BPSC AE Paper 6 (Mechanical) 2012 Official Paper (Previous Year Paper)

Explanation:

Governor Effort

• It is the force exerted by the governor at the sleeve as the sleeve tends to move.
• When the speed of the governor is constant, the force exerted on the sleeve is zero as the sleeve doesn't tend to move and hence at the constant speed, the effort of the governor is zero, but when the speed changes and the sleeve tends to move to new equilibrium position force is exerted on the sleeve.
• This force gradually diminishes to zero as the sleeves move to a new equilibrium position corresponding to the new speed.
• The mean force exerted on the sleeve during the given change of speed is known as the effort of the governor.
• The given change of speed is generally taken as 1%, hence effort is defined as the force exerted on the sleeve for 1% change of speed.

Explanation:

Degree of Freedom

• The degree of freedom (DOF) of a mechanical system is the number of independent parameters that define its configuration.
• The position and orientation of a rigid body in space are defined by three components of translation and three components of rotation, which means that it has six degrees of freedom.

Explanation:

• The addendum is the height by which a tooth of a gear projects beyond (outside for external, or inside for internal) the standard pitch circle or pitch line.
• Also, the radial distance between the pitch diameter and the outside diameter
• In the gear drive, Standard Addendum = 1 module

​

Additional Information

• Addendum: It is the radial distance between pitch circle and the addendum circle.
• Dedendum: It is the radial distance between the pitch circle and the root circle.
• Working depth: This is the distance of engagement of two mating teeth and is equal to the sum of the addendums of the mating teeth of two gears.
• Whole depth: This is the height of a tooth and is equal to the addendum plus dedendum.
• Face width: This is the width of the gear and it is the distance from one end of a tooth to the other end.
• Face of a tooth: This is the surface of the tooth between the pitch circle and the outside circle.
• Flank of tooth: This is the surface of the tooth between the pitch circle and root circle.
• Clearance: This is the radial distance between the top land of a tooth and the bottom land of the mating tooth.

Concept:

Governors are classified as:

• Centrifugal Governor
• Inertia Governor

Centrifugal Governors are classified as:

• Dead Weight Governors:
• Porter governor
• Proell governor
• Spring controlled governors:
• Hartnell governor
• Hartung governor
• Pickering governor

Explanation:-

Critical or whirling speed of a shaft

• When the rotational speed of the system coincides with the natural frequency of lateral/transverse vibrations, the shaft tends to bow out with a large amplitude. This speed is termed as critical/whirling speed.
• Whirling speed or Critical speed of a shaft is defined as the speed at which a rotating shaft will tend to vibrate violently in the transverse direction if the shaft rotates in the horizontal direction.
• In other words, the whirling or critical speed is the speed at which resonance occurs.
• Hence we can say that Whirling of the shaft occurs when the natural frequency of transverse vibration matches the frequency of a rotating shaft.
• It is the speed at which the shaft runs so that the additional deflection of the shaft from the axis of rotation becomes infinite is known as critical or whirling speed.

Deflection of the shaft due to transverse vibration of the shaft.

$y = \frac{e}{{{{\left( {\frac{{{\omega _n}}}{\omega }} \right)}^2} - 1;}}$

At critical speed,

$\omega = {\omega _n} = \sqrt {\frac{k}{m}} = \sqrt {\frac{g}{\delta }}$

where,

ω = Angular velocity of the shaft, k = Stiffness of shaft, e = initial eccentricity of the center of mass of the rotor

m = mass of rotor, y = additional of rotor due to centrifugal force.

Explanation:

• V-belts have trapezoidal shapes and the pulleys used for the V -belts are the grooved pulleys and sheaves.
• The groove angle of the sheave is made smaller than the belt cross-section angle.
• Thus, V-belts make contact at the sides of the groove of the pulley.

Standard V – belt section

Velocity ratio:

• For flat belts → 4 : 1
• For V belts → 7 : 1
• For chains → 15 : 1

Concept:

• The ropes are made from cotton, hemp, manila, synthetic jute, steel wire, etc.
• Rope drives are used for long-distance and for large amounts of power transmission.
• The rope drives are mainly employed in the mining and textile industries.
• Rope drives can be employed when there is a misalignment between the pulleys within limits and in fluctuating temperature and humidity conditions.
• One big advantage of rope drives is that a number of separate drives may be taken from the one driving pulley.
• The groove angle varies from 40° to 60° but is generally 45°.

• Stress-strain curves are an extremely important graphical measure of a material’s mechanical properties.
• Perhaps the most important test of a material’s mechanical response is the tensile test, in which one end of a rod or wire specimen is clamped in a loading frame and the other subjected to a controlled displacement δ.
• When the stress σ is plotted against the strain ϵ, an engineering stress-strain curve is obtained.

Note:

Metals have different properties. Depending on the type of application, different metals are selected.

• Physical properties of metals: Colour, Weight/Specific gravity, Structure, Conductivity, Magnetic property, Fusibility
• Mechanical properties: Ductility, malleability, hardness, brittleness, toughness, tenacity, elasticity
• Optical properties: The optical properties of material define how it interacts with light. Example: Refraction, Refraction index.
• Magnetic properties: Magnetic properties describe the behaviour of any substance under the influence of a magnetic field.

