QUESTIONS ON THEORY OF MACHINES

Q-The number of instantaneous centers of rotation in a slider-crank quick return mechanism is 
1-10
2-8
3-6
4-4

Q-A planer mechanism has 10 links& 12 rotary joints. Using Kutchback's criterion ,the number of degree of freedom of mechanism is 
a-1
b-3
c-2
d-4

Workouts on Vocabulary [SYNONYMS]

1-CONTENTIOUS                                      Litigious

2-BONVIVANT                                            Epicurean
3-BUCOLIC                                                Pastoral
4-CONTROVERT                                       Contradict
5-RELEGATE                                              Demote
6-HUDDLE                                                 Cluster
7-COARSE                                                 Uncouth
8-AQUILINE                                               Hunched
9-VERBATIM                                              Replica
10-EUPHONIC                                           Canorous

UN declares 2017 as the International Year of Sustainable Tourism for Development.

The United Nations General Assembly (UNGA) has declared the year 2017 as the International Year of Sustainable Tourism for Development. The declaration recalls the potential of tourism sector to advance the universal 2030 Agenda for Sustainable Development and 17 Sustainable Development Goals (SDGs).l

Twinning

—The second important mechanism of plastic deformation is twinning.

—It results when a portion of crystal takes up an orientation that is related to the orientation of the rest of the untwined lattice in a definite, symmetrical way.

—The twinned portion of the crystal is a mirror image of the parent crystal. The plane of symmetry is called twinning plane.

—Each atom in the twinned region moves by a homogeneous shear a distance proportional to its distance from the twin plane.

The lattice strains involved in twinning are small, usually in order of fraction of inter-atomic distance, thus resulting in very small gross plastic deformation.

—The important role of twinning in plastic deformation is that it causes changes in plane orientation so that further slip can occur.

—Twinning generally occurs when slip is restricted, because the stress necessary for twinning is usually higher than that for slip.

—Thus, some HCP metals with limited number of slip systems may preferably twin.

—Also, BCC metals twin at low temperatures because slip is difficult.

—Of course, twinning and slip may occur sequentially or even concurrently in some cases.

Slip

—Slip is the prominent mechanism of plastic deformation in metals.

—It involves sliding of blocks of crystal over one other along definite crystallographic planes, called slip planes.

—In physical words it is analogous to a deck of cards when it is pushed from one end.

—Slip occurs when shear stress applied exceeds a critical value.

—During slip each atom usually moves same integral number of atomic distances along the slip plane producing a step, but the orientation of the crystal remains the same.

—Steps observable under microscope as straight lines are called slip lines.

Plain carbon steel

Low-carbon steel: (less than 0.3%C)

—Good formability and weld ability but lack hardenability

—Used in hot-forming, cold-forming etc.

  Medium carbon steel or Mild steel (0.3% to 0.8 % carbon)

—high toughness & ductility

—Most widely used steel

—Heat treatable (austenitizing, quenching and tempering).

—Hardenability is increased by adding Ni, Cr, Mo.

—Used in various tempered conditions.

—Typical applications: gears, railway tracks, machine parts. 

  High carbon steel (more the 0.8 %C)

—Hardness & wear resistance  are high but  Toughness & formability is very low

  NoteÞ purest form of Iron i.e. wrought iron has least carbon content.

Question on iron carbon diagram

Pearlite phase in an iron-carbide phase diagram is

  (a) Eutectic phase

  (b) Hypoeutectic mixture

  (c) Eutectoidal mixture

  (d) Hypereutectic phase

Ans-c

An iron-carbon binary alloy has 0.5% C by weight. What is this alloy called?

  (a)  Eutectoid alloy

  (b)  Eutectic alloy 

  (c)  Hypo-eutectoid alloy

  (d)  Hypereutectoid alloy

Ans-c

Fe-C equilibrium diagram

—The structural form of pure iron at room temperature is called ferrite or    -iron.

—Ferrite is soft and ductile.

—Since ferrite has a body-centred cubic structure, the inter-atomic spaces are small and pronouncedly oblate, and cannot readily accommodate even a small carbon atom. Therefore, solubility of carbon in ferrite is very low, of the order of 0.006% at room temperature.

—The maximum carbon content in ferrite is 0.05% at 723 °C.

—In addition to carbon, a certain amount of silicon, manganese and phosphorous may be found in ferrite.

—The face-centred modification of iron is called austenite or    -iron. It is the stable form of pure iron at temperatures between 910°C and 1400°C. At its stable temperature austenite is soft and ductile and consequently, is well suited for manufacturing processes.

—The face-centred cubic structure of iron has larger inter-atomic spacing than in ferrite. Even so, in FCC structure the interstices are barely large enough to accommodate carbon atoms, and lattice strains are produced. As a result, not all the interstitial sites can be filled at any one time.

