Department of Mechanical Engineering

Manufacturing Technology Lab

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METAL CASTING

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I. Metal Casting Lab PATTERN DESIGN

Exp. No. 1 Date:

AIM: -

To make the pattern with allowances for the given sketch of a casting.

APPARATUS:

1. Planning tools, 2. sawing tools, 3. Marking and lay out tools.

DESCRIPTION OF PATTERN: A pattern may be a replica model of the desired casting which when packed or embedded in a suitable moulding material produces a cavity called mould. A pattern may be differing from an actual component. That is it carries additional allowance, draft allowance like shrinkage allowance, machining allowance, draft allowance, it carries additional projection like core prints etc.

SKETCH:

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CALCULATIONS: Draft angle=40

Vertical side length of casting=hIncrement in length due to draft angle =L+δL+δL =L+2δLWhere δL=L+2×h×tan 40

Increment in breadth due to draft angle=b+ δb+ δb =b+2δbWhere δb=b+2×h×tan 40

PROCEDURE:

1. Calculate the dimension considering draft angle.2. Mark the dimensions on given wooden work piece.3. Shape the work piece to the calculated size.4. Finish the surface of pattern

PRECAUTIONS: Top surface of pattern must match with parting surface.

RESULT: The pattern is prepared by considering draft angle 40.

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SAND PROPERTIES TESTING

Sand preparation: Tests are conducted on a sample of the standard sand. The moulding sand should be

prepared exactly as is done in the shop on the standard equipment and then carefully enclosed in a closed container to safeguard its moisture content.

Moisture content: Moisture is an important element of the moulding sand as it affects many properties. To

test the moisture of a moulding sand a carefully weighted test sample of 50g is dried at a temperature of 1050C to 1100C for 2 hours by which time all the moisture in the sand would have been evaporated. The sample is then weighted. The weight difference in grams when multiplied by 2 would give the percentage of moisture contained in the moulding sand. Alternatively a moisture teller can also be used for measuring the moisture content. In this the sand is dried by suspending the sample on a fine metallic screen and allowing hot air to flow through the sample. This method of drying completes the removal of moisture in a matter of minutes compared to 2 hours as in the earlier method.

Another moisture teller utilizes calcium carbide to measure the moisture content. A measured amount of carbide in a container along with a separate cap consisting of measured quantity of moulding sand is kept in the moisture teller care has to be taken before closing the apparatus that carbide and sand do not come into contact. The apparatus is then shaken vigorously such that the following reaction takes place. CaC2+2H2O→C2H2+Ca (OH) 2. The acetylene (C2H2) coming out will be collected in the space above the sand raising the pressure. A pressure gauge connected to the apparatus would give directly the amount of acetylene generated which is proportional to the moisture present. It is possible to calibrate the pressure gauge to directly read the amount of moisture.

Clay content: The clay content of moulding sand is determined by dissolving of washing it off the sand.

To determine the clay percentage a 50g sample is dried at 105 to 1100C and the dried sample is taken in a one litre glass flask and added with 475 ml of distilled water and 25ml of a one percent solution of caustic soda (NaOH 25g per litre). This sample is thoroughly stirred. After the stirring, for a period of five minutes the sample is diluted with fresh water up to a 150mm graduation mark and the sample is left undisturbed for 10 minutes to settle. The sand settles at the bottom and the clay particles washed from the sand would be floating in the water. 125mm of this water is siphoned off the flask and it is again topped to the same level and allowed to settle for five minutes. The above operation is repeated till the water above the sand becomes clear, which is an indication that all the clay in the moulding sand has been removed. Now the sand is removed from the flask and dried by heating. The difference in weight of the dried sand and 50g when multiplied by two gives the clay percentage in the moulding sand.

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Sand grain size: To find out the sand grain size, a sand sample which is devoid of moisture and clay such

as the one obtained after the previous testing is to be used. The dried clay-free sand grains are placed on the top sieve of a sieve shaker which contains a series of sieves one upon the other with gradually decreasing mesh sizes. The sieves are shaken continuously for a period of 15min. After this shaking operation, the sieves are taken apart and the sand left over on each of the sieve is carefully weighed.

