Concept of Surface Roughness
Surface roughness refers to the irregularities on a processed surface characterized by small spacing and tiny peaks and valleys. The distance between two consecutive peaks or valleys (wavelength) is very small (below 1 mm), representing micro-scale geometric errors. Specifically, it refers to the small peaks and valleys’ Z-height and spacing S condition. Generally, it is categorized based on S as follows:
- S＜1mm: Surface Roughness
- 1≤S≤10mm: Waviness
- S＞10mm: Form
Please note that the term ‘form’ is used for cases where S exceeds 10mm.
The factors contributing to surface roughness formation
The factors contributing to surface roughness formation are as follows:
- Friction between the cutting tool and the workpiece surface during machining.
- Plastic deformation of the surface metal layer during chip separation.
- High-frequency vibrations in the machining process.
- Electrical discharge craters in electrical machining.
Due to variations in machining methods and workpiece materials, the depth, density, shape, and texture of the resulting surface roughness differ.
The main effects of surface roughness on components
1. Impact on Wear Resistance:
The rougher the surface, the smaller the effective contact area between mating surfaces, leading to higher pressure and increased friction resistance, resulting in faster wear.
2. Influence on Fit Stability:
For clearance fits, rough surfaces are more prone to wear, causing the clearance to increase gradually during operation. For interference fits, the micro-peaks on the surfaces are flattened during assembly, reducing the actual effective interference and weakening the connection strength.
3. Effect on Fatigue Strength:
Rough surfaces on components contain larger valleys that act as stress concentrators, similar to sharp notches and cracks, which significantly affect the fatigue strength of the parts.
4. Impact on Corrosion Resistance:
Rough surfaces facilitate the penetration of corrosive gases or liquids into the metal’s inner layers through micro-cavities, leading to surface corrosion.
5. Influence on Sealability:
Rough surfaces cannot form a tight seal, resulting in gas or liquid leakage through the gaps between contact surfaces.
6. Effect on Contact Stiffness:
Contact stiffness is the ability of mating surfaces to resist deformation under external forces. The overall stiffness of a machine largely depends on the contact stiffness between its components.
7. Impact on Measurement Accuracy:
The surface roughness of a component’s measured surface and the measuring tool directly affects measurement accuracy, especially in precision measurements.
Additionally, surface roughness can influence the plating or coating of components, thermal conductivity, contact resistance, reflectivity, radiative performance, fluid and gas flow resistance, and conduction of surface currents in conductors.
Criteria for Surface Roughness Evaluation
Sampling Length:
The sampling length is a specified segment of the baseline used to assess surface roughness. It should be chosen based on the component’s actual surface formation and texture characteristics, reflecting the surface roughness features. When determining the sampling length, we should consider the overall direction of the actual surface profile. Specifying and selecting the sampling length aim to limit and mitigate the influence of surface waviness and form errors on the measurement results of surface roughness.
Evaluation Length:
The evaluation length is a necessary segment of the profile used to assess surface roughness and may consist of one or several sampling lengths. Since surface roughness may not be uniformly distributed on different parts of the component’s surface, a single sampling length may not adequately represent a specific surface roughness feature. Therefore, multiple sampling lengths are taken from the surface to evaluate the surface roughness. Typically, the evaluation length includes five sampling lengths.
Reference Line:
The reference line is the midline for evaluating surface roughness parameters. There are two types of reference lines: the least squares centerline and the arithmetic mean centerline. In the least squares centerline, the sum of squared deviations of points on the profile within the sampling length is minimized, representing the geometric contour shape. The arithmetic mean centerline is the one where the area above and below the centerline within the sampling length is equal. Theoretically, the least squares centerline is the ideal reference line, but it is challenging to obtain in practical applications. Therefore, the arithmetic mean centerline is commonly used as a replacement, and a straight line with a close approximation of the position can be used for measurement.
Surface Roughness Evaluation Parameters
Height Feature Parameters
Ra: Arithmetic Mean Deviation of the Profile – It represents the arithmetic average of the absolute values of profile deviations within a sampling length (lr). In practical measurements, more measuring points result in a more accurate Ra value.
Rz: Maximum Height of the Profile – The distance between the profile’s highest peak and the lowest valley.
In the commonly used range of amplitude parameters, Ra is given priority. Before 2006, another parameter called “Ry” represented the ten-point height of the micro-roughness. After 2006, the national standard eliminated the ten-point height parameter and adopted Rz to represent the maximum height of the profile.
Spacing Feature Parameter
Rsm: Mean Spacing of the Profile Elements – It represents the average width of profile micro-roughness elements within the sampling length. The micro-roughness spacing refers to a profile segment that includes a peak and its adjacent valley along the centerline. Even with the same Ra value, the Rsm value may vary, reflecting different textures. Surface evaluations emphasizing texture often focus on both Ra and Rsm as key indicators.
Rmr: Rmr is a shape feature parameter that represents the ratio of the profile supporting length, obtained by intersecting the profile with straight lines parallel to the centerline and at a distance c from the profile’s highest peak within the sampling length, to the sampling length itself.
