Experiment No 7-MQC

Metrology & Quality Control Laboratory

Experiment No: 7

Date:

Identification of Surface Pattern & Measurement of Surface Roughness

Part- 7A Aim: Identification of surfaces using optical flat/interferometers Apparatus: 1) Monochromatic Light Apparatus, 2) Optical Flats, 3) Standard Specimen etc. Theory: An optical flat is an optical-grade piece of glass lapped and polished to be extremely flat on one or both sides, usually within a few thousands of millimetres (about 25 nanometres). They are used with a monochromatic light to determine the flatness of other optical surfaces by interference. When an optical flat is placed on another surface and illuminated, the light waves reflect off both the bottom surface of the flat and the surface it is resting on. This causes a phenomenon similar to thin-film interference. The reflected waves interfere, creating a pattern of interference fringes visible as light and dark bands. The spacing between the fringes is smaller where the gap is changing more rapidly, indicating a departure from flatness in one of the two surfaces, in a similar way to the contour lines on a map. A flat surface is indicated by a pattern of straight, parallel fringes with equal spacing, while other patterns indicate uneven surfaces. Two adjacent fringes indicate a difference in elevation of one-half wavelength of the light used, so by counting the fringes differences in elevation of the surface can be measured to thousands of millimetres. Principle of Working: The apparatus required is a monochromatic light source and optical flat. If optical flat is placed on slip gauge, it will not form an intimate contact, but will be at some angle 'θ' making an inclined plane. If the optical flat is illuminated by monochromatic light and eye if placed in proper position will observe number of bands. They are produced by interference of light rays reflected from lower plane of optical flat and top surface of slip gauge.

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Metrology & Quality Control Laboratory

As shown in Figure, if 'S' is monochromatic light source. At 'C' ray is reflected in direction CDE. The two reflected components are combined by eye, having travelled path whose wavelengths differ by an amount ACD. If path lengths differ by odd number of λ/2 then interference is said to have occurred. If surface is perfectly flat then the surface will be crossed by the pattern of alternate light and dark bands which will be straight and dark line is seen passing at C. The next line occurs at 3/2 alternate dark and bright fringes are seen and variation from the straightness of the bands measure the error in the flatness of slip gauge. The pitch of the bands depends on the angle of the wedge and it can be easily seen that increase in this angle reduces the pitch. The orientation of the bands depends on the orientation of the wedge. The spherical surface can be concave or convex and a little pressure on the optical flat at the centre will spread the bands outwards in a convex way. Figure shows interference band patterns on various surfaces. This fact can be used for drawing various conclusions about the nature of the surface by applying pressure on the optical flat at various points and observing the change in the pattern of bands.

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Metrology & Quality Control Laboratory

Procedure: 1) For testing purpose switch on monochromatic light source. 2) Sufficiently polish the surface to be tested so that it will reflect light & place it below the monochromatic light source. 3) The optical flat is placed on the surface to be tested and gently pressed on the surface. 4) Observe fringe pattern and draw sketches in observation table. 5) Interpret the nature of surface from observed fringe pattern by making use of given diagrams.

Observation Table: Sample No

Observed Fringe Pattern

Nature of surface

Conclusion: Optical flats are useful to compare the flatness of given surface using monochromatic light source. We have studied various fringe pattern observed and the nature of surface. AISSMS College of Engineering Pune

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Metrology & Quality Control Laboratory

Experiment No: 7

Date:

Identification of Surface Pattern & Measurement of Surface Roughness

Part- 7B Aim: To measure surface roughness using surface roughness tester. Apparatus: 1) Surface roughness tester, 2) Standard Specimen, 3) Slip Gauge. Theory: Surface roughness, often shortened to roughness, is a component of surface texture. It is quantified by the vertical deviations of a real surface from its ideal form. If these deviations are large, the surface is rough; if they are small, the surface is smooth. Roughness is typically considered to be the high-frequency, short-wavelength component of a measured surface. However, in practice it is often necessary to know both the amplitude and frequency to ensure that a surface is fit for a purpose. Roughness plays an important role in determining how a real object will interact with its environment. Rough surfaces usually wear more quickly and have higher friction coefficients than smooth surfaces. Roughness is often a good predictor of the performance of a mechanical component, since irregularities in the surface may form nucleation sites for cracks or corrosion. On the other hand, roughness may promote adhesion. Although roughness is often undesirable, it is difficult and expensive to control in manufacturing. Decreasing the roughness of a surface will usually increase exponentially its manufacturing costs. This often results in a trade-off between the manufacturing cost of a component and its performance in application

Evaluation of Surface Roughness 1) C.L. A. Index: (Ra) The C.L.A index (Ra) means Centre Line Average index. To calculate the value of Ra, from a graph, it is necessary to have a mean line. The mean line can be drawn along the direction of the surface profile and dividing the profile in such a way that the area above the line should approximately equal to the area under the line. Then suitable length L is selected which is called sampling length for the given surface. Then average height Ha is calculated as follow, AISSMS College of Engineering Pune

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Metrology & Quality Control Laboratory



=





=



ℎ





∑

Then the C.L.A index can be calculated by considering horizontal and vertical magnification as,

. . .

=

×

× 1000

Where, V = Vertical Magnification, H = Horizontal Magnification Consider a surface having following surface profile,

The average height Then C.L.A. index



= =

(

∑

=

(

) (

) (

)

)

× 1000

2) R.M.S. Average: (Rq) R.M.S. average means Root mean square- number. It is the geometrical average of the ordinates of the profile about the mean line. The mean line or centre line is located such that the sum of the areas above the line is approximately equal to sum of the areas below the line. If n measurements are made from the mean line above and below to the points on the surface profile, which are denoted by

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Metrology & Quality Control Laboratory

Yi. Then the RM.S. Value is the positive square root of the arithmetic mean of the squares of the Yi values in the set.

∴

=

∑

For the surface profile given below, the R.M.S value can be calculated as,

∴

∴

=

=

=

+

∑

+

+. . . +

The R.M.S value is slightly greater than the Ra value for same profile. The ratio of two depends upon the shape of the profile. In general R.M.S value is equal to 1.11 times the Ra value. Rq= 1.11 Ra

Observations: 1) Ra value of Standard Specimen= 2.94 µm 2) Measuring force= 4 mN 3) Stylus tip radius= 5 µm 4) Cut-off Length=

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Metrology & Quality Control Laboratory

Observation Table: Sr. No

Reading Over

Actual Value (Ra)

1

Standard Specimen

2.94 µm

2 3

Standard Specimen (Lapped Surface) Slip Gauge

Experimental value (Ra)

Experimental value (Rq)

0.012- 0.16 0.012- 0.16

Conclusion: Hence, we have studied surface roughess measurement, various devices used for surface roughness measurement and evaluation of surface roughness. The results of measuremnet are given in observation table.

AISSMS College of Engineering Pune

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