The sand retained on each of the sieve expressed as a percentage of the total mass can be plotted against the sieve number as in figure. To obtain the grain distribution. But more important is the Grain Fineness Number (GFN) which is a quantitative indication of the grain distribution. To calculate the grain fineness, each sieve has been given a weight age factor as shown in table. The amount retained on each sieve is multiplied by the respective weight age factor, summed up, and then divided by the total mass of sample, which gives the grain fineness number. The same can be expressed as GFN=ΣMifi/ΣfiMi =Multiplying factor for the i th sieve, fi=amount of sand retained on the ith sieve.

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PERMEABILITY TESTING

The rate of flow of air passing through a standard specimen under a standard pressure is termed as permeability number.

The standard permeability test is to measure time taken by a 2000cm3 of air at a pressure typically of 980 pa to pass through a standard sand specimen confined in a specimen tube. The standard specimen size is 50.8mm in diameter and a length of 50.8mm. Then the permeability number P is obtained byP=VH/p ATWhere V= volume of air=2000cm3H= height of the sand specimen=5.08cmP= air pressure, g/cm2A= C/S area of sand specimen=20.268cm2T= time in minutes for the complete air to pass through the gapsInserting the above standard values in to the expression, we getP=501.28/pT

Specimen preparation: Since the permeability of sand is dependent to a great extent, on the degree of ramming, it

is necessary that the specimen be prepared under standard conditions. To get reproducible ramming conditions, a laboratory sand rammer is used along with a specimen tube. The measured amount of sand is filled in the specimen tube, and a fixed weight of 6.35- 7.25kg is allowed to fall on the sand three times from a height of 50.8}0.8mm. to produce this size of� specimen usually sand of 145 to 175g would be required.

After preparing a test sample of sand as described, 2000cm3 of air are passed through thesample and the time taken by it to completely pass through the specimen is noted. Then from the above equation the permeability number can be calculated.

Strength: Measurement of strength of moulding sands can be carried out on the universal sand

strength testing machine. The strength can be measured in compression, shear and tension. The sands that could be tested are green sand. Dry sand of core sand. The compression and shear test involve the standard cylindrical specimen that was used for the permeability test.

Green compression strength: Green compression strength or simply green strength generally refers to the stress

required to rupture the sand specimen under compressive loading. The sand specimen is taken out of the specimen tube and is immediately (any delay causes the drying of the sample which increases the strength) put on the strength testing machine and the force required to cause the compression failure is determined. The green strength of sands is generally in the range of 30 to 190kPa.

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Green shear strength: With a sand sample similar to the above test, a different adapter is fitted

in the universal machine so that the loading now be made for the shearing of the sand sample. The stress required to shear the specimen along the axis is then represented as the green shear strength. The green shear strengths may vary from 10 to 50 kPa.

Dry strength: The tests similar to the above can also be carried with the standard specimens dried

between 105 and 1100C for 2 hours. Since the strength greatly increases with drying, it may be necessary to apply larger stresses than the previous tests. The range of dry compression strengths found in moulding sands is from 140 to 1800 kPa, depending on the sand sample.

Mould hardness: The mould hardness is measured by a method similar to the Brinell hardness test. A

spring loaded steel ball with a mass of 0.9kg is indented into the standard sand specimen prepared. The depth of indentation can be directly measured on the scale which shows units 0 to 100. When no penetration occurs, then it is a mould hardness of 100 and when it sinks completely, the reading is zero indicating a very soft mould.

Besides these, there are other tests to determine such properties as deformation, green tensile strength, hot strength, expansion, etc.

