CNC Knowledge: What is surface roughness Ra?

1. The concept of surface roughness

Surface roughness refers to irregularities in the machined surface with small spacing and tiny peaks and valleys. The distance (wave pitch) between the two wave crests or troughs is very small (less than 1 mm), which constitutes a microscopic geometric shape error.

Specifically refers to the height and S spacing of small peaks and valleys. Generally divided into S points:

S<1mm is the surface roughness;

1≤S≤10mm corresponds to the corrugation;

S>10mm is f-shaped.

2. Comparison table VDI3400, Ra, Rmax

National standards provide that three indicators are commonly used to evaluate surface roughness (unit: μm): the arithmetic average deviation Ra of the profile, the average height of irregularities Rz and the maximum height Ry. The Ra indicator is often used in real production. The maximum microscopic height deviation Ry of the contour is commonly expressed by the symbol Rmax in Japan and other countries, and the indicator VDI is commonly used in Europe and the United States. The following is a comparison table of VDI3400, Ra and Rmax.

Comparison table VDI3400, Ra, Rmax

3. Factors causing surface roughness

Surface roughness is usually caused by the machining method used and other factors, such as friction between the tool and the workpiece surface during the machining process, plastic deformation of the surface metal during chip separation and high frequency vibration in the processing system. , electrical machining discharge pits, 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 main effects of surface roughness on parts

Affects wear resistance. The rougher the surface, the smaller the effective contact area between the contact surfaces, the higher 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 more easily it wears, resulting in a gradual increase in the gap during work; For interference fits, the actual effective interference is reduced due to the flattening of the microscopic convex peaks during assembly.

Affects fatigue resistance. There are large pits on the surface of rough parts which, like sharp corners and cracks, are susceptible to stress concentration, thereby affecting the fatigue life of the part.

Affects corrosion resistance. Rough surfaces of parts can easily allow corrosive gases or liquids to penetrate the inner metal layer through microscopic valleys on the surface, thereby causing surface corrosion.

Affects waterproofing. The rough surfaces cannot fit together tightly and gas or liquid escapes through the gaps between the mating 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 rigidity of a machine depends largely on the rigidity of the contact between the different parts.

affect the measurement accuracy. The roughness of the measured surface of the workpiece and the measuring surface of the measuring tool will directly affect the measurement accuracy, especially in precision measurements.

In addition, surface roughness will have different degrees of impact on parts coating, thermal conductivity and contact resistance, reflection ability and radiation performance, resistance to liquid and gas flow and the current flow on the surface of the conductor.

5. Basis for evaluating surface roughness

1. Sampling length

Sampling length is a reference line length specified to evaluate surface roughness. The length that can reflect the surface roughness characteristics should be selected according to the actual surface forming and texture characteristics of the workpiece. The sampling length should be measured based on the general direction of the actual surface profile. The sampling length is specified and selected to limit and reduce the effects of surface waviness and shape errors on surface roughness measurement results.

2. Duration of the assessment

The evaluation length is a length necessary to evaluate the profile, which may include one or more sampling lengths. Since the surface roughness of various parts of the workpiece surface is not necessarily uniform, a sampling length often cannot reasonably reflect a certain surface roughness characteristic. Therefore, several sample lengths must be taken from the surface to evaluate the surface roughness. The evaluation length generally includes 5 sampling lengths.

3. Reference

The reference line is the midline of the profile used to evaluate surface roughness parameters. There are two basic types of baselines: Least-squares centerline of the contour: In the sampling length, the sum of squares of the contour offsets of each contour point is the smallest and it has a geometric contour shape . Arithmetic average of the center line of the contour: In the sampling length, the areas of the contours on both sides of the center line are equal. Theoretically, the least squares centerline is the ideal baseline, but it is difficult to obtain in practical applications. Therefore, the arithmetic mean center line of the contour is usually used instead, and a straight line with an approximate position can be used instead. measures.

6. Surface roughness evaluation parameters

1. Height characteristic parameters

Ra arithmetic mean contour deviation: the arithmetic average of the absolute value of the contour deviation in the sampling length (lr). In actual measurement, the greater the number of measurement points, the more accurate Ra is. Rz maximum profile height: the distance between the crest line and the valley bottom line.

Ra is preferred in the common range of amplitude parameters. Before 2006, there was an evaluation parameter in the national standard: “The ten-point height of microscopic irregularities”, which is represented by Rz, and the maximum height of the profile is represented by Ry. After 2006, the height in ten points. Microscopic irregularities were removed in the national standard and Rz was adopted. Indicates the maximum height of the profile.

2. Spacing characteristic parameters

Rsm Average width of contour cells. Average spacing of microscopic profile irregularities over the sampling length. Micro-irregularity spacing refers to the length of the contour peak and adjacent contour valley on the midline. For the same Ra value, the Rsm value is not necessarily the same, so the reflected texture will be different. Surfaces that enhance texture generally focus on the two indicators Ra and Rsm.

