measurement and surface texture roughness value roughness comparison tracer type profilogram double micro scope light cross section method

Measurement

• The surface texture represents the combined effect of several causes,of which the roughness,waviness and the error of form are more predominant

• If only the total height is considered then idea of above three characteristics can be had by considering different sample lengths

Surface Texture

• If only surface measurement is considered then the sampling length should be neither too big as to include the waviness, nor too small as to ignore the occasional deep scratches

Determination of roughness value

• To compute the roughness value by taking mean results from the measurement of several sampling lengths taken consecutively along the profile
• So that it gives better indication and is the correct representation of the whole of surface roughness

Roughness comparison specimens

• The roughness comparison specimens are used for qualitative assessment of the surface roughness
• Qualitative assessment of the roughness of the finished product is evaluated by visual or tactile comparison with roughness comparison specimen of known roughness value finished by similar machining process

• The roughness comparison specimens are available either in flat form or segmented cylindrical form
• The flat specimens have the size:length-25mm,width-16mm,thickness-1.6 to 10mm

Tracer type profilogram

• The surface to be tested is placed on a working table
• The table can move to and fro because there is a nut at the bottom of table,which fits into a lead screw driven by a motor.
• The stylus or tracer point which is made to rest on the surface to be tested , is pivoted with mirror as shown in the fig.

• The pivot point is fixed in space with the stand and its position can be changed initially
• When the surface to be tested moves, the oscillations of the tracer are transmitted to the mirror
• The beam of light strikes on the oscillating mirror and is reflected on to the revolving drum,upon which sensitized film is arranged

• The drum is also rotated from the same lead-screw that traverses the table through bevel gears
• The trace obtained on the drum is very much magnified and can be further assessed by various methods

Light cross-section method

• Principle:
• The principle of this method is that when a thin film of light strikes the surface to be tested at an angle of 45°, a band of reflected light will appear on the surface
• The edge of this band will reproduce the profile of the surface irregularities, to say, it will show an optical cross-section.
• This profile is magnified and observed by an optical device arranged at an angle of 45°

Double microscope

• A double microscope is used to evaluate the surface finish by light cross-section method

• Surface meter based on light cross section

• The general view and optical system of the microscope is shown in the fig.
• From a light source a beam of light passes through the condenser and precision slit and is directed at an angle of 45° to the surface to be tested.

• The observing microscope having objective and eyepiece is also inclined at an angle of 45°with the tested surface.
• The field of view of the eyepiece contains reading scale called an eyepiece micrometer
• Eye piece micromate is used to measure the height of surface irregularities.