100 Questions and Answers on Surface Roughness

1． What is the sampling length? answer: A reference line length used to identify surface roughness characteristics. The rougher the surface, the greater 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. Within the sampling length range, it generally includes more than 5 contour peaks and contour valleys. For the selection value of the sampling length, please refer to GB/T 1031-1995 Surface roughness parameters and their values. 2. What is the assessment length? answer: It is a length necessary to evaluate the profile, which may include one or several sampling lengths. Due to the uneven surface processing of parts, in order to fully and reasonably reflect the roughness characteristics of the measured surface, several sampling lengths need to be used for evaluation. For the selection value of the evaluation length, please refer to GB/T 1031-1995 Surface roughness parameters and their numerical values. 3． What is a baseline? answer: A reference line for evaluating the numerical value of surface roughness parameters is called the baseline. There are two types of baselines: Contour least squares midline and contour arithmetic mean midline. 9. What is called the contour least squares midline? answer: The least squares midline of the contour is the line that minimizes the sum of squares of the contour offsets of each point on the contour within the sampling length. 10. What is called the arithmetic mean midline 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 within the sampling length and makes the upper and lower areas equal.

11. What are the basic evaluation parameters? answer: The three height parameters are the basic evaluation parameters, 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). ； The other three items are additional evaluation parameters, namely the average spacing of the microscopic unevenness of the profile (Sm), the average spacing of the single peaks of the profile (S) and the profile support length ratio (tP). 12． What is called the contour arithmetic mean deviation (Ra)? answer: Within the sampling length, the arithmetic mean of the absolute value of the distance from each point on the measured contour to the contour centerline. 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 directly measured with an electric profilometer, but it is not intuitive enough. 13. What is the ten-point height of microscopic roughness (Rz)? answer: Within the sampling length, the sum of the average of the five largest contour peak heights and the average of the five largest valley depths. The larger the Rz value, the rougher the surface. Rz is highly 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 between the peak line and the bottom line of the valley within the sampling length. The peak line and valley bottom line refer to the lines parallel to the midline and passing through the highest and lowest points of the contour within the sampling length, respectively. The parameter Ry is simple to measure. When the surface to be measured is small and it is not suitable to use Rz, Rz evaluation can be used. 15. How to determine the allowable values ​​of surface roughness height evaluation parameters (Ra, Rz, Ry))? answer: See GB/T 1031-1995 surface roughness parameters and their values. 16. The symbol and code of surface roughness is file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image001.gif时，它的意义是什么？答：基本符号、表示表面可用任何方法获得。当不加粗糙度参数值或有关说明（例如：表面处理、局部热处理状况等）时，仅适用于简化代号标注。17．表面粗糙度的符号、代号为file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image002.gif时，它的意义是什么？答：基本符号加一短划，表示表面是用去除材料的方法获得。例如：车、铣、钻、磨、剪切、抛光、腐蚀、电火花加工、气割等。18．表面粗糙度的符号、代号为file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image003.gif时，它的意义是什么？答：基本符号加一小圈，表示表面是用不去除材料的方法获得。例如：铸、锻、冲压变形、热轧、冷轧、粉末冶金等。或者是用于保持原供应状况的表面（包括保持上道工序的状况）。19．表面粗糙度的符号、代号为file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image004.gif时，它的意义是什么？答：用任伺方法获得的表面粗糙度，Ra的上限值为3.2μm。20．表面粗糙度的符号、代号为file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image005.gif时，它的意义是什么？答：用去除材料方法获得的表面粗糙度，Ra的上限值为3.2μm。

