# Engineering Metrology Questions and Answers – Precision Polygons

This set of Engineering Metrology Multiple Choice Questions & Answers (MCQs) focuses on “Precision Polygons”.

1. What is the maximum overall diameter for precision polygons?
a) 100 mm
b) 200 mm
c) 300 mm
d) 400 mm

Explanation: Precision polygons are also known as multifaced polygons. These can be used for calibration of rotary tables, dividing head. The overall diameter of polygons should not exceed 300 mm for convenient handling.

2. What are the maximum numbers of faces a precision polygon can have?
a) 30
b) 60
c) 90
d) 72

Explanation: The minimum size of the reflecting face of the precision polygons should be approximately 13 mm x 13 mm. Thus a maximum number of faces of polygons can be seventy two, corresponding to an included angle of 5 degree.

3. How many holes are provided in precision polygons for clamping purpose?
a) 1
b) 2
c) 3
d) 4

Explanation: About four holes are provided in the polygons for the clamping purposes. These holes are plain in the type-1 polygon and threaded in type-2 and type-3 precision polygon respectively.

4. What is the dimension of polygon between top and supporting surfaces for a type 2 precision polygon as per IS: 6987-1973?
a) 75 mm
b) 150 mm
c) 300 mm
d) 600 mm

Explanation: As per IS : 6987-1973, there are three types of precision polygons that is ‘type-1’, ‘type-2’ and ‘type-3’. The dimension of polygon between the top surface and the supporting surface is 75 mm for type -1,150 mm for type-2 and 300 mm for type-3.

5. What is the accuracy of top and supporting surface of type 3 precision polygon as per IS: 6987-1973?
a) 0.005 and 0.04 mm
b) 0.0025 and 0.005
c) 0.007 and 0.0035
d) 0.001 and 0.002 mm

Explanation: The top surface and supporting surface are made flat and parallel to accuracies of 0.0025 and 0.005 mm for type-1 and 0.007 mm and 0.0035 for type-2, 0.005 and 0.04 mm for type-3 and departure from flatness is permissible in concave direction only.
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6. What is the primary consideration in the accuracy of precision polygons?
a) How closely the mirror agrees with nominal angle
b) How closely the reflecting surface agrees with nominal angle
c) Primary consideration for precision polygons is how square and flat it is or its surface
d) How many sides polygon has having

Explanation: The primary consideration in accuracy for the certainty of calibration is not how closely reflecting faces or mirror agrees with the nominal angle, but how flat and square it is.

7. Which of the following option is true for the given statements about precision polygons?
Statement 1: It may have unequal angles.
Statement 2: Polygon with 12 sides is used for most of the works.
a) T, F
b) F, F
c) T, T
d) F, T

Explanation: The polygons mostly have equiangular faces but unequal angles can also be provided for some special purposes. These are generally about 15 mm in thickness and 75 mm across flats and have a central hole for mounting purposes.

8. To what accuracy all face of a precision polygon made square to the bottom supporting surface?
a) 2.5 µm
b) 3.5 mm
c) 2.5 mm
d) 3.5 µm

Explanation: All faces of polygon are made square to the bottom surface to an accuracy of 2.5µm. The error in angle between any two adjacent working faces should not be more than 10 seconds of arc.

9. What is the flatness of reflecting faces of precision polygons?
a) Within 0.0002 mm
b) Within 0.0001 mm
c) Within 0.0003 mm
d) Within 0.0004 mm

Explanation: The reflecting faces of precision polygons should be flat with in 0.0001 mm. If individual mirrors are used then they should be mounted in such a way that there is no distortion. The mirror position should be fixed or there should be no possibility of alterations of mirror position from bumping or poor mounting.

10. Which of the following is true for precision polygons?
a) It is not possible to make a full calibration of the rotary table with one polygon
b) Correction factor is applicable for reference to the exact centering
c) Reflecting surfaces should be within 6 to 7 seconds of nominal angle
d) Correction factor is not applicable when the centre of the mirror is used in calibration

Explanation: It is not possible to make a full calibration of the rotary table with only one polygon because of the limited sides. Thus all the lines of the circular scale of an optical rotary table are not inspected for error.

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