Surface roughness Ra is a symbol often used in CNC custom machining. It is basically an old friend of ours. If the drawing does not have it, it is estimated that the drawing will be scrapped. It is such a symbol that we deal with every day.
The concept of surface roughness
Surface roughness refers to the small spacing and small peaks and valleys of the machined surface.
The distance (wave distance) between two crests or troughs on the surface of Precision Milling Machined Steel OEM Component is very small (below 1mm), which is a microscopic geometric shape error.
Specifically, it refers to the level of the tiny peaks and valleys Z and the distance S. Generally divided by S: 1≤S≤10mm is the waviness; S>10mm is the f shape.
VDI3400, Ra, Rmax comparison table
National standards stipulate that three indicators are commonly used to evaluate surface roughness (unit: μm): the average arithmetic deviation of the profile Ra, the average height of unevenness Rz and the maximum height Ry. Ra indicator is often used in actual production. The maximum microscopic height deviation Ry of the contour is often represented by the Rmax symbol in Japan and other countries, and the VDI indicator is commonly used in Europe and America. The following is the comparison table of VDI3400, Ra, Rmax.
Surface roughness formation factors
Surface roughness is generally formed by the processing method used and other factors, such as the friction between the tool and the surface of the part during the processing, the plastic deformation of the surface layer metal when the chips are separated, and the high-frequency vibration in the process system, electrical processing The discharge pits and so on. Due to the difference in processing methods and workpiece materials, the depth, density, shape and texture of the traces left on the processed surface are different.
The main manifestation of the influence of surface roughness on parts
Affect wear resistance. The rougher the surface, the smaller the effective contact area between the mating surfaces, the greater the pressure, the greater the frictional resistance, and the faster the wear.
Affect the stability of coordination. For clearance fits, the rougher the surface, the easier it is to wear, which will gradually increase the clearance during work; for interference fits, the microscopic peaks are squeezed flat during assembly, which reduces the actual effective interference and reduces The connection strength.
Affect fatigue strength. There are large wave troughs on the surface of rough parts. Like sharp notches and cracks, they are very sensitive to stress concentration, which affects the fatigue strength of the parts.
Affect corrosion resistance. The surface of rough parts can easily cause corrosive gas or liquid to penetrate into the inner metal layer through the microscopic valleys on the surface, causing surface corrosion.
Affect the tightness. Rough surfaces cannot be closely attached, and gas or liquid leaks through the gaps between the contact surfaces.
Affect contact stiffness. Contact stiffness is the ability of the joint surface of the part to resist contact deformation under the action of external force. The stiffness of the machine depends to a large extent on the contact stiffness between the parts.
Affect measurement accuracy. The surface roughness of the measured surface of the part and the measuring surface of the measuring tool will directly affect the accuracy of the measurement, especially during precision measurement.
In addition, the surface roughness has varying degrees of influence on the coating, thermal conductivity and contact resistance, reflectivity and radiation performance of the parts, the resistance of liquid and gas flow, and the flow of current on the conductor surface.
Evaluation Basis of Surface Roughness：1. Sampling length 2. Evaluation length 3. Baseline 4. Measuring stroke
Surface roughness evaluation parameters：1. Height characteristic parameters 2. Spacing characteristic parameters