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SHEAR STRENGTH OF MOULDING SAND

Exp. No. Date:

Aim:-To determine the shear strength of the moulding sand containing various amounts of clay on moisture

Apparatus used:-

1. Universal sand testing machine 2. Shear shockles 3. Sand rammer specimen tubee 4. Stripper 5. Balance etc

Theory:-

It is necessary for the moulding sand to process adequate strength to enable the preparation of the mould.In addition it should retain the strength during pouring of the molten metal and also during solidification. The ability of the sand particles to resist shear stress, rupture usually occurs parallel to the axis of the specimen.

Procedure:- The experiment may be conducted in two parts a) Vary clay content and keep water content constant. b) Vary water content and keep clay content constant.

1. Weighed quantities of sand and clay is mixedthoroughly (by hand asmulles) for 3 minutes. Then to this dry mixture,calculated amount of water is added and again mixed thorouhly .The specimen is prepared as discussed earlier in the permeability test

2. remove the specimen from the specimen tubee by the strippes part and place

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the same in b/w the shear shackles which are fixed in universal testing machine

3. rotate the hand wheel of the apparatus slowly to activate the ram which applies pressure to the specimen gradually till the specimen breaks

4. the univeral sand testing machine has three scales.they are for compression strength, shear strength and tensile strength

5. read the shear strength vaule from the guage and note down the reading6. repeat the above procedure for specimen containing various percentage of

clay & water

a) Water content = 6%

b) Water content = 4%

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Sl No Weight of sand in Grams

% of caly % of water Shear strength

1 150 4 62 150 6 63 150 8 6

Sl No Weight of sand in Grams

% of caly % of water Shear strength

1 150 4 42 150 4 6

3 150 4 8

Result:- As the clay percentage increases shear strength increases and as the water percentage increases shear strength decreases

PERMEABILITY TEST Exp. No. Date:

Aim: - To find the effect of water content, dry content on green permeability of green sand

Materials used:- Base sand,clay,and water

Appratus: - 1.Beam balance2. Stop watch3. Sand rammer4. permeability meter5. weighting pan6. pecimen tubes

Theory: 1. Molten metal always contain certain amount of dissolved gases which are

evolved when the metal starts freezing 2. Also the molten coming in contact with moist sand generates steam (or) water

vapour.3. Permeability is one of the most important property affecting the

characteristics of mould which depends upon the given size, grain shape moisture level and degree of compactness.

4. Permeability is a physical property of moulding sand misoture which allows gases to pass through it easily

5. The permeability No. Pn can be found out by Pn=VH/PAT V=volume of air passing through spe P=pressure as read from the manometer gm/cm2 H=height of speacimen T=time in min for 2000c at air passed through the sand specimen

PROCEDURE:1. Conduct the experiment in two parts .To the first ease very water content

keeping clay constant

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2. Take weighed proportions of sand and clay and dry mix them together for 3min

3. Fill the sand mixture into the specimen tube and ram trice .using sand rammer the specimen containing the within limits represent the standard specimen required

4. Now the prepared standard specimen is having a diameter 50 mm and height 50 mm

5. Place the standard specimen along with the tube in the inverted position6. Operate the value and start the stop watch simultaneity when the zero mark

on the inverted jar first touches the top at water tank ,note down the manometer reading

7. Note down the lines required to pas 2000c of air through the specimen ,calculate the permeability number by using thee formula

Base sand taken = 140 gmClay = 4% constant

Sand taken = 140 gmClay = 5% constant

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Sl No % of water

Pressure (Gm / Cm2)

Time (min) P N No.

1 4 2.32 5 1.83 6 1.6

Sl No % of Clay

Pressure (gm / Cm2)

Time (min) PN No.

1 0.42 0.53 0.6

RESULT:-As the percentage of water increases permeability increases and the

percentage of clay increases permeability decreases

COMPRESSION STRENGTH TEST FOR MOULDING SAND

Exp. No. Date:

AIM: - To determine the mean compression strength of the given concrete sand sample containing different amount of water and clay.