Surface Roughness Measurement Methods
Comparison Method
This method is suitable for on-site measurements in workshops and is commonly used for measuring surfaces with medium to rough roughness. The process involves comparing the surface to be measured with a roughness specimen with known numerical values to determine the roughness value of the measured surface.
Stylus Profilometer Method
Surface roughness is measured using a stylus profilometer, where a diamond-tipped stylus with a radius of curvature of about 2 micrometers glides slowly along the surface to be measured. The vertical displacement of the diamond stylus is converted into an electrical signal by an inductive length sensor. After amplification, filtering, and calculations, the surface roughness value is indicated on a display instrument. Additionally, the measured profile curve of the cross-section can be recorded using a recording device. Instruments capable of displaying only the surface roughness value are referred to as surface roughness testers, while those capable of recording the surface profile curve are called surface roughness profilometers. Both types of measurement tools have electronic calculation circuits or electronic computers that can automatically calculate parameters such as arithmetic mean deviation (Ra), ten-point height of micro-roughness (Rz), maximum height of the profile (Ry), and other evaluation parameters. These measurement tools offer high efficiency and are suitable for measuring surface roughness with Ra ranging from 0.025 to 6.3 micrometers.
60 Questions and Answers on Surface Roughness
Q1: What is surface roughness?
A: Surface roughness refers to the micro-scale geometric features on the surface of a component, characterized by small spacing and peaks and valleys. It represents a type of micro-scale geometric error.
Q2: How is surface roughness formed?
A: The surface of a component formed through cutting processes or other methods always contains geometric errors due to material plastic deformation, mechanical vibrations, friction, and other factors during the manufacturing process.
Q3: What are the effects of surface roughness on components?
A: Surface roughness significantly influences friction, wear, fatigue strength, corrosion resistance, and the fit properties between components.
Q4: What are the main ISO standards for “surface roughness”?
- ISO 1302:2010 – Geometrical product specifications (GPS) – Indication of surface texture in technical product documentation
- ISO 4287:1997 – Geometrical Product Specifications (GPS) – Surface texture: Profile method – Terms, definitions, and surface texture parameters
- ISO 4288:1996 – Geometrical Product Specifications (GPS) – Surface texture: Profile method – Rules and procedures for the assessment of surface texture
- ISO 4289:1997 – Geometrical Product Specifications (GPS) – Surface texture: Profile method – Nominal characteristics of contact (stylus) instruments
- ISO 3274:1996 – Geometrical Product Specifications (GPS) – Surface texture: Profile method – Nominal characteristics of non-contact (optical) instruments
- ISO 13565-1:1996 – Geometrical Product Specifications (GPS) – Surface texture: Profile method – Surface texture profile method – Metrological characteristics of phase correct filters
- ISO 13565-2:1996 – Geometrical Product Specifications (GPS) – Surface texture: Profile method – Surface texture profile method – Terms, definitions, and surface texture parameters
- ISO 13565-3:1996 – Geometrical Product Specifications (GPS) – Surface texture: Profile method – Surface texture profile method – Rules and procedures for the assessment of surface texture
- ISO 13565-4:1996 – Geometrical Product Specifications (GPS) – Surface texture: Profile method – Surface texture profile method – Nominal characteristics of contact (stylus) instruments
- ISO 13565-5:1996 – Geometrical Product Specifications (GPS) – Surface texture: Profile method – Surface texture profile method – Nominal characteristics of non-contact (optical) instruments
Q5: What is meant by ‘actual profile’?
A: It refers to the profile line obtained by the intersection of a plane with the actual surface. Depending on the direction of the intersection, it can be classified as either a transverse actual profile or a longitudinal actual profile. When evaluating and measuring surface roughness, unless specifically specified otherwise, it is generally based on the transverse actual profile, which is the profile on a cross-section perpendicular to the direction of the machining texture.
Q6: What is meant by ‘sampling length’?
A: It is a segment of baseline length used to identify surface roughness characteristics. The larger the surface roughness, the greater the required sampling length. Specifying the sampling length aims to limit and mitigate the influence of other geometric errors on the measurement results of surface roughness. Within the sampling length, there should be five or more peaks and valleys of the profile. The sampling length values are selected in GB/T 1031-1995: Surface roughness – Parameters and their values.
Q7: What is meant by ‘evaluation length’?
A: It is a necessary profile segment for evaluating surface roughness and can include one or several sampling lengths. Due to the non-uniformity in surface machining, several sampling lengths are needed to fully and reasonably reflect the roughness characteristics of the measured surface. The evaluation length values are selected in GB/T 1031-1995: Surface roughness – Parameters and their values.
Q8: What is meant by ‘reference line’?
A: The reference line determines the numerical values of surface roughness parameters. There are two types of reference lines: the least squares centerline and the arithmetic mean centerline.
Q9: What is meant by ‘least squares centerline’?
A: The least squares centerline is a line within the sampling length that minimizes the sum of squared profile deviations at each point on the profile.
Q10: What is meant by ‘arithmetic mean centerline’?
A: The arithmetic mean centerline is a line within the sampling length that divides the actual profile into upper and lower portions and ensures that the areas above and below the centerline are equal.
