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Content1. Welding

2. Casting

3. Metrology

4. Unconventional

5. Other Machine tools

6. Jig Fixtures

7. NC CNC Robotics

8. Material Science

9. Metal Cutting, Metal forming, metrology

10. All PPTs

IES 2009 Conventional

Explain the three types of oxy-acetylene flames.

Indicate with the help of sketches the various

zones, respective temperature ranges and

applications of each type of flame.

[ 20 Marks]

Three types of flames can be obtained by varyingthe oxygen/acetylene (or oxygen/fuel gas) ratio.

If the ratio is about 1 : 1 to 1.15 : 1, all reactions arecarried to completion and a neutral flame is produced.

Most welding is done with a neutral flame. It ischemically neutral and neither oxidizes or carburizesthe metal being welded.

Oxy-acetylene gas welding neutral flame

A higher ratio, such as 1.5 : 1, produces an oxidizingflame, hotter than the neutral flame (about 3300oC)but similar in appearance.

Used when welding copper and copper alloys butharmful when welding steel because the excess oxygenreacts with the carbon, decarburizing the regionaround the weld.

Oxy-acetylene gas welding Oxidising flame

Excess fuel, on the other hand, produces a carburizingflame. Carburizing flame can carburize metal also.

The excess fuel decomposes to carbon and hydrogen,and the flame temperature is not as great (about3000oC).

Flames of this type are used in welding Monel (anickel-copper alloy), high-carbon steels, and somealloy steels, and for applying some types of hard-facingmaterial.

Oxy-acetylene gas welding Carburizing flame

Metal Flame

M S N

High carbon steel R

Grey cast iron N, slightly oxidizing

Alloy steel N

Aluminium Slightly carburizing

Brass Slightly oxidizing

Copper, Bronze N, slightly oxidizing

Nickel alloys Slightly carburizing

Lead N

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Always use the special wrench or key provided by thesupplier to open and close acetylene cylinder valvesnot provided with hand wheels. Such keys shouldalways be left on the cylinder while in use in case of anemergency need to shut down

Five gases used for gas welding

1. Acetelyne (C2H2) gives 50 MJ/Kg

2. Hydrogen (H2) 11 MJ/m3

3. Propane (Gasol) C3H84. Propylene (THERMOLENE) C3H6

5. LPG

Combustion of oxygen and acetylene (C2H2) in a welding torch produces a temp. in a two stage reaction.

In the first stage

+ Heat

This reaction occurs near the tip of the torch.

In the second stage combustion of the CO and H2 and occurs just beyond the first combustion zone.

2CO + O2 2CO2 + Heat

H2 + O2 H2O + Heat

Oxygen for secondary reactions is obtained from the atmosphere.

CH O CO H 2 2 2 22

12

IAS-2011 MainDraw a self explanatory sketch of oxy-acetylene gas

cutting torch. Briefly explain how cutting is

effected.

[20-Marks]

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For thicker plates with specified contour, shearingcannot be used and oxy-fuel gas cutting (OFC) isuseful.

Gas-cutting is similar to gas welding except torch tip.

Fig- differences in torch tips for gas welding and gas cutting

Contd

Oxygen Torch Cutting (Gas Cutting) Iron and steel oxidize (burn) when heated to a

temperature between 8000C to 10000C.

High-pressure oxygen jet (300 KPa) is directed againsta heated steel plate, the oxygen jet burns the metal andblows it away causing the cut (kerf).

For cutting metallic plates shears are used. These areuseful for straight-line cuts and also for cuts up to 40mm thickness.

Contd

For complete oxidation 0.287 m3 oxygen/kg of iron isrequired

Due to unoxidized metal blown away the actualrequirement is much less.

Torch tip held vertically or slightly inclined in thedirection of travel.

Torch position is about 1.5 to 3 mm vertical from plate.

Contd

The drag lines shows the characteristics of the movementof the oxygen stream.

Drag is the amount by which the lower edge of the dragline trails from the top edge.

Good cut means negligible drag.

Fig- positioning of cutting torch in oxy- fuel gas cutting

Contd

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If torch moved too rapidly, the bottom does not getsufficient heat and produces large drag so very roughand irregular-shaped-cut edges.

If torch moved slowly a large amount of slag isgenerated and produces irregular cut.

Contd

Gas cutting is more useful with thick plates.

For thin sheets (less than 3 mm thick) tip size shouldbe small. If small tips are not available then the tip isinclined at an angle of 15 to 20 degrees.

Fig. Recommended torch position for cutting thin steel

IFS-2011What is meant by low -hydrogen electrode ?

[2-marks]

Low Hydrogen Electrode The basic coatings contain large amount of

calcium carbonate (limestone) and calciumfluoride (fluorspar) and produce low hydrogen.

But it can absorb moisture therefore coated lowhydrogen electrodes are backed before use to atemperature of 200oC to 3000C and stored in anoven at 110oC to 150oC

Other types of electrode release large amount ofhydrogen, which can dissolve in the weld metaland lead to embrittlement or cracking.

IFS-2011What is the maximum output current that can bedrawn at 100% duty cycle from a welding power sourcerated at 600A at 60% duty cycle.

[3-Marks]

Duty Cycle The percentage of time in a 5 min period that a

welding machine can be used at its rated outputwithout overloading.

Time is spent in setting up, metal chipping, cleaningand inspection.

For manual welding a 60% duty cycle is suggested andfor automatic welding 100% duty cycle.

Contd

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Required duty cycle,

Where ,T = rated duty cycle

I = rated current at the rated duty cycle

Io = Maximum current at the rated duty cycle

2

aa

IT T

I

IAS-2013

Fig. The desired temperaturedistribution across theelectrodes and the workpieces in lap resistancewelding.

Fig. Typical current and

pressure cycle for resistance

welding. The cycle includes

forging and post heating

operations.

The overall resistance in the welding circuits can be

quite low, high currents are generally required to

produce a resistance weld. Power transformers convert

the high-voltage, low-current line power to the high-

current (up to 100,000 A) low-voltage (0.5 to 10 V)

power required for welding. While smaller machines

may utilize single-phase circuitry. Time for a cycle will

be 10 to 100 milliseconds.

Fig. The arrangement of the electrodes and the work in spotwelding, showing design for replaceable electrode tips.

Projection welding Limitations of spot welding.

1. Electrode condition must be maintainedcontinually, and only one spot weld at a time.

2. For additional strength multiple welds needed.

Projection welding (RPW) overcomes abovelimitations.

Contd

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Dimples are embossed on work pieces at the weldlocations and then placed between large-areaelectrodes, and pressure and current applied like spotwelding.

Current flows through the dimples and heats themand pressure causes the dimples to flatten and form aweld.

Fig. Principle of projection welding, (a) prior to application of current and pressure (b) and after formation of welds

Contd

Projections are press-formed in any shape.

Multiple welds at a time.

No indentation mark on the surface.

Bolts and nuts can be attached to other metal parts.

IES 2007What is the principle of resistance welding?

Indicate where the resistance is maximum in spot

welding operation.

[ 2 marks]

Flash Welding

It is similar to upset welding except the arc rather than

resistance heating.

One pieces is clamped with cam controlled movable

platen and other with is fixed platen.

Contd

Two pieces are brought together and the power supply isswitched on. Momentarily the two pieces are separatedto create the arc to melt the ends of the two pieces.Then again the pieces are brought together and thepower switched off while the two ends are fused underforce. Most of the metal melted would flash outthrough the joint and forms like a fin around the joint.

Faster than upset welding.

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Advantages

1. Butt welding is possible

2. Impurities and contaminants are squeezed out during this operations so good quality welding

Applications

For Butt joint only

Current and Voltage

1. Current very low (1 to 10 A)

2. Voltage very high ( 10 kV to 1500 kV)

Heat is generated from the are as the ends of the twomembers begin to make contact. An axial force isapplied at a control rate then weld is formed by plasticdeformation (Upsetting) of the joint, so flash isneeded.

IES 2007 ConventionalTwo steel sheets of thickness one mm are weldedby resistance projection welding technique. Acurrent of 30,000 A for 0005 second is made toflow. The effective resistance of joint can be takenas 100 micro ohms. The joint can be considered asa cylinder of diameter 5 mm and height 15 mm.The density of steel is 000786 gm/mm3. The heatneeded for welding steel is 10 J/mm3. Calculate theefficiency of welding. [20]

IFS-2011Discuss with figure the various steps required for

friction welding, mentioning at least two methods

of control.

[5-marks]

Friction Welding

Heat is obtained by the friction between the ends of

the two parts to be joined.

One part is rotated at a high speed and other part is

axially aligned and pressed tightly against it.

Friction raises the temperature of both the ends. Then

rotation is stopped abruptly and the pressure is

increased to join.

Contd

Machine is similar to a centre lathe.

Power requirements 25 kVA to 175 kVA.

The axial pressure depends on the strength and

hardness of the metals being joined.

Pressure 40 MPa for low-carbon steels to as high as 450

MPa for alloy steels.

Contd

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Very efficient.

Wide variety of metals or combinations of metals canbe joined such as aluminium to steel.

Grain size is refined

Strength is same as base metal.

Only round bars or tubes of the same size, orconnecting bars or tubes to flat surfaces can join.

One of the components must be ductile.

Friction welding is a solid state welding.

A low contact pressure may be applied initially topermit cleaning of the surfaces by a burnishing action.

