It is important to mention that, in both polishing and shot blasting, controlled roughness can be evaluated and measured using instruments that provide information on roughness parameters, such as average roughness (Ra), roughness height (Rz) and the distribution of irregularities on the surface.
These are the fundamental parameters used to measure roughness.
Arithmetic mean roughness (Ra)
Ra is the mean value of the absolute values of the profile heights, with respect to the midline, over the evaluation length, based on the R curve (filtered).
Ra is the most commonly used parameter to control roughness, as it provides relevant information on the following concepts:
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- Determines the quality of the manufacturing process of the part we are inspecting.
- Indicates the wear of tools (grinding wheels, inserts, etc.) in the machining process.
- Watertightness.
- Filing.
Average roughness height (Rz)
This measurement gives the average of the heights of the peaks and valleys found on the surface of the part. It represents the mean of the absolute values of the 5 highest profile ridges and the 5 deepest valleys within the evaluation length.
Root mean square root, RMS
The root mean square takes the average of the square of the distance between peak height and mean length, and then calculates the square root of that value. This value represents the surface roughness in the form of a sine wave, each of which represents the distance from the midline.
The root mean square gives an approximate value of the roughness of the surface, but does not represent an accurate view of the final surface finish. It is related to the average roughness Ra, by a factor of 1.1.
Maximum ridge height (Rp)
It is the distance from the highest point of the profile to the midline based on the P-curve (unfiltered). It is a valid parameter to evaluate the quality of pressed fits and surface wear of parts.
Maximum profile height (Ry)
Distance between the deepest valley and the highest crest of the profile in the evaluation length, based on the P curve (unfiltered).
This parameter is very sensitive to the presence of local defects such as blows, tool breakage at that point, and other defects that may falsify the value obtained.
Maximum peak-to-valley height (Rt or Rmax)
Provides the distance between the highest peak and the deepest valley in the evaluation length based on the R-curve (filtered).
The maximum roughness depth does not give an accurate value of the surface roughness, as any scratch can distort the value obtained.