Surface roughness is one of the main criteria by which the strength of a part is evaluated. The damage to which a part is subjected under the influence of variable loads is explained to a greater extent by stress concentration. The latter, in turn, occurs in the presence of irregularities. The lower the surface roughness, the less the likelihood of cracking due to metal fatigue. Processing of parts (finishing, polishing, lapping, etc.) contributes to a significant increase in the fatigue strength.
The lower the surface roughness, the better the corrosion resistance of the part. This is especially important in cases where it is not possible to use protective coatings when processing products. Such details, in particular, include engine cylinders.
Low surface roughness is of great importance in mates that meet the conditions of thermal conductivity, tightness. This increases the ability to reflect electromagnetic, light and ultrasonic waves. In addition, the loss of electromagnetic energy in resonating systems, waveguide paths becomes smaller, the capacitance of the electrodes becomes lower; gas evolution and gas absorption in electrovacuum devices are also reduced, thus cleaning of parts from adsorbed particles of dust, steam, gas is facilitated.
Assessing the quality of the surface, experts pay attention to the direction of the strokes formed after mechanical or other processing. This indicator affects the degree of strength of the press joints, the definition of fit, wear resistance. The designer needs to specify the orientation of the strokes in critical cases. Such a need may, for example, arise in connection with the direction of motion of a stream of gas or liquid over the part or with the direction of relative motion (slip) of the conjugate elements. Reducing wear to a minimum is possible when the direction of the unevenness of the parts coincides with the direction of sliding.
The surface waviness and low roughness always correspond to high accuracy, which is associated not only with the working conditions of the conjugated elements, but also with the need to obtain reliable results when measuring in production. Such processing of parts can be used to give products an attractive appearance or to provide convenience when keeping products clean.
Surface roughness is measured if necessary. Indicators are set in accordance with the functional purpose of the product. If there are no requirements for the quality of the part, then control over the level of roughness is not performed. It should be said that the requirements do not include the assessment of defects (presence of shells, for example). In this regard, when exercising control over surface roughness, control over the influence of defects is completely excluded. In individual cases, requirements for individual parts of one part may be established.
Roughness assessment is carried out according to the roughness of the profile (usually transverse). To do this, produce a section by plane. In order to separate the roughness from other irregularities with relatively large steps, its consideration is carried out within a certain, rather limited area.
Quantification and standardization are made according to six parameters. So, three altitude parameters are taken into account, by which the surface roughness (Ra, Rz, Rmax), two step (S, Sm), and also an indicator determining the relative reference profile length (tp) are estimated.