Do you really understand surface roughness?

What is Surface Roughness?

In factory communication, many people are accustomed to using the term "surface finish". However, "surface finish" is proposed from the perspective of human vision. In order to conform to the international standard (ISO), the national standard no longer uses the term "surface finish". Therefore, in formal and rigorous expressions, the word "surface roughness" should be used.

Surface roughness refers to the small spacing and the unevenness of small peaks and valleys on the machined surface. The distance (wave distance) between the two peaks or two troughs is very small (below 1mm), which belongs to the microscopic geometric shape error. The smaller the surface roughness, the smoother the surface.

Specifically, it refers to the degree of height and distance S of tiny peaks and valleys. Generally divided by S:

S<1mm is the surface roughness

1≤S≤10mm is waviness

S>10mm is f shape

Surface Roughness Forming 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 processing, the plastic deformation of the surface layer metal when the chip is separated, and the high frequency vibration in the process system, electrical machining discharge pits, etc. Due to the different processing methods and workpiece materials, the depth, density, shape and texture of the traces left on the processed surface are different.

Surface roughness evaluation basis

1)    Sampling length

The unit length of each parameter, the sampling length is the length of a reference line specified for evaluating the surface roughness. Under the ISO1997 standard, 0.08mm, 0.25mm, 0.8mm, 2.5mm, and 8mm are generally used as the reference length.

Selected values ​​of sampling length L and evaluation length Ln of Ra, Rz, Ry:

2)    Evaluation length

Consists of N reference lengths. The surface roughness of each part of the surface of the part cannot truly reflect the real parameters of the roughness on a reference length, but it is necessary to take N sampling lengths to evaluate the surface roughness. Under the ISO1997 standard, the evaluation length is generally N equal to 5.

3)    Baseline

The reference line is the center line of the profile used to evaluate the surface roughness parameters.

Surface roughness measurement method

1)    Comparative method

The comparison method is easy to measure and is used for on-site measurement in the workshop, and is often used for the measurement of medium or rough surfaces. The method is to compare the measured surface with a roughness sample marked with a certain value to determine the value of the measured surface roughness. Methods that can be used for comparison: when Ra > 1.6 μm, use visual inspection, when Ra1.6~Ra0.4 μm, use a magnifying glass, and when Ra < 0.4 μm, use a comparison microscope.

When comparing, the processing method, processing texture, processing direction, and material of the sample are required to be the same as the surface of the measured part.

2)    Stylus meansure method

The diamond stylus with a tip curvature radius of about 2 microns slides slowly along the measured surface. The up and down displacement of the diamond stylus is converted into an electrical signal by an electrical length sensor. After amplification, filtering and calculation, the display instrument indicates that the surface is rough. degree value, the recorder can also be used to record the profile curve of the measured section. Generally, the measurement tool that can only display the surface roughness value is called a surface roughness measuring instrument, and the one that can record the surface profile curve is called a surface roughness profiler. These two measurement tools have electronic calculation circuits or electronic computers, which can automatically calculate the arithmetic mean deviation Ra of the contour, the ten-point height Rz of the microscopic unevenness, the maximum height Ry of the contour and other evaluation parameters, with high measurement efficiency and suitable for The surface roughness of Ra is 0.025-6.3 microns is measured.

3)    Intervention method

Use the principle of light wave interference (see flat crystal, laser length measurement technology) to display the shape error of the measured surface as an interference fringe pattern, and use a microscope with a high magnification (up to 500 times) to magnify the microscopic part of these interference fringes Measurements are taken to obtain the roughness of the surface being measured. The surface roughness measurement tool using this method is called an interference microscope. This method is suitable for measuring surface roughness with Rz and Ry ranging from 0.025 to 0.8 microns.

The main manifestations of the influence of surface roughness on parts

1)    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.

2)    Affect the stability of coordination

For clearance fit, the rougher the surface, the easier it is to wear, so that the gap gradually increases during the working process; connection strength.

3)    Affect fatigue strength

There are large troughs on the surface of rough parts, which, like sharp notches and cracks, are very sensitive to stress concentration, thus affecting the fatigue strength of parts.

4)    Affect corrosion resistance

Rough parts surface can easily cause corrosive gas or liquid to penetrate into the inner layer of the metal through the microscopic valleys on the surface, causing surface corrosion.

5)    Affect the tightness

Rough surfaces cannot fit tightly, and gas or liquid leaks through the gaps between the contact surfaces.

6)    Affect contact stiffness

Contact stiffness is the ability of the joint surface of parts to resist contact deformation under the action of external force. The stiffness of a machine is largely determined by the stiffness of the contact between the parts.

7)    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 in precision measurement.

In addition, surface roughness will have varying degrees of influence on the plating coating, thermal conductivity and contact resistance of parts, reflection and radiation performance, resistance to liquid and gas flow, and current flow on the surface of conductors.