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Understanding surface roughness Ra

2023-05-18

一· The concept of surface roughness
Surface roughness refers to the unevenness of the machined surface with small gaps and small peaks and valleys. The distance (wave distance) between its two peaks or valleys is very small (below 1mm), which belongs to micro geometric shape error.Specifically, it refers to the height and spacing S of the small peak valley Z. Generally divided by S:
S<1mm is the surface roughness;
1 ≤ S ≤ 10mm is the waviness;
S 10mm is in the shape of f.
二·
The main impact of surface roughness on parts
Affects wear resistance. The rougher the surface, the smaller the effective contact area between mating surfaces, the greater the pressure, the greater the frictional resistance, and the faster the wear.
Affects the stability of the fit. For clearance fit, the rougher the surface, the more prone it is to wear, resulting in a gradual increase in clearance during the working process; For interference fit, because the micro convex peaks are squeezed flat during assembly, the actual effective interference is reduced and the connection strength is reduced.
Affects fatigue strength. There are large troughs on the surface of rough parts, which, like sharp notches and cracks, are sensitive to stress concentration and thus affect the fatigue strength of the parts.
Affects corrosion resistance. Rough surface of parts can easily cause corrosive gases or liquids to penetrate into the inner layer of the metal through microscopic grooves on the surface, causing surface corrosion.
Affects sealing performance. Rough surfaces cannot fit tightly together, and gas or liquid leaks through the gaps between the contact surfaces.
Affects contact stiffness. Contact stiffness is the ability of the joint surface of a component to resist contact deformation under external forces. The stiffness of a machine largely depends on the contact stiffness between various parts.
Affects measurement accuracy. The surface roughness of both the measured surface of the part and the measuring surface of the measuring tool directly affects the measurement accuracy, especially in precision measurement.
In addition, surface roughness has varying degrees of influence on the coating, thermal conductivity and contact resistance, reflection and radiation properties, resistance to liquid and gas flow, and current flow on the surface of conductors of parts.