### Technical Interview Questions for Mechanical Engineers

1. What is anisotropy ?

The phenomenon of different properties in different directions is called anisotropy.

2. What is vapour pressure and when it becomes important ?

Vapour pressure is the partial pressure created by the vapour molecules when evaporation takes place within an enclosed space. It becomes important when negative pressures are involved.

3. Explain absolute viscosity ?

It is the ratio of shear stress and the gradient of velocity with distance between a fixed plate and moving plate (Its unit is Pa . s).

4. Define surface tension ?

Surface tension of a liquid is the work that must be done to bring enough molecules from inside the liquid to the surface to form one new unit area of that surface. (Its unit is Nm/m2 or N/m).

5. What is capillarity and when liquid rises or falls ?

Capillarity : The rise or fall of a liquid in a capillary tube caused by surface tension. The magnitude of rise or fall depends on the relative magnitudes of the cohesion of the liquid and adhesion of the liquid to the walls of the containing vessels. Capillarity is of importance in tubes smaller than 10 mm diameter.

Rise of liquid : Liquids rise (in capillary tube) when they wet and adhesion of the liquid to the walls is greater than the cohesion of the liquid.

Fall of liquid : Liquids fall (in capillary tube) when they do not wet and the cohesion of the liquid is greater than the adhesion of the liquid to the walls.

6. What is the difference between perfect and real fluids ?

Perfect fluids are treated as if all tangential forces created by friction can be ignored.
Real fluids refer to the cases in which friction must properly be taken into account.

7. Which factors affect tool size ?

Following factors influence tool size :

1. Process variable (speed, feed and depth of cut)
2. Tool material
3. Tool geometry
4. Work piece material, its hardness, microstructure and surface condition
5. Cutting conditions.

8. When the motion of fluid is fully known ?

The motion of a fluid is fully known when the velocity of each of its particles can be specified.

9. Define fluid.

A fluid is a substance that can’t remain at rest under the action of any shear force.

10. How is the size of a vertical boring machine designated ?

The specifications of vertical boring machine are :

(i) Column height
(ii) Table size
(iii) Floor area
(iv) Weight of the job
(v) Spindle of the motor.

11. What do you mean by drill sleeve and a drill socket ?

The drill sleeve is suitable for holding only one size of shank. If the taper shank of the tool is smaller than the taper in the spindle hole, a taper sleeve is used.

When the tapered tool shank is larger than the spindle taper, drill sockets are used to hold the tools. Drill sockets are much longer in size than the drill sleeves.

12. What is nonbarotropic fluid ?

Fluid such as air for which the density is not a single-valued function of the pressure (aerostatics).

13. On account of which property, the falling drops of rain acquire superical shape.

Surface tension.

14. How you can definte standard atmospheric pressure ?

By international agreement, the standard atmospheric pressure is defined as 101.325 kN/m2.

15. What do you understand by acoustic velocity ?

Accoustic velocity is the speed of a small pressure (sound) wave in a fluid.

16. Explain the difference between centipoise and centistoke ?

Centipoise is the unit of dynamic viscosity. It is equal to 1/100 of poise, and poise = 0.1 Pa . s. Value of dynamic viscosity of water at 20°C is approximately equal to 1 Centipoise.

Centistoke is the unit of kinematic viscosity and is equal to 1/100 x stoke. Stoke is defined as 1 square centimeter per second.

17. What is saybolt seconds universal ?

Saybolt Seconds Universal (SSU) is the unit of viscosity and is equal to the time required for a gravity flow of 60 cc through saybolt universal viscometer.

18. Explain difference between cohesion, adhesion and capillarity ?

Cohesion is the attraction of like molecules.
Adhesion is the attraction of unlike molecules for each other.
Capillarity is the elevation or depression of a liquid surface in contact with a solid.

19. Differentiate between gear hobbing and gear shaping with reference to various relative motions and applications.

Gear hobbing is a continuous indexing process in which both cutting tool (a hob) and work piece rotate in a constant relationship while the hob is being fed into work. Hob is also imparted a radial feed.