Concept:

Prismatic bar

• It means the bar has a uniform cross-section.
• It is a structural member having a straight longitudinal axis and a constant cross-section throughout its length.
• The elongation δ of a prismatic bar subjected to a tensile load F is shown in the figure.
• The elongation is $\delta = \frac{FL}{AE}$
• The stress at cross-section is given as, $\sigma = \frac{F}{A}$

Concept:

• For ductile material under static load, material start yielding at yield point beyond which material gets deformed permanently and finally fail by cup and cone fracture at UTS.
• The allowable stress of the material is also governed by yield stress and in industries too beyond this point material is termed as failed.
• If an external force is fluctuating then material get fail at stress which is considerably lower than the normal stress of static load. This behavior is termed as fatigue.
• There are two terminologies used in fatigue fractures. These are fatigue strength and fatigue life. The graph in between these tells the fatigue strength and finally define the endurance limit of a particular material.
• Under fatigue failure ductile material will fail as like as brittle material.
• The failure of ductile material under fluctuating load is called fatigue.
• Failure of ductile material under static load occurs at yield stress.
• The endurance limit is the maximum stress amplitude of a component that can withstand. It is the strength of the material.

Concept:

Elastic Modulus (E): When the body is loaded within its elastic limit, the ratio of stress and strain is constant. This constant is known as Elastic modulus

${\rm{E}} = \frac{{{\rm{Stress}}}}{{{\rm{Strain}}}} = \frac{{\rm{\sigma }}}{\epsilon}$

S.I unit: N/m²

Rigidity modulus (G): When a body is loaded within its elastic limit, the ratio of shear stress and shear strain is constant, this constant is known as the shear modulus.

${\rm{G}} = \frac{{{\rm{Shear;stress;}}}}{{{\rm{Shear;strain}}}} = \frac{{\rm{\tau }}}{\phi }$

S.I unit: N/m ²

Bulk modulus (K): When a body is subjected to three mutually perpendicular like stresses of same intensity then the ratio of direct stress and the volumetric strain of the body is known as bulk modulus

${\rm{K}} = \frac{{{\rm{Direct;stress}}}}{{{\rm{Volumetric;strain}}}} = \frac{{\rm{\sigma }}}{{\frac{{{\rm{\delta V}}}}{{\rm{V}}}}}$

S.I unit: N/m²

Explanation:

The relation between E, G, μ is

E = 2G (1 + μ)           ---(1)

Relation Between E, K, μ is

E = 3K (1 – 2μ)         ---(2)

Equating (1) & (2)

2G (1 + μ) = 3K(1 – 2μ)

2G + 2μG = 3K – 6μK

μ (2G + 6K) = 3K – 2G

$\mu = \frac{{3K; -; 2G}}{{2G; + ;6K}}$

Important Points

Poisson’s ratio:

 When the body is loaded within its elastic limit, the ratio of lateral strain and linear strain is constant. This constant is known as Poisson's ratio.

${\rm{\mu }} = \frac{{{\rm{lateral;strain}}}}{{{\rm{linear;strain}}}}$

Other relations between elastic constants are,

1) E = 2G (1 + μ)

2) E = 3K (1 – 2μ)

3) ${\bf{E}} = \frac{{9{\bf{KG}}}}{{3{\bf{K}}; + ;{\bf{G}}}}$

Concept:

• The shear stress on the principal plane is zero.
• The plane on which normal stress attains its maximum and minimum values are called principal planes.
• The planes of maximum and minimum normal stresses are at an angle of 90° to each other.
• Planes of maximum stress occur at 45° to the principal planes.

Concept:

We know that,

$\frac{M}{I}=\frac{\sigma _{b}}{y}=\frac{E}{R}$

And MR = EI 

where, R = Radius of curvature of beam, M = Bending moment, E = Young’s modulus, I = Area moment of inertia,

Here, EI = Flexural rigidity

i.e. EI ∝ M

Flexural rigidity is a measure of the resistance of a beam to bending, that is, the larger the flexural rigidity, the smaller the curvature for a given bending moment.

Explanation:

In the case of the simply supported beam as shown in the figure below:

${\delta_c} = \frac{{P{L^3}}}{{48EI}}$

${\theta _B} = \theta _A=\frac{{w{L^2}}}{{16EI;}}$

When a central point load is acting, we have the following boundary conditions,

• Deflections at supports are assumed zero unless otherwise stated, while the slope will be maximum.
• The slope at the center of symmetrically loaded and supported beams is zero.
• The deflection will be maximum at the center of the loaded beam.

Additional Information

Deflection and slope of various beams are given by:

Where, y = Deflection of beam, θ = Slope of beam

Explanation:

We can represent a close-coiled helical spring as,

Here, we can note the following points,

• The Axis of the helix passes through the axis of the imaginary cylinder enclosing the helix.
• For a close-coiled structure, the plane of the coil will be normal to the axis of the helix.

