Specifically, it refers to the level of Z height and spacing S of micro peaks and valleys. Generally according to S: S<1mm is the surface roughness, 1≤S≤10mm is the waviness S>10mm is the f shape
VDI3400, Ra, Rmax comparison table
The national standard stipulates that three indicators are commonly used to evaluate the surface roughness (unit: μm)
The mean arithmetic deviation Ra of the contour, the mean height of roughness Rz and the maximum height Ry. Ra indicators are often used in actual production. The maximum microscopic height deviation Ry of the contour is commonly represented by the Rmax symbol in Japan and other countries, and the VDI index is commonly used in Europe and the United States. The following is a comparison table of VDI3400, Ra and Rmax.
Factors forming surface roughness
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.
The main performance of the influence of surface roughness on parts
Affect wear resistance
The rougher the surface, the smaller the effective contact area between the mating surfaces, the greater the pressure, the greater the frictional resistance, and the faster the wear.
Affect the stability of the fit
For clearance fit, the rougher the surface is, the easier it is to wear, so that the clearance gradually increases during the working process; for interference fit, due to the flattening of microscopic peaks during assembly, the actual effective interference is reduced. connection strength.
Affect fatigue strength
There are large wave valleys on the surface of rough parts, which are sensitive to stress concentration like sharp corner notches and cracks, thus affecting the fatigue strength of the parts.
Affect corrosion resistance
The rough surface of the parts easily allows corrosive gases or liquids to penetrate into the inner metal layer through the microscopic valleys on the surface, causing surface corrosion.
Affect tightness.
Rough surfaces do not fit tightly, and gas or liquid leaks through the gaps between the contact surfaces.
Affect contact stiffness
Contact stiffness is the ability of the joint surface of a part to resist contact deformation under the action of external force. The stiffness of the machine is largely determined by the contact stiffness 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 the precise measurement.
In addition, surface roughness will have varying degrees of influence on the coating, thermal conductivity and contact resistance of parts, reflection ability and radiation performance, resistance to liquid and gas flow, and current flow on the conductor surface.
Surface roughness evaluation basis
Sampling length
The sampling length is the length of a reference line specified for evaluating the surface roughness. The length that can reflect the surface roughness characteristics should be selected according to the actual surface formation and texture characteristics of the part, and the sampling length should be measured according to the overall trend of the actual surface profile.
The sampling length is specified and selected to limit and reduce the influence of surface waviness and shape errors on the measurement results of surface roughness.
Assessment length
The evaluation length is the length necessary to evaluate the profile, and it may consist of one or several sampling lengths. Since the surface roughness of each part of the surface of the part is not necessarily very uniform, it is often impossible to reasonably reflect a certain surface roughness feature on a sampling length, so it is necessary to take several sampling lengths on the surface to evaluate the surface roughness. The evaluation length generally consists of 5 sampling lengths.
Baseline
The reference line is the center line of the profile used to evaluate the surface roughness parameters. There are two kinds of reference lines: the least squares center line of the contour: within the sampling length, the sum of the squares of the contour offsets of each point on the contour line is the smallest, and it has a geometric contour shape. Arithmetic mean midline of the contour: Within the sampling length, the area of the contours on both sides of the midline is equal. Theoretically, the least squares centerline is an ideal reference line, but it is difficult to obtain in practical applications, so it is generally replaced by the arithmetic mean centerline of the contour, and a straight line with an approximate position can be used for measurement.
Surface roughness evaluation parameters
Height characteristic parameters
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.
Profile Max Height: The distance between the profile peak line and the valley bottom line.
Ra is preferred within the common range of amplitude parameters.
Spacing feature parameters
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.
Surface roughness measurement method
Comparative method
It 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 compare the measured surface with a roughness sample marked with a certain value to determine the measured surface roughness value.
Stylus method
The surface roughness uses a diamond stylus with a curvature radius of about 2 microns to slide slowly along the surface to be measured. 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, it is indicated by a display instrument. The surface roughness value can be obtained, and the profile curve of the measured section can also be recorded by the recorder.
Generally, the measuring tool that can only display the surface roughness value is called the surface roughness measuring instrument, and the one that can record the surface profile curve is called the surface roughness profiler.
These two measurement tools have electronic calculation circuits or electronic computers, which can automatically calculate the arithmetic mean deviation Ra of the contour, the ten-point height Rz of the micro-roughness, the maximum height of the contour Ry and many other evaluation parameters, with high measurement efficiency, suitable for The surface roughness of Ra is 0.025 to 6.3 microns was measured.