Surface roughness

Surface roughness is one of the main criteria by which the strength of a part is evaluated. The destruction to which a part is subjected under the influence of variable loads is explained to a greater degree by the concentration of stresses. The latter, in turn, arises in the presence of irregularities. The lower the surface roughness, the less likely the formation of cracks due to metal fatigue. The processing of parts (finishing, polishing, finishing, etc.) contributes to a significant increase in the fatigue strength limit.

The lower the surface roughness, the better the anticorrosive properties of the part. This is especially important in cases where the processing of products is not possible to apply protective coatings. To such details, in particular, it is possible to carry cylinders of engines.

The low surface roughness is of great importance also in conjugations, which correspond to the conditions of thermal conductivity, tightness. This increases the ability to reflect electromagnetic, light and ultrasonic waves. In addition, there is less loss of electromagnetic energy in the resonating systems, waveguide paths, the capacity of the electrodes becomes lower; Gas evaporation and gas absorption in electrovacuum devices also decreases, thus, cleaning of parts from adsorbed dust, vapor, gas particles is facilitated.

Assessing the quality of the surface, experts pay attention to the direction of strokes formed after mechanical or other processing. This index influences the degree of strength of press joints, the certainty of planting, wear resistance. The designer must specify the direction of strokes in critical cases. Such a need can, for example, arise in connection with the direction of the movement of a gas or liquid jet through a part or with the direction of relative motion (slip) of the conjugate elements. Reduction of wear to a minimum is possible when the direction of the unevenness of the parts coincides with the direction of sliding.

Surface roughness and low roughness always correspond to high accuracy, which is associated not only with the operating conditions of the conjugated elements, but also with the need to obtain reliable results when measuring in production. This processing of the parts can be used to give the products an attractive appearance or to provide convenience in keeping the products clean.

Measure the surface roughness if necessary. The indicators are set in accordance with the functional purpose of the product. If the quality requirements for the parts are not shown, then control over the level of roughness is not made. It should be said that the requirements do not include the evaluation of defects (the presence of shells, for example). In this connection, when controlling the surface roughness, control over the influence of defects is completely excluded. In some cases, the requirements for individual parts of one part can be established.

The roughness is estimated from the unevenness of the profile (usually transverse). For this, a section is made by a plane. For the purpose of separating the roughness from other irregularities with relatively large steps, its consideration is carried out within a certain, rather limited area.

The quantitative evaluation and rationing is performed according to six parameters. Thus, three altitude parameters are taken into account, according to which the surface roughness (Ra, Rz, Rmax), two step parameters (S, Sm), and the index determining the relative reference length of the profile (tp) are estimated.