The shape characteristic parameter Rmr is expressed by the profile support length ratio, which is the ratio between the profile support length and 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 the distance c from the crest line of the contour in the sampling length.

7. Surface roughness measurement method

1. Comparative method

Used for on-site workshop measurements, often used for measurements on medium or rough surfaces. The method involves determining the roughness value of the measured surface by comparing it with a roughness sample marked with a certain value.

2. Pen method

Surface roughness uses a diamond stylus with a radius of curvature of approximately 2 microns to slowly slide along the measured surface. Moving up and down 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. To obtain the surface roughness value, a recorder can also be used to record the profile curve of the measured section. Generally, measuring tools that can display only 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 average deviation of the profile Ra, the ten-point height of micro-irregularity Rz, the maximum height of the profile Ry and others varied evaluation parameters. measuring efficiency and are suitable for measuring surface roughness with Ra ranging from 0.025 to 6.3 microns.

The dry stuff continues——

100 questions and answers on surface roughness, don’t think it’s simple!

1. What is surface roughness?

Answer: Surface roughness refers to microgeometric features consisting of small gaps and peaks and valleys on the machined surface of the part. This is a microscopic geometric error.

2. How does surface roughness occur?

Answer: The surface of parts formed by cutting or other methods always has geometric errors due to plastic deformation of the material during processing, mechanical vibration, friction and other reasons.

3. What effect does surface roughness have on parts?

Answer: Surface roughness has a significant impact on part friction and wear, fatigue resistance, corrosion resistance and fit properties between parts.

4. What are the current main national standards for “surface roughness” in my country?

Answer: GB/T 3505 2000 Surface roughness terminology and its parameters; GB/T 1031-1995 Surface roughness parameters and their numerical values ​​GB/T 131-1993 Mechanical drawing surface roughness symbols, codes and their notation methods;

5.What is called real contour?

Answer: It is the level line obtained by the intersection of the plan and the real surface. According to the different intersection directions, it can be divided into transverse true contour and longitudinal true contour. When evaluating and measuring surface roughness, unless otherwise specified, the actual cross-sectional profile is generally used, that is, the profile of the cross-section perpendicular to the direction of the grain being processed.

6. What is the sampling length?

Answer: It is used to identify the length of a reference line with surface roughness characteristics. The rougher the surface, the longer the sampling length should be. The sampling length is specified in order to limit and weaken the influence of other geometric errors on the surface roughness measurement results. In the sampling length range, it generally includes more than 5 contour peaks and contour valleys. For the sampling length selection value, please refer to GB/T 1031-1995 surface roughness parameters and their values.

7. How long does the assessment take?

Answer: This is a length needed to evaluate the contour, which may include one or more sample lengths. Due to the uneven surface treatment of parts, in order to fully and reasonably reflect the roughness characteristics of the measured surface, multiple sampling lengths should be used for evaluation. For the evaluation length selection value, please refer to GB/T 1031-1995 surface roughness parameters and their numerical values.

8. What is a reference?

Answer: A reference line for evaluating the numerical value of surface roughness parameters is called baseline. There are two basic types of baselines: the least squares centerline of the contour and the centerline of the arithmetic mean of the contour.

9. What is called the center line of the least squares contour?

Answer: The least squares centerline of the contour is the line that minimizes the sum of squares of the contour offsets of each contour point within the sample length.

10.What is the arithmetic mean of the center line of the contour?

Answer: The arithmetic mean center line of the contour is the line that divides the actual contour into upper and lower parts in the sample length and makes the upper and lower areas equal.

11.What are the basic evaluation parameters?

Answer: The three height parameters are the basic parameters of the evaluation, namely the arithmetic mean deviation of the profile (Ra), the ten-point height of the micro-roughness (Rz) and the maximum height of the profile ( Ry); three are additional evaluation parameters, namely profile microroughness, average spacing (Sm), average profile single vertex spacing (S), and profile bearing length ratio (tP ).

12. What is the arithmetic mean contour deviation (Ra)?

Answer: In sampling length, the arithmetic mean of the absolute value of the distance between each point of the measured contour and the center line of the contour. The larger the Ra value, the rougher the surface. Ra can objectively reflect the geometric characteristics of the measured contour. The Ra value can be measured directly with an electric profilometer, but it is not intuitive enough.

13.What is the height of ten micro-roughness points (Rz)?

Answer: In sample length, the sum of the average of the five largest contour peak heights and the average of the five largest valley depths. The higher the Rz value, the rougher the surface. Rz is very intuitive for evaluating surface roughness height parameters and is easy to measure on optical instruments, but it has limitations in reflecting the geometric shape characteristics of the measured contour.

14.What is the maximum height of the profile (Ry)?

Answer: The distance from the crest line to the lower valley line over the sample length. The ridge line and valley floor line refer to the lines parallel to the center line and passing through the highest and lowest points of the contour in the sampling length, respectively. The Ry parameter is simple to measure. When the area to be measured is small and it is not appropriate to use Rz, the Rz rating can be used.

