Measurement of Surface Roughness and Finish

Measurement of Surface Roughness and Finish

Measurement of Surface Roughness and Finish - There are three general methods by which the degree of surface roughness/finish may be measured:

(i) Visual.

(ii) Optical.

(iii) Electronic.

(i) The visual method of evaluating surface roughness is the simplest and most straightforward method. It is also the least accurate. A commercial set of master precision reference specimens ranging in roughness from 50 to 3125 μmm in height are available.

The reference specimen is places adjacent to workpiece under examination and the surfaces are compared visibly with the aid of illuminated magnifiers or tactually by drawing the tip of the finger nail across each at right angle to the tool marks. The fingernail touch or feel will be the same when both finishes are identical.

(ii) Optical or area systems use optical methods for surface evaluation. Equipment ranges from exploration of the surface with simple microscopes or threedimensional microtopography to highly sophisticated techniques such as interferometry.

Area systems inspect all the surface, not simply one line across it. The surface texture in this process is clearly distinguished from the surface geometry. Sincethere is no stylus, the surface is not mechanically contacted and thus there can be no damage to the workpiece surface. Another important advantage of optical inspection methods is that tbe biasing effect of the stylus radius is eliminated.

(iii) Electronic instrument used for the measurement of surface roughness is known as profilometer. The only reading that such an instrument can provide is the average roughness height of the workpiece surface.

A profilometer has two main units, a tracer, and amplimeter. The tracer has a stylus. As the tracer is moved across the work piece surface to be measured, the stylus follows the contours of the irregularities left by the cutting tool.

These up and down movements of the tracer stylus are converted into a small fluctuating voltage. The voltage is fed into the amplimeter where it is amplified to actuate the microinch meter on the front panel. The meter shows the variations in the average roughness height in microinches (or micrometers).