01. What is surface finish?
In technical communication, many people are used to using the "surface finish" indicator. In fact, "surface finish" is put forward according to the human visual point of view, and "surface roughness" is put forward according to the actual surface micro-geometry. The term "surface finish" is no longer used in the national standard because it is in line with international standards (ISO), and the term "surface roughness" should be used for formal and rigorous expressions.
Surface roughness refers to the small spacing and small peaks and valleys of the processed surface. The distance (wave distance) between its two crests or two troughs 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:
- S＜1mm is the surface roughness
- 1≤S≤10mm is the waviness
- S＞10mm is f shape
02. Factors forming surface roughness
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 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.
03. Evaluation basis for surface roughness
1) Sampling length
The unit length of each parameter, the sampling length is the length of a reference line for evaluating the surface roughness. Generally use 0.08mm, 0.25mm, 0.8mm, 2.5mm, 8mm as the reference length under the ISO1997 standard.
Selection values for 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 component surface cannot truly reflect the true roughness parameters on a reference length, but N sampling lengths are needed to evaluate the surface roughness. Under the ISO1997 standard, the assessment length is generally N equal to 5.
3) Baseline
The baseline is the contour centerline used to evaluate the surface roughness parameters.
04. Surface roughness evaluation parameters
1) Height characteristic parameters
Ra contour arithmetic mean deviation: the arithmetic mean of the absolute value of the contour deviation within the sampling length (lr). In actual measurement, the more the number of measurement points, the more accurate Ra is.
Rz Maximum profile height: the distance between the top line of the profile peak and the bottom line of the valley.
In the common range of amplitude parameters, Ra is preferred. Before 2006, there was another evaluation parameter in the national standard: "Microscopic irregularity ten-point height" is represented by Rz, and the maximum contour height is represented by Ry. After 2006, the national standard canceled the microscopic irregularity ten-point height and adopted Rz. Indicates the maximum height of the contour.
2) Spacing feature parameters
Rsm The average width of contour elements. Within the sampling length, the average value of the profile microscopic unevenness spacing. The microscopic unevenness distance refers to the length of the contour peak and the adjacent contour valley on the midline. In the case of the same Ra value, the Rsm value is not necessarily the same, so the reflected texture will also be different. Surfaces that value texture usually pay attention to the two indicators of Ra and Rsm.
The Rmr shape characteristic parameter is expressed by the contour support length ratio, which is the ratio of the contour support length to the sampling length. The profile support length is the sum of the length of each section of the profile obtained by intercepting a straight line parallel to the center line and a distance of c from the top line of the profile within the sampling length.
05.VDI3400, Ra, Rmax comparison table
The Ra indicator is often used in actual domestic production; the Rmax indicator is commonly used in Japan, which is equivalent to the Rz indicator; the European and American countries often use the VDI3400 standard to indicate surface roughness. When making European mold orders, the VDI indicator is often used. For example, I often hear customers say "The surface of this product is made according to VDI30."