15. How to determine the permissible values ​​of the surface roughness height evaluation parameters (Ra, Rz, Ry)?

Answer: See GB/T 1031-1995 surface roughness parameters and their values.

16.The surface roughness symbol and code arewhat is its meaning?

Answer: Basic symbols and surface representations can be obtained by any method. When no roughness parameter values ​​or relevant descriptions (such as surface treatment, local heat treatment conditions, etc.) are added, only simplified code labeling is applicable.

17. The surface roughness symbol and code arewhat is its meaning?

Answer: The base symbol plus a dash indicates that the surface is obtained by material removal. For example: turning, milling, drilling, grinding, shearing, polishing, corrosion, EDM, gas cutting, etc.

18.The surface roughness symbol and code arewhat is its meaning?

Answer: A small circle is added to the base symbol to indicate that the surface is obtained without material removal. For example: foundry, forging, stamping, hot rolling, cold rolling, powder metallurgy, etc. Or an area used to maintain the original state of the supply (including maintaining the state of the previous process).

19.The surface roughness symbol and code arewhat is its meaning?

Answer: The upper limit of Ra is 3.2 μm for surface roughness obtained by any servo method.

20.The surface roughness symbol and code arewhat is its meaning?

Answer: The upper limit of Ra is 3.2 μm for surface roughness obtained by material removal.

21.The surface roughness symbol and code arewhat is its meaning?

Answer: The upper limit of Ra is 3.2 μm for the surface roughness obtained without material removal.

22.The surface roughness symbol and code arewhat is its meaning?

Answer: For surface roughness obtained by material removal, the upper limit of Ra is 3.2 μm and the lower limit of Ra is 1.6 μm.

23.The surface roughness symbol and code arewhat is its meaning?

Answer: The upper limit of Ry is 3.2 μm for surface roughness obtained by any servo method.

24.The surface roughness symbol and code arewhat is its meaning?

Answer: The upper limit of Rz is 200 μm for the surface roughness obtained without material removal.

25.The surface roughness symbol and code arewhat is its meaning?

Answer: The upper limit of Rz is 3.2 μm and the lower limit of Rz is 1.6 μm for surface roughness obtained by material removal.

26.The surface roughness symbol and code arewhat is its meaning?

Answer: For surface roughness obtained by material removal, the upper limit of Ry is 3.2 μm and the lower limit of Ry is 12.5 μm.

27. What should we pay attention to when marking the roughness of a surface?

Answer: When Ra is selected as the height parameter, its code name can be omitted when labeling. When Ry and Rz are selected, the codename cannot be omitted. The surface roughness code shown on the drawing is the requirement for the finished surface. Generally, you only need to indicate the symbol and value of the admissible parameter. If there are special requirements for the surface function of the part (additional requirements such as processing texture, processing tolerance, etc.), the relevant parameters or codes can be marked around the basic symbols.

28. How to draw surface roughness symbol?

Answer: As shown in Figure 1.

Figure 1

d’ =h/10; H=1.4h; h is the height of the font.

29.What are the methods of labeling surface roughness? Example 1.

Answer: As shown in Figure 2.

Figure 2

The surface roughness code (symbol) shall be noted on visible contour lines, dimension lines, extension lines or their extension lines. The tip of the symbol should point from the material toward the surface.

30. What are the methods of marking surface roughness? Example 2.

Answer: As shown in Figure 3.

Figure 3

The center hole working surface, keyway working surface, chamfer and rounded surface can simplify marking.

31.What are the methods of marking surface roughness? Example 3.

Answer: As shown in Figure 4.

Figure 4

The method of marking when the shape of the tooth (tooth) is not drawn on the working surfaces of gears, involute splines, threads, etc.

32. What are the methods of labeling surface roughness? Example 4.

Answer: As shown in Figure 5.

Figure 5

When the same surface has different surface roughness requirements, the dividing line should be drawn with a thin solid line, and the corresponding surface roughness symbol and size should be noted.

33. What are the methods of labeling surface roughness? Example 5.

Answer: As shown in Figure 6.

Figure 6

Where it is necessary to indicate local heat treatment or local plating, the range should be drawn with thick dotted lines and the corresponding dimensions marked. Requirements can also be written in the surface roughness symbol.

34. What are the methods of marking surface roughness? Example 6.

Answer: As shown in Figure 7.

Figure 7

The continuous surface of the part and the surface of repeated elements (holes, grooves, teeth, etc.) and the discontinuous surface connected by thin solid lines, their symbols (symbols) are marked only once.

35.What are the methods of marking surface roughness? Example 7.

Answer: As shown in Figure 8.

Figure 8

When most parts have the same area requirements, they should be marked uniformly in the upper right corner and add the word “rest”. To simplify annotation, or where localization is restricted, simplified codes may be marked, or omissions may be used, but the meaning of these simplified codes (symbols) must be explained near the title bar. When using unified labeling and simplified labeling, codes and text descriptions should be 1.4 times greater than the codes and text noted on other surfaces of the chart.

