1、 Roughness evaluation parameters
The average arithmetic deviation Ra of the contour: the arithmetic mean of the absolute value of the contour offset within the sampling length. In actual measurement, the more measurement points there are, the more accurate Ra is. Ra index is commonly used in actual production.
Maximum height Rz of the contour: the distance between the peak and valley lines of the contour. Ra is preferred within the commonly used range of amplitude parameters, and Rz is also commonly used to evaluate surface roughness height parameters.
Maximum height deviation Ry (or Rmax) of the contour: The maximum microscopic height deviation of the contour. The Rmax symbol is commonly used in countries such as Japan to represent VDI indicators, while VDI indicators are commonly used in Europe and America.
The average width Rsm of the contour unit: the average value of the micro roughness spacing of the contour within the sampling length. The micro roughness spacing refers to the length of the contour peak and adjacent contour valley on the centerline. In the case of the same Ra value, the Rsm value may not be the same, so the reflected texture will also be different. Surfaces that value texture usually focus on Ra and Rsm as two indicators.
Profile support length ratio Rmr: The shape feature parameter is represented by the profile support length ratio, which is the ratio of the profile support length to the sampling length. The contour support length is the sum of the lengths of each section of the contour obtained by cutting a straight line parallel to the centerline and at a distance of c from the contour peak line within the sampling length.
2、 Classification of roughness levels
According to national standards and relevant regulations, roughness grade is usually defined by different evaluation parameters, among which the most commonly used is the arithmetic mean deviation Ra of the contour (unit: μ m). The following are some roughness grade ranges based on Ra values (these ranges may vary depending on different industries or standards):
High precision surface: Ra value less than 0.01 μ m.
Ultra fine surface: Ra value between 0.01 μ m and 0.25 μ m.
Fine surface: Ra value between 0.25 μ m and 0.8 μ m.
Medium rough surface: Ra value between 0.8 μ m and 3.2 μ m.
Generally rough surface: Ra value between 3.2 μ m and 6.3 μ m.
Rough surface: Ra value greater than 6.3 μ m.
3、 Methods and standards for roughness assessment
Comparative method: Used for on-site measurement in workshops, commonly used for measuring medium or rough surfaces. The method is to compare the measured surface with a roughness template labeled with a certain value to determine the roughness value of the measured surface.
Touching method: Using a diamond needle with a small curvature radius to slowly slide along the measured surface, the vertical displacement of the diamond needle is converted into an electrical signal by an electrical length sensor. After amplification, filtering, and calculation, the surface roughness value is indicated by a display instrument, and the measured cross-sectional profile curve can also be recorded using a recorder. Generally, measuring tools that can only display surface roughness values are called surface roughness measuring instruments, while those that can record surface contour curves are called surface roughness profilometers.
In addition, there are evaluation methods such as microscope focusing and tape replication. The international standard ISO 8503 divides surface roughness into five parts for explanation, including technical requirements and definitions for surface roughness comparison blocks, grading and sample comparison methods for steel surface roughness after spray cleaning, verification of ISO reference blocks and methods for determining surface roughness (microscope focusing method, touch needle method), and replication tape method for determining surface profile. The national standards GB/T 13288.1 to GB/T 13288.5 in our country are equivalent translations of the ISO8503 series standards.
In summary, the roughness standard covers multiple evaluation parameters, grade classification, as well as evaluation methods and standards. In practical applications, it is necessary to select appropriate roughness standards and corresponding measurement methods based on specific needs and conditions.