Concept:

• Euler’s formula gives correct results only for very long columns.
• For short or long columns Rankine’s Formula is used.
• Load columns can be analysed with the Euler’s column formulas can be given as
• $P = \frac{{{n^2}{\pi ^2}EI}}{{{L^2}}}$
• For both end hinged, n = 1
• $P = \frac{{{\pi ^2}EI}}{{{L^2}}}$
• For one end fixed and other free, n = 1/2
• For both ends fixed, n = 2
• For one end fixed and other hinged, n = √2
• Effective length:
• $L_{eq}=\frac{L}{n}$

Explanation:

Thick Cylinder: 

A cylinder is considered to be ‘thick’ if the ratio of the inner diameter to the thickness of the walls is less than 20.

When we considered thin cylinders,

• We assumed that the hoop stress was constant across the thickness of the cylinder wall
• We ignored any pressure gradient across the wall.

When we consider thick cylinders,

• These simplifications are no longer valid.
• we have to consider the variation of both hoop and radial stresses.

If the cylinder is long in comparison with its diameter, the longitudinal stress is assumed to be uniform across the thickness of the cylinder wall.

Additional Information

Thin Cylinder: A cylinder is considered to be ‘thin’ if the ratio of the inner diameter to the thickness of the walls is greater than 20.

Explanation:

There are three types of stresses on the walls of the pressure vessel

1. The longitudinal stress
2. The circumferential (hoop) stress
3. The radial stress

The radial stress for a thick-walled cylinder is equal and opposite to the gauge pressure on the inside surface, and zero on the outside surface.

The circumferential stress and longitudinal stresses are usually much larger for pressure vessels, and so for thin-walled instances, radial stress is usually neglected.

Explanation:

The mineral graphite is the allotrope of carbon.

Graphite is a semimetal and it is the most stable form of carbon under standard conditions.

Graphite is considered as the highest grade of coal just above anthracite and it is not normally used as fuel because it is difficult to ignite.

There are three principal types of natural graphite, each occurring in different types of ore deposit  

• Crystalline flake graphite occurs as isolated, flat, plate-like particles with hexagonal edges. It contains 85-95% carbon.
• Amorphous graphite occurs as fine particles and is the result of thermal metamorphism of coal which is the last stage of coalification. Very fine flake graphite is sometimes called amorphous in the trade. It contains 70-85% carbon after refining.

Lump graphite (known as vein graphite) occurs in fissure veins or fractures and appears as massive platy intergrowths of fibrous or acicular crystalline aggregates, and is probably hydrothermal in origin. It contains 95-99% carbon without refining.

Concept:

Vanadium:

• It has a marked effect on increasing the hardenability
• It inhibits grain growth when steel is heated at high temperature
• It is a strong carbide former; Vanadium carbides possess the highest hardness and wear resistance
• It improves fatigue resistance (endurance strength)
• It is generally found in tool steels

Nickel:

• Nickel dissolves in ferrite, increasing hardness, strength and toughness without sacrificing the ductility
• It increases the impact resistance of steel at a very low temperature
• A higher amount of nickel is added to increase the corrosion resistance of high chromium steels

Explanation:

The process of increasing the hardness of the surface by the process of diffusion of carbon and nitrogen in mild steels while keeping internal material stronger and ductile is known as surface hardening or Case Hardening. The following are the important processes for surface hardening:

• Case carburizing.
• Cyaniding
• Nitriding.
• Induction Hardening.
• Flame Hardening.

For iron or steel with low carbon content which has poor to no hardenability of its own, the case hardening process infuses additional carbon after the part formed its final shape.

Explanation:

Properties of Monel Metal:

• It is an important alloy of nickel and copper.
• It contains 68% nickel, 29% copper, and 3% other constituents like iron, manganese, silicon, and carbon.
• It resembles nickel in appearance and is strong, ductile, and tough.
• It is superior to brass and bronze in corrosion-resisting properties.
• It is used for making propellers, pump fittings, condenser tubes, steam turbine blades, sea water exposed parts, tanks, and chemical and food handling plants.

Explanation:

Thermoplastic: Plastics that can be deformed easily upon heating and can be bent easily.

• Linear polymers and a combination of linear and cross-linked polymers come under thermoplastics.
• Thermoplastics have low melting points are able to be re-melted and remolded and thus the material lends itself very well to plastic welding.
• Example: PVC, nylon, polythene, etc.

Thermosetting: Plastics that cannot be softened again by heating once they are molded.

• Heavily cross-linked polymers come under the category of thermosetting plastics.
• Thermosetting plastics have high melting points and tensile strength.
• Thermosetting plastic is high in molecular weight.
• High melting points and tensile strength signifies it is not a weldable plastic.
• Example: Bakelite, melamine, etc.

​

Properties of Plastic:

• Strong and ductile.
• Poor conductors of heat and electricity.
• Easily molded into different shapes and sizes.
• Resist corrosion and are resistant to many chemicals**.**

Explanation:

Bakelite is a polymer that is made up of the monomers phenol and Formaldehyde. This phenol-Formaldehyde is a Thermosetting polymer.

Some important properties of Bakelite are listed below.

