Surface roughness refers to the small spacing and small peak valleyUnevenness[1]。The distance (wave distance) between two wave peaks or two wave troughs is very small (below 1mm), which belongs to micro geometric shape error.The smaller the surface roughness, the smoother the surface.Generally, the morphology features with the wave distance less than 1mm are attributed to the surface roughness, the morphology features with the size of 1-10mm are defined as the surface waviness, and the morphology features with the size greater than 10mm are defined as the surface morphology.
The 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, the plastic deformation of the surface layer metal during chip separation, and the high-frequency vibration in the process system.Due to different processing methods and workpiece materials, the depth, density, shape and texture of the traces left on the machined surface are different.
Surface roughness is closely related to the matching property, wear resistance, fatigue strength, contact stiffness, vibration and noise of mechanical parts, and has an important impact on the service life and reliability of mechanical products.
Our national standard GB/T 131-2006 Representation of Surface Texture specifies the expression method of surface roughness, which is applicable to the marking of surface roughness and drawing marking;GB/T 1031-2009 Surface Texture Profile Method Surface Roughness Parameters and Their Values specifies the parameters and their values of surface roughness, which is applicable to the evaluation of machined surface quality, as well as the formulation of machining process procedures and mold design.
In order to study the influence of surface roughness on part performance and the need to measure surface micro unevenness, from the late 1920s to the 1930s, some experts from Germany, the United States and Britain designed and produced contour recorders and profilers, and also produced optical instruments such as light sectioning microscope and interference microscope to measure surface micro unevenness,It creates conditions for quantitative evaluation of surface roughness.Since the 1930s, the quantitative evaluation parameters of surface roughness have been studied. For example, Abbott of the United States proposed to use the depth from the peak of the surface profile and the bearing length ratio curve to characterize the surface roughness.In 1936, he published Schmaltz's monograph on surface roughness and put forward suggestions on the standardization of evaluation parameters and values of surface roughness.
However, the use of roughness evaluation parameters and their values has really become a widely accepted standard since the corresponding national standards of various countries were issued in the 1940s.First, the United States issued the ASA B46.1 national standard in 1940. After several revisions, it became the current standard ANSI/ASME B461-1988 Surface Texture, Surface Roughness, Surface Waviness and Processing Textures, which adopts the central line system and takes the arithmetic mean deviation Ra of the profile as the main parameter;Then in 1945, the former Soviet Union issued the national standard GOCT2789-1945 Surface Finish, Surface Micro Geometry, Classification and Representation, which was revised three times to GOCT2789-1973 Surface Roughness Parameters and Characteristics. This standard also adopts the central line system, and specifies six evaluation parameters including the root mean square deviation Rq of the contour and their corresponding parameter values.In addition, most of the standards of other industrial developed countries were formulated in the 1950s. For example, the Federal Republic of Germany issued DIN4760 and DIN4762 standards on evaluation parameters and terms of surface roughness in February 1952.In 2007, China issued GB/T 131-2006 Representation of Surface Texture, and in 2009, issued GB/T 1031-2009 Surface Texture Profile Method Surface Roughness Parameters and Their Values, terminology and evaluation parameter standards related to surface roughness.
Evaluation parameters
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Height characteristic parameters
one
Arithmetic mean deviation of contourRa：The arithmetic mean of the absolute value of the contour offset within the sampling length lr.In actual measurement, the more measuring points, the more accurate Ra.[1-2]
two
Maximum height of contourRz：The distance between the contour peak line and the valley bottom line.
Ra is preferred within the common range of amplitude parameters[1]。Before 2006, the evaluation parameter "ten point height of micro unevenness" in the national standard was expressed in Rz, and the maximum height of contour was expressed in Ry. After 2006, the "ten point height of micro unevenness" was canceled in the national standard, and Ry was no longer used, and Rz was used to represent the maximum height of contour.Therefore, when using the current technical documents and drawings (GB/T 3505-2000), care must be taken, because the difference between the results obtained by using different types of instruments according to different provisions cannot be ignored.
Spacing characteristic parameters
Average width of contour unit Rsm[2]express.The average value of the contour micro unevenness spacing within the sampling length.The micro unevenness distance refers to the length of the contour peak and the adjacent contour valley on the central line.[1]
Rsm needs to distinguish between height and spacing.If not otherwise specified, the height resolution of omitted marks is 10% of Rz, and the spacing resolution of omitted marks is 1% of sampling length lr.
