01 What is surface roughness?
In technical communication, the term “surface finish” is commonly used. However, it should be noted that “surface finish” is based on human perception, while “surface roughness” is based on the actual surface microgeometry.
To align with international standards (ISO), the use of “surface finish” is no longer acceptable in national standards. The preferred and more accurate term is “surface roughness.”
Surface roughness refers to the unevenness of a machined surface, characterized by small peaks and valleys with close spacing. The distance between these peaks and valleys, known as wave distance, is typically less than 1mm, which falls under the category of microgeometry errors.
It is specifically defined by the level of high and low micro peaks and valleys (Z) and the spacing (s) between them.
Generally according to S:
- S < 1 mm – surface roughness
- 1 ≤ s ≤ 10mm- waviness
- S > 10 mm- f shape
02 Formation factors of surface roughness
Surface roughness is typically generated by various factors in the machining process, such as the friction between the tool and the workpiece surface, plastic deformation of the surface layer metal during chip separation, high-frequency vibrations in the system, and discharge pits in electrical machining.
The depth, density, shape, and texture of the traces left on the machined surface can vary depending on the processing method and material of the workpiece.
03 Evaluation basis of surface roughness
1) Sampling length
The unit length and sampling length for each surface roughness parameter are determined by a specified reference line for evaluation.
According to ISO1997 standards, the common reference lengths are 0.08mm, 0.25mm, 0.8mm, 2.5mm, and 8mm.
Sampling length Ln and evaluation length L of RA, RZ and Ry
| Ra(μm) | Rz.Ry(μm) | L(mm) | Ln=5L(mm) |
|---|---|---|---|
| ≥ 0.008-0.02 | ≥ 0.025-0.10 | 0.08 | 0.4 |
| >0.02-0.1 | >0.10-0.50 | 0.25 | 1.25 |
| >01-2.0 | >0.50-10.0 | 0.8 | 4 |
| >2.0-10.0 | >10.0-50.0 | 2.5 | 12.5 |
| >10.0-80.0 | >50.0-320 | 8 | 40 |
2) Evaluation length
Evaluating the surface roughness of a part requires multiple reference lengths, which are represented by N datum lengths.
One reference length does not accurately reflect the true roughness parameters of a surface, therefore multiple sampling lengths are necessary for an accurate evaluation.
According to ISO 1997 standards, the typical number of evaluation lengths, N, is 5.
3) Baseline
The datum line is the centerline of the contour that is used as a reference for evaluating the surface roughness parameters.
04 Evaluation parameters of surface roughness
1) Height characteristic parameters
Ra (Arithmetic Mean Deviation) is a measure of the average deviation of the contour from its centerline, within the sampling length (Lr). The greater the number of measurement points, the more accurate the Ra value becomes.
Rz (Contour Maximum Height) is the distance between the highest peak and the lowest valley of the contour.
Ra is commonly used as the preferred measure of amplitude parameters.
Prior to 2006, there was another evaluation parameter in the national standard, known as the “Ten Point Height of Micro Unevenness”, which was expressed as Rz. The maximum height of the contour was expressed as Ry.
However, in 2006, the “Ten Point Height of Micro Unevenness” was removed from the national standard, and Rz became the sole representation of the maximum height of the contour.
2) Characteristic parameters of spacing
Rsm (Average Width of Contour Cell) represents the average width of the micro roughness distance within the sampling length.
The micro roughness distance refers to the length between the peak of the contour and the adjacent valley on the centerline.
Even if two surfaces have the same Ra value, their Rsm values may not be the same, resulting in a different texture.
Surfaces that emphasize texture typically focus on both Ra and Rsm parameters.
The shape feature parameter, Rmr, is expressed as the ratio of the contour support length to the sampling length.
The support length of the profile is calculated as the sum of the lengths of each section of the profile, obtained by cutting lines parallel to the centerline and C away from the peak line of the profile, within the sampling length.
05 Comparison table of VDI3400, Ra and Rmax
The Ra index is widely used in domestic production.
In Japan, the Rmax index is commonly used and is equivalent to the Rz index.
European and American countries often use the VDI 3400 standard to indicate surface roughness.
European mold manufacturing factories often use VDI indicators, and customers may request that the surface be made to VDI30 specifications.
The VDI3400 surface has a corresponding relationship with the widely used Ra standard. Many people find it necessary to consult reference data to determine the corresponding value. The following table is comprehensive and it is recommended to keep it for reference.
Comparison table of VDI3400 standard and Ra
| VDI | Ra | VDI | Ra |
|---|---|---|---|
| 3400 | μm | 3400 | μm |
| 0 | 01 | 23 | 1.4 |
| 1 | 0.112 | 24 | 1.6 |
| 2 | 0.126 | 25 | 1.8 |
| 3 | 0.14 | 26 | 2 |
| 4 | 0.16 | 27 | 2.2 |
| 5 | 0.18 | 28 | 2.5 |
| 6 | 0.2 | 29 | 2.8 |
| 7 | 0.22 | 30 | 3.2 |
| 8 | 0.25 | 31 | 3.5 |
| 9 | 0.28 | 32 | 4 |
| 10 | 0.32 | 33 | 5 |
| 11 | 0.35 | 34 | 5 |
| 12 | 0.4 | 35 | 5.6 |
| 13 | 0.45 | 36 | 6.3 |
| 14 | 0.5 | 37 | 7 |
| 15 | 0.56 | 38 | 8 |
| 16 | 0.63 | 39 | 9 |
| 17 | 0.7 | 40 | 10 |
| 18 | 0.8 | 41 | 11.2 |
| 19 | 0.9 | 42 | 12.6 |
| 20 | 1 | 43 | 14 |
| 21 | 1.12 | 44 | 16 |
| 22 | 1.26 | 45 | 18 |
Comparison table of RA and Rmax
| Ra (μm) | Rmax (μm) |
|---|---|
| 0.1 | 0.4 |
| 0.2 | 0.8 |
| 0.4 | 1.5 |
| 0.56 | 2.4 |
| 0.8 | 3.3 |
| 1.12 | 4.7 |
| 1.6 | 6.5 |
| 2.2 | 10.5 |
| 3.2 | 12.5 |
| 4.5 | 17.5 |
| 6.3 | 24 |