• It can be easily Molded.
• Very smooth molding can be obtained from this polymer.
• Bakelite moldings are heat resistant and scratch-resistant.
• Owing to its low electrical conductivity, bakelite is resistant to electric current.

Bakelite structure:

Important Points

• Bakelite is used for making electrical switches.
• Melamine is used for floor tiles.

Explanation:

The Cutting Speed of Tool is given by 

$V~=~\frac{\pi DN}{1000}$

where, D = diameter of the cutting tool, N = Revolution per min, 

From the above equation ew can assume that

V ∝ D

∵ velocity is directly proportional to the diameter.

Concept:

From the merchant first analysis

$2\phi + \beta - \alpha = \frac{\pi }{2}$

Where, ϕ = shear angle, β = friction angle, α = cutting rake angle

There are several theories for the relationship between α (rake angle), β (friction angle), and ϕ (shear angle).

Ernst – Merchant theory:

$\phi = \frac{\pi }{4} - \frac{\beta }{2} + \frac{\alpha }{2}$

Stabler theory (Modified Ernst – Merchant theory):

$\phi = \frac{\pi }{4} - \beta + \frac{\alpha }{2}$

Lee and Shaffer theory:

$\phi = \frac{\pi }{4} - \beta + \alpha$

Explanation:

Boring is also known as Internal Turning.

Boring: 

• Boring is the process of enlarging and locating previously drilled holes with a single-point cutting tool.
• It is used to achieve greater accuracy of the diameter of a hole and can be used to cut a tapered hole as shown in the figure,

Additional Information

Milling: 

• Milling is the most common form of machining, a material removal process, which can create a variety of features on a part by cutting away the unwanted material.
• Milling is typically used to produce parts that are not axially symmetric and have many features, such as holes, slots, pockets, and even three-dimensional surface contours

Facing: 

• Facing is the operation of generating a flat surface on the LATHE machine.
• In this process, there is a removal of metal from the end of a workpiece to produce a flat surface as shown in the below figure.

Tapping: 

• The process of cutting inside thread is known as Tapping Operation.
• It is used to insert the screw or bolt can be threaded inside the hole.
• Also, it is used to make threads inside the nuts.
• Most of the Industry uses Hand tapping.

Explanation:

Zinc Alloys are preferably cast by Die Casting.

Casting: It is a manufacturing process in which liquid material is filled into the mold, which contains a hollow cavity of the desired shape and then allowed to solidify. The solidified part is known as a casting, which injected or broken out of the mold to complete the process.

Die Casting:  Die casting is a manufacturing process for producing accurately dimensioned, sharply defined, smooth, or textured surface metal parts. It is accomplished by forcing molten metal under high pressure into reusable metal dies

• Die casting produces parts that are durable and dimensionally stable while maintaining close tolerances. They are also heat resistant
• Die-cast parts are stronger than plastic injection moldings having the same dimensions.
• In Die Casting major form of Alloy used is Zinc Alloy.

Additional Information

Investment Casting: 

• It is a wax or suitable polymer pattern is coated by dipping into the refractory material slurry.
• Once the refractory material coating is hardened then this dipping process is repeated several times to increase the coating thickness and its strength.
• Once the final coating is hardened the wax is melted out and molten metal is poured into the cavity created by the wax pattern.
• Once the metal solidifies within the mould, metal casting is removed by breaking the refractory mould.

Centrifugal Casting: This is a process where the mold is rotated rapidly about its central axis as the metal is poured into it. Because of the centrifugal force, continuous pressure will be acting on the metal as it solidifies.

• The slag, oxides, and other inclusions being lighter gets separated from the metal and segregates toward the center.
• There are three main types of centrifugal casting processes. They are,

1. True centrifugal casting,
2. Semi-centrifugal casting, and
3. Centrifuging.

Shell-mould Casting: It is a process in which the sand mixed with a thermosetting resin is allowed to come into contact with a heated metallic pattern plate so that a thin and strong shell of mould is formed around the pattern.

• Then the shell is removed from the pattern and the cope and drag are removed together and kept in a flask with the necessary back-up material and the molten metal is poured into the mould.
• Generally, dry and fine sand that is completely free of the clay is used for preparing the shell moulding sand.
• The grain size to be chosen depends on the surface finish desired on the casting.
• Too fine a grain size requires a large amount of resin which makes the mould expensive.

Concept:

• Chaplets are used to support the core inside the mould cavity.
• Chaplets are often required to be placed between the mould wall or base and core, in order to avoid deflection of the core and achieve the exact section thickness of the casting.
• As far as possible, the chaplet should be of the same composition as the cast metal so that a homogeneous structure is obtained and no internal flows are developed.
• Chaplet also takes care of its own weight and overcomes the metallostatic forces.

Explanation:

Nose radius:

• It is the curvature of the tooltip.
• It provides strengthening of the tool nose and better surface finish.
• It is expressed in mm.
• Proper tool nose radius improves machinability to some extent through an increase in tool life by increasing mechanical strength and reducing the temperature at the tooltip.