Gear shaping uses a pinion type of cutter which is reciprocated with required cutting speed along the face of workpiece and is gradually fed radially to plunge. Continuous generation motion is obtained by feeding cutter to full depth and rotating the cutter and workpiece slowly.

20. What is bulk modulus of elasticity ?

It represents the compressibility of a fluid. It is the ratio of the change in unit pressure to the corresponding volume change per unit of volume.

21. What is is entropic exponent ?

It is the ratio of the specific heat of a gas at constant pressure to the specific heat at constant volume.

22. The best shape of a runner in sand casting is an inverted frustum of a cone why ?

Tapered shape with bigger diameter at top and smaller at bottom ensures avoidance of entrainment or absorption of air/gases into the metal while passing through the runner.

23. Explain difference between ideal fluid and non-newtonian fluid.

Ideal fluid is one for which resistance to shearing deformation is zero.
Non-netwtonian fluids deform in such a way that shear stress is not proportional to the rate of shearing deformation.

24. Explain the difference between poise and stoke.

Poise is the unit of viscosity in CGS unit and is measured in dyne sec/cm2.1 poise = 10_1 Pa s.
Stoke is the unit of kinematic viscosity in CGS units and is measured in cm2/s.

25. How does the pressure in an isothermal atmosphere behave ?

It increases exponentially with elevation.

26. The centre of pressure for a plane surface immersed vertically in a static liquid compared to centroid of area is always … ?

Below.

27. What is buoyant force equal to ? Define centroid of the area.

Volume of liquid displaced.

Centroid of the area : The point at which the area might be concentrated and still leave un¬changed the first moment of the area around any axis. It is also the centre of gravity.

28. Define vapour pressure and on what parameters it depends ?

The pressure exerted when a solid or liquid is in equilibrium with its own vapour is called vapour pressure. It is a function of the substance and its temperature.

29. Explain the difference between steady flow, one dimensional flow, two dimensional flow and three dimensional flow ?

If at every point in the continuum, the local velocity and other fluid property, remains unchanged with time, it is referred to as steady flow.

One dimensional flow is one in which a line is necessary to describe the velocity profile.
Two dimensional flow is one in which an area is necessary to describe the velocity profile.
Three dimensional flow is one in which a volume is necessary to describe the velocity profile.

30. What is aquifer and explain difference between confined and unconfined aquifers.

Groundwater occurs in permeable, water-bearing geologic formations known as aquifers.

Confined aquifer : It is a relatively high-permeable, water-bearing formation.
Unconfined aquifer : It is a water-bearing formation with a free water table, below which the soil is saturated.

31. What do you understand by flurial hydraulics ?

The channels may be classified as rigid boundary or mobile boundary channels. Mobile boundary channels include rivers and unlined alluvial canals, the boundaries of which are made of loose soil which can be easily eroded and transported by flowing water. The study of flow of water in mobile boundary channels is dealt in flurial hydraulics.

32. By which instruments the shear stress in fluids can be measured directly ?

By Stanton tube or Preston tube.

33. On what factors does the pressure at a point as a static mass of liquid depends upon?

Specific weight of liquid and the depth below the free liquid surface.

34. What is the difference between hoop or longitudinal tension and circum-ferencial tension ?

Hoop tension is created in the walls of a cylinder subjected to internal pressure. For thin walled cylinder (t < 0.1 d), hoop stress = pressure x radius/thickness.
Longitudinal tension in thin-walled cylinders closed at the ends is equal to half the hoop tension.

35. How much force is exterted by liquid ?

Force exerted by a liquid on a plane area A is equal to the product of the unit weight pg of the liquid, the depth hcg of the centre of gravity of the area, and the area.

36. State standard point angle and helix angle of a twist drill.

Standard point angle of twist drill is 118° and helix angle varies from 16° to 30° depending as diameter of hole.

37. Explain the difference between horizontal and vertical components of hydrostatic force ?

The horizontal component of the hydrostatic force on any surface is equal to the normal force on the vertical projection of the surface and acts through the center of pressure for the vertical projection.