APPARATUS:1.Measuring jar2.AFF-Hammer3.Specimen4.Compression & halks5.Universal sand testing machine

PROCEDURE: The experiment is conducted in two partsi. Vary the clay content and keep water content constant.ii. Vary water content and keep clay content constant1. Weighed quantity of sand and clay in mixwd thoroughly for 3min. Then to this dry mixture calculated amount of water is added and again mixed thoroughly.2. Remove the specimen from specimen tube before stripper part and place the same in between the shaleels which all mixed in the universal testing machine.3. Rotate the hand wheel of the apparatus slowly to activate the which applies pressure to the specimen gradually till the specimen breaks.4. The universal testing machine has 3 scales.These all compression shear and tensile strength.5. Read the compression strength value from the gauge and note down the reading.6. Repeat the above procedure for specimen containg various percentage of clay and water.

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a) Water content = 6%

b) Water content = 4%

RESULT: Compression strength decreases with increase in the % of water and increase with increase in the ………% of clay

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Sl No Weight of sand in Grams

% of caly % of water Shear strength

1 145 3 92 145 6 93 145 9 9

Sl No Weight of sand in Grams

% of caly % of water Shear strength

1 145 6 32 145 6 6

3 145 6 9

WELDING

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II. WELDING SPOT WELDING

Exp. No. Date:

Aim: - To study the working of the spot welding machine and weld the sheets

given.Introduction: -

Electric arc welding process is a fusion welding process where only heat is applied in the joint by creating an arc between the weld metal and the electrode (filler rod) tip.

In contrast, resistance welding process is a fusion welding process where both heat and pressure are applied on the joint but no filler metal or flux is added. The heat necessary for the melting of the joint is obtained by the heating effect of the electrical resistance of the joint and hence, the name resistance welding. Spot welding is one of the resistance welding processes.

Principle: -

In resistance welding a low voltage and very high current (about 1500A) is passed through the joint for a very short period (Ex 0.25s). This high amperage heats the joints due to the contact resistance at the joint and melts it. The pressure on the joint is continuously maintained and the metal fuses together under this pressure. The heat generated in resistance welding can be expressed as:

H = K I2 R t Where H = the total heat generated in the work, JI = electric current, At = time in seconds for which the current is passing through the joint, SR = the resistance of the joint, ohms

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and K = a constant to account for the heat losses from the welded joint.

Electrode materials

The electrode in resistance welding carry very high current required for fusion, as also transmit the mechanical force to keep the plates under pressure and in alignment during fusion. They also help to remove the heat from the weld zone thus preventing overheating and surface fusion of the work. For both these purposes, the electrodes should have higher electric conductivity as well as higher hardness.

Steels though strong do not have the conductivity required for electrodes. Hence copper in alloyed form is generally used for making electrodes. Though pure copper has high electrical and thermal conductivities it is poor in mechanical properties.

Copper-Cadmium (0.5 to 1%) alloys have the highest electrical conductivity with moderate strengths and therefore are used for welding non ferrous materials such as aluminum and magnesium alloys.

Copper -Chromium (0.5 to 0.8%) alloys have slightly lower electrical conductivity than above but better mechanical strength. These are used for low strength steels such as mild steel, G I steel and low alloy steels.

Cobalt and Beryllium alloys are used for stainless steel, Tungsten and other alloy steels.

Electrode tips are available in number of different shapes.

Result:-

Hence studied the working of the spot welding machine and how to weld the given sheets

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ARC WELDING-1Exp. No. Date:

AIM: Preparation of double welded lap joint as shown in fig using shielded metal arc welding.

MATERIAL REQUIRED: 2 M.S. flat of given size.

TOOLS REQUIRED: 1. Welding transformer, 2. Connecting cables, 3. Electrode holder, 4. Ground clamp, 5. Electrodes, 6. Chipping hammer, 7. Welding shield etc.

THEORY:Arc welding is fusion welding process. Arc welding is a process of joining two metallic pieces by the application of heat is obtained from the electric arc between two electrodes. In this process two metallic pieces will act as base metal or parent metal and electrode will act as filler metal. The electrode is coated with flux which prevents oxidation of parent metals.

