What is Surface Finish?- Units, Symbols & Chart

What is Surface Finish?

Surface finish, also known as surface texture or surface topography, is the nature of a surface as defined by the three characteristics of lay, surface roughness, and waviness. It comprises the small, local deviations of a surface from the perfectly flat ideal (a true plane).

Surface texture is one of the important factors that control friction and transfer layer formation during sliding. Considerable efforts have been made to study the influence of surface texture on friction and wear during sliding conditions.

Surface textures can be isotropic or anisotropic. Sometimes, stick-slip friction phenomena can be observed during sliding, depending on surface texture.

Each manufacturing process 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 latter process may be grinding (abrasive cutting), polishing, lapping, abrasive blasting, honing, electrical discharge machining (EDM), milling, lithography, industrial etching/chemical milling, laser texturing, or other processes

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Definition of Surface Finish

Surface finish, also known as surface texture or surface topography, is the nature of a surface as defined by the three characteristics of lay, surface roughness, and waviness. It comprises the small, local deviations of a surface from the perfectly flat ideal (a true plane).

Surface Finish is a measure of the overall texture of a surface that is characterized by the lay, surface roughness, and waviness of the surface. Surface Finish when it is intended to include all three characteristics is often called Surface Texture to avoid confusion since machinists often refer to Surface Roughness as Surface Finish. Another term, analogous to Surface Texture, is Surface Topology.

This diagram gives an idea of how to think of the relationship of Waviness, Lay, and Roughness:

Surface Finish

Lay

Lay is the term used to describe the dominant pattern on a surface and the orientation of that pattern.  Lay is generally produced by the manufacturing process and can be parallel, perpendicular, circular, crosshatched, radial, multi-directional, or isotropic (non-directional).  We will discuss the symbols and interpretation of lay in the Symbols section below.

Waviness

Waviness is the term used for the most broadly spaced surface finish variations.  These periodic imperfections in the surface are larger than the roughness sampling length but small, short, and regular enough that they are not considered flatness defects. 

Common causes of surface waviness include warping from heating and cooling and machining defects from chatter or deflection.

Waviness is measured over an evaluation length, and a waviness profile for that length is generated.  The waviness profile does not include any irregularities in the surface due to roughness, flatness, or form variations. 

Waviness spacing (Wsm) is the peak-to-peak spacing of the waves, while the wave height is defined by the average waviness (Wa) or total waviness (Wt) parameters. 

Waviness requirements are less common than roughness requirements, but they can be important for certain parts, such as bearing races or sealing surfaces.

Surface Roughness

Surface roughness, frequently shortened to roughness, refers to small irregularities in surface geometry.  Roughness is the most commonly specified, measured, and calculated aspect of surface finish, and many people use the term “Surface Finish” to only describe roughness.

Surface Finish Symbols

The basic surface finish symbol is a checkmark with the point resting on the surface to be specified.  Variations of this symbol provide additional instructions as described in the table below.

Surface Finish Symbols

Surface Finish Units

Ra – Average Roughness

Ra is also known as Arithmetic Average (AA) or Center Line Average (CLA). It is the average roughness in the area between the roughness profile and its mean line. Graphically, Ra is the area between the roughness profile and its centerline divided by the evaluation length. The evaluation length is normally five sample lengths where each sample length is equal to one cutoff length.

Ra is by far the most commonly used Surface Finish parameter. One reason it is so common is that it is fairly easy to take the absolute value of a signal and integrate the signal using analog electronics, so Ra could be measured by instruments that contain no digital circuits.

Ra, while common, is not sufficient to completely characterize the roughness of a surface. Depending on the application, surfaces with the same Ra can perform quite differently. Here are 4 surfaces with the same Ra and quite different shapes.

To distinguish these differences, more parameters are needed.

Rmax – Vertical distance from highest peak to lowest valley

Rmax is particularly sensitive to anomalies such as scratches and burrs that may not be obvious from measures such as Ra that rely on averages.

Rz – Preferred by many Europeans

Rz is often preferred to Ra in Europe and particularly Germany. Instead of measuring from centerline like Ra, Rz measures the average of the 5 largest peaks to valley differences within five sampling lengths. While Ra is relatively insensitive to a few extremes, Rz is quite sensitive since it is the extremes it is designed to measure.

Surface Finish Chart

Surface Finish Chart

Key Take-Aways

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