Vertical component of the hydrostatic force on any surface is equal to the weight of the volume of liquid abaove the area and passes through the e.g. of the volume.

38. What should be diameter of capillary tube to avoid correction for effect of capillarity in manometer ?

Greater than 6 mm.

39. How much hydrostatic pressure acts on a curved surface ?

The horizontal component of the total hydrostatic pressure force on any surface is always equal to the total pressure on the vertical projection of the surface, and can be located through the center of pressure of this projection.

The vertical components of the total hydrostatic pressure force on any surface is always equal to the weight of the entire water column above the surface extending vertically to the free surface, and cvan be located through the cemntroid of this column.

40. How much is the horizontal component of force on a curved surface ?

Force on a vertical projection of the curved surface.

41. State Archimedes principle.

Any weight, floating or immersed in a liquid, is acted upon by a buoyant force equal to the weight of the liquid displaced. This force acts through the center of buoyancy, i.e. the e.g. of the displaced liquid.

42. What do you understand by center of buoyancy ?

Center of buoyancy is the center of gravity of the displaced liquid and buoyant force acts through it.

43. How one can establish stability of submerged body and floating body ?

A submerged body floats in stable equilibrium when the center of gravity of the body lies directly below the center of buoyancy.

44. What is neutral equilibrium ?

A submerged body is in neutral equilibrium for all positions when the center of gravity of body coincides with the center of buoyancy.

45. On what factors the stability of floating objects depends ?

Stability of floating objects depends upon whether righting or overturning moment is developed when the center of gravity and center of buoyancy move out of vertical alignment due to the shifting of position of center of buoyancy.

46. State two products each produced by forward extrusion and reverse impact extrusion.

Two examples of products made by direct extrusion are : gear profile, solder wire.
Two examples of reverse impact extrusion are : Short tubes of soft alloys, tooth paste containers.

47. What are the best geometrical shapes suggested for sheet metal drawing and spinning?

Cup shape is best suited by drawing operation. Conical shape is easiest to produce by spinning operation.

48. What are the conditions for stability of buoyant bodies ?

A floating body is stable if the center of gravity is below the metacentre.
A submerged body is stable if the center of gravity is below the center of buoyancy.

49. Define pipe flow ?

It refers to full water flow in closed conduits of circular cross section under a certain pres¬sure gradient.

50. How much is vertical component of pressure force on a submerged curved surface ?

The weight of liquid vertically above the curved surface.

51. Multiple coated, disposable carbide tips have more-or-less replaced brazed carbide tipped tools in CNC applications. Why ?

Disposable tips are usually indexable type and have a number of cutting edges. Very convenient gripping devices for holding such inserts on tool holders are available. A wide variety of ready made inserts are available which require no grinding or adjustment.

52. State, sequentially elements of a canned CNC drilling cycle.

Canned cycles are fixed cycles for general sequences of operations. For drilling operation, sequence of operations will be movement of drill or workpiece to position of actual drilling, movement of drill downwards till it is in the proximity of workpiece, then movement of drill at desired feed rate to sufficient depth of hole, retracting the drill above the workpiece.

54. Explain the difference between one and two dimensional flow ?

True one-dimensional flow occurs when the direction and magnitude of the velocity at all points are identical. Velocities and accelerations normal to the streamline are negligible. Two dimensional flow occurs when the fluid particles, move in planes or parallel planes and the stream-line patterns are identical in each plane.

55. What is the difference between irrotational and rotational flow ?

Irrotational flow : An ideal fluid flow in which no shear stresses occur and hence no torques exist. Rotational motion of fluid particles about their own mass centers can not exist. It can be represented by a flow net.
Rotational flow occurs when the velocity of each particle varies directly as the distance from the center of rotation.

