Surface finish measurement

Surface finish measurement

 The Surface finish, also defined as Surface Texture or Surface Topography is the nature of a surface. It comprises the small local deviations of a surface from the perfectly flat ideal (a true plane).

The surface of every component has some form of texture, which varies according to its structure and the way it has been manufactured.

In order to control the manufacturing process or predict a component’s behaviour during use, it is necessary to quantify surface characteristics by using surface finish parameters.


Surface Finish and Surface Finish Measurement Parameters

Example of Surface Finish (Surface Texture)

Figure – Lay, Waviness, and Roughness

The Basics of Surface Finish Measurement

Diagram Showing a Contact Profilometer
Contact Profilometer Tip Shape


What are the Types of Surface Finish Measurement Parameters?

Surface Texture parameters or Surface Finish Measurement parameters can be separated into three basic types, these include:

  • Roughness
  • Waviness
  • Form

The seven possible lay directions are indicated in the table below. 

Surface Finish Symbols – Basics

Lay Symbols
Where a represents the average roughness value (Ra), and b represents the production method, coating, note, or other additional information.  The letter c provides the roughness sampling length in millimeters or inches, while d gives the direction of the surface lay.  The value of e indicates a minimum material removal requirement in millimeters.  Finally, if an alternate surface finish parameter is provided, the parameter symbol and value are provided in location f (ie: Rz 0.4). 

Average Roughness (Ra) produced by common manufacturing processes

Within the capability range of the equipment, some manufacturing processes allow a degree of control over the surface finish.  For example, when milling or turning, shallow cutting depths and slow feed speeds typically provide smoother finishes, while deeper cuts and faster speeds provide rougher finishes.  Cutting tool wear can also affect surface finish.  The chart below shows the range of surface roughness values typically produced by various manufacturing processes. 

Key Take-Aways

  1. Surface Finish consists of waviness, lay, and roughness, but it is common for only roughness to be specified on technical drawings.
  2. Ra is average roughness, and it under-estimates surface height variations.
  3. Rz is mean roughness depth, and it approximates the size of the most severe surface height variations.
  4. Ra < Rz in most cases. General conversion: 7.2 x Ra = Rz (rough estimation only)
  5. It is important to know if roughness is specified in SI units (micrometers) or English units (micro-inches).
  6. Smoother surfaces are more expensive because more manufacturing processes are required.  Therefore, the roughest acceptable finish should be specified to minimize cost.


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