What is surface roughness, do you really understand?

- 2022-03-09-

What is surface roughness, do you really understand? Let the editor from Sunbright reveal it. 

01 What is surface roughness?

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 microscopic geometry of the surface. Because of the need to be in line with international standards (ISO), the term "surface finish" has long been no longer used in the national standard, and the term "surface roughness" should be used for formal and rigorous expressions.

Surface roughness refers to the small spacing and the unevenness of small peaks and valleys that the machined surface has. The distance (wave pitch) between the two wave crests or two wave troughs is very small (below 1mm), which belongs to the micro-geometric error.

Specifically, it refers to the level of Z height and spacing S of micro peaks and valleys. Generally, according to S points:

S<1mm is the surface roughness

1≤S≤10mm is the waviness

S>10mm for f shape

02 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 metal when the chips are separated, and the high-frequency vibration in the process system, electrical machining. discharge pits, etc. Due to different processing methods and workpiece materials, the depth, density, shape and texture of marks left on the machined surface are different.

03 Surface roughness evaluation basis

1) Sampling length

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

2) Assessment length

It consists of N reference lengths. The surface roughness of each part of the surface of the component cannot truly reflect the true parameters of the roughness on a reference length, but N sampling lengths need to be taken 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.

04 Surface Roughness Evaluation Parameters

1) Height characteristic parameters

Ra Contour arithmetic mean deviation: the arithmetic mean of the absolute value of contour deviation within the sampling length (lr). In actual measurement, the more the number of measurement points, the more accurate Ra is.

Rz Profile Maximum Height: The distance between the profile peak line and the valley bottom line.
Ra is preferred within the common range of amplitude parameters. Before 2006, there was another evaluation parameter in the national standard: "ten-point height of micro-roughness", which was expressed by Rz, and the maximum height of the contour was expressed by Ry. After 2006, the ten-point height of micro-roughness was cancelled in the national standard, and Rz was used. Indicates the maximum height of the contour.

2) Spacing feature parameters

Rsm Average width of contour cells. The average value of the microscopic roughness spacing of the profile within the sampling length. The micro-roughness spacing refers to the length of a profile peak and an adjacent profile 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 attach importance to texture usually pay attention to the two indicators of Ra and Rsm.

The Rmr shape feature parameter is expressed by the contour support length ratio, which is the ratio of the contour support length to the sampling length. The contour support length is the sum of the lengths of the sections obtained by intersecting the contour with a straight line parallel to the midline and at a distance c from the contour peak line within the sampling length.

05 VDI3400, Ra, Rmax comparison table

Ra index is often used in actual domestic production; Rmax index is commonly used in Japan, which is equivalent to Rz index; European and American countries often use VDI3400 standard to indicate surface roughness, and factories that make European mold orders often use VDI index. "The surface of this product is made according to VDI30".