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Roughness parameters
GOST 2789-73 provides for the evaluation of six roughness parameters: three height (R _z , R _a , R _max ), two step (S and S _m ) and relative reference profile length t _p .
Let's draw a profilogram (Fig. 3.2.) and introduce such notation.
The base line is the line relative to which the parameters of the profilogram are determined. Base length l is the length of the base line used to highlight bumps. The middle line of the profile is located on the profilogram so that the areas bounded by the contour of the profile are equal on both sides of it, i.e.:
or (1.1)
ledge lines and troughs are parallel to the midline and pass through the highest and lowest points of the profile.
R _a is the arithmetic mean of the absolute values of the profile deviation from the midline within the base length l :
or (1.2)
where y _i is the distance from some point of the profile to the midline.
R _z - the arithmetic mean of the sum of the absolute values of the height
five largest depressions within l :
(1.3)
where H _imax and H _imin are the height and depth of the largest i-th protrusions and depressions.
R _max - the highest height of the profile irregularities, i.e. the distance between the lines of protrusions and depressions within the base length.
S is the average step of the profile protrusions, i.e. the average value of the step Si of the protrusions of the profile within L :
(1.4)
S _m is the average step of profile irregularities, i.e. the average value of the pitch S _mi profile irregularities along the midline within l :
, (1.5)

Fig 3.2 Surface roughness parameters
Roughness designations in the drawings are regulated by GOST 2.309-73. The sign has the form of a mathematical root, with its tip touching the line depicting the surface of the part in the drawing (Fig. 3.3).