + helium

20-80% mixture

High heat input

Minimum of porosity

Table 7.4 Shielding mixtures for MIG

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Copper Argon + nitrogen

25-30% mixture

Good heat input on copper

Carbon steels

Low alloy steels

Argon + oxygen

3-5% mixture

Stabilizes arc

Reduces spatter

Causes weld metal to flow

Eliminates undercut

May require electrode to

contain deoxidizers

Low alloy steels Mixture of argon,

helium and carbon

dioxide

Increases toughness of weld

deposit

7.3.2. Selection of Gas.

The primary purpose of the inert gas is to shield the weld

crater from contamination. Shielding gas may also affect (1) the

transfer of

metal across the arc, (2) fusion and penetration, (3) the shape of weld

deposit, (4) the speed of completing the weld, (5) the ability of filler

metal to flow over the surface without undercutting, and (6) the cost of

the finished weld.

No single inert gas is satisfactory for all welding conditions. Some specific

jobs are more efficiently welded with a mixture of gases.

For example, low alloy steels are welded with a mixture of argon,

helium, and carbon dioxide (Table 7.4).

7.3.3. Filler Metal.

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Electrodes used for filler metal with the MIG process are much

smaller in diameter than those used with the metal-arc process. Sizes

may range from 0.4 mm to 5.5 mm in diameter. Small diameter

electrodes require high feed rates, from 100 to 1,400 inches per

minute. The composition of the electrode usually matches that of the

base metal, but for welding high-strength alloys, the composition of

the electrode may vary widely from that of the base metal.

For example, an aluminium-zinc-magnesium alloy (7039) is

welded with an aluminium-magnesium alloy (5356).

7.4. JOB PROCEDURES

High-quality welds are obtained by controlling process variables

which include current, voltage, travel speed, electrode extension, cleanliness,

and type of joint.

7.4.1. Current.

Welding current varies with the melting rate of the electrode.

Extreme values of current tend to promote defects, but a high current

(1.1 mm. electrode at 220 amp) reduces the drop size of the transfer,

improves arc stability, and improves penetration.

7.4.2. Voltage.

With the MIG welding process, the voltage control determines

the arc length. The higher the voltage setting, the longer the arc. A

desirable voltage range to establish a short arc is 19-22 volts; defects

are more likely to occur outside this range (Fig. 7.14).

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Position of welding will determine voltage needed. For example, a

higher voltage is more desirable for flat-position welding than for vertical or

overhead welding. Table 7-5 indicates typical voltage values.

Metal Argon Helium Ar-O2 Mixture

1-5%O2

CO2

Aluminium 25 30 * *

Carbon Steel * * 28 30

Low-alloy Steel * * 28 30

Stainless Steel 24 * 26 *

Nickel 26 30 * *

Copper 30 36 * *

*Not recommended.

Sev

erit

y o

f d

efec

t (

Incr

ease

)

Sev

erit

y o

f d

efec

t (

Incr

ease

)

Fig. 7.13. Defects related to voltage settings.

Voltage Voltage

Curve representing

undercutting

Curve representing

porosity

Table 7-5 Typical arc voltage for MIG using drop transfer and 1/16 inch

diameter electrode.

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7.4.3. Travel Speed.

After selecting a current and voltage setting, select the rate of

travel. A typical example is 0.6m – 0.76m per minute (in./min). If the

rate is changed more than a few mm per minute, weld quality will be

greatly affected (Fig. 7.15).

Position of welding will affect the travel speed. For example, if

the weld direction is dropped 15 degrees from flat so that the position

is slightly downhill, travel speed can be increased.

7.4.4. Electrode Extension.

Electrode extension is important. The further the electrode

extends from the gun to the arc, the greater the electrical resistance

between the output terminals. Higher resistance increases the

temperature of the electrode, and the resistance-heated electrode uses

less current in the weld puddle. In the spray-arc process, the electrode

Fig. 7.15. Undercutting of horizontal fillet on 6.3mm thick aluminium as

affected by travel speed. Gas metal arc process was used.

No undercut.

Travel speed 26 in/min

Undercutting. Travel speed

32 in/min

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extension should be about 12 mm to 25 mm, for short-circuiting

transfer; it should be approximately half this distance.

7.5. MIG WELDING TECHNIQUES

There are three methods of initiating the arc.

i. The gun switch operates the gas and water solenoids and

when released the wire drive is switched on together

with the welding current.

ii. The gun switch operates the gas and water solenoids and

strikes the wire end on the plate operates the wire drives

and welding current (known as „scratch start‟).

iii. The gun switch operates the gas and water solenoids and

wire feed with welding current known as „scratch start‟.

As a general rule dip transfer is used for thinner sections up to 6.4

mm and for positional welding, whilst spray transfer is used for thicker

sections.

The gun is held at an angle of 80o or slight less to the line of the weld

to obtain a good view of the weld pool, and welding proceeds from right to

left with nozzle held 6 – 12 mm from the work.