Concept:

• As the name implies, the work is not supported on the centres, as in cylindrical grinding machine, but is held against the face of the grinding wheel by the combined action of a supporting rest and a control wheel for regulating the wheel.

• Centreless grinders are basically cylindrical grinders only, but they differ from center-type cylindrical grinders in that the work, instead of being held between centers, is supported by a combination of a grinding wheel, a regulating wheel, and a work rest blade

• Work is revolved and traversed across the face of to grinding wheel while being supported on the work rest blade

• The cutting pressure of the grinding wheel and regulating wheel keeps the workpiece in contact with the work rest blade

• The workpiece is not only revolved but also simultaneously given an axial movement by the regulating wheel and guides to pass between the wheels.

• Centreless grinding is a high – production process for continuously grinding cylindrical surface in which the workpiece is supported not by centres (hence the term centreless) or chucks, but by a blade.

Explanation:

Resistance welding is a group of welding processes in which coalescence is produced by the heat obtained from resistance of the workpiece to electric current in a circuit of which the workpiece is a part and by the application of pressure.

Seam welding

• Seam welding can be defined as a series of continuous spot welds.
• This process is employed for making a continuous joint between two overlapping pieces of sheet metal.
• In this process, the workpiece to be welded is placed between the two wheels which apply sufficient mechanical pressure and also carry sufficient current for producing continuous welds.
• The mechanical pressure applied is kept constant and the current is regulated by a timer.
• This type of welding is very important as it provides pressure-tight or leak proof joint.

Additional Information

Spot Welding

• Spot welding is a form of resistance welding in which the parts or pieces are joined in spots, accompanied by heated relatively small sections of the parts or pieces between suitable electrodes under pressure.
• Spot welding is the most widely used form of resistance welding.
• It is characterized by low cost, speed, and dependability making it a common electrical resistance welding process.
• Spot welding is usually used for joining or fabricating sheet metal structures.
• This type of welding only provides mechanical strength and is neither airtight nor watertight.

Projection welding

• Projection welding is a resistance welding process that produces coalescence by the heat obtained from the resistance to the flow of the welding current.
• The resulting welds are localized at predetermined points by projections, embossments, or intersections.
• Projection welding uses the same equipment as spot welding.
• The only difference is that the electrodes used are flat on the ends and larger in diameter than the flat electrodes used in spot welding.
• Successful projection welding depends greatly on the surface preparation of the pieces to be welded.

Percussion welding

• It is a resistance welding process wherein coalescence is produced simultaneously over the entire area of abutting surfaces by the heat obtained from an arc produced by a rapid discharge of stored electrical energy.
• This is a self-timing spot welding method.
• In this process, the pieces to be welded are held apart and backed up against heavy spring pressure.
• When the movable clamp is released, it moves rapidly carrying with it the piece to be welded.

Explanation:

Twist Drill Bit

• A twist drill is a metal rod of a specific diameter that has two, three, or four spiral flutes running most of its length.
• Two-flute drills are for primary drilling, whereas three- and four-flute drills are only for enlarging cast or punched holes in a production situation.
• Twist drills are the most widely used of all drill bit types.
• They will cut anything from wood and plastic to steel and concrete.
• They are most frequently used for metal cutting.
• Twist drills are not only the cheapest of all bits a woodworker could use, they offer the widest selection of sizes.
• Although they are designed for cutting metal, they will work quite well in wood and other materials.

Additional Information

Step Drill Bit

• A step drill bit is a drill bit that has the tip ground down to a different diameter.
• The transition between this ground diameter and the original diameter is either straight, to form a counter-bore, or angled, to form a countersink.

Spot Drill Bit

As the name specifies a spot drill to make a spot in the workpiece so that the twist drill does not deflect and "walk" off-center and the hole is drilled in the correct location.

Spade Drill Bit

• Spade bit ais for wood, plywood, and some plastics.
• It cuts quickly with surprising precision.
• Use them only at high speeds, to prevent the bit from catching in the workpiece and the drill from trying to wrench itself from your grasp.

Explanation:

Reaming

• Reaming is a process to make a hole smooth.
• Reamers are generally used for reaming a hole.
• Reamers are edge cutting tools and help to cut and smoothen the edge of a hole already drilled in the workpiece.
• Reaming is done after drilling a hole in your workpiece.
• The drilling diameter of the hole is kept a little less than the target diameter when you drill.
• To have the target diameter after reaming.
• There is a difference between the drill's diameter and the reamer's diameter. This is known as an allowance.
• There are many types of reamers such as the straight reamer, rose reamer, shell reamer, etc.
• By drilling, gives a hole that may not be as perfect as you expect. But after boring the hole and then reaming the hole gets the perfect shape.

​Tapping is the process of cutting a thread inside a hole

The Differences between drilling, boring, and reaming​

The process of boring always comes after drilling. And reaming comes at last. The sequence is always the same.