36. How to determine the location of surface roughness parameters and different regulations?

Answer: As shown in Figure 9.

Figure 9

37. How to choose surface roughness?

Answer: The selection of surface roughness should not only meet the functional requirements of the workpiece surface, but also consider processing economy.

38. When using the analog method to determine surface roughness, what are the general principles for selecting height parameters?

Answer: On the same part, the roughness value of the working surface must be lower than that of the non-working surface. The roughness value of the friction surface should be lower than that of the frictionless surface; the roughness value of the rolling friction surface should be lower than that of the sliding friction surface; little. The roughness value of the surface subjected to cyclic loading and parts easily causing stress concentration (such as fillets and grooves) should be selected smaller. Combined surfaces with high requirements for contact properties, contact surfaces with small contact gaps and contact surfaces with interference that require reliable connections and are subjected to heavy loads must all have roughness values smaller surface area. With the same fitting properties, the smaller the part size, the lower its surface roughness value should be. For the same level of accuracy, the surface roughness value of a small size and a shaft is lower than that of a large size and a hole. For the contact surface, its dimensional tolerance, shape tolerance and surface roughness must be coordinated, and there is generally a certain corresponding relationship.

39. When the surface roughness Ra is between 50 and 100 μm, what are the characteristics of the surface shape and how to apply it?

Answer: The surface shape is characterized by obvious tool marks. It is applied to rough machined surfaces and is generally rarely used. Foundry, forging and gas cutting blanks can meet this requirement.

40. When the surface roughness Ra is 25 μm, what are the characteristics of the surface shape and how to apply it?

Answer: Surface form is characterized by visible tool marks, applied to rough machined surfaces and generally rarely used. Foundry, forging and gas cutting blanks can meet this requirement.

41. When the surface roughness Ra is 12.5 μm, what are the characteristics of the surface shape and how to apply it?

Answer: The surface shape is characterized by micro knife marks. It is used in the first level of rough machining, which has a wide range of applications, such as shaft end surfaces, chamfers, surfaces of screw holes and rivet holes, surfaces of contact of washers, etc.

42. When the surface roughness Ra is 6.3 μm, what are the characteristics of the surface shape and how to apply it?

Answer: Surface shape features are visible machining marks, which are applied to semi-rough machined surfaces, non-contact surfaces such as carriers, boxes, clutches, pulley sides, pulley sides, cam, surfaces in contact with bolt heads and rivet heads, all shafts and hole undercut, joint surface of general cover, etc.

43. When the surface roughness Ra is 3.2 μm, what are the characteristics of the surface shape and how to apply it?

Answer: The surface shape is characterized by micromachining marks. It is applied to semi-finished surfaces, surfaces such as boxes, racks, lids, sleeves, etc. which are connected to other rooms without corresponding requirements, surfaces which must be blue. , and knurling Pre-machined surfaces, all non-contacting outer surfaces of the spindle, etc. This is a surface roughness value that can be obtained more economically by basic cutting methods such as turning.

44. When the surface roughness Ra is 1.6 μm, what are the characteristics of the surface shape and how to apply it?

Answer: The surface shape is characterized by unclear processing marks. It is used for surfaces with high surface quality requirements, medium-sized machine tool worktables (ordinary precision), spindle boxes and combined cover surfaces of machine tools, flat pulleys and rollers. medium sized triangles. pulleys. The working surface, the ring sliding bearing pressing hole and the journal which generally rotates at low speed. Unfitted surfaces of some important parts of aeronautical and aerospace products.

45. When the surface roughness Ra is 0.8 μm, what are the characteristics of the surface shape and how to apply it?

Answer: The surface shape characteristic is the direction in which machining marks can be discerned. It is used in sliding guide surfaces of medium-sized machine tools (normal precision), guide rail pressure plates, cylindrical and tapered spindle surfaces, general precision dials. and exterior surfaces that require chrome plating and polishing. Quick-turning trunnion, positioning pin insertion hole, etc. This is a commonly used value for contact surfaces and is an important contact point for medium and heavy equipment. Its grinding is economical.

46. ​​When the surface roughness Ra is 0.4 μm, what are the characteristics of the surface shape and how to apply it?

Answer: The surface shape characteristic is the micro-discrimination direction of machining marks, which is used in medium-sized machine tools (to improve precision), sliding guide surfaces, working surfaces of plain bearings, the main surfaces of the fixing positioning elements and the drilling rings. , working journals of crankshafts and camshafts, surface of indexing plate, working surface of journal and bushing in high-speed operation, etc.

47. When the surface roughness Ra is 0.2 μm, what are the characteristics of the surface shape and how to apply it?

Answer: The surface shape is characterized by the direction of indistinguishable machining marks. It is used in precision machine tool spindle taper holes and tapered top surfaces; the mating surface of precision chucks and small diameter rotating shafts, as well as the piston pin holes of pistons. , which require airtight surfaces and supports. The surface, leaf basin and rear surface of the aircraft engine blade.

