Surface Texture – Characteristics and Calculation Method
Surface Texture is defined as a degree of finish conveyed to the machinist by a system of symbols devised by a Standards Association. e.g ASA – American Standard Association, BS – British Standards.
Surface Texture is the nature of the surface defined by four main characteristics which are Surface roughness, waviness, lay and flaw.
Surface Texture is also known as Surface topography.
We will discuss these four characteristics in more detail below.
The four main characteristics of surface texture are:-
Waviness is a recurrent deviation from a flat surface, much like waves on the surface of water.
Waviness width is the distance between the adjacent crests of the wave.
Waviness height is the height between the crests and valleys of the waves (waviness height).
The main causes of Waviness are:-
a) defection of tools, dies or the workpiece.
b) force or temperature sufficient to cause warping.
c) vibration during machining.
d) Improper or uneven lubrication.
e) any periodic mechanical or thermal vibration on the system during manufacturing operations.
Roughness is defined as closely spaced, the irregular deviation on a scale smaller than that of waviness. It is caused by cutting tool or abrasive grain action and the machine feed. Roughness may be superimposed on waviness.
Roughness Height – Roughness height ( Ra ) is the deviation to the centerline in micro inch or micrometers.
Roughness width – Roughness width is the distance between successive roughness peaks parallel to the normal surface in inches or millimeters.
Lay is the direction of the predominant surface pattern. It is also known as directionally. It is caused by the machining process and is usually visible to the naked eye.
Flaws are random irregularities caused during the machining or production process such as molding, drawing, forging, etc. Any scratches, cracks, holes, depressions, seams, tears or inclusions can be called a flaw.
Mean Line –
A straight line that is generated on the profile resulting in equal areas above and below the line. It is also known as the centre line.
A two-dimensional slice through an area is called profile.
The area selected for assessment and evaluation of the roughness parameter having a cutoff wavelength. Any surface irregularities spaced farther part than sampling length are considered waviness. It is also known as cutoff length.
Methods to Evaluate Surface Roughness:
Surface roughness is measured in microns. In 1 mm = 1000 microns.
There are mainly three methods to evaluate surface roughness:
1) CLA method
2) RMS method
3) Ten-point height method
1) CLA Method:
CLA method stands for Centre Line Average method. CLA method is also denoted Ra.
To calculate surface roughness using this method it is necessary to have a mean line in the graph.
The mean line is drawn in such a way that the area above the line should be equal to the area under the line.
In this method, the height of peaks and valleys in a surface is measured. We get the average height of these peeks and valleys by adding the height of sample numbers peeks and valleys and dividing them by the number of peeks and valleys available.
2. RMS method:-
RMS method stands for Root Mean Square method. Surface. RMS is the square root of the mean square ( arithmetic mean of the square of a set of numbers ).
In this method, the height of peaks and valleys are measured from the mean line and their root means square is calculated.
The mean line is taken in such a way that the area above the line should be equal to the area under the line.
3. Ten Point Height Method:-
Ten point height is also denoted Rz. In this method, the average difference of heights of five highest peak and five lowest valleys withing sampling length is calculated.
The heights of peaks and valleys are measured from a line parallel to the mean line and not crossing the profile.