# Metrology and Precision instruments - Mechanical Engineering Interview Questions

41. Define Zero Line?

Ii is the graphical representation of the basic size. The deviations are measured from this line.
42. What is the actual deviation?

It is algebraic difference between an actual size and corresponding basic size.

43. What is the necessity of giving tolerance?

• It saves the labour charges.
• It saves the material from wastage.
• It saves the time.
• Production can be increased .
• It helps to reduce the manufacturing cost of the parts.
44. Describe the shaft basis and hole basis system of limits ?
• Shaft basis system - In this system, the size of shaft is and various classes of fits can be obtained by giving tolerance on hole
• Hole basis system - In this system, the size of hole is constant and various classes of fits can be obtained by giving tolerance on shafts
45. Why the Hole basis system is more popular between the manufacturers?
Hole basis system is more popular, because it has many advantages over the Shaft basis system, such as,
• It is easier to vary the size of the shaft than that of hole
• More adaptable to produce hole of standard size than that of shaft, because of majority of hole in engineering works is finished to its nominal size by using drills and reamers of standard size.
46. How many classifications are there in Indian Standard System for Fits?
• Clearance Fit - It is a kind of fit which always provides clearance with positive allowances. The tolerance region of the hole is above than that of the shaft.
• Transition Fit - This type of fit always has close providing by either a clearance or an interference. The tolerance region of the hole and shaft overlaps.
• Interference Fit - It is a fit which always provides interference with negative allowances. The tolerance region of the hole is below the size than that of the shaft.
47. What is the difference between the Free Fit and the Snug Fit?

The Free and the Snug are the fits come under the clearance fit but Snug Fit has close contact than that of Free Fit.

48. Define the term ‘Interchangeability’?

Interchangeability is that the manufacturing of similar components under mass production can be assembled and replaced without any further treatment as well as without affecting the functioning of the component.

49. What is the significance of Interchangeability in mass production?

• It saves time and labor charges.
• It gives more employment due to mass production.
• Improves the efficiency of workers.
• Easy to assemble due to International trade mark.
• Minimize overall production cost.

50. Explain briefly on IS: 919

• Scope. This standard makes recommendations relating to limits of size of parts or components and to fits constituted by their assembly.
• Symbols. Following symbols are used in this standard,
• Upper deviation of a hole ES (Ecart Superieur)
• Lower deviation of a hole EI (Ecart Inferieur)
• Upper deviation of a shaft es
• Lower deviation of a shaft ei
• Fits. When the parts are to be assembled, the relation resulting from the differences between the sizes before assembly is known as Fit. Depending up on the limits of the hole or shaft, the fit may be a clearance, a transition or an interference.
• Hole Basis System. In this standard, the different clearances and interferences are obtained in associating various shafts with a single hole, whose upper deviation is zero and the symbol is H.
• Shaft Basis System. In this, the different interferences and clearances are obtained in associating various shafts with a single shaft, whose upper deviation is zero and the symbol is h.
• Indication of Fit. A fit is indicated by the basic size common to both components followed by symbols corresponding to each component, the hole being quoted first.
• Eg : 40 H7/g 6 or 40 H 7 – g 6

51. Identify the fit indicated by the following,(a) H8/g7 (b) H7/p6

• H8/g7 indicates sliding fit or close running fit which includes the group of clearance fit.
• H7/p6 indicates light press fit which includes the group of interference fit.

52. What is mass production and point out its advantages?

• Production of a unit, component or part in large scale or number is called mass production.
• Time consumption of manufacturing.
• Reducing the manufacturing cost.
• Interchangeability of products.

53. Discuss the terms Selective Assembly and Non-Selective Assembly?

• Selective Assembly - This assembly is slow and expensive, also the maintenance is difficult to manage since the spares are not easily available. For example special threaded nut and bolt in which each nut fits only one bolt.
• Non-Selective Assembly - It is quiet rapid and less expensive, maintenance is very easy since the spares are widely available. For example any nut fits on any bolt of same size and the type of thread.

54. What is Lapping?

Lapping is an operation of finishing the flat and the cylindrical surfaces to a fine degree of accuracy by means of a lap and lapping compound. It may be done by hand or by machine.

55. What is the necessity of Lapping?

• To finish the job up to the last digit of accuracy.
• To get good surface finish.
• To control the size.
• To get desired fit.

56. What is Honing?

Honing is an operation of finishing cylindrical surfaces to a fine degree of accuracy by means of abrasive sticks.

57. Write the three main coating methods done on metal to prevent corrosion?

• Temporary coating, like grease, oil or varnish etc.
• Semi-permanent coating, like painting, coloring of brass, bluing of steel, black finish of steel, tinning, galvanizing etc.
• Permanent coating, like chromium plating, nickel plating, silver plating etc.

58. Differentiate between Plug gauge and Snap Gauge?

• Plug gauge. It is used for checking the diameter of the hole. It may be found in single ended, double end or progressive type.
• Snap gauge. It is used for checking the external dimension of the parts. It may be found in solid, adjustable or double ended type.

59. Find out the height of the slip gauges if the sine angle is 20 degree using a 100 mm sine bar?

• Value of sine 20 degree = 0.3420 (from sine table)
• Hypotenuse is the length of sine bar i.e 100 mm.
• Height of slip gauges = Hypotenuse * sine 20 = 100* o,3420 mm =34.20 mm

60.Find the least count of metric vernier outside micrometer?

• 10 vernier divisions = 9 thimble divisions
• 1 thimble division = 0.01 mm
• 10 vernier divisions =0.09 mm
• and 1 vernier division =0.09/ 10 = 0.009 mm
• so the difference is the value of one thimble division – value of 1 vernier division = 0.01-0.009 mm = 0.001 mm

61.How to find out the least count of a universal bevel protractor?

• 1 degree= 60 minutes
• 1 minute= 60 seconds
• 12 division on vernier scale = 23 degree
• one division on vernier scale = 23/12 = 23*60/ 12= 115 minutes
• 1 vernier scale division is less than 2 degree
• so, value of 2 degree on dial = 2*60 = 120
• Value of one vernier scale division = 115
• Least count = 120-115= 5 minutes

• 15 .00 mm - 15 main scale divisions (15*1 mm)
• 0.50 mm - 1 subdivision (1* 0.5 mm)
• 0.30 mm - 15 vernier divisions (15* 0.02 mm)
• So the total reading is 15.80 mm

• 25 .00 mm - Zero reading of 25-50 micrometer
• 4.00 mm - 4 main divisions (4 * 1 mm)
• 0.50 mm - 1 subdivision (1 * 0.5 mm)
• 0,21 mm - 21 thimble divisions (21* 0.01 mm)
• So the total reading is 29.71 mm
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