# Do you really know all about dealing with surface roughness Ra every day?

First, the concept of surface roughness
Surface roughness refers to the roughness of the machined surface with small spacing and small peak valley. The distance between the two peaks or two wave valleys is very small (below 1mm), and it belongs to the microscopic geometric error.
It refers to the S level of Z. Generally according to S:
S < 1mm is surface roughness;
1 < S < < 10mm > waviness;
S > 10mm is the shape of F.

Two, VDI3400, Ra, Rmax comparison table
The national standard stipulates that three indexes are commonly used to evaluate the surface roughness (unit m): the average arithmetic deviation of the contour Ra, the average height Rz of the unevenness and the maximum height Ry. Ra index is used in actual production. The maximum height deviation of the contour Ry is commonly expressed in Rmax symbols in Japan and other countries, and VDI is commonly used in Europe and the United States. The following are the

VDI3400, Ra, Rmax control table

 VDI3400 Ra（μm） Rmax（μm） 0 0.1 0.4 6 0.2 0.8 12 0.4 1.5 15 0.56 2.4 18 0.8 3.3 21 1.12 4.7 24 1.6 6.5 27 2.2 10.5 30 3.2 12.5 33 4.5 17.5 36 6.3 24

Three. Formation factors of surface roughness
The surface roughness is usually formed by the processing methods and other factors, such as the friction between the tool and the surface of the parts during the processing, the plastic deformation of the surface metal when the chip separation is separated, and the high frequency vibration in the process system and the discharge pits of the electric machining. Because of the difference between processing methods and workpiece materials, the depth, density, shape and texture of the machined surface are different.
Four. The influence of surface roughness on parts is mainly manifested.
It affects wear resistance. The coarser the surface is, the smaller the effective contact area is, the greater the pressure, the greater the friction resistance, and the faster the wear.
It affects the stability of the coordination. For the gap coordination, the roughness of the surface, the more easy to wear and the increase of the middle gap in the working process; for the interference fit, the actual effective interference is reduced and the connection strength is reduced because of the extrusion of the micro convex peak in the assembly.
It affects the fatigue strength. The surface of rough parts has large trough, which is very sensitive to stress concentration, such as sharp notches and cracks, which affects the fatigue strength of parts.
It affects corrosion resistance. The surface of rough parts can cause corrosive gas or liquid to penetrate into the inner layer of metal through the surface concave Valley, causing surface corrosion.
Influence the seal. The rough surface can not be tightly adhered, and the gas or liquid leaks through the gap between the contact surfaces.
Influence the contact stiffness. Contact stiffness is the ability of the joint surface to resist contact deformation under external force. The stiffness of a machine depends largely on the contact stiffness between the parts.
It affects the accuracy of measurement. The surface roughness of the measured surface of the parts and the measuring tools will directly affect the accuracy of the measurement, especially when the precision is measured.
In addition, surface roughness has varying degrees of influence on the coating, thermal conductivity and contact resistance, reflection and radiation, resistance of liquid and gas flow, and flow of current on the surface of the conductor.
Five. The basis for evaluating the surface roughness
1. Sampling length
The length of sampling is the length of a reference line for evaluating the surface roughness. According to the formation and texture features of the actual surface of the parts, the length of the section which can reflect the surface roughness should be selected and the length of the sample should be taken according to the general direction of the actual surface profile. The length of sampling is specified and selected to limit and weaken the effect of surface waviness and shape error on the measurement results of surface roughness.
2. Evaluation length
The length of assessment is a length necessary to evaluate the contour, which can include one or several sampling lengths. Because the surface roughness of the parts of the parts is not very uniform, it is often not reasonable to reflect the characteristics of a surface roughness on a sampling length, so several sampling lengths need to be taken on the surface to evaluate the surface roughness. The evaluation length generally contains 5 sampling lengths.
3. Datum line
The datum line is the contour line used to evaluate the surface roughness parameters. There are two kinds of datum lines: the least square middle line of the Outline: within the sampling length, the square of the contour of the contour of each point on the contour is the smallest, and has the shape of the geometric contour. The arithmetic mean midline of a contour: the length of the upper and lower sides of the middle line is equal in the sampling length. In theory, the least square middle line is an ideal datum line, but it is difficult to obtain in practical application, so it is usually replaced by the arithmetic mean line of the contour and can be replaced by a straight line with an approximate position in the measurement.
Six. Evaluation parameters of surface roughness
1. Height characteristic parameters
Ra arithmetic mean deviation: the arithmetic mean of the absolute value of the contour offset in the sampling length (LR). In actual measurement, the more the number of measuring points, the more accurate Ra is.
The maximum height of the Rz contour: the distance between the contour peak line and the valley bottom line.
Ra is preferred in the range of amplitude parameters. In the national standard before 2006, one of the evaluation parameters was the “ten point height of micro unevenness”, which was expressed by Rz. The maximum height of the contour was expressed by Ry. The ten point height of the micro unevenness was cancelled in the national standard after 2006, and the maximum height of the contour was expressed by Rz.

2. Spaced feature parameters
The average width of the Rsm contours. The mean value of the microcosmic roughness interval in the sampling length. The microscopic roughness spacing 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 texture will be different. The emphasis on the texture surface usually pays attention to the two indexes of Ra and Rsm.

The RMR shape feature parameter is expressed by contour support length ratio, which is the contour support length.
Seven. Measurement of surface roughness
1. comparison method
It is commonly used in the measurement of the medium or rough surface. The method is to compare the measured surface with the roughness sample with a certain value to determine the roughness value of the measured surface.
2. Touch needle method
The surface roughness using the needle tip radius of 2 microns is slowly sliding along the measured surface. The upper and lower displacement of the diamond touch needle is converted to the electrical signal by the electrical length sensor. The surface roughness value is indicated by the display instrument after amplification, filtering and calculation, and the measured cross section can be recorded by the recorder. Contour curve. Generally, the measuring tool that can only show the surface roughness value is called the surface roughness measuring instrument, and the surface profile curve can be recorded as the surface roughness profilometer. Both of these two measuring tools have electronic computing circuits or electronic computers. It can automatically calculate the arithmetic average deviation Ra of the contour, the ten point height of the microcosmic unflatness, the maximum height Ry of the contour and the other kinds of evaluation parameters, and the measurement efficiency is high. It is suitable for measuring the surface roughness of 0.025 to 6.3 microns in Ra.
Source: China Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)