ContdFig- friction welding process

IAS-2014

Plasma Arc Weld (PAW)

Similar to GTAW except the plasma caused by the arc is

constricted by a water-cooled orifice

Uses ionized gas jet (plasma) to cut materials resistant tooxy-fuel cutting,

High velocity electrons generated by the arc impact gasmolecules, and ionize them.

The ionized gas is forced through nozzle, and the jet heatsthe metal, and blasts the molten metal away.

Capable of high welding speeds where size permits

Argon is used as the shielding gas.

Advantage

Plasma arc has directional Stability, work to torch distance is not critical and arc length can vary.

Lower heat input and lower filler metal needed

No edge preparation needed

Limitation

Expensive equipment

Restricted to flat and horizontal positions only

Maximum thickness limited 25 mm

Large amount of ultraviolet and infrared rays are emitted.

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Application

Stainless steel

Nickel based alloy

Suitable for refractory metal coating like alumina on graphite nozzles for rockets.

Welding design and defectWelding Problem Causes

Cracking of weld metal High joint rigidity

Cracking of base metal Excessive stresses

Spatter Arc blow

Distortion Poor joint selection

Slag inclusion Improper cleaning in multi-

pass welding

Porosity Excessive H2, O2, N2, in the

welding atmosphere or Damp

electrodes

Lamellar Tearing inclusions such as Mn Fe and S in the base metal and/or residual stress

Cracks occur when localized stresses exceed theultimate tensile strength of material.

These stresses are developed due to shrinkage duringsolidification of weld metal.

Cracks may be developed due to poor ductility of basemetal, high sulphur and carbon contents, high arctravel speeds i.e. fast cooling rates, too concave orconvex weld bead and high hydrogen contents in theweld metal

Cold-cracking in steel weldments depends on

1. Carbon equivalent

2. Heat input

3. Effective thickness

3. Hydrogen content in weld pool

PREHEATING is done to prevent hot cracking.

2 216060 23.4375100a a

IT T T TI

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IES 2011 Conventional Enumerate four defects caused due to residual stresses in

welded joints.

[2 Marks]

Ans.

1. Distortion

2. Cracking in the base metal

3. Lamellar Tearing

4. Reduction of fatigue strength

What are the advantages and disadvantages of a- c welding machine?

Advantages:1. Welding transformer and its controller is very much cheaper as compared to D.C

set.2. No rotating parts so less of wear and tear.3. Troublesome magnetic fields causing arc blow is eliminated.4. Efficiency is slightly more than DC setup.

Disadvantages:1. Covered electrodes must be used. The AC arc cannot be used satisfactorily for bare

wire or lightly coated rods as the DC arc.2. Higher voltage is to be used , consequently risk of shock is also more as compared

to DC Welding.3. AC welding machines have moderate penetration.4. More diamter is required to have more AC current to get more filler material

deposit rates and faster welding speeds.5. Welding of cast iron, bronze and aluminium cannot be done using AC set up.

If the pressure is not applied properly or insufficient pressure is applied then porosity may develop at the center of the nugget or cracks may be formed.Stuck welds may be formed i.e. welds with unacceptable low bond strength.

If excessive pressure is applied then results in Weld expulsion Welds with low structural strength and low cosmetic quality.

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Effect of welding speed on grain structure:Low speed tend to allow growing columnar grain tofollow the arc, curving in behind the moving heatsource also has grain refining effect.High speed welds tend to produce solidificationpattern in which columnar crystals grow in parallel,straight rows to the weld centreline. This grainstructure tends to be weaker under stress.High weld speed produce finer cell spacing than slowspeed welding.

A consumable electrode in a gas shield.Arc is between workpiece and an automatically fed bare-wire electrode.Argon, helium, and mixtures of the two can be used.Any metal can be welded but are used primarily with thenon-ferrous metals.When welding steel, some O2 or CO2 is usually added toimprove the arc stability and reduce weld spatter.

ApplicationsFast and economical.A reverse-polarity dc arc is generally used because of itsdeep penetration, spray transfer, and ability to producesmooth welds with good profile

TIGArc is established between a non-consumable tungsten electrodeand the workpiece.Tungsten is alloyed with thorium or zirconium for better current-carrying and electron-emission characteristics.Arc length is constant, arc is stable and easy to maintain.With or without filler.

Very clean welds.

All metals and alloys can be welded. (Al, Mg also)

Straight polarity is used.

Weld voltage 20 to 40 V and weld current 125 A for RPDC to 1000

A for SPDC.

Shielded Gas: Argon

Torch is water or air cooled

DIFFUSION WELDINGIt is a solid state welding process which producescoalescence of the faying surfaces by the applicationof pressure and elevated temperatures (about 50 to80% of absolute melting point of the parentmaterials) for a time ranging from a couple of minutesto a few hours.Produces high quality bonds with good strength withlittle or no distortion.Can join very dissimilar materials.A solid filler metal may or may not be inserted.Materials welded for aircraft and rocket industry:Boron, Titanium, Aluminium, Ceramic, Composite,Graphite, Magnesium etc.

IAS-2011

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Submerged Arc welding (SAW)

A thick layer of granular flux is deposited just ahead of

a bare wire consumable electrode, and an arc is

maintained beneath the blanket of flux with only a few

small flames being visible.

A portion of the flux melts. Molten flux and flux

provides thermal insulation, slows cooling rate and

produce soft, ductile welds.

Contd

Most suitable for flat butt or fillet welds in low

carbon steel (< 0.3% carbon).

The process is not recommended for high-carbon

steels, tool steels, aluminum, magnesium,

titanium, lead, or zinc.

Characteristic of submerged arc welding

High speeds,

High deposition rates,

Deep penetration,

High cleanliness (due to the flux action).

Advantages

Wire electrodes are inexpensive.

No weld spatter.

Nearly 100% deposition efficiency.

Lesser electrode consumption.

Limitations

Extensive flux handling,

Contamination of the flux by moisture.

Large-grain-size structures.

Welding is restricted to the horizontal position.

Chemical control is important

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How do zirconium and thorium affect the tungstenelectrode in the GTAW process? What is weld decayin Ni Cr steels ?

IES-2010

WELD DECAYDuring welding of steel ; formation of chromium carbidealong the grain boundaries may take place.This results in the depletion of chromium percentage inthe adjoining region of grain boundary.If this depletion of Chromium percentage is more than12% which is needed to maintain a passive layer then theregion will be susceptible to corrosion, resulting inintergranular attack.Intergrannular corrosion causes loss of metal in theregion that parallels the weld deposit. This corrosionbehaviour is called as weld decay.

The tungsten electrode which is often alloyed with thorium orzirconium to provide better current-carrying and electron-emissioncharacteristics .

Is it possible to weld tantalum to steel, if yes, bywhich method ? Explain the term hot cracks inwelding and write four important causes.

IES-2010 Yes, tantalum can be welded to steel by using explosivewelding.Hot cracks are caused byJoint designRestraint imposed on weldHot cracks are also caused by low melting constituents,that extend the temperature range of low hot strength andlow ductility to temperatures below that of the alloy. Eg, insteel presence of phisphides and sulfides and copper canseggregate grainboundaries and cause cracking.Weld beads with high depth to width ratio can promotethe build up of low melting phases at pool centerline andthus cause hot cracking.

Explosion Welding

Done at room temperature in air, water or vacuum.

Surface contaminants tend to be blown off the surface.

Typical impact pressures are millions of psi.

Well suited to metals that is prone to brittle joints

when heat welded, such as,

Aluminum on steel

Titanium on steel

Contd

Important factors are,

Critical velocity

Critical angle

The cladding plate can be supported with tack weldedsupports at the edges, or the metal inserts.

Contd

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Typically the detonation velocity should not exceed120% of the sonic velocity in the metal.

Contd

High velocity explosives, 4572-7620 m/s. TNT RDX PETN Composition B Composition C4 Datasheet Primacord

Medium velocity explosives, 1524-4572 m/s Ammonium nitrate Ammonium perchlorate Amatol Nitroguonidine Dynamites diluted PETN

Contd

Advantages,

Can bond many dissimilar, normally unweldablemetals

The lack of heating preserves metal treatment

The process is compact, portable, and easy to contain

Inexpensive

No need for surface preparation

Contd

Disadvantages,

The metals must have high enough impact resistance,and ductility (at least 5%)

The cladding plate cannot be too large.

Noise and blast can require worker protection, vacuumchambers, buried in sand/water.

Contd

Typical applications:

Very large plates can be cladded.

Joins dissimilar metals.

(titanium to steel, Al to steel, Al to Cu etc.)

Join tube to tube sheets of large heat exchangers.

Contd

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Discuss short circuiting metal transfer in GMAWmentioning its suitability. Also define the termtransition current, with figure.

IES-2010

Fig (a) shows the initiation of arc. Under the intense heat of arcelectrode tip melts away and forms a globule of molten metal at the tip.Fig(b) As the electrode wire is fed towards the work piece, the moltentip touches the weld metal pool andFig (c) when the tip touches the metal pool short circuiting takes place,that short circuits electrode to the workpiece. This reduces Voltageacross the arc.Fig (d) The metal tip gets pinched by the surface tension of the weldmetal pool as well as the magnetic force due to current flow. Finally themetal is pinched away and the arc gets ignited again, and the cycle isrepeated all over again.

TRANSITION CURRENT:At a current above the criticalvalue called the transitioncurrent transfer higlhlydirected stream of discretedroplets of metal in the form ofspray occurs.Below transition currenttransfer mode becomesglobular and above tansitioncurrent it is spray transfer.Spray Transfer is achieved byhigh current and largerdiameter of electrode wire.