twenty one. The symbol and code of surface roughness is file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image001.gif, what is its meaning? Answer: The upper limit of Ra is 3.2μm for surface roughness obtained without material removal. twenty two. What is the meaning of the surface roughness symbol and codename file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image002.gif? Answer: For the surface roughness obtained by removing material, the upper limit of Ra is 3.2 μm and the lower limit of Ra is 1.6 μm. twenty three. What is the meaning of the surface roughness symbol and codename file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image003.gif? Answer: The upper limit of Ry is 3.2μm for the surface roughness obtained by any servo method. twenty four. What is the meaning of the surface roughness symbol and codename file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image004.gif? Answer: The upper limit of Rz is 200μm for surface roughness obtained without material removal. 25. What is the meaning of the surface roughness symbol and codename file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image005.gif? Answer: The upper limit of Rz is 3.2μm and the lower limit of Rz is 1.6μm for the surface roughness obtained by removing material. 26． What is the meaning of the surface roughness symbol and codename file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image006.gif? Answer: For the surface roughness obtained by removing material, 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 surface roughness? Answer: When Ra is selected as the height parameter, its code name can be omitted when labeling. When Ry and Rz are selected, the code name cannot be omitted. The surface roughness code given on the drawing is the requirement for the finished surface. Generally, you only need to indicate the symbol and the allowable parameter value. If there are special requirements for the surface function of the part (additional requirements such as processing texture, processing allowance, etc.), relevant parameters or codes can be marked around the basic symbols. 28． How to draw the surface roughness symbol? Answer: See Figure 1 file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image007.gif Figure 1d' =h/10 ； H=1.4h ； h is the font height. 29． What are the methods for labeling surface roughness? Example 1 answer: See Figure 2 file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image008.gif图2表面粗糙度代(符)号应注在可见轮廓线、尺寸线、尺寸界线或它们的延长线上。符号的尖端必须从材料外指向表面。30．表面粗糙度标注方法有哪些？示例2答：见图3file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image009.gif图3中心孔工作表面、键槽工作面、倒角、圆角的表面，可以简化标注。

31． What are the methods for labeling surface roughness? Example 3 answer: See Figure 4 file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image002.jpg图4齿轮、渐开线花键、螺纹等工作表面没有画出齿(牙)形时的标注方法。32．表面粗糙度标注方法有哪些？示例4答：见图5file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image003.gif图5同一表面有不同的表面粗糙度要求时，须用细实线画出其分界线，并注出相应的表面粗糙度的符号和尺寸。 33. What are the methods for labeling surface roughness? Example 5 Answer: See Figure 6 file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image004.gif图6需要表示局部热处理或局部镀涂时，应用粗点划线画出其范围并标注相应的尺寸，也可将其要求注写在表面粗糙度符号内。 34. What are the methods for labeling surface roughness? Example 6 answer: See Figure 7 file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image005.gif图7零件连续表面及重复要素(孔、槽、齿……等)的表面和用细实线联接不连续的一表面，其代(符)号只标注一次。35．表面粗糙度标注方法有哪些？示例7答：见图8file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image006.gif图8零件大部分表面要求相同时，在右上角统一标注，并加注“其余”二字。为了简化标注，或位置受到限制时，可标注简化代号，也可采用省略注法，但必须在标题栏附近说明这些简化代(符)号的意义。采用统一标注和简化标注时，其代号和文字说明均应是图形上其它表面所注代号和文字的1.4倍。36．表面粗糙度参数和各项规定注写的位置如何规定？file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image007.giffile:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image008.gif答：见图9 file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image009.giffile:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image010.giffile:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image011.giffile:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image013.jpg file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtml1/01/clip_image014.gif Figure 937. How to choose surface roughness? answer: The selection of surface roughness must not only meet the functional requirements of the part surface, but also consider the economy of processing. 38. When using the analogy method to determine surface roughness, what are the general principles for selecting height parameters? answer: On the same part, the surface roughness value of the working surface should be smaller than that of the non-working surface. The surface roughness value of the friction surface should be smaller than that of the non-friction surface ； The surface roughness value of the rolling friction surface should be smaller than that of the sliding friction surface ； The surface roughness value should be small when the movement speed is high and the unit pressure is high. The surface roughness value of the surface subject to cyclic load and the parts easily causing stress concentration (such as fillets and grooves) should be selected smaller. Combining surfaces with high requirements for mating properties, mating surfaces with small mating gaps, and interference mating surfaces that require reliable connections and are subject to heavy loads should all have smaller surface roughness values. With the same fit properties, the smaller the size of the part, the smaller its surface roughness value should be. For the same accuracy level, the surface roughness value of small size and shaft is smaller than that of large size and hole. For the mating surface, its dimensional tolerance, shape tolerance, and surface roughness should be coordinated, and generally there is a certain corresponding relationship. 39． When the surface roughness Ra is 50-100μm, what are the characteristics of the surface shape and how to apply it? answer: The surface shape is characterized by obvious tool marks, which are used on rough machined surfaces and are generally rarely used. Casting, forging and gas cutting blanks can meet this requirement. 40. When the surface roughness Ra is 25μm, what are the characteristics of the surface shape and how to apply it? answer: The surface shape is characterized by visible tool marks, which is applied to rough machined surfaces and is generally rarely used. Casting, forging and gas cutting blanks can meet this requirement.