48. When the surface roughness Ra is 0.1 μm, what are the characteristics of the surface shape and how to apply it?

Answer: The surface shape is characterized by a dark and shiny surface. It is used in holes where the main spindle housing of precision machine tools matches the sleeve. Surfaces on which the instrument must withstand friction during use, such as guide rails, groove surfaces. , etc., the surface of holes used for hydraulic transmission and valves. Working surface, cylinder inner surface, piston pin surface, etc. General mechanical design limitations. Grinding is very uneconomical.

49. When the surface roughness Ra is 0.05 μm, what are the characteristics of the surface shape and how to apply it?

Answer: The surface shape is characterized by a bright and shiny surface, which is used in particularly precise bearing ring raceways, ball and roller surfaces, working surface of medium precision game pieces in measuring instruments, measuring surface of working gauges, etc.

50. When the surface roughness Ra is 0.025 μm, what are the characteristics of the surface shape and how to apply it?

Answer: The surface shape is characterized by a shiny, mirror-like surface, which is used in the raceways of rolling rings, the surfaces of particularly precise balls and rollers, as well as the contact surfaces of pistons and piston sleeves in high pressure oil pumps to ensure high pressure. airtight bonding surface, etc.

51. When the surface roughness Ra is 0.012 μm, what are the characteristics of the surface shape and how to apply it?

Answer: The shape characteristic of the surface is a hazy mirror surface, which is applied to the measuring surface of instruments, the working surface of high-precision clearance parts in measuring instruments, the working surface of blocks standards with a size greater than 100 mm, etc.

52. When the surface roughness Ra is 0.008 μm, what are the characteristics of the surface shape and how to apply it?

Answer: The surface shape characteristic is the mirror surface, which is used in the working surface of gauge blocks, the measuring surface of high-precision measuring instruments and the surface of the metal mirror in optical measuring instruments, etc.

53. When the surface roughness Ra is >10~40μm, what are the economical processing methods?

Answer: The economical processing methods are rough turning, rough planing, rough milling, drilling, rasping and sawing.

54. When the surface roughness Ra is >5~10μm, what are the economical processing methods?

Answer: Economical processing methods are turning, planing, milling, boring, drilling and rough boring.

55. When the surface roughness Ra is >2.5~5μm, what are the economical processing methods?

Answer: Economical processing methods are turning, planing, milling, boring, grinding, drawing, rough scraping and rolling.

56. When the surface roughness Ra is >1.25~2.5 μm, what are the economical processing methods?

Answer: Economical processing methods are turning, planing, milling, boring, grinding, drawing, scraping, pressing and tooth milling.

57. When the surface roughness Ra is >0.63~1.25 μm, what are the economical processing methods?

Answer: Economical processing methods are turning, boring, grinding, drawing, scraping, fine boring, gear grinding and rolling.

58. When the surface roughness Ra is >0.32 ~ 0.63 μm, what are the economical processing methods?

Answer: Economical processing methods are fine boring, fine boring, grinding, scraping and rolling.

59. When the surface roughness Ra is >0.16 ~ 0.32 μm, what are the economical processing methods?

Answer: Economical processing methods are fine grinding, sharpening, grinding and super-finishing.

60. When the surface roughness Ra is >0.08 ~ 0.16 μm, what are the economical processing methods?

Answer: Economical processing methods are fine grinding, ordinary grinding and polishing.

61. When the surface roughness Ra is >0.01 to 0.08 μm, what are the economical processing methods?

Answer: Economical processing methods are super fine grinding, fine polishing and mirror grinding.

62. When the surface roughness Ra is ≤0.01 μm, what are the economical processing methods?

Answer: Economical processing methods are mirror grinding and super precision grinding.

63. How to choose the value of the thread surface roughness parameter Ra?

Answer: When the precision level of ordinary coarse thread is level 4, Ra is 0.4-0.8 μm.

When the precision level of ordinary coarse thread is level 5, Ra is 0.8 μm.

When the precision level of ordinary coarse thread is level 6, Ra is between 1.6 and 3.2 μm.

When the precision level of ordinary fine thread wire is level 4, Ra is between 0.2 and 0.4 μm.

When the precision level of ordinary fine thread wire is level 5, Ra is 0.8μm.

When the precision level of ordinary fine thread wire is level 6, Ra is between 1.6 and 3.2 μm.

64. How to choose the value of the bond surface roughness parameter Ra?

Answer: The linkage shape is a key and when moving along the hub groove, Ra is 0.2-0.5 μm.

The bond shape is a key and when moving along the axis groove, Ra is 0.2-0.4 μm.

The bond shape is bond and the fixed position, Ra is 1.6 μm.

The combined shape is a shaft groove. Where it travels along the hub groove, Ra is 1.6 μm.

The combined shape is an axial groove, and when moving along the axial groove, Ra is 0.4-0.8 μm.

The combined shape is a shaft groove and the fixed position, Ra is 1.6 μm.

The combined shape is a hub groove. Where it travels along the hub groove, Ra is 0.4 to 0.8 μm.