LAP JOINT: The lap joint is used in joining two overlapping plates so that edge of each plate iswelded to the surface of the other. The overlapping portion is called lap. The width of lap may be 3 to 5 times the thickness of the plates to be welded. Welds usually run each side of the lap. No edge preparation is required for a lap joint.

Sketch : -

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PROCEDURE:1. The given metallic pieces are prepared to given sizes by filling.2. The metallic pieces are thoroughly cleaned from rest, grease, oil etc.3. Now given metallic pieces were assembled as shown in fig. select the electrodes, based

on thickness of metal piece and hold it in the electrode holder. 4. Switch on the supply and initiate the arc by either striking arc method or drag.5. Tack welding to be done before full welding.6. The full welding process is carried after completion of one pass slag is removed from

the full weld bed with help of chipping hammer and metallic wire brush.7. Then the above process is repeated until to reach desired height of the weld.

PRECAUTIONS:1. Use goggles, gloves in order to protect the human being. 2. Maintain constant arc length to have uniform weld bead.

RESULT: Lap joint is prepared as shown in fig by using arc welding process.

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ARC WELDING-2

Exp. No. Date:

AIM: preparation of butt joint as shown in figure using shielded metal arc welding process.

Sketch: -

MATERIAL REQUIRED: 2M.S Flat pieces of given size.

TOOLS REQUIRED: welding transformer, connecting cables, electrode holder, ground clamp,

electrodes, hipping hammer, welding shield etc.

THEORY: Arc welding is a fusion welding process. Arc welding is the process of joining two

metallic pieces by the application of heat, where heat is obtained from the electric arc between

two electrodes.

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In this process two metallic pieces will act as base metal or parent metal and electrode

will act filler metal. The electrode is coated with flux which prevents oxidation of parent metals.

Butt joint: the butt joint is join the ends or edges of plates or surfaces located approximately in

the same plate with each other. Preparation of edge varies according to the thickness of the

material and welding process used. Light gauge section requires only 90 sheared edges with no

spacing between them. Materials ranging from 9 to 13mm thick that can be welded only from

one side should be reduced either as a single V or single U. However thicker plates are prepared

from both sides. The U shaped type of joint is more satisfactory and requires less filler material

than V type groove. However it is more expensive to prepare U shape.

PROCEDURE:

1. The given metallic pieces filled to the desired size.

2. On both pieces beveled in order to have V groove.

3. The metallic pieces are thoroughly cleaned from rust grease, oil, etc.

4. The metallic pieces are connected to terminals of Trans former.

5. Select electrode dia based on thickness of work piece and hold it on the electrode holder.

Select suitable range of current for selected dia.

6. Switch on the power supply and initiates the arc by either striking arc method or touch and

drag method.

7. Take welding to be done before full welding.

8. In full welding process after completion one part before going to second part. Slag is removed

from the weld bed. With the metal wire brush or chipping hammer.

9. Then the above process will be repeated until to fill the groove with weld bed or weld metal.

PRECAUTIONS:

1. Use goggles, gloves in order to protect the human body.

2. Maintain the constant arc length.

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RESULT: butt joint is prepared as shown in figure by using arc-welding process.

TUNGSTEN INERT GAS WELDING (TIG)Tungsten inert gas (TIG) welding or gas tungsten arc welding (GTAW) is an inert gas

shielded arc welding process using non-consumable electrode. The electrodes may also contain 1 to 2 % thoria (thorium oxide) mixed along with the core tungsten or tungsten with 0.15 to 0.40 % zirconia (zirconium oxide).the pure tungsten electrodes are less expensive but will carry less current. The thoriated tungsten electrodes carry high currents and are more desirable because they can strike and maintain a stable arc with relative ease. The zirconia added tungsten electrodes are better than pure tungsten but inferior to thoriated tungsten electrodes.

A Typical tungsten inert gas welding setup is shown in fig. it consists of a welding torch at the centre of which is the tungsten electrode. The inert gas is supplied to the welding zone through the annular path surrounding the tungsten electrode to effectively displace the atmosphere around the weld puddle. The smaller weld torches may not be provided with circulating cooling water.

