Updated: Aug 22, 2020
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 surface finishing process is Grinding, Polishing, Lapping, Honing, etc.
Reasons for controlling the surface texture:
Improve the service life of the components.
Improve fatigue resistance.
Reduce frictional wear.
Have a close dimensional tolerance on the parts.
Reduce corrosion by minimizing the depth of irregularities.
Roughness consists of surface irregularities which result from the various machining process. These irregularities combine to form surface texture.
2. Roughness Height
It is the height of the irregularities with respect to a reference line. It is measured in millimeters or microns or microinches. It is also known as the height of unevenness.
3. Roughness Width
The roughness width is the distance parallel to the nominal surface between successive peaks or ridges which constitute the predominant pattern of the roughness. It is measured in millimeters.
4. Roughness Width Cut Off
Roughness width cut off is the greatest spacing of respective surface irregularities to be included in the measurement of the average roughness height. It should always be greater than the roughness width in order to obtain the total roughness height rating.
Lay represents the direction of the predominant surface pattern produced and it reflects the machining operation used to produce it.
6. Waviness Height
Waviness height is the peak to valley distance of the surface profile, measured in millimeters.
Indication of Surface texture
The basic symbol consists of two legs of unequal length inclined at approximately 60 degrees to the line representing the considered surface The symbol must be represented by a thin line
If the removal of material by machining is required, a bar is added to the basic symbol,
If the removal of material is not permitted, a circle is added to the basic symbol.
When special surface characteristics have to be indicated, a line is added to the longer arm of any of the above symbols,
Indication of Surface Roughness
The value or values defining the principal criterion of roughness are added to the symbols
If it is necessary to impose maximum and minimum limits of the principal criterion of surface roughness, both values shall be shown
It is the length of Profile necessary for the evaluation of irregularities to be taken in account. Also called as “Cut-off” length.
It is measured parallel to the direction of the profile.
Analysis of Surface Traces
1-Root Mean Square (R.M.S) Value
It is defined as the square root of the arithmetic mean of values of the square of ordinates of the surfaces measured from the mean line.
2-Center Line Average (C.L.A) value (Ra-roughness average)
It is defined as the average value of the ordinates from the mean line, regardless of the arithmetic sign of ordinates
if the area can be measured by planimeter
3-Maximum height （Ry）
it is the maximum peak to valley distance. the vertical distance between the top of the highest peak and the bottom of the deepest valley within the sampling length
4-Ten-spot average roughness （Rz）
it is the average peak to valley roughness based on one peak and one valley per sampling length. the single largest deviation is found in five sampling length and then average