In steady flow, at any point, the velocity of successive fluid particles is the same at successive periods of time.
Flow is unsteady when conditions at any point in a fluid change with time,

57. What is the difference between uniform and non-uniform flows ?

Uniform flow occurs when the magnitude and direction of the velocity do not change from point to point in the fluid.
Non uniform flow occurs when velocity, depth, pressure, etc. change from point to point in the fluid flow.

58. Explain the difference between stream lines and stream tube.

Stream lines are imaginary curves drawn through a fluid to indicate the direction of motion in various sections of the flow of the fluid system. There can be no flow across a streamline at any point.
Stream tube represents elementary portions of a flowing fluid bounded by a group of stream¬lines which confine the flow.

59. What is the advantage of independent jaw chuck ?

Irregular job can be fixed.

60. Define vorticity ?

It equals twice the angular velocity.

61. What is irrational How ?

If a flow is uniform in one region (without rotation), this property is conserved as the fluid moves into a region of space where the motion is not uniform, and the flow is termed irrotational flow.

62. What is one-seventh power law ?

According to one-seventh power law of turbulent, velocity variation corresponds to a much favoured velocity profile and the mean velocity increases as the one-seventh power of the distance from the boundary wall.

63. Mention the major difference between drilling and boring ?

Drilling Boring
(a) It is the operation of initiating a hole Boring is the operation of enlarging a drilled hole
(b) It can produce only standard holes We can produce non-standard hole
(c) Drill is a multi point tool A single point tool is used

64. What is the magnitude of buoyant force and where does the line of action of buoyant force act ?

It is equal to the volume of the liquid displaced. The line of action of buoyant force acts through the centroid of the displaced volume of the fluid.

65. What is metacentre ?

Metacentre is the point at the intersection of the buoyancy with the vertical axis of the body.

66. Define Prandtl’s pitot-static tube.

It is a combination of pressure probe and pitot tube. It is widely used for pressure measurements in wind-tunnel installations.

67. Define the term fluid friction.

The deformation of real fluids is resisted by forces caused by internal friction or viscosity. Viscosity is that property of a real fluid which creates shear forces (fluid friction) between two fluid selements.

68. Why are hydrodynamic bearings used for grinding machines in preference to ball or roller bearings ?

Hydrodynamic bearings are more suitable at high speed, better tolerances, quiet operation, higher capacity to withstand shock, lesser space requirement, better life under fatigue conditions etc.

69. Distinguish between a jig and fixture.

Jig clamps and locates parts in positive manner and guides cutting tools (drills, reamers, taps etc). Fixture is bolted or fixed securely to the machine table so that work is in correct location ship to the cutter. Fixtures are used for mass milling, turning and grinding, etc.

70. State the similarities and differences between a fixture and a jig.

Both jig and fixture positively locate, hold and clamp the workpiece. Jigs guide the tool for drilling etc but fixture has no facility to guide the cutting tools. Jigs are used for drilling etc and fixture for mass milling, turning, grinding, etc.

71.Explain the difference between relative density, viscosity and kinematic viscosity ?

Relative density is that pure number which denotes the ratio of the mass of a body to the mass of an equal volume of a substance taken as a standard. This standard is usually water (at 4°C) for solids and liquids, and air free or C02 or hydrogen (at 0°C and 1 atmosphere = 1.013 x 105 Pa pressure) for gases.

Viscosity of a fluid is that property which determines the amount of its resistance to a shearing force. It is due primarily to interaction between fluid molecules.

Kinematic viscosity is the ratio of absolute viscosity and mass density. (Its unit is m2/s).

72. A right hand helical gear is being cut on a milling machine. What changes in machine settings have to be made to cut a left hand helical gear of same pitch and number of teeth ?

For cutting right hand helical gear right side of milling table is tilted up from normal position by the helix angle. For cutting left hand helical gear, right side of table needs to be tilted down at helix angle, all other settings remaining unchanged.

73. Explain what you understand by the term hydrostatic paradox ?

Hydrostatic paradox is the phenomenon that the floor load of water in vessels of different shapes carrying water at same level depends only on the area of the bottom surface and not on the shape of the vessel.