The further the nozzle is held from the work less the efficiency of the

gas shield, leading to porosity. If the nozzle is held too close to the work

spatter may build up, necessitating frequent cleaning of the nozzle, while

acting between nozzle and work can be caused by a bent wire guide tube

allowing the wire to touch the nozzle, or by spatter build-up short-circuiting

wire and nozzle. If the wire burns back to the guide tube it may be caused

by a late start of the wire feed, fouling of the wire in the feed conduit or the

feed rolls being too tight. Intermittent wire feed is generally due to

insufficient feed rolls pressure or looseness wire due to wear in the rolls.

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Excessively sharp bends in the flexible guide tubes can also lead to this

trouble.

Root run is performed with no weave and filler runs with as little

weave as possible consistent with good fusion since excessive weaving tends

to promote porosity. The amount of wire projecting beyond the contact tube

is important because the greater the projection, the greater the I2R effect

and the greater the voltage drop which may reduce the welding current and

affect penetration. The least projection commensurate with accessibility to

the joint being welded should be aimed at.

Backing the strips which are welded permanently on to the reverse

side of the plate by the root run are often used to ensure sound root fusion.

Backing bars of copper or ceramics with grooves of the required penetration

bead profile can be used and are removed after welding. It is not necessary

to back-chip the root run of the light alloys but with stainless steel this is

often done and a sealing run put down. The importance of fit-up in securing

continuity and evenness of the penetration bead cannot be over-emphasized.

Flat welds may be slightly tilted to allow the molten metal to flow

against the deposited metal and thus give a better profile. If the first run

has a very convex profile poor manipulation of the gun may cause cold laps

in the subsequent run.

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7.6. DIRECT CURRENT STRAIGHT POLARITY

The welding circuit shown in figure 7.16, is known as a straight

polarity circuit. It is understood that the electrons are flowing from the

negative terminal (cathode) of the machine to the electrode. The electrons

continue to travel across the arc into the base metal and to the positive

terminal (anode) of the machine.

Approximately two-thirds of the total heat produced with DCSP is

released at the base metal while one-third is released at the electrode. The

choice of direct current straight polarity depends on many variables such as

material of the base metal, position of the weld, as well as the electrode

material and covering.

Electrode

Reactor

Cathode

d

Field Holder

Anode

Arc gap

Work piece

Figure 7.16. Wiring diagram of a direct current, straight polarity (DCSP) arc circuit

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7.7. DIRECT CURRENT REVERSE POLARITY ARC WELDING

It is possible, and sometimes desirable, to reverse the direction of

electron flow in the arc welding circuit. When electron flow from the

negative terminal (cathode) of the arc welder to the base metal, this circuit is

known as direct current reverse polarity (DCRP). In this case, the electron

returns to the positive terminal (anode) of the machine from the electrode

side of the arc, as shown in Figure 7.17.

When using DCRP, one-third of the heat generated in the arc is

released at the base-metal and two-thirds is liberated at the electrode. With

two-thirds of the heat released at the electrode in DCRP, the electrode metal

and the shielding gas are super-heated. This superheating causes the

molten metal in the electrode to travel across the arc at a very high rate of

speed. Deep penetration results due to the force of the high velocity arc.

There is theory that, with a covered electrode, a jet action and/or expansion

of gases in the metal at the electrode tip causes the molten metal to be

propelled with great impact across the arc.

The choice of direct current reverse polarity depends on many

variables such as material of the base metal, position of the weld, as well as

the electrode material and covering.

Anode

Electrode

Reactor

Cathode

d

Field Holder

Arc gap

Work piece

Figure 7.17. Wiring diagram of a direct current, reverse polarity (DCRP) arc circuit

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1. Explain the term nonconsumable electrode.

2. What does the term inert signify?

3. List the gases used for shielding a welding arc.

4. Explain how TIG welding electrodes are shaped.

5. How far should the electrode extend beyond the nozzle of the TIG

torch?

6. Explain why MIG welding is classified as a semiautomatic process.

7. From the standpoint of operation, how are TIG and MIG processes

different? How are they similar?

8. What polarity does anode signify?

9. In what direction do the electrons travel when using straight polarity?

10. How much of the heat used for arc welding is liberated at the

electrode when using straight polarity?

11. Why is it recommended that a tungsten electrode arc be started on a

scrap tungsten surface?

12. What would happen if the tungsten electrode were bent off centre?

13. Name two defects that could occur with gas shielded-arc welding

processes and explain how each could be avoided.

ACTIVITY 7

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RAPID PROTOTYPING

CONTENT

1 Introduction

1.1 : Introduction of rapid prototyping

1.2 : History of rapid prototyping

1.3 : The advantages of rapid prototyping

2 Classification of rapid prototyping

2.1 : Three major group of rapid prototyping

2.1.1 : Subtractive process

2.1.2 : Additive process

2.1.3 : Virtual process

2.1.3.1 : Fused deposition modeling

2.1.3.2 : Stereolithography

2.1.3.3 : Selective laser sintering

2.1.3.4 : Ballistic

2.1.3.5 : Laminated object manufacturing

3 Understanding Direct Manufacturing And Rapid Tooling

3.1 : Basic methodology of rapid tooling

3.2 : Rapid tooling

3.2.1 : Benefits of rapid injection tool molding

3.2.2 : Advantages of rapid tooling for manufacturing

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Topic 1 : Introduction

1.1 : Introduction of Rapid Prototyping

Rapid prototyping is a revolutionary and powerful technology with wide

range of applications. The process of prototyping involves quick building up

of a prototype or working model for the purpose of testing the various design

features, ideas, concepts, functionality, output and performance. The user is

able to give immediate feedback regarding the prototype and its

performance. Rapid prototyping is essential part of the process of system

designing and it is believed to be quite beneficial as far as reduction of

project cost and risk are concerned.