Casting

IES 2010 In metal casting define the terms chaplet and resin

binder. Write the merits and demerits of shellmoulding process.

Chaplet: Chaplets are used to support cores inside themould cavity to take care of its own weight and overcomethe metallostatic forces. Since some of this metal willmelt during the operation. Since they ultimately becomepart of the final casting, chaplets must be made from thesame alloy as that being cast.

Resin binder : Resign binder is a thermosetting phenolicresin (phenol formaldehyde) which acts as a binder. Whenheated pattern is come in contact with fine sand and resignmixture a shell is produced.

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Merits

Good dimensions tolerance

Good surface finishing

High production rate.

Demerits

Part size limited

Expensive pattern and equipment required.

IES-2010Explain why the strength to weight ratio of die cast parts increases with decreasing wall thickness.

Density is another property engineers may considersespecially for compact, high efficiency and light weightapplication. Material strength helps determine if adesign made out of die cast metal can be slimmeddown for weight saving, and hence when decrease asthickness increase. Strength to weight ratio of die cast part.

IES-2010Explain the term stack molding.

Stack moulding are high production plastic injectionmoulds with multiple parting line, with stackmoulding we can produce multiple injection mouldedplastic parts more economically over a largeproduction run, stack moulding uses small machinesthat stack vertically take up less space, reduce run cost.

Advantage of stack moulding

Increase output efficiency

Fewer machines requires

Multiple parts designs produces simultaneously.

IAS2011

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Runner: A runner is commonly a horizontal channel

which connects the sprue with gates, thus allowing the

molten metal to enter the mould cavity. The runners

are of larger cross-section and often streamlined to

slow down and smooth out the flow, and are designed

to provide approximately uniform flow rates to the

various parts of the mould cavity. Runners are

commonly made trapezoidal in cross-section.

Contd

Liquid shrinkage and solid shrinkage Liquid shrinkage refers to the reduction in volume

when the metal changes temperature from pouring tosolidus temperature in liquid state. To account for this,risers are provided in the moulds.

Solidification shrinkage refers to the reduction involume when metal changes from liquid to solid stateat the solidus temperature. To account for this, risersare provided in the moulds.

Solid shrinkage is the reduction in volume caused,when a metal loses temperature in the solid state. Theshrinkage allowance is provided to take care of thisreduction.

Gray CI with a carbon equivalent of 4.3% has

negative shrinkage, that is, it actually expands

upto 2.5% because of graphite precipitation. So,

for this, no riser is needed.

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During solidification nucleation event produces a crystal or grain and latent heat removed.

During cooling thermal vibrations reduces.

Metallurgical defects Hot tears or hot cracking, cause of this defect is that

stresses and strains built up during solidification are toohigh compared to the actual strength of the semisolidmaterial. This type of defects occurs in the lower part ofthe solidification range, close to the solidus, when thealloy has a wide solidification temperature range and asmall amount of liquid, when the solid fraction is morethan 0.9, the hot tearing is easy to occur. Proper moulddesign prevents this type of defect.

The mis-run and cold shut defects are caused either bya lower fluidity of the mold or when the sectionthickness of the casting is very small. Fluidity can beimproved by changing the composition of the metaland by increasing the pouring temperature of themetal.

Principle of riser design

Riser size, shape and location, as well as the type ofconnection between the riser and casting.

Riser Size: Freezing time or riser or casting depends upon theamount of heat in a casting (Directly) and depends inversely uponthe surface area of the casting. Based on this facts many relationshave been suggested by different scientists. The riser should alsobe designed to conserve the metal.

Riser Shape: Riser should tall enough so that any shrinkage cavityin the riser ( pipe formation ) does not penetrate into the castings.The shrinkage cavity must lie above the neck. The neck should beas short as possible and also solidify longer than the casting.

Riser location: It should be located so that directionalsolidification occurs from the extremities of the mould cavity backtowards riser. Since the thickest regions of a casting will be last tofreeze, the riser should be feed directly into these locations.

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IES 2011 Conventional A round casting is 20 mm in diameter and 50 mm in

length. Another casting of the same metal is elliptical in

cross section, with a major to minor axis ratio of 2, and

has the same length and cross-sectional area as the

round casting. Both pieces are cast under the same

conditions. What is the difference in the solidification

times of the two castings ? [10 Marks]

2 2

Areaofellipse

Circumference 3 3 3

2 /2 (approx.)

ab

a b a b a b

a b

Conventional Question ESE 2003

Compare the solidification time of two optimum side

risers of the same volume with one has cylindrical shape

and other is parallopiped. [30 Marks]

Modulus Method It has been empirically established that if the modulus

of the riser exceeds the modulus of the casting by a

factor of 1.2, the feeding during solidification would be

satisfactory.

MR = 1.2 Mc

Modulus = volume/Surface area

In steel castings, it is generally preferable to choose a

riser with a height-to-diameter ratio of 1.Contd

22

4D

D

Conventional Question IES-2008 Calculate the size of a cylindrical riser (height and diameter

equal) necessary to feed a steel slab casting of dimensions

30 x 30 x 6 cm with a side riser, casting poured horizontally

into the mould.

[Use Modulus Method]

[10 - Marks]

Caines MethodFreezing ratio = ratio of cooling characteristics of casting to the riser.

The riser should solidify last so x > 1

According to Caine X =

Y = and a, b, c are constant.

ac

Y b

riser

casting

VV

Casting

Riser

AV

XAV

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Table: Constants in Caines Method

Conventional Question IES-2007 Calculate the size of a cylindrical riser (height and

diameter equal) necessary to feed a steel slab

casting of dimensions 25 x 25 x 5 cm with a side

riser, casting poured horizontally into the mould.

[Use Caines Method]

[ For steel a = 0.10, b = 0.03 and c = 1.00 ]

This method is a simplification of Caine'smethod. In this method, freezing ratio is replacedby Shape Factor.The shape factor is defined asShape Factor= (Length+Width)/ThicknessThe underlying argument is that calculatingvolumes and surface areas is too complicated andtherefore simplification would be desirable. Thelength, width and thicknesses are computed fromthe maximum dimensions of the casting section.

Naval Research Laboratory Method

Procedure for getting riser size is as follows:1. Calculate the shape factor for the given casting.2. Obtain riser volume to casting volume ratio from the

graph . (or the table provided in questions)3. Calculate riser volume Vr .4. For cylindrical riser (h=D), Vr = (.D

3 )/45. Obtain the diameter.

For circular plates, the length and width are same as thatof the diameter.But for cylinders, the width and thickness are same as thediameter for calculating the shape factor.But for calculating the riser volume actual casting volumeis to be used.

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The other shape of interest is hollow cylindrical shape. In these the heat removal is restricted , a correction factor k, needs to be applied to get the effective plate thickness. If T is the true wall thickness

Corediameter

0.5T T 2T 4T

Correctionfactor

1.17 1.14 1.02 1.00

Shape Factor = (Length+ width)/(k.T)

Calculate the height of cylindrical riser(height=diameter) necessary to feed the steel slab casting 25 x 25 x 5 with a side riser, casting poured horizontally into the mold.Solution:Shape Factor = (25+25)/5 = 10From graph at shape factor 10 (riser volume/casting volume) is 0.47.Riser Volume(Vr ) = (riser volume/casting volume) x casting volumeRiser Volume = 0.47 x 25 x 25x 5 =1468.75 cm3

For cylindrical riser of height = diameterVr = (.D

3 )/41468.75= (.D3 )/4D = 12.32 cm

Example

The dimensions of a cylindrical sideriser(height = diameter) for a 25 cm x 15 cm x 5cm steel casting are to be determined. For thetabulated shape factor values given below, thediameter of the riser (in cm)________

Shape Factor 2 4 6 8 10 12

Riser volume / Casting Volume

1.0 0.70 0.55 0.50 0.40 0.35

GATE-2015

25 15Shape Factor = 85

From the given table, for shape factor of 8,ratio of Riser volume to casting volume is 0.5,

Volume of riser = 0.5 x casting volume0.5 x 25 x 15 x 5 =937.

length widththickness

3

3

3

5 cmFor a cylindrical riser with height = diameter

4

937.5 10.604

DV

D D cm

Normally the risers are located at the heaviest sections and they themselves act as feeders for thin sections. But when smaller sections are connected to thicker sections, the riser should have larger volume to cater this appendage. The total volume of the casting is taken as the volume of the main section plus the effective percentage of the appendage volume, called the parasitic volume.

GRAPH-2

Calculate the risering requirement for the casting shown in fig.

Example

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First neglect the branch and calculate shape factor for main plate ;

Shape Factor = (25+12.5)/5 = 7.5

From graph at shape factor 7.5 (riser volume/casting volume) is 0.575

Riser Volume(Vr ) = (riser volume/casting volume) x casting volume

Riser Volume = 0.575 x 25 x 12.5x 5 =898.437 cm3

The branch Volume=2.5 x 2.5 x 10 =62.5cm3

This is plate feeding the bar with thickness ratio (2.5/5) of 0.5.

From the graph we get parasitic volume of 30%

Hence, riser volume = 0.3 x 62.5 + 898.437=917.185cm3

Vr = (.D3 )/4 or 917.185= (.D3 )/4 or D = 10.53 cm

Grain fineness test

Permeability test

Sand mould strength test

Moisture Content test

Clay content test

hardness test

Types of Gate or In-gateTop gate: Causes turbulence in the mould cavity, it is prone

to form dross, favourable temperature gradient towards the

gate, only for ferrous alloys.