The combined shape is a hub groove. Where it travels along the shaft groove, Ra is 1.0 μm.

The combined shape is a hub groove and the fixed position, Ra is 1.6 to 3.2 μm.

Note: The non-working surface Ra is 6.3 μm.

65. How to choose the value of the surface roughness parameter Ra of a rectangular spline?

Answer: Internal spline, at the outer diameter, Ra is 6.3 μm.

Internal spline, inner diameter, Ra is 0.8 μm.

Internal spline, key side, Ra is 3.2 μm.

External splines, at the outer diameter, Ra is 3.2 μm.

External spline, inner diameter, Ra is 0.8 μm.

External spline, key side, Ra is 0.8 μm.

66. How to choose the value of the gear surface roughness parameter Ra?

Answer: When the workpiece is the tooth surface and the precision level is level 5, Ra is 0.2-0.4 μm.

When the tooth surface precision level is level 6, Ra is 0.4 μm.

When the precision level of the tooth surface is level 7, Ra is between 0.4 and 0.8 μm.

When the tooth surface precision level is 8, Ra is 1.6 μm.

When the tooth surface precision level is level 9, Ra is 3.2 μm.

When the tooth surface precision level is level 10, Ra is 6.3 μm.

When the part is an outer circle and the precision level is level 5, Ra is between 0.8 and 1.6 μm.

When the part is an outer circle and the precision level is level 6, Ra is between 1.6 and 3.2 μm.

When the part is an outer circle and the accuracy level is level 7, Ra is between 1.6 and 3.2 μm.

When the part is an outer circle and the precision level is level 8, Ra is between 1.6 and 3.2 μm.

When the part is an outer circle and the accuracy level is level 9, Ra is between 3.2 and 6.3 μm.

When the part is an outer circle and the accuracy level is level 10, Ra is between 3.2 and 6.3 μm.

When the end face precision level is level 5, Ra is between 0.4 and 0.8 μm.

When the tip precision level is level 6, Ra is between 0.4 and 0.8 μm.

When the end face precision level is level 7, Ra is between 0.8 and 3.2 μm.

When the end face precision level is level 8, Ra is between 0.8 and 3.2 μm.

When the end face precision level is level 9, Ra is between 3.2 and 6.3 μm.

When the end face precision level is level 10, Ra is between 3.2 and 6.3 μm.

67 How to choose the value of the surface roughness parameter Ra of the worm gear?

Answer: When the endless part is the tooth surface and the precision level is level 5, Ra is 0.2 μm.

When the endless part is a tooth surface with a precision level of 6, Ra is 0.4 μm.

When the endless part is a tooth surface with a precision level of 7, Ra is 0.4 μm.

When the endless part is a tooth surface with a precision level of 8, Ra is 0.8 μm.

When the endless part is a tooth surface with a precision level of 9, Ra is 1.6 μm.

When the endless part is the tooth tip and the precision level is level 5, Ra is 0.2 μm.

When the endless part is the tip of the tooth and the precision level is level 6, Ra is 0.4 μm.

When the endless part is the tooth tip and the precision level is level 7, Ra is 0.4 μm.

When the endless part is the tip of the tooth and the precision level is level 8, Ra is 0.8 μm.

When the endless part is the tip of the tooth and the precision level is level 9, Ra is 1.6 μm.

Note: The worm part is the tooth root and Ra is 6.3 μm.

When the worm gear part is a toothed surface with a precision level of 5, Ra is 0.4 μm.

When the precision level of the worm gear tooth surface is level 6, Ra is 0.4 μm.

When the worm gear part is a toothed surface with a precision level of 7, Ra is 0.8 μm.

When the worm gear part is a toothed surface with a precision level of 8, Ra is 1.6 μm.

When the worm gear part is a toothed surface with a precision level of 9, Ra is 3.2 μm.

Note: The worm gear part is the root of the tooth and Ra is 3.2 μm.

68. How to choose the value of the gear surface roughness parameter Ra?

Answer: When the precision of the working surface of the pinion teeth is medium, Ra is between 1.6 and 3.2 μm.

When the working surface of the pinion teeth has high precision, Ra is between 0.8 and 1.6 μm.

When the tooth base is of medium precision, Ra is 3.2 μm.

When the location is the tooth base with high precision, Ra is 1.6 μm.

When the tooth top precision is medium, Ra is between 1.6 and 3.2 μm.

When the workpiece is a high-precision tooth tip, Ra is between 1.6 and 6.3 μm.

69. How to choose the value of the pulley surface roughness parameter Ra?

Answer: The location is the working surface of the pulley. When the pulley diameter is ≤120mm, Ra is 0.8μm.

When the pulley diameter is ≤300mm on the working surface of the pulley, Ra is 1.6μm.

When the pulley diameter is greater than 300mm on the working surface of the pulley, Ra is 3.2μm.

70. How to choose the value of the surface roughness parameter Ra of hydraulic components?

Answer: The part is the piston pump crank. At the piston, Ra is 1.6 to 0.8 μm.