Rapid prototyping is known by many terms as per the technologies involved,

like SFF or solid freeform fabrication, FF or freeform fabrication, digital

fabrication, AFF or automated freeform fabrication, 3D printing, solid

imaging, layer-based manufacturing, laser prototyping and additive

manufacturing.

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1.2 : History of Rapid Prototyping:

Sixties: The first rapid prototyping techniques became accessible in the later

eighties and they were used for production of prototype and model parts. The

history of rapid prototyping can be traced to the late sixties, when an

engineering professor, Herbert Voelcker, questioned himself about the

possibilities of doing interesting things with the computer controlled and

automatic machine tools. These machine tools had just started to appear on

the factory floors then. Voelcker was trying to find a way in which the

automated machine tools could be programmed by using the output of a

design program of a computer.

Seventies: Voelcker developed the basic tools of mathematics that

clearly describe the three dimensional aspects and resulted in the earliest

theories of algorithmic and mathematical theories for solid modeling. These

theories form the basis of modern computer programs that are used for

designing almost all things mechanical, ranging from the smallest toy car to

the tallest skyscraper. Volecker‟s theories changed the designing methods in

the seventies, but, the old methods for designing were still very much in use.

The old method involved either a machinist or machine tool controlled by a

computer. The metal hunk was cut away and the needed part remained as

per requirements.

Eighties: However, in 1987, Carl Deckard, a researcher form the University

of Texas, came up with a good revolutionary idea. He pioneered the layer

based manufacturing, wherein he thought of building up the model layer by

layer. He printed 3D models by utilizing laser light for fusing metal powder

in solid prototypes, single layer at a time. Deckard developed this idea into a

technique called “Selective Laser Sintering”. The results of this technique

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were extremely promising. The history of rapid prototyping is quite new and

recent. However, as this technique of rapid prototyping has such wide

ranging scope and applications with amazing results, it has grown by leaps

andbounds.

Voelcker‟s and Deckard‟s stunning findings, innovations and researches

have given extreme impetus to this significant new industry known as rapid

prototyping or free form fabrication. It has revolutionized the designing and

manufacturingprocesses. Though, there are many references of people

pioneering the rapid prototyping technology, the industry gives recognition

to Charles Hull for the patent of Apparatus for Production of 3D Objects by

Stereolithography. Charles Hull is recognized by the industry as the father

of rapid prototyping.

Present-day Rapid Prototyping: Today, the computer engineer has to simply

sketch the ideas on the computer screen with the help of a design program

that is computer aided. Computer aided designing allows to make

modification as required and you can create a physical prototype that is a

precise and proper 3D object.

1.3 : The Advantages Of Rapid Prototyping

CAD data files can be manufactured in hours.

Tool for visualization and concept verification.

Prototype used in subsequent manufacturing operations to

obtain final part.

Tooling for manufacturing operations can be produced.

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TOPIC 2 : CLASSIFICATION OF RAPID PROTOTYPING

2.1 : Three Major Group Of Rapid Prototyping.

2.1.1 : Subtractive Process

The subtractive process is the prevalent process in the history of model

making. Model makers once utilized materials like clay and wood or other

hard material, to whittle, carve, or sculpt a model component. A complex

part could be made in a number of pieces and assembled to create the final

product. The excess material was basically chiseled, cut, and sanded to

expose the design within the carving medium. This process was

understandably time-intensive and resulted in a finished product that was a

one-of-a-kind and could not be easily replicated without remaking the part

from scratch. Once a part was roughed out in the desired material, hand

finishing, applying colors, textures and graphics allowed model makers to

achieve a unique part that often closely mimicked the desired future

product.

Today CAD/CAM programs make the replication of these parts much

simpler and provide high tolerances for part specifications. Architectural

model makers use laser cutting technology to precisely incise materials like

foam core, high- density papers and other materials to replicate panels used

in the construction of structural models. Product design model makers may

use molds and castings, CNC routers or milling machines to electronically

carve parts out of the desired medium.

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2.1.2 : Additive Process

Additive fabrication refers to a class of manufacturing processes, in

which a part is built by adding layers of material upon one another. These

processes are inherently different from subtractive processes or

consolidation processes. Subtractive processes, such as milling, turning, or

drilling, use carefully planned tool movements to cut away material from a

workpiece to form the desired part. Consolidation processes, such as casting

or molding, use custom designed tooling to solidify material into the desired

shape. Additive processes, on the other hand, do not require custom tooling

or planned tool movements. Instead, the part is constructed directly from a

digital 3-D model created through Computer Aided Design (CAD) software.