Bottom gate: No mould erosion, used for very deep moulds,

higher pouring time, Causes unfavourable temperature

gradients.

Parting Gate: most widely used gate, easiest and mosteconomical in preparation.

Step Gate: Used for heavy and large castings, size of ingatesare normally increased from top to bottom.

Pouring Metal DefectsThe likely defects in this category are

Mis-runs and

Cold shuts

A mis-run is caused when the metal is unable to fillthe mold cavity completely and thus leaves unfilledcavities.

A cold shut is caused when two streams while meetingin the mold cavity, do not fuse together properly thusforming a discontinuity in the casting.

Contd

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The mis-run and cold shut defects are caused either bya lower fluidity of the mold or when the sectionthickness of the casting is very small. Fluidity can beimproved by changing the composition of the metaland by increasing the pouring temperature of themetal.

Cupola Cupola has been the most widely used furnace for

melting cast iron.

In hot blast cupola, the flue gases are used to preheat theair blast to the cupola so that the temperature in thefurnace is considerably higher than that in aconventional cupola. Coke is fuel and Lime stone(CaCO3) is mostly used flux.

Cost of melting low.

Main disadvantages of cupola is that it is not possible toproduce iron below 2.8% carbon.

Steel can be also prepared in cupola by employingduplexing and triplexing operations.

IES 2007What is permeability? Permeability is more important

in the basic process of sand casting than porosity. Give

one important reason for this feature.

[2 marks]

Permeability: Gases evolving from the molten metaland generated from the mould may have to gothrough the core to escape out of the mould. Hencecores are required to have higher permeability.

Permeability Number: The rate of flow of air passingthrough a standard specimen under a standard pressure istermed as permeability number.

The standard permeability test is to measure timetaken by a 2000 cu cm of air at a pressure typically of980 Pa (10 g/cm2), to pass through a standard sandspecimen confined in a specimen tube. The standardspecimen size is 50.8 mm in diameter and a length of50.8 mm.

Then, the permeability number, R is obtained by

Where V= volume of air = 2000 cm3

H = height of the sand specimen = 5.08 cm

p = air pressure, g/cm2

A = cross sectional area of sand specimen = 20.268 cm2

T = time in minutes for the complete air to pass through

Inserting the above standard values into the expression, we get

VHRpAT

501.28.

RpT

Calculate the permeability number of sand if it takes 1 min 25 s to pass 2000 cm3 of air at a pressure of5 g/cm2 through the standard sample.

25.0 /1min25 1.417min501.28

70.755 1.417

p g cmT s

R

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Collapsibility: At the time of cooling, casting shrinks, and

unless the core has good collapsibility (ability to decrease

in size) it is likely to provide resistance against shrinkage

and thus can cause hot tears.

Friability: The ability to crumble should be a very

important consideration at the time of removal.

Carbon Dioxide Moulding Sodium silicate (water glass, SiO2:Na2O) is used as a binder.

This is essentially a quick process of core or mouldpreparation.

The mould is prepared with a mixture of sodium silicate andsand and then treated with carbon dioxide for two to threeminutes such that a dry compressive strength of over 1.4MPa is arrived.

The carbon dioxide is expected to form a weak acid, whichhydrolyses the sodium silicate resulting in amorphous silica,which forms the bond.

The introduction of CO2 gas starts the reaction by forminghydrated sodium carbonate (Na2CO3 + H2O).

Contd

The compressive strength of the bond increases with

standing time due to dehydration.

Because of the high strength of the bond, the core need not

be provided with any other reinforcements.

It does not involve any distortions due to baking and also

better dimensional accuracies are achieved.

The sand mixture does not have good shelf life and

therefore should be used immediately after preparation.

Purpose

Muller's are normally used in foundries to mix the sands.

Type

1. Batch muller for small foundries

2. Continuous muller for large scale productions

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Distortion Allowance

A metal when it has just solidified is very weak andtherefore is likely to be distortion prone.

This is particularly so for weaker sections such as longflat portions, V, U sections or in a complicated castingwhich may have thin and long sections which areconnected to thick sections.

The foundry practice should be to make extramaterial provision for reducing the distortion.

Single Crystal CastingThe process is effectively:

1. Prepare a mold so that one end is a heated oven, andthe other end chilled. The part should be oriented sothat the cooling happens over the longest distance.

2. Cast metal into the mold

3. Solidification will begin at the chill plate. Thesedendrites will grow towards the heated end of thepart as long dendritic crystals. The part is slowlypulled out of the oven, past the chill plate.

4. Remove the solidified part.

Creep and thermal shock resistance properties.

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Investment CastingInvestment casting process or lost wax process

Basic steps:

1. Produce expendable wax, plastic, or polystyrene patterns.

2. Assemble these patterns onto a gating system

3. Investing or covering the pattern assembly with refractoryslurry

4.Melting the pattern assembly to remove the pattern material

5. Firing the mould to remove the last traces of the patternmaterial

6.Pouring molten metal

7. Knockout, cutoff and finishing.

Fig. Investment flask-casting procedure

Ceramic Shell Investment Casting

In ceramic shell investment casting a ceramic shell is

built around a tree assembly by repeatedly dipping a

pattern into a slurry (refractory material such as

zircon with binder).

After each dipping and stuccoing is completed, the

assembly is allowed to thoroughly dry before the next

coating is applied.

IES 2009 2 marks

Slush Casting Slush casting is a variation of the permanent mold process

in which the metal is permitted to remain in the mold onlyuntil a shell of the desired thickness has formed.

The mold is then inverted and the remaining liquid ispoured out.

When the mold halves are separated, the resulting castingis a hollow shape with good surface detail but variable wallthickness.

Frequently used to cast low-melting-temperature metalsinto ornamental objects such as candlesticks, lamp bases,and statuary.

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IES 2010What is meant by interchangeable manufacture ? Discuss a Go gauge.

Errors Systematic errors or fixed errors (Bias): Due to faulty

or improperly calibrated instruments. These may be

reduced or eliminated by correct choice of instruments.

Eg. calibration errors, Errors of technique etc.

Random errors: Random errors are due to non-specific

cause like natural disturbances that may occur during

the experiment. These cannot be eliminated.

Eg. Errors stemming from environmental variations, Due to Insufficient sensitivity of measuring system

Accuracy & Precision Accuracy - The ability of a measurement to match the actual

(true) value of the quantity being measured. The expectedability for a system to discriminate between two settings.Smaller the bias more accurate the data.

Precision - The precision of an instrument indicates itsability to reproduce a certain reading with a given accuracyOR it is the degree of agreement between repeated results.

Precision data have small dispersion ( spread or scatter ) butmay be far from the true value.

A measurement can be accurate but not precise, precise butnot accurate, neither, or both.

A measurement system is called valid if it is both accurateand precise.

Repeatability It is the ability of a measuring system to reproduce

output readings when the same input is applied to itconsecutively, under the same conditions, and in thesame direction.

Imperfections in mechanical systems can mean thatduring a Mechanical cycle, a process does not stop at thesame location, or move through the same spot eachtime. The variation range is referred to as repeatability.

Reliability of measurement It is a quantitative characteristic which implies

confidence in the measured results depending onwhether or not the frequency distributioncharacteristics of their deviations from the true valuesof the corresponding quantities are known. It is theprobability that the results will be predicted.

Which of these targets representsaccurate shooting? Preciseshooting? Reliable shooting?

A change in one variable, such as wind,alters the results as shown. Dose thisshow which shooting was the mostreliable?

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Calibration It is the setting or correcting of a measuring device

usually by adjusting it to match or conform to adependably known value or act of checking.

Calibration determines the performance characteristicsof an instrument, system or reference material. It isusually achieved by means of a direct comparison againstmeasurement standards or certified reference materials.

It is very widely used in industries.

A calibration certificate is issued and, mostly, a sticker isprovided for the instrument.

IAS 2013

IFS 2013 IAS 2014

Why is a unilateral tolerance preferred over bilateral tolerance ?

This system is preferred for Interchangeable manufacturing.

It is easy and simple to determine deviations.

It helps standardize the GO gauge end

Helpful for operator because he has to machine the upper

limit of the shaft and the lower limit of the hole knowing

fully well that still some margin is left for machining before

the part is rejected.

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Methods of Measurement

1. Direct method

2. Indirect method e.g density

3. Absolute method or Fundamental method e.glenghth form definition

4. Comparison method e.g comparator

5. Deflection method e.g. Dial Indicator

American Standard Association Tolerance System

1. Heavy force shrunk fit

2. Medium force fit

3. Tight fit

4. Wringing fit

5. Snug fit

6. Medium fit

7. Free fit

8. Loose fit

Snug fit1/3

Tolerance = 0.0004D Deviation 0

Medium force fit : 1/3 1/3

Tolerance = 0.0006D Deviation 0.0005 0.0006

and

and D D

Snug fit is applicable where no shake is permissible

Medium force fit is applicable for shrink fit on cast iron

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IFS 2012 Drift: It is a slow change of a metrological characteristics of a

measuring instruments

Resolution: It is the smallest change of the measuredquantity which changes the indication of a measuringinstruments

Sensitivity: The smallest change in the value of themeasured variable to which the instrument respond issensitivity. It denotes the maximum changes in an inputsignal that will not initiate a response on the output.

Rule of 10 or Ten-to one rule: That the discrimination(resolutions) of the measuring instrument should divide thetolerance of the characteristic to be measured into ten parts.In other words, the gauge or measuring instrument should be10 times as accurate as the characteristic to be measured.