Shape feature parameters
Support length ratio with contour Rmr (c)[2]Is the ratio of the contour support length to the sampling length at the given horizontal position c.The supporting length of the contour is the sum of the length of each section obtained by cutting the contour parallel to the centerline at a given horizontal section height c within the sampling length, expressed in Ml (c)
Assessment basis
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Sampling length
Sampling lengthlrIs the length of a reference line specified for evaluating surface roughness[2]。The sampling length shall be selected according to the formation and texture characteristics of the actual surface of the part, and the length that can reflect the surface roughness characteristics shall be measured according to the overall trend of the actual surface contour.The sampling length is specified and selected to limit and weakenSurface wavinessAnd the influence of shape error on the measurement results of surface roughness.
Assessment length
Assessment lengthlnIt is a length necessary for evaluating the contour, and it can include one or several sampling lengths.Since the surface roughness of each part of the part surface is not necessarily uniform, and the characteristics of a certain surface roughness can not be reasonably reflected in a sampling length, it is necessary to take several sampling lengths on the surface to evaluate the surface roughness.The evaluation length ln generally includes five sampling lengths lr.[2]
Baseline
The reference line is the centerline of the contour used to evaluate the surface roughness parameters[1]。There are two kinds of datum lines:
The least square centerline of the contour: the contour of each point on the contour within the sampling lengthOffsetThe sum of squares of is the smallest and has a geometric contour shape.
Arithmetic mean centerline of the contour: within the sampling length, the area of the contour on both sides of the centerline is equal.
In theory, the median line of the least squares is an ideal datum line, but it is difficult to obtain in practical applications, so it is generally replaced by the arithmetic mean median line of the contour, and can be replaced by a straight line with approximate position when measuring.
Symbolic annotation
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Surface roughness symbol
Symbol and significance of surface roughness
The surface roughness code specified in the national standard is composed of specified symbols and relevant parameters[3]。
Figure 1 Basic Marking Method of Surface Roughness Code
1) Surface roughness symbol
There are five symbols representing surface roughness according to the national standard, see Figure 1 on the right.
2) Surface roughness code
Surface roughness code shall be marked, such as roughness parameter value, sampling length value during measurement, processing texture, processing method, etc[1]。
Marking of surface roughness on drawings
The annotation direction of codes and parameters is shown in Figure 1.When most surfaces of parts have the same surface roughness, the most commonly used symbol and code can be uniformly marked on the upper right corner of the drawing, and the word "rest" should be added. The uniformly marked code and text height should be 1.4 times of that of other surfaces on the drawing.
Figure 2 Marking of Surface Roughness Codes on Surfaces at Different Positions
For the annotation of surface codes at different locations, the tip of the symbol must point from the outside of the material to the surface, and the direction of the number in the code is consistent with the direction of the size number, as shown in Figure 2.
Fine turning, fine planing, fine milling, rough reaming, rough grinding
No processing trace can be seen, and the processing direction can be identified
Ra1.6、Ra0.8、Ra0.4、
Fine turning, fine grinding, fine reaming, grinding
Dark glossy surface
Ra0.2、Ra0.1、Ra0.05、
Grinding, honing, superfinishing, polishing
measuring method
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comparative law
The comparison method is simple, and is used for workshop field measurement. It is often used for measurement of medium or rough surfaces.The method is to compare the measured surface with the roughness template marked with a certain value to determine the roughness value of the measured surface.Methods for comparison: visual inspection when Ra>1.6 μ m, magnifying glass when Ra 1.6~Ra 0.4 μ m, and comparison microscope when Ra<0.4 μ m.
The processing method, texture, direction and material of the sample plate shall be the same as the surface of the measured part.
Stylus method
Surface roughness
The diamond stylus with a tip radius of curvature of about 2 μ m slides slowly along the measured surfaceLength sensorIt is converted into electrical signal, and the surface roughness value is indicated by the display instrument after amplification, filtering and calculation, and the measured profile curve can also be recorded by the recorder.Generally, measuring tools that can only display surface roughness values are calledSurface roughness measuring instrument, which can record the surface profile curve at the same time is called surface roughnessProfilometer。Both measuring tools have electronic computing circuits or computers, which can automatically calculate the contourArithmetic mean deviationRa，The ten point height Rz of the micro unevenness, the maximum height Ry of the contour and other various evaluation parameters have high measurement efficiency and are suitable for measuring the surface roughness with Ra of 0.025-6.3 μ m.