The 3-D CAD model is converted into many thin layers and the

manufacturing equipment uses this geometric data to build each layer

sequentially until the part is completed. Due to this approach, additive

fabrication is often referred to as layered manufacturing, direct digital

manufacturing, or solid freeform fabrication.

The most common term for additive fabrication is rapid prototyping. The

term "rapid" is used because additive processes are performed much faster

than conventional manufacturing processes. The fabrication of a single part

may only take a couple hours, or can take a few days depending on the part

size and the process. However, processes that require custom tooling, such

as a mold, to be designed and built may require several weeks. Subtractive

processes, such as machining, can offer more comparable production times,

but those times can increase substantially for highly complex parts. The

term "prototyping" is used because these additive processes were initially

used solely to fabricate prototypes. However, with the improvement of

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additive technologies, these processes are becoming increasingly capable of

high-volume production manufacturing, as will be explored in the section on

applications.

Additive fabrication offers several advantages, listed below.

Speed - As described above, these "rapid" processes have short build

times. Also, because no custom tooling must be developed, the lead time in

receiving parts is greatly reduced.

Part complexity - Because no tooling is required, complex surfaces and

internal features can be created directly when building the part. Also, the

complexity of a part has little effect on build times, as opposed to other

manufacturing processes. In molding and casting processes, part

complexity may not affect the cycle times, but can require several weeks

to be spent on creating the mold. In machining, complex features directly

affect the cycle time and may even require more expensive equipment or

fixtures.

Material types - Additive fabrication processes are able to produce

parts in plastics, metals, ceramics, composites, and even paper with

properties similar to wood. Furthermore, some processes can build parts

from multiple materials and distribute the material based on the location

in the part.

Low-volume production - Other more conventional processes are not

very cost effective for low-volume productions because of high initial costs

due to custom tooling and lengthy setup times. Additive fabrication

requires minimal setup and builds a part directly from the CAD model,

allowing for low per-part costs for low-volume productions.

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With all of these advantages, additive fabrication will still not replace more

conventional manufacturing processes for every application. Processes such

as machining, molding, and casting are still preferred in specific instances,

such as the following:

Large parts - Additive processes are best suited for relatively small parts

because build times are largely dependent upon part size. A larger part in

the X-Y plane will require more time to build each layer and a taller part

(in the Z direction) will require more layers to be built. This limitation on

part size is not shared by some of the more common manufacturing

methods. The cycle times in molding and casting processes are typically

controlled by the part thickness, and machining times are dependent upon

the material and part complexity. Manufacturing large parts with

additive processes is also not ideal due to the current high prices of

material for these processes.

High accuracy and surface finish - Currently, additive fabrication

processes can not match the precision and finishes offered by machining.

As a result, parts produced through additive fabrication may require

secondary operations depending on their intended use.

High-volume production - While the production capabilities of additive

processes are improving with technology, molding and casting are still

preferred for high-volume production. At very large quantities, the per-

part cost of tooling is insignificant and the cycle times remain shorter

than those for additive fabrication.

Material properties - While additive fabrication can utilize various

material types, individual material options are somewhat limited. As a

result, materials that offer certain desirable properties may not be

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available. Also, due to the fabrication methods, the properties of the final

part may not meet certain design requirements. Lastly, the current prices

for materials used in additive processes are far greater than more

commonly used materials for other processes.

2.1.3 : Virtual Process

Virtual prototyping is becoming a cost-effective method used in testing new

products and systems. It is an integral part of current rapid prototype

Shenzhen methods wherein virtual designs created from computer aided

design (CAD) or animation modeling software are used and then

transformed into cross sections in a still virtual environment.A special

machine is then used to create each virtual cross section in then takes

physical form layer after layer until an identical prototype model is created.

The whole process enables the virtual model become a physical model with

corresponding identical features.

In the additive fabrication of virtual prototypes, the rapid prototyping (RP)

machine reads the data from a CAD drawing, and forms successive layers of

liquid or powdered material according to the virtual data received. It slowly

builds up a physical model from a series of cross sections.These different

layers, which match up to the virtual cross sections created from the CAD

model, are then glued or fused together to create the final three dimensional

prototype model.All the rapid prototyping technologies in current use have

many things in common. All make use of additive processes. Rapid

prototyping makes use of additive construction as the means of creating

solid prototype objects which has the distinct advantage of creating almost

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any shape or form that even the best machining and tooling methods may

not be able to achieve. During the ensuing development, virtual prototyping

goes through a number of stages that eventually turns designs into fully

testable three dimensional models.All the rapid prototyping machines being

used slowly form the three dimensional models by putting together thin,

two-dimensional layers one at a time. The three dimensional manifestation

of the virtual design is formed from the bottom up. Models are formed on an

elevator-like platform from virtual CAD designs. The platform is lowered a

layer-height at a time once a layer is completed. The thinner the layer, the

smoother the finish will be on the completed prototype model. Once the

model is completely formed, it may be sanded, plated or painted, depending

on material used.Rapid prototyping technologies can either be a "dry" or a

"wet" process. Most machines create prototype models by solidifying some

sort of loose powder, liquid, or semi-liquid material. A machine may be able

to cut through adhesive-coated sheets of prototype fabrication material. The

dry powdered materials can either be some sort of polymer, powdered metal,

or wax. Some machines may even be able to use starch as the building

material for forming the prototype model.Some of the powders used may also

require a binder. The liquid materials mainly used are usually

photosensitive polymers that solidify when exposed to either a laser or

ultraviolet (UV) light. Wet rapid prototype Shenzhen methods generally

require a curing phase.