Explanation:

Process of Coining

• Coining is a closed die forging process, in which pressure is applied on the surface.
• The objective is to obtain closer tolerances, smoother surfaces, and eliminate drafts.
• Closed die forging is a process in which forging is done by placing the workpiece between two shaped dies.
• This process may be done in hot or cold working conditions but is predominantly a cold work process.
• In coining, the pressure is applied to a portion of the entire surface of a forging.
• This process is considered a method of precision stamping
• In precision stamping, the metal workpiece is subjected to high stress to induce plastic deformation in the shape of the die.
• Flashless forging is true closed die forging.

​

​Flashless forging 

• It is a Closed-die forging.
• the raw workpiece is completely contained within the die cavity during compression, and no flash is formed.
• The term flashless forging is appropriate to identify this process.
• Most important is that the work volume must equal the space in the die cavity within a very close tolerance.
• If the starting blank is too large, excessive pressures may cause damage to the die or press.
• if the blank is too small, the cavity will not be Filled.
• Flashless forging is often classified as a precision forging process.
• Forces in flashless forging reach values comparable to those in impression-die forging.
• Coining is a special application of closed-die forging in which line details in the die are impressed into the top and bottom surfaces of the workpiece.
• There is little flow of metal in coining, yet the pressures required to reproduce the surface details in the die cavity are high.
• The process sequence is illustrated in following Figure.

Additional Information

1. In open-die forging, a hammer strikes and deforms the workpiece, which is placed on a stationary anvil

2. Swaging is a forging process in which the dimensions of an item are altered using dies into which the item is forced.

3. Upset forging is a manufacturing process that decreases the length of a work-piece in order to increase the diameter of the material in this area with the help of force using a die.

Explanation:

Rotary transfer device

• The rotary transfer device moves the workpieces in a circular path, which usually has a maximum diameter of about 3 m.
• Most of the rotary transfer devices are dial indexing machines, but some other rotary mechanism, generally more rudimentary, can also be considered for rotary transfer.
• The dial indexing machine is an ideal choice for a transfer device when the total number of production operations required is rather low and not too complicated.

Rotary table:

• This is the least expensive of the rotary device.
• The rotary table is driven by a pneumatic or hydraulic cylinder, which actuates a rack and gear mechanism to convert linear motion to rotary motion.
• As the gear rotates, the Ratchet-Pawl is driven into teeth of the Ratchet gear attached to the dial table, causing the dial to index.
• The rotary table gives an accuracy of 0.125 mm, the index table gives better accuracy depending on the specific type of drive, reaching in some cases an accuracy close to ± 0.25 mm.

​

Floating position:

• To improve the accuracy in both these types of tables, a floating position of the holding fixtures could be used.
• In order to obtain the floating position, the holding fixture is built with a floating side in which holes for securing pins are drills.
• A set of securing pins is attached to the tools in such a way that they reach the holding fixture before the tool. When the pins reach the fixture, they will enter the holes drilled for them, these securing the holding fixture.

Concept:

• The diamond shape is a critical feature that helps with machining inaccuracies and smoother locating operations.
• When two round headpins are installed onto one plate, the distance between two mounting holes must be extremely precise and even then, the workpiece will not be placed on the base as easily as it would with the use of a diamond-shaped pin.
• Diamond pin location is used in fixtures because it takes care of any variation in center distance between two holes.

Explanation:

The PERT (Project Evaluation and Review Technique) technique is used, when activity time estimates are stochastic in nature. For each activity, three values of time (optimistic, most likely, pessimistic) are estimated.

• Optimistic time (to) estimate is the shortest possible time required for the completion of an activity
• Most likely time (tm) estimate is the time required for the completion of activity under normal circumstances
• Pessimistic time (tp) estimate is the longest possible time required for the completion of an activity

In PERT expected time of an activity is determined by using the below-given formula:

${t_e} = \frac{{\left( {{t_o} + 4{t_m} + {t_p}} \right)}}{6}$

Explanation:

Critical Path

• Critical path activities are the project tasks that must start and finish on time according to schedule.
• A delay in any critical path activity will delay the completion of the scheduled project.

Float or Slack in critical activity

• Slack or float refers to the amount of time that various related tasks can be delayed without pushing the schedule.
• Float is a measure of schedule flexibility.
• The critical path, by definition, has no slack. But other tasks not on the critical path may have a built-in float.
• If the Start date and Finish date for an activity are the same as the schedule, the activity is said to have zero floats.
• Activities that have zero floats must start on time to prevent the schedule from slipping.
• Critical activities can also have a negative float.
• Negative float occurs when an activity finishes before the scheduled date.

Additional Information

Critical Path Method (CPM):

It is commonly used for those projects which are repetitive in nature and where one has prior experience of handling similar projects. Its is a deterministic model and places emphasis on time and cost for activities of a project.

• A project can be defined as a set of large numbers of activities or jobs that are performed in a certain sequence determined.
• A network is a graphical representation of a project, depicting the float as well as the sequence of well-defined activities and events.
• An activity is any portion of a project which consumes time or resources and has a definable beginning and end.
• Event is the beginning and ending point of activity or group of activities.

B-D-E-H is the critical path in the diagram below.

Explanation:

Forecast value in Smoothing constant method is given by-

F_t = F_t-1 + α [ D_t-1 - F_t-1 ]

where F_t = Current period forecast, F_t-1 = last period forecast, D_t-1 = last period demand, α = smoothing constant

for α = 0 

Ft = Ft-1 

Hence for α = 0 only the current period forcast becomes equal to last period forecast.

