How to choose the surface roughness value of different parts?

01 Concept of surface roughness

surface roughness refers to the micro-geometric errors composed of small spacing and tiny peak-valley on the processed surface, which can be understood as the roughness of fine spacing and tiny peak-valley in the process of processing products. The smaller the geometrical error of the machined surface is, the higher the surface accuracy is. Otherwise, the lower it is.

Surface geometry error:

Roughness: Surface roughness is the term that reflects the micro-geometric errors of the machined surface of the part. Surface geometric errors with wave spacing less than 1mm belong to surface roughness.

Waviness: The geometric shape error with wave distance of 1~10mm belongs to surface waviness.

Shape error: Geometric shape error with wave spacing greater than 10mm belongs to shape error (macroscopic geometric error).

02 Factors of surface roughness formation

The formation of surface roughness is caused by the machining process of the workpiece. Due to plastic deformation, mechanical vibration, friction and other reasons, the surface of parts formed by cutting or other methods, there are always geometric errors. Due to the difference in processing methods and workpiece materials, the depth, density, shape and texture of the marks left by the machined surface are different.

03 Influence of surface roughness on workpiece

Surface roughness has an important impact on the service life and reliability of mechanical products, specifically as follows:

1. Surface roughness affects the wear resistance of parts. The rougher the surface, the smaller the effective contact area between the mating surfaces, and the greater the pressure, the faster the wear.

2. Surface roughness affects the stability of matching properties. For clearance fit, the rougher the surface, the easier it is to wear, so that the clearance gradually increases in the working process; For interference fit, the actual effective interference is reduced and the bonding strength is reduced because the micro-convex peaks are flattened during assembly.

3, surface roughness affects the fatigue strength of parts. There are large troughs on the surface of rough parts, which are sensitive to stress concentration like sharp notch and crack, thus affecting the fatigue strength of parts.

4, surface roughness affects the corrosion resistance of parts. Rough surface, easy to make corrosive gas or liquid through the surface of the micro concave into the inner layer of metal, resulting in surface corrosion.

5, surface roughness affects the sealing of parts. Rough surfaces can not fit tightly, gas or liquid through the gap between the contact surface leakage.

6. Surface roughness affects the contact stiffness of parts. Contact stiffness is the ability of the joint surface of a part to resist contact deformation under the action of external forces. The stiffness of the machine depends to a large extent on the stiffness of the contact between the parts.

7, affect the measuring accuracy of parts. The roughness of the surface measured by the part and the surface measured by the measuring tool will directly affect the precision of the measurement, especially in the precision measurement.

In addition, surface roughness has different degrees of influence on parts coating, thermal conductivity and contact resistance, reflection ability and radiation performance, liquid and gas flow resistance, current flow on conductor surface, etc.