IAS 2012 IFS 2011

IAS 2011 IAS 2010

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IES 2010 Why is a unilateral tolerance preferred over bilateral

tolerance ?

In surface roughness, discuss with a figure Root Mean Square method.

Need for Unconventional Processes New materials having high strength and hardness, such as

nimonic alloys and alloys with alloying elements such as

tungsten, molybdenum, and columbium are difficult to

machine by the traditional methods.

By conventional machining the MRR reduces with an

increase in the work material hardness.

Need for development of non-traditional machining

processes which utilize other methods such as

electrochemical processes for the material removal.

Need for Unconventional Processes

Complex shapes.

A very high accuracy is desired besides the complexity of

the surface to be machined.

Classification of NTMMThe Non-traditional Machining Methods are classifiedaccording to the major energy sources employed inmachining.

1. Thermal Energy Methods

2. Electro - Chemical Energy Method

3. Chemical Energy Methods

4. Mechanical Energy Methods

1. Thermal Energy Methods

Electrical discharge machining (EDM)

Laser beam Machining (LBM)

Plasma Arc Machining (PAM)

Electron Beam Machining(EBM)

Ion Beam Machining (IBM)

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2. Electro - Chemical Energy Method

Electro-Chemical Machining (ECM)

Electro-Chemical grinding (ECG)

Electro-Chemical Honing (ECH)

Electro-Chemical Deburring (ECD)

3. Chemical Energy MethodsThese methods involve controlled etching of theworkpiece material in contact with a chemical solution.

Chemical Machining Method (CHM).

4. Mechanical Energy Methods

Ultra Sonic Machining (USM)

Abrasive Jet Machining (AJM)

Water Jet Machining (WJM)

Some Observations EDM has the lowest specific power requirement and can

achieve sufficient accuracy.

ECM has the highest metal removal rate, MRR.

USM and AJM have low MRR and combined with hightool wear, are used for non-metal cutting.

LBM and EBM have high penetration rates with lowMRR and, therefore, are commonly used for microdrilling, sheet cutting, and welding.

CHM is used for manufacturing PCB and other shallowcomponents.

PAM can be used for clean, rapid cuts and profiles inalmost all plates upto 20 cm thick with 5o to 10o taper.

Shapes Cutting CapabilityThe various NTMM have some special shape cuttingcapability as given below:

1. Micro-machining and Drilling : LBM and EBM

2. Cavity sinking and standard Hole Drilling: EDM and

USM

3. Fine hole drilling and Contour Machining: ECM

4. Clean, rapid Cuts and Profiles: PAM

5. Shallow Pocketing: AJM192

Water Jet Machining Narrow jet of water directed, at high pressure and

velocity, against surface of workpiece

Jet of water erodes surface of workpiece, thereby

cutting workpiece

Computer control to achieve shape

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193

Water Jet MachiningIAS2014

Physical Principle Basic process

Physical Principle An arc jumps between two points along the path of least

resistance.

Physical Principle The energy of the arc is so concentrated that it causes the

electrode, and the work to melt. But the electrodematerial is chosen so that it melts less.

Physical Principle The metal and dielectric fluid is partly vaporized,

causing sudden expansion.

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Physical Principle The blast from the expanding vapors knocks some

molten particles loose, and the remaining molten metalhardens.

Advantages1. Hardness, toughness or brittleness of the material poses no

problems. Due to this EDM can be used for machiningmaterials that are too hard or brittle to be machined byconventional methods.

2. The method does not leave any chips or burrs on the workpiece.

3. Cutting forces are virtually zero, so very delicate and finework can be done.

4. The process dimension repeatability and surface finishobtained in finishing are extremely good.

5. The characteristic surface obtained, which is made up ofcraters, helps in better oil retention. This improves die life.

6. Because the forces between the tool and the workpiece andvirtually zero, very delicate work can be done.

Disadvantages1. Only electrically conductive materials can be machined

by EDM. Thus non - metallic, such as plastics, ceramicsor glass, cannot be machined by EDM.

2. Electrode wear and over-cut are serious problems.

3. A re-hardened, highly stressed zone is produced on thework surface by the heat generated during machining.This brittle layer can cause serious problems when thepart is put into service.

4. Perfectly square corners cannot be made by EDM.

5. High specific energy consumption (about 50 times thatin conventional machining)

6. MRR is quite low

EDM ToolThe usual choices for tool (electrode) materials are

Copper,

brass,

alloys of zinc and tin,

hardened plain carbon steel,

copper tungsten,

silver tungsten,

tungsten carbide,

copper graphite, and graphite.

IAS2011 Electrochemical Machining Electrochemical machining is the reverse of electro

plating

The work-piece is made the anode, which is placed inclose proximity to an electrode (cathode), and a high-amperage direct current is passed between them throughan electrolyte, such as salt water, flowing in the anode-cathode gap.

Metal is removed by anodic dissolution and is carriedaway in the form of a hydroxide in the electrolyte forrecycling or recovery.

MRR in ECM depends on atomic weight of work material

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Fig- Electrochemical Machining process

Electrochemical Machining Variation in the current density will result in work

taking the electrodes shape.

The electrode is fed with a constant velocity, and theelectrolyte is fed through the tool.

Advantages1. Complex three-dimensional surfaces can be machined

accurately. Good for low machinability or complicatedshapes.

2. As ECM leads to atomic level dissolution, the surfacefinish is excellent (Ra 0.2 to 0.6 m) with almost stressfree machined surface and without any thermaldamage.

3. The tool wear is practically nil which results in a largenumber of components produced per tool.

4. MRR is highest (1600 mm3/min) among NTMM andcomparable with conventional machining.

Disadvantages1. Use of corrosive media as electrolytes makes it difficult to

handle.

2. Sharp interior edges and corners (< 0.2 mm radius) aredifficult to produce.

3. Very expensive machine.

4. Forces are large with this method because of fluid pumpingforces.

5. Very high specific energy consumption (about 150 timesthat required for conventional processes),

6. Not applicable with electrically non-conducting materialsand jobs with very small dimensions

7. Lower fatigue strength

IAS2012 Refer note

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IFS2012

Tool Tool materials: Copper, brass, bronze, Al, Stainless

Steel, Cupro nickel, etc.

For laser beam machining

Materials: All materials except those with high thermal

conductivity and high reflectivity.

IAS2011

216

Abrasive Jet Machining (Dry) It is similar to sand blasting, except that a very narrow jet of

gas and abrasive particles achieves localized cutting. It removes material through the eroding action of a high

velocity stream of abrasive-laden gas. The gas is first compressed and mixed with the abrasive

powder in a mixing chamber and passed through outletnozzle.

Computer is used to position the jet. Gas Pressure about 7 atm Velocity of jet about 300 m/s Jet Diameter 0.12 mm to 1.25 mm Abrasive used: Al2O3 , SiC with particle size 10 to 50 m Tool (nozzle) material tungsten carbide or sapphire Tool (nozzle) Life about 30 hours

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217

Abrasive Jet Machining Advantages of AJM Can be used in any material, conductive, non-

conductive, ductile or brittle

Good dimensional accuracy (0.05 mm)

Good Surface finish 0.25 to 1.25 m

Due to cooling action of gas stream no thermal damage

on the work surface

Due to negligible force delicate workpiece can be

machined.

Disadvantages of AJM Low MRR

Possibility of stray cutting

Embedding of abrasive particles in soft workpiece

Dust control needed

Application of AJM Cutting and drilling on metal foils and thin

sections of ceramics and glass

Intricate holes in electronic components such as

resistor paths in insulation

Engraving of characters on toughened glass

automobile windows

Cleaning, polishing and deburring the surface

IFS-2011Write the advantages, limitations and applications of

electron beam machining. What is the safety problem

connected with EBM?

[5-Marks]

222

Electron Beam Machining Workpiece placed in vacuum chamber

High-voltage electron beam directed toward

workpiece

Energy of electron beam melts/ vaporizes selected

region of workpiece

Electron beam moved by deflection coils

Similar process to EB welding

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223

Electron Beam Machining Advantage There is no effect of local heat on workpiece as the

temperature of surrounding material (25 50 m away from the machining spot) is not raised

Disadvantage Necessity of vacuum

Not suitable for large workpiece

Little taper produced on holes

Very high specific power consumption

Application All materials can be machined

Drill small holes for thin plates

Cutting narrow slots

Safety High velocity electron may produce X-ray

IFS2014 IAS2013

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229

Laser Beam Machining

230

Laser Beam Machining Direct laser beam against surface of workpiece, as in

laser welding

Successive pulses from laser gun vaporize tiny bits of

workpiece

Location of laser beam controlled by computer

Workpiece need not be conductive

Cuts are tapered

Gotta trap overshoot from laser beam

231

Laser Beam Machining Produces large remelt zone

Can produce holes as small as 0.0005 mm diameter

Can produce deep holes

Used to produce cooling holes in blades/vanes for jet

engines

IES 2011 ConventionalWhat is creep feed grinding? Discuss its salientfeatures, advantages, and application.

[10 marks]

Creep feed grinding This machine enables single pass grinding of a surface

with a larger down feed but slower table speed than that adopted for multi-pass conventional surface grinding.

In creep-feed grinding, the entire depth of cut is completed in one pass only using very small in-feed rates.

State the basic advantage of a creep feed grinder over a conventional surface

Productivity is enhanced and life of the grinding wheel is extended.

Economic

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Application Grinding shaped punches

Key seats

Twist drill flutes

Roots of turbine blades

Various complex superalloy parts

IAS2014

G Ratio The grinding ratio or G ratio is defined as thee cubic mm

of stock removed divided by the cubic mm of wheel lost.

