Surface finish

• Surface regularities can be quantified in terms of surface roughness values as it is concerned with the size and shape of the surface. The appearance, wear resistance, fatigue resistance, corrosion resistance, initial tolerance, hardness, colour, absorption capacity, etc., are some of the important characteristics of the product which are influenced by surface texture.

• Any manufacturing process cannot obtain an absolutely smooth and flat surface. This is because, on the machined part, the surface irregularities are left after manufacturing. The practical alternative is to produce a surface within acceptable tolerance values. Many factors effect the surface texture, e.g., type of machining process, material of work piece, cutting conditions, viz., speed, feed, depth of cut, tool material and its geometry, machine tool rigidity, internal and external vibrations, etc.

Terms used in surface roughness measurements

The factors affecting surface roughness are:

• Type of coolant used• Cutting parameters such as feed, speed and

depth of cut• Type of machining• Rigidity of the system, consisting of machine tool,

fixture, cutting tool and work• Vibrations• Type of machining• Material of tool and work piece.

Final surface roughness consist of :

1. Ideal surface roughness (function of feed and geometry)

2. Natural surface roughness (result of irregularities in the cutting operation)

RMAX =f/cotφ+cotβ

Surface roughness measurement method

• Comparison – based method• Direct measurement method:

a) typical stylus probe surface measuring instrument

a) typical stylus probe surface measuring instrument

Surface texture

• The purpose of the parameter is to generate a number that can characterize a certain aspect of the surface with respect to a datum, removing the need for subjective assessment. However it is impossible to completely characterize a surface with a single parameter. Therefore, a combination of parameters are normally used . Parameters can be separated into three basic types:a) amplitude parameters—measures of vertical characteristics of the surface deviations.b) spacing parameters—measures of horizontal characteristics of surface deviations.c) hybrid parameters—combination of both.

Any material being machined by conventional machining process cannot be finished perfectly. The surface generated will have some irregularities and these geometrical irregularities could be classified as follows:

• First Order: It includes the irregularities developed due to the inaccuracies in the machine tool such as Iack of straightness of guide ways, on which tool post is moving.

• Second Order: It includes tile irregularities developed due to the vibrations and rigidity of tile machine tools.Third Order: It includes the irregularities due to the cutting parameters such as cutting speed, feed and depth of cut.

• Fourth Order: It includes the irregularities developed due to the rupture of the material during the separation of the chip from the already finished surface of the work piece.

Ra –average roughness

Rt , Rp , Rv