Surface Texture or Finishing-Definition, Surface Roughness Methods, PDF

Surface Texture or Finishing Definition:

Surface finish, Also known as 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:

  1. Improve the service life of the components.
  2. Improve fatigue resistance.
  3. Reduce frictional wear.
  4. Have a close dimensional tolerance on the parts.
  5. 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.

The 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.

Surface Texture

Evaluation of surface roughness:

There are three types of 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)

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 centerline 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)

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.
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Anup Kumar Dey

A Mechanical Engineer with 19 years of working experience in various renowned MNCs.

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