In conventional grinding, the G ratio is in the range 20: 1to 80: 1.

The G ratio is a measure of grinding production andreflects the amount of work a wheel can do during itsuseful life.

As the wheel losses material, it must be reset orrepositioned to maintain workpiece size.

Grade The worn out grit must pull out from the bond and make

room for fresh sharp grit in order to avoid excessive riseof grinding force and temperature.

A soft wheel should be chosen for grinding hardmaterial.

A hard wheel should be chosen for grinding softmaterial.

IES2009

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IAS2012

Fig- cutting action of abrasive grains

Interaction of the grit with the workpiece

Shape of grit is very important because it determines thegrit geometry e.g. rake and clearance angle.

The grits do not have definite geometry unlike a cuttingtool.

Interaction of the grit with the workpiece

Grit with favourable geometry can produce chip in shearmode.

However, grits having large negative rake angle orrounded cutting edge do not form chips but may rub ormake a groove by ploughing leading to lateral flow of theworkpiece material.

Fig- Grits engage shearing, ploughing and rubbing

Why is aluminium oxide preferred to silicon carbide in grinding steel?

Al2O3 is tougher than SiC. Therefore it ispreferred to grind material having high tensilestrength like steel. Moreover, Al2O3 shows higherchemical inertness than SiC towards steel leadingto much improved wear resistance duringgrinding.

Why diamond is not used for steel? On ferrous materials, diamonds are not suitable

because of the diffusion of carbon atoms fromdiamond to the work-piece material.

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IES 2010Draw the typical configuration of Internal Centre lessgrinding mentioning main advantage and use.

Centerless Grinding

Centerless Grinding Centerless grinding makes it possible to grind both

external and internal cylindrical surfaces withoutrequiring the workpiece to be mounted between centersor in a chuck.

This eliminates the requirement of center holes in someworkpieces and the necessity for mounting theworkpiece, thereby reducing the cycle time.

Two wheels are used. The larger one operates at regulargrinding speeds and does the actual grinding. Thesmaller wheel is the regulating wheel. It is mounted atan angle to the plane of the grinding wheel.

Centerless Grinding The regulating wheel controls the rotation and

longitudinal motion of the workpiece and usually is aplastic- or rubber-bonded wheel with a fairly wide face.

The workpiece is held against the work-rest blade by the cutting forces exerted by the grinding wheel and rotates at approximately the same surface speed as that of the regulating wheel.

State the disadvantages of centrelesscylindrical grinding machine?

It does not grind concentrically with centres.

Large diameter short workpiece are difficult to control in the process

It may not improve workpiece perpendicularity.

Loading

Some grinding chips get lodged into the spaces between

the grits resulting in a condition known as loaded wheel.

Loading is generally caused during the grinding of soft

and ductile materials.

A loaded grinding wheel cannot cut properly and need

dressing.

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Dressing Dressing is the conditioning of the wheel surface which

ensures that grit cutting edges are exposed from thebond and thus able to penetrate into the workpiecematerial.

In dressing attempts are made to splinter the abrasivegrains to make them sharp and free cutting and also toremove any residue left by material being ground.

Dressing therefore produces micro-geometry.

Truing Truing is the act of regenerating the required geometry

on the grinding wheel.

Truing is also required on a new conventional wheel toensure concentricity with specific mounting system.

Truing and dressing are commonly combined into oneoperation for conventional abrasive grinding wheels, butare usually two distinctly separate operation for superabrasive wheel.

IFS-2011What is the main difference between rose reamer

and chucking reamer ? Write in short about shell

reamer.

[5-marks]

Rose ReamerRose chucking reamersare ground cylindricaland have no reliefbehind the outer edgesof the teeth. All cuttingis done on the beveledends of the teeth

Chucking ReamerFluted chuckingreamers have reliefbehind the edges of theteeth as well as beveledends. They can cut onall portions of the teeth.Their flutes arerelatively short and theyare intended for lightfinishing cuts.

Shell ReamerShell reamers often areused for sizes over 20mm to save cutting-toolmaterial. The shell,made of HSS for smallersizes and with carbideedges for larger sizes orfor mass-productionwork.

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Advantages of Down Milling1. Suited to machine thin and hard-to-hold parts sincethe workpiece is forced against the table or holdingdevice by the cutter.

2. Work need not be clamped as tightly.

3. Consistent parallelism and size may be maintained,particularly on thin parts.

4. It may be used where breakout at the edge of theworkpiece could not be tolerated.

5. It requires upto 20% less power to cut by this method.

6. It may be used when cutting off stock or when millingdeep, thin slots.

Disadvantages of Down Milling

1. It cannot be used unless the machine has a backlash

eliminator and the table jibs have been tightened.

2. It cannot be used for machining castings or hot rolled

steel, since the hard outer scale will damage the cutter.

What are the superabrasivematerials for grinding wheel? Hardest material diamond and cubic boron nitride

second hardest materials.

1. Work materials-Tensile strength

2. Helix angle-increase in helix angle reduce thrust

3. Point angle Increases with in point angle

4. Web thickness- Axial thrust will reduce by thinning the web

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Gear Hobbing The HSS or carbide cutter having teeth like gear milling

cutter and the gear blank apparently interact like a pairof worm and worm wheel.

The hob (cutter) looks and behaves like a single ormultiple start worms.

(a) Straight (b) helical tooth and (c) worm wheel

Gear Hobbing

Having lesser number (only three) of tool work

motions, hobbing machines are much more rigid, strong

and productive than gear shaping machine.

But hobbing provides lesser accuracy and finish and is

used only for cutting straight or helical teeth (single) of

external spur gears and worm wheels.

Advantages of Gear Hobbing(a) The method is versatile and can generate spur,helical, worm and worm wheels.

(b) Since gear hobbing is a continuous process, it israpid; economical and highly productive.

(c) The method produces accurate gears and is suitablefor medium and large batch production.

(d) The cutter is universal, because it can cut all gears ofsame module, irrespective of number of teeth on thegear.

Disadvantages of gear Hobbing(a) Gear hobbing cannot generate internal gears and

bevel gears.

(b) Enough space has to be there in component

configuration for hob approach.

Applications of Hobbing The gears produced by gear hobbing are used in

automobiles, machine tools, various instruments, clocks

and other equipments.

IFS 2013

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Jig Both jigs and fixtures hold, support, and locate the

work piece.

A jig also guides the cutting tool.

Fixtures Both jigs and fixtures

hold, support, andlocate the work piece.

A fixture has areference point forsetting the cutting toolwith reference to thework piece.

3-2-1 Locating Principle A workpiece, just like any free solid body, has six

degrees of freedom (some researchers have referredthis to the twelve degrees of freedom by consideringthe +/- movements in each category)

For locating it is necessary to arrest all these six degreesof freedom to ensure the mechanical stability.

A single locator in Plane 1 would arrest the linearmotion along the X-axis.

A second locator in the same plane would arrest therotary motion about the Z-axis.

Another locator placed in the same plane would arrestthe rotary motion about the Y-axis.

Adding one more locator in Plane 1 would not serve anypurpose.

So fourth locator is placed in Plane 2 which isperpendicular to Plane 1. This would restrict the linearmotion along the Y-axis.

The fifth locator is placed in the Plane 2 which canarrest the rotational motion about the X-axis.

The sixth locator placed in Plane 2 would not serve anypurpose.

So, sixth locator is placed in Plane 3 which isperpendicular both the planes 1 and 2. This wouldarrest the linear motion along the Z-axis.

Fig. A component with six locators

IES - 2007According to the principle of location in jigs andfixtures, how many degrees of freedom are to beeliminated to have a body fixed in space?

(a) 3

(b) 4

(c) 5

(d) 6

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Considering 12 DOF You must fix all the 12 degrees of freedom except the three

transitional degrees of freedom (-X, -Y and -Z) in order tolocate the work piece in the fixture. So, 9 degrees offreedom of the work piece need to be fixed.

Rest the work piece on three non-collinear points of thebottom surface (XY), and you will be able to fixthe +Z, CROT-X, ACROT-X, CROT-Y and ACROT-Y degrees of freedom.

Now, rest the work piece at two points of side surface (XZ),and you will be able to fix the +Y and CROT-Z and ACROT-Z degrees of freedom.

Now, rest the work piece at one point of the adjacentsurface (YZ), and you will be able to fix the +X degrees offreedom.

Points to ponder When more than one locator is placed on a surface

(plane), they should be distributed as far apart as

possible on the surface.

While selecting the surface for the largest locators,

consideration should be given to the largest area of the

workpiece.

GATE - 2005When 3-2-1 principle is used to support and locate athree dimensional work-piece during machining,the number of degrees of freedom that arerestricted is

(a) 7

(b) 8

(c) 9

(d) 10

GATE - 20013-2-1 method of location in a jig or fixture wouldcollectively restrict the workpiece in n degrees offreedom, where the value of n is

(a) 6

(b) 8

(c) 9

(d) 12

GATE-2013 (PI)

In the 3-2-1 principle of fixture design, 3 refers to the

number of

(a) Clamps equired

(b) Locators on the primary datum face

(c) Degrees of freedom of the workpiece

(d) Operations carried out on the primary datum face

IES 2011In the 3-2-1 principle of fixture 3 refers to number of

(a) Setups possible

(b) Clamps required

(c) Positions on primary face

(d) Locating positions

4/1/2015

48

IES 1999Assertion (A): Spherical washers are used to locatethe job in the fixtures.