Light sectioning
Double tube microscopeThe surface roughness can be measured for Ry and Rz parameter evaluation, and the measurement range is 0.5~50.
Interferometry
Using the principle of light wave interference (see plain crystalLaser length measurement technology）The shape error of the measured surface is displayed as interference fringe pattern, and the microscopic part of these interference fringes is magnified by a microscope with high magnification (up to 500 times) for measurement to obtain the roughness of the measured surface.The surface roughness measurement tool using this method is calledInterference microscope。This method is suitable for measuring surface roughness with Rz and Ry of 0.025-0.8 μ m.
Application principle
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Surface roughness has a great influence on the use of parts.Generally speaking, small surface roughness will improve the fitting quality, reduce wear and prolong the service life of parts, but the processing cost of parts will increase.Therefore, the surface roughness value should be selected correctly and reasonably.When designing a part, the selection of surface roughness value is determined by the role of the part in the machine.
The general principle is to select a larger surface roughness value on the premise of meeting the technical requirements.The following principles can be referred to for specific selection:
(1) The roughness value of the working surface is smaller than that of the non working surface.[4]
(2) The roughness value of friction surface is smaller than that of non friction surface.The higher the friction speed of the friction surface, the greater the unit pressure, the higher it should be;The roughness of rolling friction surface is smaller than that of sliding friction surface.
(3) For clearance fit, the smaller the fit clearance, the smaller the roughness value;For interference fit, in order to ensure the firm and reliable connection strength,loadThe larger the value, the smaller the roughness value is required.General situationClearance fitthanInterference fitThe roughness value should be small.
(4) The roughness of the mating surface shall be equivalent to its dimensional accuracy requirements.At the same fit property, the smaller the part size, the smaller the roughness value;At the same accuracy level, the roughness value of small size is smaller than that of large size, and the roughness value of shaft is smaller than that of hole (especially the accuracy of IT8~IT5).
(5) Surfaces subject to periodic load and possible occurrenceStress concentrationThe roughness value of fillets and recesses shall be small.
influence factor
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The influence of surface roughness on parts is mainly shown in the following aspects:
1. ImpactAbrasion 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.
2. ImpactcoordinationStability.yesClearance fitFor example, the rougher the surface is, the easier it is to wear, and the gap in the working process gradually increases;yesInterference fitFor example, the micro convex peaks are flattened during assembly, which reduces the actual effective interference and reduces theconnectstrength.
3. Impactfatigue strength。There are large troughs on the surface of rough parts. They are like sharp notches and cracksStress concentrationVery sensitive, thus affecting the fatigue strength of parts.
4. ImpactCorrosion resistance。Rough partssurfaceIt is easy for corrosive gas or liquid to penetrate into the inner layer of metal through the micro valley on the surface, causing surface corrosion.
5. Impactseal upSex.Rough surfaces cannot fit tightly, and gas or liquid leaks through the gap between the contact surfaces.
6. Affect exposurerigidity。Contact stiffness refers to the contact stiffness between theexternal forceResistance to contact deformation under action.The stiffness of the machine depends to a large extent on the contact stiffness between the parts.
7. The measurement accuracy is affected.The surface roughness of the measured surface of the part and the measuring surface of the measuring tool will directly affect the measurement accuracy, especially in the precision measurement.
In addition, surface roughness will have varying degrees of influence on the coating of parts, thermal conductivity and contact resistance, reflectivity and radiation performance, resistance to liquid and gas flow, and current flow on conductor surface.
Assessment basis
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Surface finish
Surface finishIt is another name for surface roughness.The surface finish is proposed according to the human visual point of view, while the surface roughness is proposed according to the actual micro geometric shape of the surface.Because it is in line with international standards (ISO), China adopted surface roughness and abolished surface finish after the 1980s.After the promulgation of the national standards of surface roughness GB3505-83 and GB1031-83, the surface finish is no longer used.
