Material Science

Surface Texture or Surface Finishing-Definition, Controlling, Surface Roughness, Surface Waviness

Surface Texture:

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Definition:

  • Surface finish, Also known as a surface texture or surface topography, is the nature of the surface, as defined by three characteristics of lay, surface roughness, and waviness.
  • The surface texture is one of the important factors that control friction.
  • Each manufacturing process, (such as the many kinds of machining) produces a surface texture.
  • The process is usually optimized to ensure that the resulting texture is usable. If necessary, an additional process will be added to modify the initial texture.
  • the surface finishing process is Grinding, Polishing, Lapping, Honing etc.

Reasons for controlling the surface texture:

  • Improve the service life of the components.
  • Improve the fatigue resistance.
  • Reduce frictional wear.
  • Have a close dimensional tolerance on the parts.
  • Reduce corrosion by minimizing the depth of irregularities.

Surface Roughness:

  • Surface roughness often shortened to roughness, is a component of surface texture.
  • It is Quantified by the deviation in the direction of the normal vector of a real surface from its ideal form.   
  • It is caused due to the irregularities in the surface roughness.
  • If these deviations are large, the surface is rough; If bey is small, the surface is smooth.
  • Roughness plays an important role in determining, How a real object will interact with its environment.
  • Rough surface usually wears more quickly and have higher friction coefficients than the smooth surface.
  • roughness is often a good predictor of the performance of mechanical components, Since irregularities in the surface may form nuclear sites for cracks or corrosion.
  • Roughness is also known as a primary texture.

Surface waviness :

  • Surface waviness is known as the secondary texture.
  • It results from the factor such as a machine or work deflections, vibrations, chatter, heat treatment or working strains.
  • waviness is the component of surface roughness upon which roughness is superimposed.

                            Texture                                        Surface Texture                                         

Evaluation of surface roughness:

There are three types of the methods

  1. CLA method
  2. RMS method and
  3. Ten point height method

1. CLA Method:

  • To calculate the value of Ra, from a graph it is necessary to have a mean line.
  • The mean line can be drawn along the direction of the surface profile and dividing the profile in such a way. That the area above the line should approve equal to the area under the line.
  • The average height Ha is calculated as:

Ha = summation of all area above and below a line ⁄ Sampling length

Ha = ΣA ⁄ L

  • Then the CLA index can be calculated horizontal & vertical magnification.

CLA = [ Ha ⁄ (V×H) ] × (1000)

Surface Texture

Example 1. Consider a surface having the following surface profile (as shown in fig.)

The average height Ha = ∑A ⁄ L

=[ (A + A+ A) +(B+B) ] ⁄ L

Where areas are in mm² & length in mm.

CLA index = [ Ha ⁄ (V × H) ] × 1000 μm

Ra = { [ (A+A+A)+(B+B) ] ⁄ L } ×1000 μm

2. RMS Methods :

  • RMS average means Root mean square-number.
  • It is the geometrical average of the ordinates of the profile about the mean line.
  • The mean line or center line is located Such that the sum of areas the line is approximately equal to the sum of the areas below the line.
  • If n measurements are made from the mean line above & below to the points on the surface profile, which is denoted by (Hi)², values in the 

Rrms = √(∑H²i ⁄ n)

Surface Texture

Example: for the given surface profile given below, the RMS value can be calculated.

3. Ten point height method:

  • In this method, the average difference between the fine highest peaks and fine lowest valleys of surface texture within sampling length measured from a line parallel to the mean line and not crossing the profile is used to denote the amount of surface roughness.

Surface Texture

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