01 Concept of surface roughness
Surface roughness refers to the micro geometric form error composed of small spacing and small peaks and valleys on the machined surface, which can be understood as the unevenness of small spacing and small peaks and valleys during product processing. The smaller the micro geometric error of the machined surface is, the higher the surface accuracy is; On the contrary, it is lower.
Surface geometry error:
Roughness: the term reflecting the micro geometric shape error of the machined surface of the part is called surface roughness. The surface geometric shape error with wave distance less than 1mm belongs to surface roughness.
Wavelength: geometric shape error of wave distance between 1~10mm belongs to surface waviness.
Shape error: geometric shape error with wave distance greater than 10mm belongs to shape error (macroscopic geometric shape error)
02 Factors forming surface roughness
The formation of surface roughness is caused by the machining process of the workpiece. Due to plastic deformation, mechanical vibration, friction and other reasons of materials in processing, the surface formed by cutting or other methods of parts always has geometric shape errors. Due to different processing methods and workpiece materials, the depth, density, shape and texture of the traces left on the machined surface are different.
03 Effect of surface roughness on workpiece
The surface roughness has an important influence on the service life and reliability of mechanical products, as follows:
1. The surface roughness affects the wear resistance of parts. The rougher the surface, the smaller the effective contact area between mating surfaces, the greater the pressure, and the faster the wear.
2. Surface roughness affects the stability of fit properties. For clearance fit, the rougher the surface is, the easier it is to wear, which gradually increases the 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.
3. The surface roughness affects the fatigue strength of parts. The surface of rough parts has large wave troughs, which, like sharp notch and crack, are sensitive to stress concentration, thus affecting the fatigue strength of parts.
4. The surface roughness affects the corrosion resistance of parts. The rough surface is easy to make corrosive gas or liquid penetrate into the inner layer of metal through the micro valley of the surface, causing surface corrosion.
5. The surface roughness affects the sealing performance of parts. Rough surfaces cannot fit tightly, and gas or liquid leaks through the gap between contact surfaces.
6. The surface roughness affects the contact stiffness of the part. Contact stiffness is the ability of the joint surface of a part to resist contact deformation under external forces. The stiffness of the machine depends to a large extent on the contact stiffness between the parts.
7. It affects the measurement accuracy of parts. The surface roughness of the measured surface of the part and the measuring surface of the measuring tool will directly affect the measurement accuracy, especially in the precision measurement.
In addition, the surface roughness will affect the coating, thermal conductivity and contact resistance, reflective ability and radiation performance, resistance of liquid and gas flow, and current flow on the conductor surface to varying degrees.
04 Surface roughness achieved by different processing methods
05 Is the surface roughness the same as the surface finish?
Surface finish is another term for surface roughness. The surface finish is proposed according to the human visual point of view, while the surface roughness is proposed according to the actual micro geometric shape of the surface. Because of the conformity with the international standard (ISO), our country adopted the surface roughness and abolished the surface finish after the 1980s. After the promulgation of the national standards for surface roughness GB3505-83 and GB1031-83, the surface finish is no longer used.
There are corresponding comparison tables for surface finish and surface roughness. There is a calculation formula for roughness measurement, while the finish can only be compared with a sample gauge. Therefore, roughness is more scientific and rigorous than finish.
06 Comparison Table of Surface Finish and Surface Roughness
07 Selection of surface roughness
The selection of surface roughness parameters should not only meet the functional requirements of the part surface, but also consider the economic rationality. The specific selection can be determined by analogy with reference to the existing drawings of similar parts. On the premise of meeting the functional requirements of parts, the larger surface roughness parameter value should be selected as far as possible to reduce the processing cost.
Generally speaking, the working surface, mating surface, sealing surface, friction surface with high movement speed and high unit pressure of parts require high smoothness of the surface, and the parameter value should be smaller.