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Lecture number 17.
Methods for measuring the roughness and wear of surfaces.
Any surface of a part, processed even in the most careful way, cannot be completely perfectly even. The value of the smoothness and evenness of the surface of the part in any case will differ from the value specified by the drawing, i.e. from the nominal value. In this case, the deviation can be either macrogeometric or microgeometric. Macro geometric deviations can be characterized by part waviness and shape mismatch. Microgeometric deviations, in turn, are determined by nothing more than surface roughness.

Roughness is a set of microroughnesses that appear on the surfaces of finished products or parts. In this case, the step of unevenness, taken as roughness, must be very small, relative to the base length of the entire surface.
In general, it is customary to distinguish three types of object roughness:
- Initial roughness - resulting from the technological processing of the product with various abrasives.
- Operational roughness is the roughness acquired during operation as a result of wear and working friction.
- Equilibrium roughness is a type of operational roughness that can be reproduced in stationary friction conditions.
Roughness parameters are defined in GOST 2789-73 “Surface roughness. Parameters, characteristics and designations". According to this document, such roughness indicators are distinguished as:

• 
  Ra is the arithmetic mean of the profile deviation.
  
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  Rz is the height of the profile irregularities taken at 10 points.
  
• 
  S is the average step of the local protrusions of the profile;
  
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  Sm is the arithmetic mean of the roughness pitch;
  
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  Rmax is the maximum profile height;
  
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  tp is the relative length of the profile (reference), p is the level of the profile section.
  

When setting the roughness, as a rule, the cp parameter is used. arithm th. profile deviations (Ra).
It should be noted that it is roughness that has the strongest impact on the performance of machine engines, as well as parts and assemblies of various equipment. Perhaps that is why, the exact determination of the roughness value is one of the most important tasks of metrology.
Roughness can be assessed in two ways:
either element by element, by comparing individual parameters, or in combination, using a comparative analysis of the test sample with the standard.

The most accurate, at the time of this writing, is the element-by-element method, which can be carried out by various methods for determining the roughness:
1) The probe method for measuring surface roughness is a contact method, in which measurements are made using a profilometer. The profilometer is a sensitive sensor equipped with a thin, sharply sharpened diamond needle, with a so-called probing head.
The diamond needle is pressed and moved parallel to the surface being examined. In places where microroughnesses (protrusions and depressions) occur, mechanical vibrations of the measuring head of the needle occur. These vibrations are transmitted to a sensor that converts the mechanical energy of the vibration into an electrical signal, which is amplified by the transducer and measured. The recorded parameters of this signal exactly repeat the irregularities on the rough surface of the part.