msharizanJJ204 1 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.
This note is reference to basic of mechanical engineering where student available to follow the knowledge in Industrial Revolution era, a workshop may be a room or building which provides both the area and tools.
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msharizanJJ204
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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
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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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.