74. Define buoyancy in brief.

Buoyancy is the vertical force exerted on the body by the fluid at rest. The buoyancy of a submerged body in a fluid at rest is equal to the weight of the fluid displaced by the body and it acts upward through the center of gravity of the displaced volume (the center of buoyancy).

75. What is the necessary condition for a body to float in stable equilibrium ?

Meta center should be above the center of gravity.

76. If liquid rotates at constant angular velocity about a vertical axis as a rigid body, then how its pressure varies at various radial distances ?

As square of the radial distance.

77. How does the velocity vary along radius in a few vortex ?

78. What type of flow will occur when liquid discharges at constant rate through a long, straight tapering pipe ?

79. What does continuity equation represent ?

It relates mass rate of flow along a stream tube.

80. Under what condition steady flow occurs ?

When conditions do not change with time at any point.

81. Why a drill can not drill deeper than its flute length ?

If flute submerges in a drilled hole, the chips disposal passage is blocked and the drill will get jammed in the hole.

82. When do you recommend the use of straight fluted drill ?

Drills used for brass and other soft materials and thin sheets need not be provided with any helix angle, to prevent the lips digging into the workpiece.

83. Explain velocity distribution for laminar and turbulent flows.

In case of laminar flow, the velocity distribution follows a parabolic law of variation. The maximum velocity at center of pipe is twice the average velocity. For turbulent flow, more uniform velocity distribution results.

84. In which type of flow the stream lines, streak lines and path lines are identical ?

85. Under what condition a drill may not cut ?

When the cutting lips are not provided with clearance angles. It will cut if 12° clearance angle is ground.

86. Some drills have straight shanks and some have tapered shank. Why ?

Small drills (less than 12 mm diameter) are provided with straight shanks which can be held in Dniren chucks. Bigger drills have tapered shank using self holding Morse tapers which prevent gravitational fall of drill.

87. What are the main factors that are responsible for the formation of built up edge ?

The main factors that are responsible for the formation of built up edge are :

1. Extreme pressure in the cutting zone
2. High friction in the tool chip interface
3. High local temperature.

88. In which type of flow the Navier-strokes equation is useful ?

Viscous flow.

89. Explain the difference between energy line and hydraulic grade line ?

The energy line is a graphical representation of the energy at each section.
Hydraulic grade line lies below the energy line by an amount equal to the velocity head at the section.

90. What is the characteristic of equipotential line ?

It has no velocity component tangent to it.

91. How specific cutting pressure behaves with feed rate and what conclusion can be drawn from this ?

As feed rate increases, the specific cutting pressure decreases. It leads to conclude that feed rates be maximised in metal cutting process.

92. What is hydraulically rough pipe ?

When Reynolds number is very small, friction factor f becomes independent of the Reynolds number and depends only on the relative roughness height. Such a pipe behaves as hydraulically rough pipe.

93. What are the qualities of ideal tool material ?

It should be hard to resist flank wear and deformation, have high toughness to resist fracture, be chemically inert to the workpiece, be chemically stable to resist oxidation and dissolution, and have good resistance to thermal shocks.

94. What are the factors to be considered while evaluating the machinability ?

1. Tool life
2. Rate of metal removal
3. Power required
4. Surface texture and size of component
5. Temperature of tool (or) chip.

95. Which equation is satisfied by ideal or real, laminar or turbulent flow ?

Continuity equation.

96. How to judge the unsatisfactory performance of cutting tool ?

Unsatisfactory performance of cutting tool leads to loss of dimensional accuracy, increase in surface roughness and increase in power requirements.

97. What are the criteria for judging machinability ?

Cutting speed, tool life, surface finish, cutting force/energy required, teasperature rise at cutting point.

99. What is critical velocity ?

It is the velocity below which all turbulence is damped out by the viscosity of the fluid.

100. What is laminar flow ?

In laminar flow the fluid particles move along straight, parallel paths in layers or laminae. Laminar flow is governed by the law relating shear stress to rate of angular deformation i.e. the product of the viscosity of the fluid and velocity gradient.