41. When the surface roughness Ra is 12.5μm, what are the characteristics of the surface shape and how to apply it? answer: The surface shape is characterized by micro knife marks, which is used in the first level of rough machining, and has a wide range of applications, such as shaft end surfaces, chamfers, surfaces of screw holes and rivet holes, and contact surfaces of washers, etc. 42． When the surface roughness Ra is 6.3μm, what are the characteristics of the surface shape and how to apply it? answer: The surface shape characteristics are visible processing marks, which are applied to semi-rough machined surfaces, non-contact free surfaces such as brackets, boxes, clutches, pulley sides, cam sides, surfaces in contact with bolt heads and rivet heads, undercuts of all shafts and holes, and the joint surface of general shutters, etc. 43. When the surface roughness Ra is 3.2μm, what are the characteristics of the surface shape and how to apply it? answer: The surface shape is characterized by micro-machining marks, which are applied to semi-finished surfaces, surfaces where boxes, brackets, covers, sleeves, etc. are connected to other parts without matching requirements, surfaces that require bluing, pre-processed surfaces that require knurling, all non-contact outer surfaces of the spindle, etc. It is a surface roughness value that can be achieved more economically by basic cutting processing methods such as turning. 44. When the surface roughness Ra is 1.6μm, what are the characteristics of the surface shape and how to apply it? answer: The surface shape is characterized by indistinct machining traces, and is used for surfaces with high surface quality requirements, medium-sized machine tool worktables (normal precision), combined machine tool spindle boxes and cover surfaces, working surfaces of medium-sized flat pulleys and triangle pulleys, press-in holes of bushing sliding bearings, and journals that generally rotate at low speeds. Non-fitting surfaces of some important parts of aviation and aerospace products. 45. When the surface roughness Ra is 0.8μm, what are the characteristics of the surface shape and how to apply it? answer: The surface shape feature is the direction in which machining traces can be discerned. It is used in medium-sized machine tools (normal precision) sliding guide surfaces, guide rail pressure plates, surfaces of cylindrical pins and conical pins, general precision dials, outer surfaces that require chrome plating and polishing, medium-speed rotating journals, positioning pin press-in holes, etc. It is a commonly used value for mating surfaces and is an important mating point for medium and heavy equipment. It is economical to grind. 46. When the surface roughness Ra is 0.4μm, what are the characteristics of the surface shape and how to apply it? answer: The surface shape is characterized by the direction of micro-discrimination machining marks, which is used in medium-sized machine tools (to improve accuracy) sliding guide surfaces, working surfaces of sliding bearings, main surfaces of fixture positioning components and drill bushings, working journals of crankshafts and camshafts, surfaces of index plates, working surfaces of journals and bushings under high-speed operation, etc. 47. When the surface roughness Ra is 0.2μm, what are the characteristics of the surface shape and how to apply it? answer: The surface shape is characterized by the direction of indiscernible machining marks, which is used in precision machine tool spindle taper holes and top conical surfaces. ； The joint surface between the small diameter precision spindle and the rotating shaft, and the piston pin hole of the piston require airtight surfaces and supporting surfaces. The leaf basin and the back side of the aero-engine blade. 48. When the surface roughness Ra is 0.1μm, what are the characteristics of the surface shape and how to apply it? answer: The surface shape is characterized by a dark glossy surface, which is used in the holes where the main spindle box of precision machine tools matches the sleeve, the surfaces where the instrument is subject to friction during use, such as guide rails, groove surfaces, etc., the surface of the hole for hydraulic transmission, the working surface of the valve, the inner surface of the cylinder, the surface of the piston pin, etc. General mechanical design limits. Grinding is very uneconomical. 49. When the surface roughness Ra is 0.05μm, what are the characteristics of the surface shape and how to apply it? answer: The surface shape is characterized by a bright glossy surface, which is used in particularly precise rolling bearing ring raceways, ball and roller surfaces, working surfaces of medium-precision clearance fitting parts in measuring instruments, and measuring surfaces of working gauges, etc. 50. When the surface roughness Ra is 0.025μm, what are the characteristics of the surface shape and how to apply it? answer: The surface shape is characterized by a mirror-like glossy surface, which is used in particularly precise rolling bearing ring raceways, balls and roller surfaces, and the mating surfaces of plungers and plunger sleeves in high-pressure oil pumps to ensure a highly airtight bonding surface.

Thank you sir, my long-standing question has finally been clarified. The specific selection of Ra50 and Ra25 in the design document does not seem to have much impact during construction.