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2.1.3.1 : Fused-Deposition Modeling

The Fused Deposition Modelling (FDM) process constructs three-

dimensional objects directly from 3D CAD data. A temperature-controlled

head extrudes thermoplastic material layer by layer.

The FDM process starts with importing an STL file of a model into a pre-

processing software. This model is oriented and mathematically sliced

into horizontal layers varying from +/- 0.127 - 0.254 mm thickness. A

support structure is created where needed, based on the part's position

and geometry. After reviewing the path data and generating the

toolpaths, the data is downloaded to the FDM machine.

The system operates in X, Y and Z axes, drawing the model one layer at a

time. This process is similar to how a hot glue gun extrudes melted beads

of glue. The temperature-controlled extrusion head is fed with

thermoplastic modelling material that is heated to a semi-liquid state.

The head extrudes and directs the material with precision in ultrathin

layers onto a fixtureless base. The result of the solidified material

laminating to the preceding layer is a plastic 3D model built up one

strand at a time.

Once the part is completed the support columns are removed and the

surface is finished.

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FDM process

Figure : 2.1.3.1

2.1.3.2 : Stereolithography

Stereolithography is an additive manufacturing process using a vat of

liquid UV-curable photopolymer "resin" and a UV laser to build parts a layer

at a time. On each layer, the laser beam traces a part cross-section pattern

on the surface of the liquid resin. Exposure to the UV laser light cures,

solidifies the pattern traced on the resin and adheres it to the layer below.

After a pattern has been traced, the SLA's elevator platform descends by a

single layer thickness, typically 0.05 mm to 0.15 mm (0.002" to 0.006").

Then, a resin-filled blade sweeps across the part cross section, re-coating it

with fresh material. On this new liquid surface, the subsequent layer

pattern is traced, adhering to the previous layer. A complete 3-D part is

formed by this process. After building, parts are cleaned of excess resin by

immersion in a chemical bath and then cured in a UV oven.

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Stereolithography requires the use of support structures to attach the part

to the elevator platform and to prevent certain geometry from not only

deflecting due to gravity, but to also accurately hold the 2-D cross sections in

place such that they resist lateral pressure from the re-coater blade.

Supports are generated automatically during the preparation of 3-

D CAD models for use on the stereolithography machine, although they may

be manipulated manually. Supports must be removed from the finished

product manually; this is not true for all rapid prototyping technologies.

2.1.3.3 : Selective Laser Sintering, SLS

Selective Laser Sintering (SLS) is a layer additive production process that

creates three dimensional objects using a CO2 laser to melt, or sinter, and

fuse selective powder molecules based on information supplied by a

computer aided design (CAD) file. Selective laser sintering (SLS) was born

out of the University of Texas and is a popular process used in rapid

prototyping and product development. The selective laser sintering (SLS)

technology was brought to the forefront of commercialization by DTM

Corporation which is now called 3D Systems. The powder material that is

fused during the selective laser sintering (SLS) process is commonly called

thermoplastic material or, in some cases, thermoplastic binders for use in

metals. The selective laser sintering (SLS) technology allows for these

materials to be fused together in tiny layers ranging between .003” and

.006”. This allows selective laser sintering (SLS) to create parts with

accurate details and tolerances comparable to stereolithography (SLA).

However, selective laser sintering (SLS) has an added benefit in that the

strength and durability of the parts it creates is much better. Additionally,

the selective laser sintering (SLS) process makes parts that have longer

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stability than stereolithography (SLA) and selective laser sintering (SLS)

parts won‟t lose their shape or post cure over time.

There are a variety of different types of materials available for use in the

selective laser sintering (SLS) process. The most beneficial characteristic of

selective laser sintering (SLS) is how durable and functional the materials

are. These durable and functional selective laser sintering (SLS) materials

include DuraForm and DuraForm glass-filled (GF) which are nylon based

materials that create plastic prototypes. Other selective laser sintering

(SLS) materials are CastForm used for investment casting patterns,

selective laser sintering (SLS) Flex for elastomeric, rubber-like parts, and

selective laser sintering (SLS) LaserForm which makes metal prototypes.