The TIG welding process can be used for the joining of a number of materials though the most common ones are aluminium, magnesium and stainless steel. The typical combination of TIG setups to be used with these and other metals are presented in table.

Sl No

Material Electrodes Power supply used

Preferred Shielding gas

1 Stainless steel thoriated tungsten DCEN Argon

2Aluminium Alltypes AC Argon

3 Magnesium Tungsten AC Argon

4 DeoxidisedCopper Argon

5 Monel Thoriated tungsten DCEN Argon

6 High carbon steel Thoriated tungsten AC or DCEN Argon

Power sources:

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The power sources used are always the constant current type. Both direct current (DC) and alternating current (AC) power supplies can be used for TIG welding. When DC is used, the electrode can be negative (DCEN) or positive (DCEP). With DCEN more heat is generated near the work piece and consequently the electrode does not get heated to a great extent. But when DCEP is used, a large amount of heat is liberated at the electrode itself thereby limiting the maximum current that can be carried by an electrode. Roughly, the current carrying capacity of a DCEN electrode is about 10 times as high as that of a DCEP electrode.

The DCEP is sometimes utilized to break down the oxides on the surface of the metals such as aluminium. The electrons from the oxide layer move towards the positive electrode weakening the surface layer. The positively charged ions from the electrode would then be able to easily break the surface layer and thus would help in obtaining the fusion. Similarly, when AC is used, the half cycle during which the electrode is positive, the electrons from the oxide layer would be moving towards the electrode, whereas in the other half the electrons from the electrode would be able to easily break the oxide layer on the work piece surface. Thus, an AC arc welding is likely to give rise to a higher penetration than that of DCEP. Hence, DCEP is normally used for welding thin metals whereas for deep penetration welds DCEN is used. DCEPalso causes larger heat affected zones and more weld distortion than DCEN.

The DC power supply used for TIG can be either a steady one or more often a step pulsed one. In the case of the step pulsed current machine, the current level is maintained at two levels, as shown in fig. the low level is called background currents which is used for cooling the weld metal. The other is the peak current used when the actual melting (welding) takes place. During the background current period the arc is maintained but very small heat input goes to the weld and, as a result, the arc crater cools. This type of step pulsed DC source is, particularly, useful for welding in out of positions (other than flat position) since it allows for the controlled heating and cooling. Otherwise, the electrode is to be flipped away slightly from the arc crater to allow for the cooling of the puddle before it is moved forward again. But the pulsed DC arc welding provides for proper solidification during the background current period when the torch is moved forward for forming the next spot (bead), as shown in fig.

When the alternating current (AC) is used for TIG welding, the current continuously changes its direction. It changes its direction 50 times every second (in 50Hz power supply) such that half the time it is operating as DCEN and rest, as DCEP. A typical AC wave form is shownin fig. which is termed as balanced wave since the positive side and the negative side are identical in magnitude. But the TIG welding machine would not behave as normal AC. During the period when the electrode is positive, the electron move from flat work piece surface to the small sized tip of the electrode which restricts the flow of electrons. This is termed as ‘Rectification’ and is responsible for the reduced current flow during DCEP portion of the AC wave as shown in fig. This is known as unbalanced wave.

This rectification is an AC cycle during the time when the electrode becomes positive will make the AC arc, a highly unstable one. To maintain a steady arc in an un balanced AC welding machine, a very high voltage, very high frequency and low current power supply is superimposed on the unbalanced wave. This maintains the shielding gas ionized during the period, when the electrode is positive and thus maintains the arc continuously. There are quite afew advantages of an un balanced AC arc welding machine compared to a balanced wave machine. It is less expensive. Since less current flows when the electrode is positive, less heat is liberated near the electrode. This permits a higher current carrying capacity for the electrodes, which results in better penetration.

