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Fig 3.3 Designation of roughness.
1 - zone where R _a and R _{z are indicated} , 2 - type of surface treatment, 3 - base length, 4 - symbol for the direction of irregularities
When calculating friction and wear, in addition to the previously mentioned six standard parameters characterizing surface roughness, a number of additional (non-standard) parameters are used. This is explained by the fact that the listed parameters do not fully estimate the microgeometry of the surface. As noted, with the same value of Rq, the distribution of irregularities in height, their shapes can be different, which also causes different performance characteristics of the mating surfaces.
The control of surface micro deviations, as a rule, is carried out by three different methods: visual - by comparing the surface roughness of the processed samples with surface roughness samples or parts with certified roughness (organoleptic control); contact - using special groping devices and non-contact using various optical devices. The state of microgeometry is qualitatively assessed by the visual method, and quantitatively by the other two methods. The classification of the most common roughness control methods is shown in fig. 3.4.

Figure 3.4 Classification of roughness control methods

Surface roughness is qualitatively determined by comparison with roughness samples. Roughness samples are bars 10 mm thick and 20x30 mm in size. A set of samples for comparison, completed by types of processing (turning, planing, milling, grinding) and the materials from which they are made, are installed in special mandrels and visual comparison is carried out. The method allows to estimate the roughness in the range from Ra=40...20 µm to Ra=1.25...0.63 µm.

To improve the accuracy of the method, probes and comparison microscopes are used, for example, of the MS-48 type.
Probe-type devices have been widely used to determine the roughness parameters. They are divided into three groups: profilometers, profilographs and profilometers-profilographs.
Profilers record the values of R _a and R _q in the form of indications of a pointer instrument or indications of a light board
Profilers present information about the surface microrelief in the form of profilograms - a graphical representation of the profile of a real surface. Deciphering the profilogram in a real scale of values allows determining the values of the surface microgeometry characteristics.
To determine the roughness in mechanical engineering, profilers-profilometers of models 201 and 202, 250, 252 are used; portable profilometer models 253, 283; shop type profilometer model 240, 296,
manufactured by the plant "Caliber". To measure the roughness of rolling bearing rings, the Kalibr plant produces special devices of models 994.995.
In practice, probe instruments of foreign companies are widely used, such as Hommeltester, Penth-O-Meter, Teleserf, Phillips, Taylor-Hobson, Brush, etc.
The design of non-contact devices for monitoring surface microgeometry is based on the methods of light section, shadow projection, interference, etc.
The method of light section consists in the fact that a narrow light flux is projected onto the surface under study. The beam projection angle is 45 ⁰ . The beam projection reproduces the shape of the profile under study with sufficient accuracy. The assessment of the relief of the surface under study is carried out by a special measuring system, which is called a double microscope. The measuring system of the microscope allows you to register the roughness parameters from R _z =80...40 µm to R _a =0.32...0.16 µm.
This method has a relatively small resolution, because unable to cover the required base length. The measurement error is at least 25%. In practice, devices of the PSS series, in particular models PSS-2, MIS-11, have found application.
The shadow projection method is based on the fact that a sharp blade is pressed against the surface under study at an angle of 60 ⁰ . The pressed blade is illuminated from one side in such a way that its shadow falls on the surface, which reproduces the irregularities of the profile. The profile parameters are evaluated using a microscope with a reading device. This method is used to measure the roughness of relatively rough surfaces.
The interference method for determining the roughness parameters is based on the division of a monochromatic light flux into two parts
translucent mirror. One part of the light hits the mirror, the other part falls on the surface under study. The beams of light reflected from the mirrors and the surface add up and give an interference pattern, which is observed on the screen in the form of alternating light and dark stripes. The stream of light reflected from the peaks of irregularities travels a shorter path than the stream reflected from the troughs. Therefore, surface irregularities change the interference pattern, causing distortion (bending) of the fringes. By determining the ratio of distortion to the width of the band interval, the heights of the irregularities are calculated. The method is suitable for determining the parameters of surface roughness with high quality processing
In mechanical engineering, to measure the surface roughness parameters of the 10-14th grade, microinterferometers of the MII-4, MII-5, MII-9, MII-11, MII-12 brands are used.
To measure the roughness of internal and hard-to-reach surfaces of parts, an immersion-replica interferometer of the MII-10 brand is used. In this case, it is not the true surface of the part that is analyzed, but its imprint (replica).
Scanning microscopes of the ORIM-1 type are also used to obtain the characteristics of microgeometry.
The main disadvantage of optical instruments for estimating roughness parameters is the relatively high laboriousness of the measurement. Methods for measuring roughness parameters using a laser are free from this shortcoming. The most accurate and convenient of the laser measurement methods is the measurement frequency shift method.