Additionally, there is continual research and development going on to bring

new selective laser sintering (SLS) materials to market. Each of these

selective laser sintering (SLS) materials requires little to no post build

processing to be ready to use which cuts out several steps in post processing

of selective laser sintering (SLS) parts as compared to stereolithography

(SLA). However, all of the selective laser sintering (SLS) materials can be

finished in multiple ways to meet the desire or needs of selective laser

sintering (SLS) users. Among other types of post processing, selective laser

sintering (SLS) parts can be sanded, painted, plated, tapped, or even

machined. This allows for a higher grade of smoothness and appearance to

selective laser sintering (SLS) parts and assemblies and also gives users an

unlimited number of ways to use selective laser sintering (SLS) parts.

Other advantages of selective laser sintering parts (SLS) are:

Parts and/or assemblies that move and work that have a good surface

finish and feature detail

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Selective laser sintering (SLS) gives the capability of flexible snaps

and living hinges as well as high stress and heat tolerance

Wide variety of materials such as flexible and rigid plastics,

elastomeric materials, fully dense metals and casting patterns

Tight dimensional tolerances all the way down to thousandths of an

inch

Finishing capabilities that include painting for presentations, tapping

or threading for use and inserts for assemblies

Rapid delivery time of most parts and/or patterns in a few days

Selective Laser Sintering (SLS) process has evolved into a common option for

the creation of end-use production parts. The large assortment of different

plastics and metals have made it quicker and less costly to use selective

laser sintering (SLS) as opposed to other methods of manufacturing such as

tooling. Selective laser sintering (SLS) is especially attractive when a design

is complex or customized and the total part production requirements are low.

Finally, the selective laser sintering (SLS) technology is well suited for use

in rapid tooling. Rapid tooling, or RT, is generally different from

conventional tooling in the following key areas:

Rapid tooling is generally produced faster than conventional tooling,

taking off as much as 80-90% of the time it takes to create first parts.

This is where the speed of selective laser sintering (SLS) comes into

play as parts can be created in days as opposed to weeks

Rapid tooling is typically delivered at a lower cost compared to

conventional tooling – as much as 90-95% less

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Tolerances for rapid tooling are usually not as accurate as

conventional tooling but selective laser sintering (SLS) allows for

customization of specs to meet customer needs

Rapid tooling life is considerably less than a conventional tool

In spite of these differences, for many applications, rapid tooling using the

selective laser sintering (SLS) process is ideal for first run parts or short run

prototype production until conventional tooling methods can be obtained.

Selective laser sintering (SLS) is an avenue that cuts a products time to

market down considerably and, since the process can be customized, allows

for design changes to be made without having to create a new tool. Harvest

Technologies has multiple options for selective laser sintering (SLS) rapid

tooling, as well as others, so please allow us to consult with you on the best

choice for your requirements.

2.1.3.4 : Ballistic

The BPM personal modeler came with all hardware and software

enclosed in one compact unit.

The BPM is controlled by a DOS based 486 powered PC which is

housed within the unit.

The BPM utilized ink jet or droplet based manufacturing techniques,

where it builds the models by firing micro-droplets of molten wax

material from a moving nozzle or jet onto a stationary platform, the

platform then lowers and the process is repeated for each layer of the

model.

The part is built as a hollow shell.

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The parts can be scaled, rotated, or translated to a desired

orientation. This is performed on a 5 axis workstation

BPM parts are not intended to be finished, however they may be

painted.

BPM Process

BPM employs a technology called digital Microsynthesis.

1.) In the first step of the process, molten plastic is fed to a piezoelectic

jetting mechanism, almost like those of inkjet printers.

2.) Next a multi-axis controlled NC (Numerical Control) system shoots

tiny droplets of material onto the target, using the jetting mechanism.

3.) Last, small droplets freeze upon contact with the surface, forming

the surface particle by particle.

BPM Uses

BPM parts are mainly used for concept visualization. Due to the

weakness of the material, the parts aren‟t well equipped for use as

functional components.

BPM parts are useful during the design process

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BPM Advantages

Requires minimal post-processing.

Low toxicity.

Minimal power consumption.

Low cost of cost and materials.

Ability to perform in microgravity and vacuum environments.

BPM has no size constraints.

The process allows use of virtually any thermoplastic. Because of this,

there are no heath hazards involved.

BPM Disadvantages

Parts produced lack strength and durability.

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2.1.3.5 : Laminated

A manufacturing process that uses a carbon-dioxide laser to create

successive cross-sections of a three-dimensional object from layers of paper

with a polyethylene coating on the backside. The first step is to create a base

on which the paper can attach itself to. This is done by placing a special tape

down onto the platform. A sheet of paper is fed through with the aid of small

rollers. As the paper is fed through, a heated roller is used to melt the

coating on the paper so that each new layer will adhere to the previous layer.

The carbon-dioxide laser then cuts the outline of the cross-sectional pattern

into the top layer of paper. Once the laser is done cutting the pattern, it

creates a border around the build that contains the desired part. This

enables the part to stay intact as each new layer is created. Once the border

has been cut, the laser then proceeds to create hatch marks, or cubes that

surround the pattern within the border. The cubes behave as supports for

the part to ensure that no shifting or movement takes place during the

entire build.