—The maximum solubility is only 2% of carbon at 11 30°C.

—Above 1400°C, austenite is no longer the most stable form of iron, and the crystal structure changes back to a body-centred cubic phase called delta iron. This is the same phase as the    -iron except for its temperature range.

—The solubility of carbon in   -ferrite is small, but it is appreciably larger than In   -ferrite, because of higher temperature. The maximum solubility of carbon in &iron is 0.1% at 1490°C.

—In iron-carbon alloys, carbon in excess of the solubility limit must form a second phase, which is called iron carbide or cementite.

—Iron carbide has the chemical composition of       . This does not mean that iron carbide forms molecules of   but simply that the crystal lattice contains iron and carbon atoms in a three -to- one ratio.

—The compound   has an orthorhombic unit cell with twelve iron atoms and four carbon atoms per cell, and thus has a carbon content of 6.67%.

—As compared to austenite and ferrite, cementite being an inter-metallic compound, is very hard and brittle.

—The presence of iron carbide with ferrite in steel greatly increases the strength of steel.

—In the reaction, the simultaneous formation of ferrite and cementite from austenite results at the temperature of 723°C and composition of 0.80% carbon.

—There are nearly 12% of iron carbide and slightly more than 88% of ferrite in the resulting mixture.

—Since the ferrite and cementite are formed simultaneously, they are intimately mixed. Characteristically, the mixture is lamellar, i.e., it is composed of alternate layers of ferrite and cementite.

—This micro-structure is called pearlite which is very important in iron and steel technology, because it can be formed in almost all steels by means of suitable heat treatments.

—The alloy containing 0.80% of carbon is called the eutectoid steel.

—Upon cooling the eutectoid steel below 723°C, all of the austenite is transformed into pearlite.

—Alloys with less than 0.80% C are called hypo-eutectoid steels and those with higher composition are called hyper-eutectoid steels. 

Broaching

—Broaching is a multiple-tooth cutting operation with the tool reciprocating.

—Since in broaching the machining operation is completed in a single-stroke as the teeth on the cutting tool, called broach, are at gradually increasing height corresponding to the feed per tooth of a milling cutter.

—The shape of the broach determines the shape of the machined part.

—Broaching was originally developed for machining internal keyways, but looking at the advantages, it has been extensively used in the mass production of automobile component manufacture for various other surfaces as well.

Questions on BORING

The main purpose of boring operation, as compared to drilling is to:

  (a)  Drill a hole  

  (b)  Finish the drilled hole

  (c)  Correct the hole 

  (d)  Enlarge the existing hole

Ans-d

Enlarging an existing circular hole with a rotating single point tool is called   

  (a)  Boring        (b)  Drilling

   (c)  Reaming   (d)  Internal turning.

Ans-a

—Boring

—Boring always involves the enlarging of an existing hole, which may have been made by a drill or may be the result of a core in a casting.

—An equally important and concurrent purpose of boring may be to make the hole concentric with the axis of rotation of the workpiece and thus correct any eccentricity that may have resulted from the drill drifting off the centerline. Concentricity is an important attribute of bored holes.

—When boring is done in a lathe, the work usually is held in a chuck or on a faceplate. Holes may be bored straight, tapered, or to irregular contours.

—Boring is essentially internal turning while feeding the tool parallel to the rotation axis of the workpiece.

Question on REAMER

For reaming operation of blind hole, the type of reamer required is

  (a)  Straight flute reamer

  (b)  Right hand spiral fluted reamer

  (c)  Left hand spiral fluted reamer

  (d)  None of the above

Ans-b

Shell reamers are mounted on

  (a)  Tool holders  (b)  Amour plates

  (c)  Arbor              (d)  Shanks

Ans-c

Types of Reamers

The principal types of reamers are:

  1.  Hand reamers

       a.  Straight

  b.  Taper

  2.  Machine or chucking reamers

  a.  Rose

  b.  Fluted

  3.  Shell reamers

  4.  Expansion reamers

  5.  Adjustable reamers

Reamer Flutes

—The reamer flutes are either straight or helical.

—The helical flutes promote smoother cutting and should be used specifically for holes that are not continuous, such as those with keyways parallel to the axis of the hole.

—The cutting action of the helical flutes is smoother and helps in preventing chatter.

—The reamers are termed as left hand or right hand, depending upon the direction in which they are moved, looking from the shank to the cutting portion.

—The right-hand reamer with right-hand helix is used for roughing cuts, since the tool tends to go into the workpiece more efficiently and thereby promotes the material removal.

—A right-hand reamer with left-hand flutes is used for finishing cuts.