Reason (R): 3-2-1 principle should be adopted tolocate the job.

(a) Both A and R are individually true and R is thecorrect explanation of A

(b) Both A and R are individually true but R is not thecorrect explanation of A

(c) A is true but R is false

(d) A is false but R is true

IFS 2012

Clamping To restrain the workpiece completely a clamping device

is required.

Holds the workpiece securely in a jig or fixture against

the forces applied over it during on operation.

Device should be incorporated into the fixture, proper

clamp in a fixture directly influence the accuracy and

quality of the work done and production cycle time.

IAS 2012

Cam Clamps Provide clamping force because of the contour of the cam

surface that comes into contact with the plate used for theclamping.

Plate is pushed down by the cam against the springpressure to hold the part in place.

Cam clamps are quick in operation. Cam clamps are of three types, eccentric cam, flat spiral

cam and cylindrical cam. The design shown in Fig. is flat spiral and is the most

commonly used clamp.

Fig. A cam clamp used for quick and easy clamping a part

The design shown is indirect pressure clamping wherethe pressure is transmit to the part through the plate.This is more stable and the vibrations duringmachining do not affect the a part clamping.

Fig. An example of a fixture held by a cam clamp

4/1/2015

49

IFS-2011What are the functions of jig ? Draw a jig to machine

four holes in a plate. What are two reasons for not

having drill bushings actually touching the workpiece

? What is a duplex fixture ?

[10-marks]

Duplex Fixture It is a type of multi-station fixtures used primarily for

high speed, high volume production runs where themachining cycle must be continuous.

It uses only two stations. Once the machining operationis complete at station one, the fixture is revolved and the

machining is started atstation two. During thisperiod, the machined partis unloaded from stationone and a fresh part isloaded there, and so on

Diamond Pin Locator Diamond pins are often used for radial location .

One cylindrical locator (Pin A) arrests five degrees offreedom, second cylindrical locator at the position Bwill arrest the sixth degree of freedom.

If the two holes are identical in size then any pin can bemade the principal locator. However, if one of the holesis larger then the principal locator will be placed in thelarger hole.

The second locator is made slightly smaller than thehole and relieved from both sides to take care of thevariation in the X direction. The cylindrical surfaceswill locate the part in the Y direction.

1

2.1 NC CNC DNC

What is NC/CNC?

NC is an acronym for Numerical Control and CNC is an

acronym for Computer Numerical Control.

What is the difference between NC and CNC ?

The difference between NC and CNC is one of age andcapability.

The earliest NC machines performed limited functionsand movements controlled by punched tape or punchcards.

As the technology evolved, the machines were equipedwith increasingly powerful microprocessors (computers)with the addition of these computers, NC machinesbecome CNC machines.

CNC machines have far more capability than theirpredecessor. contd..

2

What is the difference between NC and CNC ?

Some of the enhancements that came along with CNC

include: Canned Cycles, Sub Programming, Cutter

Compensation, Work coordinates, Coordinate system

rotation, automatic corner rounding, chamfering, and B-

spline interpolation.

What is a Conversational Control

CNC machine tool builders offer an option what is

known as the conversational control. This control lets

the operator/programmer use simple descriptive

language to program the part. The control then

displayed a graphical representation of the instructions

so the operator/programmer can verify the tool path.

3

Are CNC machines faster than conventional machines?

Yes, No, Sometimes. When it comes to making a single,

simple part it is hard to beat a conventional mill or lathe.

CNC machines move faster in rapid travel than

conventional machines.

Are CNC machines more accurate than conventional machines?

Yes, they can be. But like anything else it depends on

who is running the machine, how well the machines has

been maintained, quality of setup and so on.

4

NC/CNC Machines-Advantages High Repeatability and Precision e.g. Aircraft parts

Volume of production is very high

Complex contours/surfaces need to be machined. E.g.Turbines

Flexibility in job change, automatic tool settings, lessscrap

More safe, higher productivity, better quality

Less paper work, faster prototype production, reductionin lead times

NC/CNC Machines-Disadvantages

Costly setup, skilled operators

Computers, programming knowledge required

Maintenance is difficult

5

Stepper Motor

The stepper motor is special type of synchronous motor

which is designed to rotate through a specific angle

(Called step) for each electrical pulse received from the

control unit.

6

7

IAS-2010 MainIllustrate with the help of neat sketches the differences

between open-loop and closed-loop control in NC

system. Why is feedback not possible in open-loop

control system ?

[22- Marks]

Basic CNC Principles

8

IAS2010

IAS2013

9

IAS2014

Basic Length Unit (BLU) In NC machine, the displacement length per one pulse

output from machine is defined as a Basic Length Unit(BLU).

In the CNC computer each bit (binary digit) represents 1BLU.

Bit = BLU

Example: If one pulse makes a servo motor rotate by onedegree and the servo motor moves the table by 0.0001mm, one BLU will be 0.0001 mm.

The lead of a ball screw is related to the displacementunit of the machine tool table.

10

GATE 2008 (PI)A stepper motor has 150 steps. The output shaft of the

motor is directly coupled to a lead screw of pitch 4 mm,

which drives a table. If the frequency of pulse supply to

the motor is 200 Hz, the speed of the table (in mm/min)

is

(a) 400 (b) 320 (c) 300 (d) 280

Example A DC servomotor is coupled directly to a leadscrew

which drives the table of an NC machine tool. Adigital encoder, which emits 500 pulses perrevolution, is mounted on the other end of theleadscrew. If the leadscrew pitch is 5 mm and themotor rotates at 600 rpm, calculate

(a) The linear velocity of the table

(b) The BLU of the NC system

(c) The frequency of pulses transmitted by the encoder.

11

IES 2011 Conventional The table of a CNC machine is driven by a Lead screw which

is rotated by a DC servomotor. A digital encoder which emits

1000 pulses per second is mounted on the lead screw as a

feedback device. If the lead screw pitch is 6 mm and motor

rotates at 500 rpm, find

1. Basic length Units of the system

2. Linear velocity of the table.

3. Frequency of pulses generated by the feedback device.

[5 Marks]

IAS-2010 MainIn open-loop NC system the shaft of a stepping motor isconnected directly to the lead screw x-axis of themachine table. The pitch of the lead screw is 3.0 mm.The number of step angles on the stepping motor is 200.

Determine how closely the position of the table can becontrolled, assuming that there are no mechanical errorsin the positioning system.

Also, what is the required frequency of the pulse trainand the corresponding rotational speed of the steppingmotor in order to drive the table at a travel rate of 100mm/min? [8- Marks]

Q.6.(b) (ii) What is meant by Part Programming ? Discuss point to point control,

and its applications.

Part programs for simple components can be carried out manually. However, if the component has complex

features which require too many repetitive and/or tedious calculations for preparing its program for cutter

path description, then it is recommended that computer-aided part programming be resorted to.

To be a good CNC programmer, one should have a fair knowledge about the machine tools, cutting tools

and fixtures to be used and the manufacturing process. He also should have a good understanding of

geometry, algebra and trigonometry. In fact, machine shop experience is the pre-requisite for a good

12

programmer as only careful process planning can lead to efficient and practical programs.

The following are the steps to be followed while developing the CNC part programs.

Process planning

Axes selection

Tool selection

Cutting process parameters planning

Job and tool setup planning

Machining path planning

Part program writing

Part program proving

Control Systems possible in CNC Machine

Point to point mode:

13

Programming Key Letters O - Program number (Used for program identification)

N - Sequence number (Used for line identification)

G - Preparatory function

X - X axis designation

Y - Y axis designation

Z - Z axis designation

R - Radius designation

F Feed rate designation

S - Spindle speed designation

H - Tool length offset designation

D - Tool radius offset designation

T - Tool Designation

M - Miscellaneous function

Table of Important G codes

Code Meaning Format

G00 Rapid Transverse N__G00 X___ Y___ Z___

G01 Linear Interpolation N__G01 X___ Y___ Z___ F___

G02 Circular Interpolation, CW

N__G02 X__ Y__ Z___ R___ F___

N__G02 X___ Y__Z__I ___J __K __ F __

G03 Circular Interpolation,CCW

N__G03 X___ Y___ Z__R__F___

N__G03 X__ Y__Z__I __J __K __ F __

G04 Dwell N__G04P___

G17 XY Plane

G18 XZ Plane

G19 YZ Plane

14

Table of Important G codes

Code Meaning Format

G20/G70 Inch Unit

G21/G71 Metric Unit

G28 Automatic Return to Reference Point

G40 Cutter compensation cancel

G41 Cutter compensation left N__G41D__

G42 Cutter compensation right N__G42D__

G43 Tool length compensation (plus)

N__G43H__

Table of Important G codes

Code Meaning Format

G44 Tool length compensation (minus)

N__G44H__

G49 Tool length compensation cancel

G80 Cancel canned cycles

G81 Drilling cycle N__G81 Z__R__F__

G90 Absolute positioning

G91 Incremental positioning

G92 Absolute preset, change the datum position

N__G92X__Y__Z__

15

Rapid traverse: G00

G00: to make the machine move at maximum speed.

It is used for positioning motion.

G90 G00 X20.0 Y10.0

Start

EndG90:

absolute coordinates

(0,0)

(10,10)

(20,10)

Linear interpolation: G01 G01: linear interpolation at feed speed.