When the build is completed, the part, attached to the platform, needs to be

removed from the LOM. Depending on the size of the part, the block to be

removed may take more than one person to remove the build from the LOM.

After the part has been successfully removed from the LOM, it must then be

removed for the actual platform. Again this may take the work of more than

one individual. A wire is used and placed between the part and the platform

to "cut" the part away from the metal platform.

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The border, or frame of the part is then removed. The next step involves

decubing. or removing the supports. Often times the supports can be

removed from simple shaking the part; other times it is necessary to use a

chisel to pry the cubes away from the part.

When all of the cubes have been removed, the unfinished part is sanded

down and a lacquer is used to seal the part. Being that LOM parts are made

for paper, humidity and temperature affect the structure and composure of

the part if it is not coated; hence, the lacquer serves as a protective measure.

The LOM is very useful in manufacturing large parts quickly.

Highlights of Laminated Object Manufacturing

• Layers of glue-backed paper form the model.

• Low cost: Raw material is readily available.

• Large parts: Because there is no chemical reaction involved, parts can be

made quite large.

• Accuracy in z is less than that for stereolithography and selective laser

sintering.No milling step.

• Outside of model, cross-hatching removes material

• Models should be sealed in order to prohibit moisture.

• Before sealing, models have a wood-like texture.

• Not as prevalent as stereolithography and selective laser sintering.

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Laminated Object Manufacturing

The figure below shows the general arrangement of a Laminated Object

Manufacturing (LOM™, registered trademark by Helisys of Torrance,

California, USA) cell:

General arrangement of a Laminated Object Manufacturing

Figure : 2.1.3.5

Material is usually a paper sheet laminated with adhesive on one side, but

plastic and metal laminates are appearing.

1. Layer fabrication starts with sheet being adhered to substrate with

the heated roller.

2. The laser then traces out the outline of the layer.

3. Non-part areas are cross-hatched to facilitate removal of waste

material.

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4. Once the laser cutting is complete, the platform moves down and out

of the way so that fresh sheet material can be rolled into position.

5. Once new material is in position, the platform moves back up to one

layer below its previous position.

6. The process can now be repeated.

The excess material supports overhangs and other weak areas of the part

during fabrication. The cross-hatching facilitates removal of the excess

material. Once completed, the part has a wood-like texture composed of the

paper layers. Moisture can be absorbed by the paper, which tends to expand

and compromise the dimensional stability. Therefore, most models are

sealed with a paint or lacquer to block moisture ingress.

UNDERSTANDING DIRECT MANUFACTURING AND RAPID TOOLING

3.1:Basic Methodology for RP Process

Methodology of Rapid Prototyping

The basic methodology for all current rapid prototyping techniques can be

summarized as follows:

1. A CAD model is constructed, then converted to STL format. The

resolution can be set to minimize stair stepping.

2. The RP machine processes the .STL file by creating sliced layers of the

model.

3. The first layer of the physical model is created. The model is then

lowered by the thickness of the next layer, and the process is repeated

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until completion of the model.

4. The model and any supports are removed. The surface of the model is

then finished and cleaned.

Method

A general procedure for adopting the rapid prototyping method is outlined

below.

1. Allow enough time to create the prototype. If the prototype is to be

evaluated with users then allow time to design relevant tasks, recruit

the users, evaluate the prototype and report the results.

2. Assemble the necessary equipment, including the hardware and

software tools necessary to create the interactive prototype.

3. Develop the prototype itself.

4. Select appropriate users to test the prototype, trying to cover the

range of users within the target population. A facilitator will also be

required to instruct the users and run the evaluation.

5. Prepare realistic tasks to occupy the users as they work with the

prototype.

6. Pilot the evaluation procedure and ensure the prototype can be used

to accomplish the tasks.

7. Ensure recording facilities are available and functioning.

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8. Conduct each session. The facilitator instructs the user to work

through the allocated tasks, interacting with, and responding to, the

system as appropriate.

9. If necessary additional information can be obtained by interviewing

users following their use of the prototype. Debrief and thank the user.

10. Analyse the obtained information and then summarise the

observations and user evaluations. Determine the themes and

severity of the problems identified.

11. Summarise design implications and recommendations for

improvements and feed back to design team. Video recordings can

support this.

12. Where necessary refine the prototype and repeat the above process.

When using this method, avoid spending too long on the development of

initial prototypes as user evaluation may result in substantial changes. Also,

avoid making the prototype too polished as this may force users to accept it

as finished. Do not put in features that will raise the users expectations but

which are unlikely to be achieved with the real system (e.g. too fast response

times, too sophisticated graphics) and do not put too much effort into

particular features (e.g. animations) which may not be required.

Be aware that the method requires software development skills. Also,

although rapid, the method can often be more time consuming than other

approaches and that resources required are greater than paper and pens due

to the need for software and hardware.

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3.2:Rapid Tooling

Rapid Tooling (RT) describes a process that is the result of combining Rapid

Prototyping techniques with conventional tooling practices to produce a mold

quickly or parts of a functional model from CAD data in less time and at a

lower cost relative to traditional machining methods. Rapid Tooling can act

as a bridge to production injection molded parts.