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It is possible to provide a balanced wave as shown in fig. by incorporating a large number of capacitors in series to provide the necessary current discharges during the time when the electrode is positive. These capacitors get charged during the time when the electrode is negative. A balanced wave maintains a steady arc and therefore is preferred for better removal ofthe oxide layer is possible. However, these machines are more expensive compared to the un balanced type.

ELECTRODES:The tungsten electrodes used for the welding should be clean and completely free from any kind of contamination such as molten filler metal.

If the arc is started by first touching the base metal and withdrawn, the electrode tip may pick up the base metal which causes the subsequent sputtering and loss of metal in the electrode tip. Also, the electrode may get consumed quickly if it is allowed to be oxidized, since tungsten oxide has a lower melting temperature. The oxidation occurs when the electrode is allowed to cool in the atmosphere after welding. Hence, the shielding gas flow should be maintained for some time after extinguishing the arc so that the electrode gets sufficiently cooled in a protective atmosphere rather than in the oxidizing normal atmosphere.

The tungsten electrode tip should be prepared for proper weld penetration. The typical shapes that can be used are shown in fig. Though it is possible to use these electrodes without any tip preparation, it would be better to prepare the tip since it enhances the weld quality. For AC welding with high frequency (AC-HF) un balanced machines ,the tip should be pencil pointed as shown in fig. so that the HF current gets concentrated and the arc is easily initiated (high frequency current tend to flow through the surface). Also, once the arc is formed which reduces the effect of current rectification and thus, stabilises the AC arc.

With DCEN, the electrode would be made conical as in fig. while grinding, the tip concentricity should be maintained , otherwise the gas flow would become uneven making some part of the puddle not properly shielded and thereby, causing contamination of the weld joint in that portion. Pure tungsten electrodes are never made into conical point since the end is likely to melt and contaminate the weld metal. Instead, it is better to make full round ball at the tip, as shown in fig.WELDING TECHNIQUE:The welding technique used for TIG is essentially similar to that of the gas welding. The edge preparation is also similar to that of gas welding. Backing of the joint is sometimes preferable, to provide good performance and uniformity of the weld. The metallic backing plates used are provided with a small groove of a depth of the order of 0.4 mm near the root, with the width being about 3 to 4 times the depth. The backing plate is removed after the welding is over.

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Mechanical Press Working

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III. Mechanical Press Working HYDRAULIC PRESS WORK

BENDINGBending refers to the operation of deforming a flat sheet around a straight axis where the

neutral plane lies. The disposition of the stresses in a bent specimen is shown in fig 1. Here, due to the applied forces, the top layers are in tension and bottom layers are in compression as shown. The plane with no stresses is called the neutral axis. The neutral axis should be at the centre when the material is elastically deformed. But when the material reaches the plastic stage, the neutral axis moves downward, since the materials oppose compression much better than tension. The nomenclature normally used with bending is shown in figure: 2. in a bent specimen, since neutral axis remains constant, it is the required length. Beyond the bend lines, the material is not affected. Hence to calculate the length required, it is necessary to find out the bend allowance which is the arc length of the neutral axis between the bend lines.

Bend allowance B=α(R+Kt)Where α=bend angle, in radiansR=inside radius of the bend, in mmK=location of neutral axis from bottom surface= 0.33 when R<2t= 0.5 when R>2tt = sheet thickness, in mm

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The outer layers which are under tension should not be stretched too much; otherwise there is likelihood of rupture taking place. The amount of stretching depends on the sheet thickness and the radius. Lower the bend radius. Lower the bend radius, higher is the strain in this zone. Hence there is a minimum bend radius to be specified, depending on the material characteristics. Minimum bend radius = 0.5 t soft materials.