G91 G0l X200.0 Y100.0 F200.0

Y

100.0

XStart

End

200.0

G91: incremental coordinates

16

Circular interpolation: G02, G03 G02, G03:

For circular interpolation, the tool destination and the circle center are programmed in one block

G02 is clockwise interpolation, G03 is counterclockwise interpolation

__;____

____0302

19

__;____

____0302

18

__;____

____0302

17

FKJ

RZY

GG

G

FKI

RZX

GG

G

FJI

RYX

GG

G

End point

Circle center, radius

Circular interpolation: G02, G03

G91 G02 X60.0 Y20.0 R50.0 F300.0G91 G02 X60.0 Y20.0 R-50.0 F300.0

Start

EndR=-50mm

Y

X

R=50mm

Specify R with sign before it:

180 +R

>180 -R

17

Circular interpolation: G02, G03N0010 G92 X200.0 Y40.0 Z0 ;

N0020 G90 G03 X140.0 Y100.0 I -60.0 F300N0030 G02 X120. 0 Y60.0 I- 50.0

Or

N0010 G92 X200.0 Y40.0 Z0N0020 G90 G03 X140.0 Y100.0 R60.0 F300N0030 G02 X120.0 Y60.0 R50.0

R60

R50

90 120 140 200

100

6040

O

Y

X

G90: absolute

coordinates

G92:To define working

coordinate

Tool-Radius Compensation Tool-radius compensations make it possible to

program directly from the drawing, and thus eliminate the tool-offset calculation

G41 (G42) D D: the radius of tool to compensate is saved in a memory unit that

is named D G41/G42 is directly related with direction of tool movement and

which side of part is cut.

18

Tool-Height Compensation

G43 (G44) H

H: specified memory unit used to save height compensation of tool.

Positive compensation (G43):

real position = specified position + value saved in H

Negative compensation (G44):

real position = specified position - value saved in H

Tool-Height Compensation Example: N0010 G91 G00 X12.0 Y80.0

N0020 G44 Z-32.0 H02

If we put 0.5mm into H02,

real position = -32.0 - 0.5 = -32.5

Cancel tool-height compensation: G49

G91: incremental coordinates

19

Table of Important M codes M00 Program stop

M01 Optional program stop

M03 Spindle on clockwise

M04 Spindle on counterclockwise

M05 Spindle stop

M06 Tool change

M08 Coolant on

M09 Coolant off

M10 Clamps on

M11 Clamps off

M02 or M30 Program stop, reset to start

Example of CNC Programming

What Must Be Done To Drill A Hole On A CNC Vertical Milling Machine

20

Top View

Front View

Tool Home

1.) X & Y Rapid To Hole Position

Top View

Front View

2.) Z Axis Rapid Move

Just Above Hole

3.) Turn On Coolant

4.) Turn On Spindle

.100

21

Top View

Front View

5.) Z Axis Feed Move to

Drill Hole

Top View

Front View

6.) Rapid Z Axis Move

Out Of Hole

22

Top View

Front View

9.) X&Y Axis Rapid

Move Home

7.) Turn Off Spindle

8.) Turn Off Coolant

Top View

Front View

Tool At Home

O0001N005 G54 G90 S600 M03N010 G00 X1.0 Y1.0N015 G43 H01 Z.1 M08

N020 G01 Z-.75 F3.5

N030 G91 G28 X0 Y0 Z0N035 M30

N025 G00 Z.1 M09

Heres The CNC Program!

23

IAS2011

APT language form PPTs

IES-2008Name the four types of statements in a complete APTpart program. Prepare part program for geometrydescription of the contour shown in the figure below:

[15-Marks]

R 20

R 20

135

L3L2

20

20

30 40

L180

L5

L4

C2

P1P2

Y

X

C1

24

Answer:PARTNO CONTOUR

MACHIN/MILL, 1

CLPRNT

UNITS/MM

P0 = POINT/0.0, 0.0, 0.0

P1 = POINT/110.0, 20.0, 0.0

P2 = POINT/20.0, 20.0, 0.0

P3 = POINT/90.0, 110.0, 0.0

P4 = POINT/20.0, 100.0, 0.0

P5 = POINT/50.0, 130.0, 0.0

L1 = LINE/P2, ATANGL, 90, XAXIS

L2 = LINE/P4, ANTNGL, 45, XAXIS

L3 = LINE/P5, ATANGL, 135, L2

L4 = LINE/P1, PERPTO, L3

L5 = LINE/P1, PERPTO, L4

C1=CIRCLE/CENTER, P3, RADIUS, 20.0

C2=CIRCLE/CENTER, P1, RADIUS, 20.0

PL1=PLANE/P1, P2, P3

CUTTER/25.0

TOLER/0.1

INTOL/0.05

OUTTOL/0.05

FEDRAT/200

SPINDL/500, CLW

COOLNT/ON

FROM/P0

GO/TO, L1, TO, PL1, TO, L5

GOLFT/L1, PAST, L2

GORGT/L2, PAST, L3

GORGT/L3, TANTO, C1

GOFWD/C1, PAST, L4

GOFWD/L4, PAST, C2

GORGT/C2, PAST, L5

GORGT/L5, PAST, L1

Contd.

25

RAPID

GOTO/P0

COOLNT/OFF

SPINDL/OFF

END

FINI

Contd.

IES-2007Prepare part using APT language for milling the contourshown in Fig. in a single pass. [20-Marks]

110

R30

Q

+

+

+

+

40

40

120

100

A

B

F

P

E

110

R40

DC

8 mm

Material : M S.

26

Answer:PARTNO CONTOUR

MACHIN/MILL, 2

CLPRNT

UNITS/MM

P0 = POINT/0.0, 0.0, 10.0

PTA = POINT/0.0, 0.0, 0.0

PTB = POINT/0.0, 120.0, 0.0

PTC = POINT/30.0, 150.0, 0.0

PTD = POINT/140.0, 150.0, 0.0

PTE = POINT/140.0, 40.0, 0.0

PTF = POINT/100.0, 0.0, 0.0

PTQ = POINT/30.0, 120.0, 0.0

PTP = POINT/140.0, 0.0, 0.0

LAB = LINE/PTA, PTB

LCD = LINE/PTC, PTD

LDE = LINE/PTD, PTE

LAF = LINE/PTA, PTF

CBC = CIRCLE/CENTRE, PTQ, RADIUS, 30.0

CEF = CIRCLE/CENTRE, PTP, RADIUS, 40.0

PL1=PLANE/PTA, PTB, PTC

27

CUTTER/25.0

TOLER/0.1

INTOL/0.05

OUTTOL/0.05

FEDRAT/200

SPINDL/500, CLW

COOLNT/ON

FROM/P0

GO/TO, LAB, TO, PL1, TO, LAF

GOLFT/LAB, TANTO, CBC

GOFWD/CBC, PAST, LCD

GORGT/LCD, PAST, LDE

GORGT/LDE, PAST, CEF

GORGT/CEF, PAST, LAF

GORGT/LAF, PAST, LAB

Contd.

RAPID

GOTO/P0

COOLNT/OFF

SPINDL/OFF

END

FINI

Contd.

28

IES-2006Prepare part program to machine the contour shown inthe figure using APT on CNC milling machine.

[15-Marks]

R20

80

R30

100 mm

200 mm

60 50

Material: MS Thickness: 8.0 mm

R20

80

R30

100 mm

200 mm

60 50

P3

P2

L3

P4

L1

L2

P1 L4

C1

C2

X

Y

29

R20

80

R30

100 mm

200 mm

60 50

P3

P2

L3

P4

L1

L2

P1 L4

C1

C2

X

Y

Answer:PARTNO CONTOUR

MACHIN/MILL, 3

CLPRNT

UNITS/MM

P0 = POINT/0.0, 0.0, 10.0

P1 = POINT/0.0, 0.0, 0.0

P2 = POINT/60.0, 80.0, 0.0

P3 = POINT/150.0, 100.0, 0.0

P4 = POINT/200.0, 0.0, 0.0

C1 = CIRCLE/ CENTER, P2, RADIUS, 20

C2 = CIRCLE/CENTER, P3, RADIUS, 30

L1 = LINE/P1, LEFT, TANTO, C1

L2 = LINE/LEFT, TANTO, C1, LEFT, TANTO, C2

L3 = LINE/P4, RIGHT, TANTO, C2

L4 = LINE/P1, P4

PL1=PLANE/P1, P2, P3

CUTTER/25.0

TOLER/0.1

INTOL/0.05

OUTTOL/0.05

COOLNT/ON

SPINDL/500, CLW

FEDRAT/200

FROM/P0

GO/TO, L1, TO, PL1, TO, L4

GOLFT/L1, TANTO, C1

GOFWD/C1, PAST, L2

GOFWD/L2, TANTO, C2

GOFWD/C2, PAST, L3

GOFWD/L3, PAST, L4

GORGT/L4, PAST, L1

Contd.

30

RAPID

GOTO/P0

COOLNT/OFF

SPINDL/OFF

END

FINI

Contd.

Home WorkWrite a complete part program in APT for machiningthe product which is given in the diagram. Thickness ofthe workpiece is 6 mm. All dimensions are in mm.

[15]

31

PARTNO CONTOUR

MACHIN/MILL, 1

CLPRNT

UNITS/MM

P0 = POINT/-25.0,-25.0, 25.0

P1 = POINT/0.0, 0.0, 6.0

P2 = POINT/117.0, 32.0, 6.0

P3 = POINT/117.0, -32.0, 6.0

C1=CIRCLE/CENTER, P1, RADIUS, 10.0

C2=CIRCLE/CENTER, P2, RADIUS, 12.5

C