Rapid Tooling (RT) typically, either uses a Rapid Prototyping (RP) model as

a pattern or uses the Rapid Prototyping process directly to fabricate a tool

for a limited volume of prototypes.

Expensive tooling cost can be well justified just when the production

quantity is massive. Actually the way to produce tooling quicker and more

economically, especially for small quantity manufacturing becomes a

significant question. Additionally, in the product development cycle, requires

always some intermediate tooling to produce a small quantity of prototypes

or functional tests, samples for marketing, evaluation purpose, or production

process design. RT becomes more and more important to nowadays

manufacturing industry.

The main advantages are tooling time is much shorter than for a

conventional tool. Time to first articles can be less than one-fifth that of

conventional tooling; tooling cost is much less than for a conventional tool.

Cost can be below five percent of conventional tooling cost.

The main challenges are tool life is less than for conventional tools and

tolerances are wider than for conventional tools.

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3.2.1 : BENEFITS OF RAPID INJECTION TOOL MOLDING

¤ Prototypes in production material in little time.

¤ Allows for full fit and function testing.

¤ Discover any design imperfection on early stage.

¤ Low cost tooling allows for facilitate design refinement and modification.

¤ Preceding production process- molds can produce thousands of parts.

New stations rapid milling CNC -

CharlyRobot

presenting attractive costs can produce

molds or prototypes in Fiberglass,

aluminum and Composites or fast

production of products in extremely short

time in dimensions:

3100 mm x 2100 mm x 450 mm

even to fraction larger molds modeling.

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3.2.2:Advantages Of Rapid Tooling For Manufacturing

Shorten the Tooling Lead-Time - Normal development time is

shortened from months to a few days or weeks.

Low Cost - reduced Cost allowing real trials affordable.

Allows functional test of parts on initial design stage.

Data CAD Direct Transfer - Many imperfections due to drawings

misinterpretation can be avoided using the original CAD model all

through the RP process and then along to RT process.

Due to short tooling manufacture time and low cost in using RT,

many engineers prefer to produce parts for functional test in the early

design stage. As a result, many design faults are debugged before

production, so many design failures are avoided.

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3.2.3:RAPID TOOLING CONSTRAINTS

The most rapid RP systems are still too slow and are limited as they can't

produce parts in a wide range of materials, at a fast enough rate, to perform

the enormous range of industry requirements. Despite the vast progress in

direct part fabrication, even conventional processes such as molding and

casting are still the only means available.

RP is frequently the technological path making these manufacturing

processes faster, cheaper and better. Certainly, the tooling fabrication Is

actually the most important application of direct manufacturing.

The two main ways to make tooling molds using Rapid Prototyping are

directly fabricated by an RP system, or indirect or secondary processes RP-

generated parts that can be used as patterns for a mold fabricating.

3.2.4:DIRECT FABRICATION PROCESSES

Molding and casting are specialized rapid prototyping processes that have

been developed to meet specific application and material requirements.

Stereolithography or selective laser sintering are normally forms of basic RP

processes and methods developed for specific applications.

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3.2.5:INDIRECT OR SECONDARY PROCESSES

Despite RP materials properties improve and expand continuously, a

limitless array of applications means will always request to transfer parts

fabricated in a material employed in an RP process into another material. As

usually it is necessary to use very specific materials to make most tools,

several material transfer technologies have been developed.

3.2.6:RAPID TOOLING CHOICE METHODS

The clear result is that there are a different number of paths to obtain a

final functional part or tool starting from a CAD definition. The choice

depends on:

¤ The purpose

¤ Volume to be produced

¤ Final material and accuracy requirements

¤ Applied rapid prototyping process

Numerous other factors may influence choices since most technologies are

emergent, have significant limitations, and there are usually several

competing alternatives.

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3.2.7:TYPES OF RAPID TOOLING PROCESS

Low Volume (from tens to hundreds)

¤ Soft Tooling

¤ Reaction injection molding

¤ Bridge Tooling - Direct Access Injection Molding

Intermediate (from hundreds to thousands)

¤ Metal filled Epoxy Tooling

¤ Powdered Metal Tooling

¤ Space Puzzle Molding

3.2.8:CONNECTED SERVICES

¤ Simulation

¤ Consulting and information about simulation techniques

¤ Numerical simulation of manufacturing processes

¤ Numerical simulation of the mechanical and thermal component behavior

¤ Topology and shape optimization

¤ Rapid Prototyping

¤ Consulting and technology transfer

¤ Prototyping of parts

¤ Manufacturing of mold insert.

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Enclosure

figure 5.1.1:Example of Rapid Prototyping

Figure 5.1.2:Example of Parts of Rapid Prototyping

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Figure 5.1.3:Example of Fused Deposition Modelling

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Figure 5.1.4:Example Stereolithiography

Figure 5.1.5: 3-D Printing Process

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Figure 5.1.6:Laminated Object Material

Figure 5.1.7:Example of Rapid Tooling

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Figure 5.1.8:Example of Rapid Tooling