Important Points

For α = 1 current period forecast will become equal to the last period demand.

Explanation:

Lean production:

• Lean production means supplying the customer with exactly what the customer wants, when the customer wants it, without waste, through continual improvement.
• Waste is anything that does not add value to the product.
• Lean production is driven by the "pull" system of the customer's order.
• These waste reduction efforts improve productivity and processes.

Customized Production:

• In this flow of material and parts from one location to another is intermittent or discontinuous.
• Each job order is different from the previous as regards its type, specifications, quality, and quantity.
• Product design takes a lot of time.
• Prior planning becomes difficult.
• General-purpose machinery and a flexible layout are preferred.
• The number of items to be manufactured is very small.

Mass production:

• Mass production means the production of items on large scale.
• In this type of production, there is a continuous and steady flow of materials.
• It offers the lowest production cost per unit.
• Material handling is reduced to a minimum since very little time is spent on the resetting of the machine.
• Work cycles are short and of a repetitive nature.
• Plant layout and facilities are designed to suit production requirements.
• Mass production is preferred where there is a continuous and regular demand for products.

Batch Production:

• It is a common type of production.
• The flow of material is intermittent.
• The plant layout is of the process type.
• Product planning is done for each batch.
• Proper maintenance of equipment and machinery is essential.
• A good production control system must be developed.
• Articles are manufactured in batch as per the specific order procured.

Explanation:

Process layout:

• A process layout also called a job-shop or Functional layout.
• In this layout, similar equipment or machines are grouped together.
• This layout is useful to produce High varieties of products with low volume.
• Part travels, according to the established sequence of operations, from area to area, where the proper machines are located for each operation.
• Hospitals, where the areas are dedicated to a particular type of care, is an example of this.

Product layout:

• A product layout is also called a flow-shop layout.
• This layout is very useful to produce a very high volume of products with very low variety.
• In this layout equipment or work-processes are arranged according to the requirement of a specific product.
• The path for each part is, in effect, a straight line.
• Automobiles manufacturing is an example of the product layout industry.

Cellular layout:

• This layout is also called a Hybrid layout.
• This layout is similar to a process layout in that cells are designed to perform a specific set of processes, and it is similar to a product layout in that the cells are dedicated to a limited range of products.
• This layout is can be used to produce High volume with High variety.
• In this layout grouping of dissimilar machines into work centers or cells to work on products that have similar shapes and processing requirements has been performed.
• A coding system is used to specify machine types that go into a cell in this layout.

Fixed-position layout:

• ​This layout is just opposite to the process layout.
• Very low volume and very low variety products are made in this layout.
• In a fixed-position layout, the product remains at one location & manufacturing equipment is moved to the product rather than vice-versa.
• Very high weight and bulky product which can not be moved from one place to another place conveniently like Ships, Rockets, Submarine are made with this layout.

Explanation:

Group Technology:

• Group technology (GT ) is a concept to increase production efficiency by grouping various parts and products with similar design and production process.
• Similarities among parts permit them to be classified into part families.
• In each part family, processing steps should be of a similar type.
• The improvement is typically achieved by organizing the production facilities into Manufacturing cells, that specialize in 
  

production of certain part families.

• Grouping the production equipment into machine cells where each cell specializes in the production of a part family is called cellular manufacturing.

Hence cellular layout is followed to achieve advantages of Group Technology.

Cellular layout:

• This layout is also called a Hybrid layout.
• This layout is similar to a process layout in that cells are designed to perform a specific set of processes, and it is similar to a product layout in that the cells are dedicated to a limited range of products.
• This layout is can be used to produce High volume with High variety.
• In this layout grouping of dissimilar machines into work centers or cells to work on products that have similar shapes and processing requirements has been performed.
• A coding system is used to specify machine types that go into a cell in this layout.

Process layout:

• A process layout also called a job-shop or Functional layout.
• In this layout, similar equipment or machines are grouped together.
• This layout is useful to produce High varieties of products with low volume.
• Part travels, according to the established sequence of operations, from area to area, where the proper machines are located for each operation.
• Hospitals, where the areas are dedicated to a particular type of care, is an example of this.

Product layout:

• A product layout is also called a flow-shop layout.
• This layout is very useful to produce a very high volume of products with very low variety.
• In this layout equipment or work-processes are arranged according to the requirement of a specific product.
• The path for each part is, in effect, a straight line.
• Automobiles manufacturing is an example of the product layout industry.

Fixed-position layout:

• ​This layout is just opposite to the process layout.
• Very low volume and very low variety products are made in this layout.
• In a fixed-position layout, the product remains at one location & manufacturing equipment is moved to the product rather than vice-versa.
• Very high weight and bulky product which can not be moved from one place to another place conveniently like Ships, Rockets, Submarine are made with this layout.

Explanation:

Line balancing:

Line balancing is a production strategy that involves balancing operator and machine time to match the production rate to the Takt time. Takt time is the rate at which parts or products must be produced in order to meet customer demand.