The parts are the connecting rod journal, bushing and center journal. Ra is 0.4 μm.

The location is the outer cylindrical surface of the piston. On the side surface, Ra is 0.8 μm.

The parts are the piston pump connecting rod hole, cylinder body, spool ring, piston, and the Ra of the piston is 0.8-0.4μm.

The parts are slide valve, high pressure pump piston valve and valve seat. The Ra is 0.2 to 0.1 μm.

71. How to choose the surface roughness parameter Ra of the contact surface of the plain bearing?

Answer: The location is the tolerance level of IT7-IT9 shaft, Ra is 0.2-3.2 μm.

The location is the IT11-IT12 shaft tolerance level, Ra is 1.6-3.2 μm.

The location is hole tolerance level IT7-IT9, Ra is 0.4-1.6 μm.

The location is IT11-IT12 hole tolerance level, Ra is 1.6~3.2μm.

72. How to choose the value of the conical combined surface roughness parameter Ra?

Answer: The location is the seal of the outer surface of the cone, Ra is ≤0.1 μm.

The location is the centering joint of the outer surface of the cone, Ra is ≤0.2 μm.

The location corresponds to other joints on the outer surface of the cone, Ra is ≤1.6 ~ 3.2 μm.

The location is the seal of the inner surface of the cone, Ra is ≤0.2 μm.

The location is the centering joint of the inner surface of the cone, Ra is ≤0.8 μm.

The location matches other joints on the inner surface of the cone, Ra is ≤1.6~3.2μm.

73.What are the methods of labeling surface roughness? Specification 1.

Answer: The direction of numbers and symbols in the surface roughness code should be marked as specified in the figure below. As shown in Figure 10.

Figure 10

74.What are the methods of labeling surface roughness? Specification 2.

Answer: Surface roughness symbols with horizontal lines should be marked as shown below. See Figure 11.

Figure 11

75.What are the methods of labeling surface roughness? Specification 3.

Answer: The tip of the symbol should point from the outside of the material toward the surface. The most commonly used roughness code is uniformly noted in the upper right corner of the drawing, with the word “rest” in front of it. As shown in Figure 12.

Figure 12

76.What are the methods of labeling surface roughness? Specification 4.

Answer: When all surface roughness requirements are the same, they can be uniformly noted in the upper right corner of the drawing. As shown in Figure 13.

Figure 13

77.What are the methods of labeling surface roughness? Specification 5.

Answer: When there are different surface roughness requirements on the same surface, the dividing line should be drawn with a thin continuous line. As shown in Figure 14.

Figure 14

78.What are the methods of labeling surface roughness? Specification 6.

Answer: The roughness code for continuous surfaces and surfaces with repeated elements (holes, grooves, teeth, etc.) and discontinuous surfaces connected by thin solid lines is marked only once. As shown in Figure 15.

Figure 15

79.What are the methods of labeling surface roughness? Specification 7.

Answer: When space is small or it is not practical to label, codenaming can lead to labeling. As shown in Figure 16.

Figure 16

80.What are the methods of labeling surface roughness? Specification 8.

Answer: In order to simplify the labeling or when the position of the labeling is restricted, the simplified code name can be marked, or the omitted annotation method can be used (see the figure below), but the meaning of the simplified code name must be explained. near the title bar. As shown in Figure 17.

Figure 17

81.What are the methods of labeling surface roughness? Standard 9.

Answer: When parts are to be partially heat treated or plated (coated), the range should be drawn with thick dotted lines and the corresponding dimensions should be marked. Requirements can also be written in the surface roughness symbol. As shown in Figure 18.

Figure 18

82.What are the methods of labeling surface roughness? Standard 10.

Answer: The surface roughness codes of center hole working surface, keyway working surface, chamfers and fillets can be simplified as shown below. As shown in Figure 19.

Figure 19

83.What are the methods of labeling surface roughness? Specification 11.

Answer: When there is no tooth shape (tooth) drawn on the working surface of gears, involute splines, threads, etc., the surface roughness code can be marked as shown below below. As shown in Figure 20.

Figure 20

84. Other marking codes for surface roughness arewhat is its meaning?

Answer: The sampling length should be marked below the horizontal line on the long side of the symbol, indicating the sampling length I = 2.5 mm.

85. Other marking codes for surface roughness arewhat is its meaning?

Answer: When the roughness requirements are obtained by a specified processing method, the text may be marked on the horizontal line on the long side of the symbol.

86. Other marking codes for surface roughness arewhat is its meaning?

Answer: When you need to control the texture direction of the surface treatment, you can add the texture direction symbol to the right of the symbol.

87. Other marking codes for surface roughness arewhat is its meaning?

Answer: The machining allowance should be marked on the left side of the symbol, indicating that the machining allowance is 2mm.

88. Other marking codes for surface roughness arewhat is its meaning?

Answer: It indicates the roughness value of the surface before plating (coating).