There are corresponding comparison tables for surface finish and surface roughness.There is a calculation formula for roughness measurement, while the finish can only be compared with the sample gauge.Therefore, roughness is more scientific and rigorous than finish.
Comparison Table of Surface Finish and Surface Roughness
Finish
level
(Old bid)
Roughness
Ra
（μm）
1) Surface condition, 2) processing method and 3) application examples
▽1
400~800
▽2
200~400
1) Obvious tool marks 2) Rough turning, boring, planing and drilling 3) Rough machined surface, 2) Welds before welding, rough drilling wall, etc.
▽ 3
100~200
1) Visible tool marks 2) Rough turning, planing, milling and drilling 3) General non bonding surfaces, such as shaft end face, chamfer, side face of gear and pulley, non working surface of keyway, and surface of weight reduction hole
▽4
50~100
1) Visible machining traces 2) Turning, boring, planing, drilling, milling, filing, grinding, rough reaming, and gear milling 3) Matching surfaces of unimportant parts, such as the end faces of pillars, supports, shells, bushings, shafts, covers, etc.The free surface of fastener, the surface of fastener through-hole, the non centering surface of internal and external splines, and the circular surface of gear top ring that is not used as the measurement reference
▽5
25~50
1) Slightly visible machining traces 2) Turning, boring, planing, milling, scraping 1~2 points/cm ^ 2, pulling, grinding, filing, rolling, milling teeth 3) and other parts do not form a matching surface, such as the end face of the box, housing, end cover and other parts.Fixed bearing surfaces with centering and matching characteristics are required, such as centering between shafts, working surfaces of keys and keyways.The surface of unimportant fastening thread.Surfaces requiring knurling or oxidation treatment
▽6
12.5~25
1) Can't see the machining trace 2) Turning, boring, planing, milling, reaming, pulling, grinding, rolling, scraping 1~2 points/cm ^ 2 to mill teeth 3) Housing holes for installing Class G bearings with a diameter of more than 80mm, tooth surfaces of ordinary precision gears, locating pin holes, surfaces of V-belt wheels, external diameters of internal splines centering, and centering shoulder surfaces of bearing caps
▽7
6.3~12.5
1) Direction of discernible machining trace 2) Turning, boring, pulling, grinding, end milling, scraping 3~10 points/cm ^ 2, rolling 3) Surfaces required to ensure centering and matching characteristics, such as the surface of taper pin and cylindrical pin, shaft diameter and housing hole matched with G precision rolling bearing, shaft diameter at medium speed, EThe shaft diameter and housing hole matched with grade D rolling bearing, the centering inner diameter of internal and external splines, the side and centering outer diameter of external splines, the hole (H7) of interference fit grade IT7, the hole (H8, H9) of clearance fit grade IT8~IT9, the ground gear surface, etc
▽8
3.2~6.3
1) The direction of micro machining trace 2) Reaming, grinding, boring, pulling, scraping 3~10 points/cm ^ 2, rolling 3) The matching surface required to keep the matching property stable for a long time, IT7 level shaft and hole matching surface, gear surface with high accuracy, important parts subject to variable stress, and EShaft diameter surface matched with Grade D bearing, surface of shaft in contact with rubber seal, IT13-IT16 hole with size greater than 120mm and measuring surface of shaft gauge
▽9
1.6~3.2
1) Direction of unrecognizable machining trace 2) cloth wheel grinding, grinding, grinding and super machining 3) surface of important parts subject to variable stress during operation.Ensure the fatigue strength, corrosion resistance and durability of parts, and do not damage the surfaces with matching properties during operation, such as the shaft diameter surface, the surface requiring air tightness and bearing surface, and the cone centering surface.IT5 and IT6 fitting surfaces, high-precision gear surfaces, shaft diameter surfaces matching with Class G rolling bearings, IT7~IT9 holes larger than 315mm, IT10~IT12 holes larger than 120~315mm and measuring surfaces of shaft gauges, etc
▽10
0.8~1.6
1) Dark gloss surface 2) Super machining 3) Surface of important parts subject to large variable stress during operation.Cone surface to ensure accurate centering.Bore surface for hydraulic transmission.The inner surface of the cylinder liner, the outer surface of the piston pin, the instrument guide rail surface, and the working surface of the valve.IT10~IT12 holes with size less than 120mm and measuring surface of shaft gauge
