Methods of Measuring Surface Finish (Measurement)

The methods used for measuring the surface finish are classified into

Inspection by comparison
Direct Instrument Measurements

Inspection by comparison methods

In these methods of measuring, the surface texture is assessed by observation of the surface. The surface to be tested is compared with known value of roughness specimen and finished by similar machining process.

The various methods which are used for comparison are

Touch Inspection.
Visual Inspection.
Microscopic Inspection.
Scratch Inspection.
Micro Interferometer.
Surface photographs.
Reflected Light Intensity.
Wallace surface Dynamometer.

Touch Inspection

It is used when surface roughness is very high and the fingertip is moved along the surface at a speed of 25mm/second and the irregularities as up to 0.0125mm can be detected in this method.

Visual Inspection

In this method, the surface is inspected by naked eye and this measurement is limited to rough surfaces. measurement

Microscopic Inspection

In this method, a finished surface is placed under the microscopic and compared with the surface under inspection. The light beam also used to check the finished surface by projecting the light about 60° to the work.

Scratch Inspection

The materials like lead, plastics rubbed on surface are inspected by this method. The impression of this scratches on the surface produced is then visualized.

Micro-Interferometer

Optical flat is placed on the surface to be inspected and illuminated by a monochromatic source of light.

Surface Photographs

Magnified photographs of the surface are taken with different types of illumination. The defects like irregularities appear as dark spots and flat portion of the surface appears as bright.

Reflected light Intensity

A beam of light is projected on the surface to be inspected and the light intensity variation on the surface is measured by a photocell and this measured value is calibrated

Wallace surface Dynamometer

It consists of a pendulum in which the testing shoes are clamped to a bearing surface and a predetermined spring pressure can be applied and then, the pendulum is lifted to its initial starting position and allowed to swing over the surface to be tested.

Direct Instrument Measurements

Direct methods enable to determine a numerical value of the surface finish of any surface. These methods are quantitative analysis methods and the output is used to operate recording or indicating instrument. Direct Instruments are operated by electrical principles.

These instruments are classified into two types according to the operating principle. In this, one is operated by carrier modulating principle and the other is operated by voltage generating principle, in
both types, the output is amplified.

Some of the direct methods of measuring instruments are

Stylus probe instruments
Tomlinson surface meter
Profilometer
Talyor Bobson Talysurf.

Stylus probe type instrument

When the stylus is moved over the surface to be measured, the irregularities in the surface texture are measured and it is used to assess the surface finish of the work piece.

Working:

The stylus type instruments consist of skid, stylus, amplifying device and recording device. The skid is slowly moved over the surface by hand or by motor drive. The skid follows the irregularities of surface and stylus moves along with skid.

When the stylus moves vertically up and down, the stylus movements are magnified, amplified and recorded to get a trace. Then the trace is analyzed by an automatic device.