89. Other marking codes for surface roughness arewhat is its meaning?

Answer: It indicates the surface roughness value after plating (coating) or other surface treatment.

90. What functions does the choice of surface roughness affect?

Answer: Surface roughness affects various functions: such as friction coefficient, wear, fatigue resistance, impact resistance, corrosion resistance, contact stiffness and vibration resistance, clearance in clearance fits, bond strength in interference fits, measurement accuracy, thermal conductivity, electrical conductivity and contact resistance, tightness, bond strength, painting performance, coating quality, resistance to fluid flow, reflection performance light, food hygiene, appearance, quality of sprayed metal, lubrication while waiting for stamping of steel plates.

91. What effect does the choice of surface roughness have on the corresponding properties?

Answer: Affects the reliability and stability of the corresponding performance. For clearance fits, due to initial wear, the picks will wear out quickly, increasing the gap; For tight fits, the peaks will be flattened during assembly and pressing, reducing the actual effective interference, especially for small fits. significant. Therefore, the Ra value of a joint surface that requires high stability of fit properties, a surface with a small clearance of dynamic fit, a static fit that requires a firm and reliable connection and large bearing capacity should be lower, and the small size of the same tolerance level is better than the large size (especially tolerance level 1 ~ level 3), the shaft with the same tolerance level is smaller than the hole value and the corresponding properties are the same. of the part, the lower its Ra value.

92. What effect does the choice of surface roughness have on wear resistance?

Answer: Due to the presence of peaks and valleys on the surface of the processed parts, the contact surface only has a certain contact peak, thereby reducing the contact area, increasing the specific pressure and intensifying wear . Therefore, the friction surface moves faster than the non-friction surface, the rolling friction surface moves faster than the sliding friction surface, and the Ra value of the friction surface with unit pressure higher is smaller.

93. What effect does the choice of surface roughness have on contact stiffness?

Answer: When two surfaces are in contact, since the actual contact area is part of the ideal contact area, the compressive stress per unit area increases and contact deformation is likely to occur when an external force is applied . Therefore, it is possible to reduce the Ra value. improve the contact stiffness of the joint.

94. What effect does the choice of surface roughness have on fatigue life?

Answer: The rougher the surface of the part, the more susceptible it will be to stress concentration, leading to fatigue damage of the part. Therefore, the Ra value of the surface is subject to cyclic loads and the parts subject to stress concentration. , such as fillets and grooves, must be lower. The impact of surface roughness on the fatigue resistance of parts varies depending on the material. The impact on cast iron parts is not obvious. For steel parts, the higher the resistance, the greater the impact.

95. What effect does the choice of surface roughness have on impact resistance?

Answer: The impact resistance of the steel surface increases as the Ra value of the surface roughness decreases, especially at low temperatures.

96. What impact does the choice of surface roughness have on measurement accuracy?

Answer: Due to the microscopic irregularities on the surface of the workpiece, the measuring rod comes into contact with the pick during measurement. Although the measuring force is not large, the contact area is small and the force per unit area is not small, resulting in. a certain amount of contact deformation. Since microscopic surface irregularities have certain peaks and valleys, for example when measuring, the measuring head and the measured surface must slide relative to each other, causing the measuring rod to fluctuate up and down with the peaks and valleys of the measured surface, affecting the indication value.

97. What impact does the choice of surface roughness have on waterproofing?

Answer: For static sealing surfaces without relative sliding, the bottom of the microscopic irregularities is too deep, and the preloaded sealing material cannot be completely filled, leaving gaps, causing leaks. The rougher the surface, the greater the leaks. For relatively slippery dynamic seal surfaces, due to relative movement, the microscopic roughness is generally 4-5 µm, which is more beneficial for the storage of lubricating oil. If the surface is too smooth, it will not only be unfavorable for the storage of lubricating oil, but also. will cause friction and wear. In addition, the quality of the seal is also related to the direction of the processing texture.

98.What effect does the choice of surface roughness have on corrosion resistance?

Answer: If the surface is rough, corrosive gas or liquid on the surface of the workpiece will easily accumulate and penetrate into the surface layer of the workpiece, thereby aggravating corrosion. Therefore, the Ra value of the surface of parts operating under corrosive gas conditions. or the liquid should be smaller.

99. What impact does the choice of surface roughness have on the quality of the metal surface coating?

Answer: After the part is plated with zinc, chrome or copper, the depth of microscopic irregularities on the surface will be doubled compared to before plating, while after nickel plating it will be reduced by half. And because the rough surface can absorb the tensile stress generated when the sprayed metal layer is cooled, it is not easy to produce cracks. The surface should have some roughness before spraying the metal.

100. What effect does the choice of surface roughness have on vibration and noise?

Answer: The surface of moving pairs of mechanical equipment is rough and uneven, which will produce vibration and noise during operation. This phenomenon is most obvious for bearings, gears, engine crankshafts, camshafts and other parts operating at high speeds. Therefore, the smaller the Ra value of the surface roughness of the moving pair, the smoother and quieter the moving parts will be.