Methods of Measuring Surface Finish (Metrology)


There are two methods used for measuring the finish of machined part:

(i) Surface Inspection of Comparison Methods.
(ii) Direct Instrument Measurements.
In comparative methods, the surface texture is assessed by observation of the surface. But these methods are not reliable as they can be misleading if comparison is not made with surfaces produced by same techniques. The various methods available under comparison method are :
(i) Touch Inspection, (ii) Visual Inspection, (iii) Scratch Inspection, (iv) Microscopic Inspection, (v) Surface Photographs, (vi) Micro-Interferometer, (vii) Wallace Surface Dynamometer and (viii) Reflected Light Intensity.

Touch Inspection.

The main limitation of this method is that the degree of surface roughness can’t be assessed. Also the minute flaws can’t be detected. This method can simply tell which surface is more rough. In this method, the finger-tip is moved along the surface at a speed of about 25 mm per second and the irregularities as small as 0.01 mm can be easily detected. A modification of it is possible by using a table tennis ball, which is rubbed over the surface, and vibrations from the ball

transmitted to hand and surface roughness judged thereby.

Visual Inspection.

Visual inspection by naked eye is always likely to be misleading particularly when surfaces having high degree of finish are inspected. The method is, therefore, limited to rougher surfaces and results vary from person to person. More accurate inspection can be done by using illuminated magnifiers.

Scratch Inspection.

In this method, a softer material like lead babbit or plastic is rubbed over the surface to be inspected. By doing so it carries the impression of the scratches on the surfaces which can be easily visualised.

Microscopic Inspection.

This is probably the best method for examining the surface finish but suffers due to limitation that only a small portion of the surface can be inspected at a time. Thus several readings are required to get an average value. In this method, a master finished surface is placed under the microscopic and compared with the surface under inspection. In another method a straight edge is placed on the surface to be inspected and a
beam of light projected at about 60° to the work. Thus the shadows cast into the surface scratches are magnified and the surface irregularities can be studied.

Surface Photographs.

In this method magnified photographs of the surface are taken with different types of illumination. In case we use vertical illumination, then defects like irregularities and scratches appear as dark spots and flat portion of the surface appears as bright area. In case of oblique illumination, reverse is the case. Photographs with different illumination are compared and the results assessed.

Micro Interferometer.

In this method, an optical flat is placed on the surface to be inspected and illuminated by a monochromatic source of light. Interference bands are studied through a microscope. Defects, i.e. scratches in the surface appear as interference lines extending from the dark bands into the bright bands. The depth of the defect is measured in terms of the fraction of the interference band. This has been explained in detail in the chapter of ‘Measurement by Light Wave Interference’.

Wallace Surface Dynamometer.

This is a sort of friction meter and consists of a pendulum in which the testing shoes are clamped to a bearing surface and a predetermined spring pressure can be applied. In this method the pendulum is lifted to its initial starting position and allowed to swing over the surface to the tested. If the surface is smooth, then there will be less friction and pendulum swings for a longer period. Thus time of swing is a direct measure of surface finish.

Reflected Light Intensity.

In this method a beam of light of know quantity is projected upon the surface. This light is reflected in several directions as beams of lesser intensity and the change in light intensity in different directions is measured by a photocell. The measured intensity changes are already calibrated by means of reading taken from surface of known roughness by some other suitable method.

Other Methods of Measuring Surface Roughness

Taper Sectioning—In this method, a section is cut through the surface to be examined at a shallow angle 6, thus magnifying height variations by a factor of cot 6, and the section is examined by optical microscope. It is very accurate method but is a destructive method.
Gloss measurement—There is a variation in the optical properties of different rough surfaces as judged subjectively by the human eye and various instruments have been developed to try and quantify this variation more objectively. In one instrument the reflection of a series of vertical bars in the surface is reflected at the specular angle. This angle is gradually decreased until the bars can no longer be distinguished. The roughness is inversely proportional to the cosine of the angle of extinction.
Diffraction techniques—Speckle is produced when a rough surface is illuminated with spatially coherent light and observed with an optical system of finite aperture. Light is received at each point in the image from several different points on the object because of the resolution limitations of the system. The path length of the light from each point on the surface depends on the height of the surface contour at that point. If this height varies significantly across the width of the point spread function, interference effects will appear as speckle. Surface roughness can be deduced from measurements of speckle pattern.
Other methods are tactile tests, pneumatic gauging, optical sections, thermal comparator, capacitance gauges etc.

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