Surface roughness analysis

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1. The concept of surface roughness Surface roughness refers to the unevenness of the processed surface with small spacing and tiny peaks and valleys. The distance (wave pitch) between the two wave crests or two wave troughs is very small (less than 1mm), which is a microscopic geometric shape error. Specifically refers to the height and spacing S of small peaks and valleys. Generally divided into S points: S＜1mm is surface roughness ； 1≤S≤10mm is waviness ； S>10mm is f shape. 2. VDI3400, Ra, Rmax comparison table * * Standards stipulate that three indicators are commonly used to evaluate surface roughness (unit: μm): The average arithmetic deviation Ra of the profile, the average height of unevenness Rz and the maximum height Ry. The Ra indicator is often used in actual production. The maximum microscopic height deviation Ry of the contour is commonly expressed by the Rmax symbol in Japan and other countries, and the VDI indicator is commonly used in Europe and the United States. The following is a comparison table of VDI3400, Ra and Rmax. VDI3400, Ra, Rmax comparison table 3. Surface roughness formation factors Surface roughness is generally formed by the processing method used and other factors, such as the friction between the tool and the part surface during the processing process, the plastic deformation of the surface metal during chip separation, high-frequency vibration in the process system, discharge pits in electrical machining, etc. Due to different processing methods and workpiece materials, the depth, density, shape and texture of the marks left on the processed surface are different. 4. The impact of surface roughness on parts mainly affects wear resistance. The rougher the surface, the smaller the effective contact area between mating surfaces, the greater the pressure, the greater the friction resistance, and the faster the wear. Affects the stability of the fit. For clearance fits, the rougher the surface, the easier it is to wear, causing the gap to gradually increase during work. ； For interference fit, due to the flattening of microscopic convex peaks during assembly, the actual effective interference is reduced and the connection strength is reduced. Affects fatigue strength. There are large troughs on the surface of rough parts, which, like sharp corners and cracks, are sensitive to stress concentration, thus affecting the fatigue strength of the part. Affects corrosion resistance. Rough parts surfaces can easily allow corrosive gases or liquids to penetrate into the inner metal layer through microscopic valleys on the surface, causing surface corrosion. Affect sealing. Rough surfaces cannot fit tightly together, and gas or liquid leaks through the gaps between the contact surfaces. Affects contact stiffness. Contact stiffness is the ability of the joint surface of parts to resist contact deformation under the action of external forces. The stiffness of a machine depends largely on the stiffness of the contact between the various parts. affect the measurement accuracy. The surface roughness of the measured surface of the part and the measuring surface of the measuring tool will directly affect the accuracy of the measurement, especially in precision measurement. In addition, surface roughness will have varying degrees of impact on the parts' coating, thermal conductivity and contact resistance, reflective ability and radiation performance, resistance to liquid and gas flow, and conductor surface current flow. 5. Basis for surface roughness evaluation 1. Sampling length The sampling length is the length of a reference line specified for the evaluation of surface roughness. The length that can reflect the surface roughness characteristics should be selected based on the actual surface formation and texture characteristics of the part. The sampling length should be measured based on the general direction of the actual surface profile. The sampling length is specified and selected in order to limit and reduce the effects of surface waviness and shape errors on the surface roughness measurement results. 2. Assessment length The assessment length is a length necessary to assess the profile, which may include one or several sampling lengths. Since the surface roughness of various parts of the part surface is not necessarily uniform, one sampling length often cannot reasonably reflect a certain surface roughness feature. Therefore, several sampling lengths need to be taken on the surface to evaluate the surface roughness. The evaluation length generally includes 5 sampling lengths. 3. Base line The base line is the center line of the profile used to evaluate surface roughness parameters. There are two types of baselines: least squares midline of contour: Within the sampling length, the sum of squares of the contour offsets of each point on the contour line is the smallest, and it has a geometric contour shape. Arithmetic mean center line of contour: Within the sampling length, the areas of the contours on both sides of the midline are equal. Theoretically, the least squares center line is the ideal baseline, but it is difficult to obtain in practical applications. Therefore, the arithmetic mean center line of the contour is generally used instead, and a straight line with an approximate position can be used instead during measurement. 6. Surface roughness evaluation parameters 1. Height characteristic parameter Ra, contour arithmetic mean deviation: The arithmetic mean of the absolute value of the contour offset within the sampling length (lr). In actual measurement, the greater the number of measurement points, the more accurate Ra is. Rz maximum height of profile: The distance between the peak line and the bottom line of the valley. Ra is preferred within the common range of amplitude parameters. Before 2006 * * There is also an evaluation parameter in the standard, which is the "ten-point height of microscopic unevenness" represented by Rz, and the maximum height of the profile represented by Ry. After 2006 * * The ten-point height of microscopic unevenness is canceled in the standard, and Rz is used to represent the maximum height of the profile. 2. Spacing characteristic parameter Rsm is the average width of the contour unit. The average spacing of microscopic irregularities in the profile within the sampling length. The micro-irregularity spacing refers to the length of the contour peak and the adjacent contour valley on the center line. For the same Ra value, the Rsm value is not necessarily the same, so the reflected texture will be different. Surfaces that value texture usually focus on the two indicators of Ra and Rsm. The Rmr shape characteristic parameter is expressed 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 the sections obtained by intersecting the contour with a straight line parallel to the center line and distance c from the contour peak line within the sampling length. 7. Surface roughness measurement method 1. The comparison method is used for on-site measurement in the workshop, and is often used for the measurement of medium or rough surfaces. The method is to determine the roughness value of the measured surface by comparing it with a roughness sample marked with a certain value. 2. The surface roughness of the stylus method uses a diamond stylus with a tip radius of curvature of about 2 microns to slowly slide along the measured surface. The up and down displacement of the diamond stylus 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. A recorder can also be used to record the profile curve of the measured section. Generally, measuring tools that can only display surface roughness values ​​are called surface roughness measuring instruments, while those that can record surface profile curves are called surface roughness profile meters. Both measuring tools have electronic calculation circuits or computers, which can automatically calculate the arithmetic mean deviation of the profile Ra, the ten-point height of microscopic unevenness Rz, the maximum height of the profile Ry and a variety of other evaluation parameters. They have high measurement efficiency and are suitable for measuring surface roughness with Ra ranging from 0.025 to 6.3 microns.