= t other materials= 3 t spring materialsThe other aspect to be considered in designing parts from bending is the grain orientation

of the sheet which is bent. As far as possible, the bending is to be done in a direction perpendicular to the grain direction in the metal. By virtue of the rolled sheets being used for bending, the grain direction usually is along the length axis, being the direction of rolling. There is a possibility of cracks appearing at the time of bending if the bending is done along the grains as shown in figure:3. But if two bending are to be done on the same sheet at right angles, then it may be desirable to make them at 450 to the grain direction so that the risk of cracking is minimized. Spring back in bending is difficult to estimate theoretically .but it is essential to compensate it, because the bend geometry gets affected by the spring back directly. The types of bending methods used are shown in figure:4. The first one is the wiping die which is used for simple 900 bends only. Here the work is held firmly to the die, and the punch bends the extended portion of the blank as shown in figure 4. The V bending shown in fig b, can be used fro a wide range of angles as also the 900. These are the ones that are most generally used.

The bending load may be calculated from the knowledge of material properties and the die characteristics as shown below.Fb= KLst2/WWhere Fb=bending force, NK= 1.33 for die opening of 8t= 1.20 for die opening of 16t= 0.67 for U bending= 0.33 for a wiping dieL = Length of the bent part, mms = ultimate tensile strength, MPat = blank thickness, mmW = width between the contact points, mm

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Exp. No. 9 Date:Aim To study the working of the press and to do bending of job

Sketch

1) Sketch of the equipment 2) Sketch of the job

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Procedure

Write the theory of bending with sketches

Result: -

Hence studied the study the working of the press and make the bending of given job material.

Questions

1) List various types of presses and explain the working of hydraulic press with line diagram.

2) What do you understand by punching operation?

Processing of Plastics

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IV. Processing of Plastics INJECTION MOULDING

The polymer analogue of die casting for metals is the most widely used technique for fabricating thermoplastic materials.

The correct amount of pelletized material is fed from a loading hopper in to a cylinder by the motion of a plunger or a ram. This charge is pushed forward in to a heating chamber, at which the thermoplastic material melts to form a viscous liquid. Next , the molten plastic is impelled, again by ram motion ,through a nozzle in to the enclosed mould cavity ;pressure is maintained until the molding has solidified .Finally ,the mould is opened ,the piece is ejected ,the mould is closed .and the entire cycle is repeated .Probably y the most outstanding feature of this technique is the speed with which pieces may be produced.

Thermosetting polymers may also be injection moulded; curing takes place while the material is under pressure in a heated mould, which results in a longer cycle times than for thermoplastics.

BLOW MOULDING

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Blow moulding process for the fabrication of plastic containers is similar to that used for blowing glass bottles (Fig ).First a length of polymer tubing is extruded. While still in a semi molten state, the length is placed in to a two piece mould having the desired container configuration. The hallow piece is formed by blowing or steam under pressure in to the parsion, forcing the tube walls to conform to the counters of the mould.

Casting like metals, polymeric materials may be cast, as when a molten plastic material is poured in to a mould and allowed to solidify. Both thermoplastic and thermosetting plastics may be cast .For thermoplastics, solidification occur upon cooling from the molten state; however, for thermosets, harding is a consequence of the actual polymerization or curing process.

Exp. No. 10 Date:

Aim: -

To study the process of injection moulding of plastics and make a desired casting.

Introduction

Plastic materials are classified into two types (1) Thermoplastics and (2) Thermosetting plastics based on the processing methods.

Thermoplastics

These are the plastics that can be softened and melted by heat and can be formed into the required shape when they are hot. It is possible to recycle thermoplastics.

Example: Polyamides’ . Polyethylene

Acetyls Polypropylene.

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Cellulosic’s Polystyrene

Polycarbonate P V C

Thermosetting plastics

These are the plastics that cannot be melted once they are solidified. The raw materials for thermosetting are usually called resins. They are mixed and placed in the mould, and heated and compressed during which process the materials achieve the strength and hardness. These materials cannot be recycled.

Example: Name of Plastic Trade Name

Epoxy Melamine Araldite Phenol formaldehyde Urethane Bakelite

Sketch

Description of the equipment and the process with a sketch of the machine, die and job done.

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Result: -

Hence studied study the process of injection moulding of plastics and make desired casting .

Questions

1) Explain working of screw type injection moulding machine.2) Explain blow moulding of plastics with a sketch.

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