Benefits of Line Balancing

1. Reduce waiting waste.
2. Reduce inventory waste.
3. Absorb internal and external irregularities.
4. Reduce production costs and increase profits.

Line balancing is used in Product Layout as in this layout rate of production is very high and requires proper balancing between machine and operator for better efficiency.

Explanation:

Lead time:

• Lead time measures how long it takes to complete a process from beginning to end. In manufacturing, lead time often represents the time it takes to create a product and deliver it to a consumer.

Delivery time or Delivery lead time:

• It is defined as the amount of time it takes for goods that have been brought to arrive at the place where they are wanted.

MTS:

• Make to stock (MTS) is a traditional production strategy that is used by businesses to match the inventory with anticipated consumer demand. An MTS approach requires a business to redesign operations at specific times, instead of keeping a steady level of production year-round.

MTO:

• Make to order (MTO), or made to order, is a business production strategy that typically allows consumers to purchase products that are customized to their specifications. It is a manufacturing process in which the production of an item begins only after a confirmed customer order is received.

ATO:

• Assemble-to-order (ATO) is a business production strategy where products that are ordered by customers are produced quickly and are customizable to a certain extent. It typically requires that the basic parts of the product are already manufactured but not yet assembled.

ETO:

• Engineer to order (ETO) is a process used by manufacturers. It begins as the company starts to gain orders for various items of sale. Manufacturers outline the design, engineer and do the production as required for the unique products from beginning to end for customers.
• The order from shortest to longest delivery lead time is MTS-ATO-MTO-ETO

Explanation:

Just In Time (JIT) Production system:

• Just in time (JIT) manufacturing is a workflow methodology aimed at reducing flow times within production systems, as well as response times from suppliers and to customers.

Important characteristics of JIT Production system

• Daily or hourly deliveries of small quantities of parts from suppliers.
• Certification of supplier quality, so that no receiving inspections are needed.
• The use of Kanbans to drive the demand at each workstation.
• Production stops as soon as the immediate demand level has been fulfilled.
• On-site inspection of each in-process product from the preceding work center, so that flaws are discovered at once.
• The use of rapid machine setups, so that production runs can be as short as one unit.
• The cross-training of employees, so that they are certified to work on multiple tasks.
• Immediate shipment of completed goods to customers as soon as an order has been fulfilled.

Calculation of optimal Batch size:

It is based on the simplistic calculation. We have to minimize the Total cost to get the optimal batch size. Total cost includes Setup cost, Holding cost and production cost but as production cost is constant so we consider only Setup and Holding cost.

In ideal case i.e. setup cost is Zero batch size of 1 is optimal or best.

Concept:

• Total Project Cost and Optimum Duration: The total project cost is the sum of the direct cost and indirect costs. It is U-shaped curve.
• We find that the minimum total cost is obtained at some duration known as the optimum duration t_o, the corresponding cost is known as the minimum cost.
• If the project duration is increased, total cost will increase, while if project duration is decreased to the crash value project cost will be the highest.

Explanation:

Material handling:

It is the art and science of moving, packaging and storing of substances in any form in a protective and efficient manner.

Material Handling system includes:

• AS/RS: Automated Storage and Retrieval Systems (abbreviated as AS/RS) are large automated structures that involves racks, aisles and shelves that are accessible by a type of mechanized shuttle system (like a cherry picker) for the quick retrieval of items.
• Conveyor systems: Automated conveyor systems carry heavy materials to specified destinations using belts, flexible chain, or live rollers. It is a highly efficient equipment to move large volumes of material quickly.
• Robotic delivery systems: These automated systems are ideal for moving products on an assembly line or transporting goods throughout a plant or warehouse.
• Automatic guided vehicles: These vehicles are mobile robots that follow specific markers or wires in the floor to move large materials around a manufacturing facility or warehouse. Vision, magnets, or lasers can also be used as methods for AGV navigation.

Additional Information

Explanation:

Mass production:

• Manufacture of discrete parts or assemblies using a continuous process are called mass production. The machineries are arranged in a line or product layout. It is also called as continuous production.
• Mass manufacturing is characterized by very high production rates and low variety or flexibility

Advantages of Mass Production

• Higher rate of production with reduced cycle time.
• Higher capacity.
• Less skilled operator can manage the process.
• Low in process inventory.
• Production cost per unit item will come down due to economies of scale.

Additional Information

Explanation:

Therbligs:

• It is the 17 basic fundamental motion elements which is used in the study of motion economy in the workplace. The term Therbligs is an anagram of its inventor's name (Gilbreths spelled backwards) and coined by Frank.

Note:- Initially there are 18 Therbligs but now it is 17. Find Therblig is removed as it has similar function as Search Therblig

Therbligs are used to

• examine smallest of motions
• categorize motions
• Identify unnecessary / fatigue producing motions

Additional Information

Motion study: 

• Analysis of the basic hand, arm, and body movements of workers as they perform work

Work design:

• Design of the methods and motions used to perform a task
• Work consists of tasks, Tasks consist of work elements and Work elements consist of basic motion elements.