Reaming

—Reaming removes a small amount of material from the surface of holes.

—It is done for two purposes: to bring holes to a more exact size and to improve the finish of an existing hole.

—Multiage cutting tools that has many flutes, which may be straight or in a helix are used. 

—No special machines are built for reaming. The same machine that was employed for drilling the hole can be used for reaming by changing the cutting tool.

—Only a minimum amount of materials should be left for removal by reaming. As little as 0.1 mm is desirable, and in no case should the amount exceed 0.4 mm.

—A properly reamed hole will be within 0.025 mm of the correct size and have a fine finish.

Question on BUFFING

Buffing wheels are made of 

  (a)  Softer metals  (b)  Cotton fabric

  (c)  Carbon             (d)  Graphite

Ans- b

Question on cutting fluid

Dry and compressed air is used as cutting fluid for machining

  (a)  Steel         (b)  Aluminium 

  (c)  Cast iron  (d)  Brass

Ans-c

Question on grinding

Tool life in the case of a grinding wheel is the time

  (a)  Between two successive regrinds of the wheel

  (b)  Taken for the wheel to be balanced

  (c)  Taken between two successive wheel dressings

  (d)  Taken for a wear of 1mm on its diameter

Ans-c

question on grinding.

In the grinding wheel of A 60 G 7 B 23, B stands for

  (a)  Resinoid bond         (b)  Rubber bond

  (c)  Shellac bond           (d)  Silicate bond

Honing

—Honing is a finishing process, in which a tool called hone carries out a combined rotary and reciprocating motion while the workpiece does not perform any working motion.

—Most honing is done on internal cylindrical surface, such as automobile cylindrical walls. The honing stones are held against the workpiece with controlled light pressure. The honing head is not guided externally but, instead, floats in the hole, being guided by the work surface.

—It is desired that

1. Honing stones should not leave the work surface

2. Stroke length must cover the entire work length.

3. In honing rotary and oscillatory motions are combined to produce a cross hatched lay pattern.

—The honing stones are given a complex motion so as to prevent every single grit from repeating its path over the work surface. 

Is transverse feed provided in vertical spindle reciprocating table surface grinder?

—Usually no transverse feed is provided in such machine. The wheel diameter is kept larger than the width of the workpiece surface to be ground.

State the disadvantages of centreless cylindrical grinding machine?

  •  It does not grind concentrically with centres.

  •  Large diameter short work piece are difficult to   control in the process

  •  It may not improve work piece perpendicularity.

Centerless Grinding

—Centerless grinding makes it possible to grind both external and internal cylindrical surfaces without requiring the workpiece to be mounted between centers or in a chuck.

—This eliminates the requirement of center holes in some workpieces and the necessity for mounting the workpiece, thereby reducing the cycle time.

—Two wheels are used. The larger one operates at regular grinding speeds and does the actual grinding. The smaller wheel is the regulating wheel. It is mounted at an angle to the plane of the grinding wheel.

—The regulating wheel controls the rotation and longitudinal motion of the workpiece and usually is a plastic- 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.

What are the characteristic features of a universal cylindrical grinder?

—Characteristic features of a universal cylindrical grinder not possessed by plain cylindrical grinder are:

  •  Swivelling wheel head

  •  Swivelling wheel head slide

  •  Swivelling head stock

Is dressing necessary for single layer wheel?

—Conventional macro level dressing is not required because the wheel inherently has an open structure. However, touch dressing is carried out to obtain better uniformity in grit height in order to improve surface finish of the workpiece.

Can a resin bonded CBN wheel be electrochemically dressed?

—Electrochemical dressing is not possible with resin bonded wheel because it is not electrically conducting.

What is the main short coming of vitrified bond?

Vitrified bond is brittle and can not with stand high impact loads. This bond can not be used for high wheel speed due to risk of wheel breakage under centrifugal force.

Why is coarse grain and open structured wheel is preferred for stock removal grinding?

—Coarse grit allows large grit protrusion and open structure provides large inter grit chip space. Thus in combination those two provide large space for chip accommodation during stock removal grinding and risk of wheel loading is minimized.

Why does single layer grinding wheel show progressive rise of force during grinding of high speed steel?

—The geometry of grit undergoes irreversible change in the form of rounding or flattening due to wear caused by rubbing action of hard carbides present in high speed steel.

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

Al2O3 is tougher than SiC. Therefore it is preferred to grind material having high tensile strength like steel. Moreover, Al2O3 shows higher chemical inertness than SiC towards steel leading to much improved wear resistance during grinding.

TYPES OF DRILL PRESSES

—Vertical or pillar type

—Radial Arm type

—Gang drill

—Multi Spindle drill

—Numerical Control drill