Q11: What are the basic evaluation parameters?
A: The basic evaluation parameters consist of three height parameters: arithmetic mean deviation of the profile (Ra), ten-point height of micro-roughness (Rz), and maximum height of the profile (Ry). Additionally, there are three supplementary evaluation parameters: average spacing of the profile micro-roughness (Sm*), mean spacing of single peaks in the profile (S), and profile supporting length ratio (tP).
Q12: What is the arithmetic mean deviation of the profile (Ra)?
A: Ra represents the arithmetic average of the absolute distances between each point on the measured profile and the profile’s centerline within the sampling length. A larger Ra value indicates a rougher surface. Ra objectively reflects the geometric characteristics of the measured profile. While Ra can be directly measured using a profilometer, it may not provide a complete visual representation.
Q13: What is the ten-point height of micro-roughness (Rz)?
A: Rz is the average of the five largest peak heights and the five largest valley depths within the sampling length. A higher Rz value indicates a rougher surface. Rz provides an excellent visual representation for evaluating the height parameter of surface roughness and can be easily measured using optical instruments. However, it has limitations in reflecting the geometric characteristics of the measured profile.
Q14: What is the maximum height of the profile (Ry)?
A: Ry represents the distance between the profile’s highest peak and lowest valley within the sampling length. The peak line and valley line refer to the lines passing through the highest and lowest points of the profile, respectively, within the sampling length and parallel to the centerline. The parameter Ry is easy to measure and is suitable when the measured surface is small, and Rz may not be practical. Ry can be used as an alternative to Rz for surface evaluation.
Q15: How are the permissible surface roughness height evaluation parameters (Ra, Rz, Ry) determined?
A: Industry Standards: Different industries have specific standards and guidelines for evaluating surface roughness. For example, ISO 1302 and ISO 4287 are commonly used international standards that define surface texture parameters and evaluation methods.
Q16: When the symbol and code of surface roughness are √, what does it mean?
A: When the symbol √ is used as the notation for surface roughness, the basic symbol represents the surface roughness obtainable by any method. When no roughness parameter value or relevant information is provided (such as surface treatment or localized heat treatment conditions), it only applies to simplified notation purposes.
Q17: When the symbol and code of surface roughness are 
, what does it mean?
A: The basic symbol with a short dash represents the surface obtained using material removal methods. For example: turning, milling, drilling, grinding, shearing, polishing, etching, EDM (Electric Discharge Machining), oxy-fuel cutting, etc.
Q18: When the symbol and code of surface roughness are 
, what does it mean?
A: The basic symbol with a small circle represents that the surface is obtained using methods that do not involve material removal. For example, casting, forging, stamping deformation, hot rolling, cold rolling, powder metallurgy, etc. It can also indicate surfaces that maintain the original supply condition (including the condition of the previous process).
Q19: When the symbol and code of surface roughness are 
, what does it mean?
A: The upper limit for Ra, the surface roughness obtained by any method, is 3.2 μm.
Q20: When the symbol and code of surface roughness are 
, what does it mean?
A: The upper limit for Ra is 3.2 μm, obtained using material removal methods.
Q21: When the symbol and code of surface roughness are 
, what does it mean?
The upper limit for Ra, obtained using methods that do not involve material removal, is 3.2 μm.
Q22: When the symbol and code of surface roughness are 
, what does it mean?
A: The upper limit for Ra, obtained using material removal methods, is 3.2 μm, and the lower limit is 1.6 μm.
Q23: When the symbol and code of surface roughness are 
, what does it mean?
A: The upper limit for Ry, the surface roughness obtained by any method, is 3.2 μm.
Q24: When the symbol and code of surface roughness are 
, what does it mean?
A: The upper limit for Rz, obtained using methods that do not involve material removal, is 200 μm.
Q25: When the symbol and code of surface roughness are 
, what does it mean?
The upper limit for Rz, obtained using material removal methods, is 3.2 μm, and the lower limit is 1.6 μm.
Q26: When the symbol and code of surface roughness are 
, what does it mean?
The upper limit for Ry, obtained using material removal methods, is 3.2 μm, and the lower limit is 12.5 μm.
Q27: What should be paid attention to when marking the surface roughness?
When indicating surface roughness, certain considerations should be taken into account. When using the height parameter Ra, its symbol can be omitted in the annotation. However, the symbols must not be omitted when using Ry or Rz. The surface roughness symbols specified on the drawing represent the requirements for the finished surface. Generally, it is sufficient to annotate the symbol and the permissible values of the parameters. If there are specific requirements for the surface functionality of the component (such as machining texture or machining allowance) or additional requirements, relevant parameters or symbols can be annotated around the basic symbol.
Q28: How to draw surface roughness symbols?
As shown in Figure 1. d‘ =h/10；H=1.4h；h=font height.
29. What are the methods for surface roughness notation?
1)The surface roughness code (symbol) shall be marked on the visible contour line, dimension line, dimension extension line or their extensions. The tip of the symbol must point from outside the material to the surface. As shown in Figure 2.
2) Center hole working surfaces, keyway working surfaces, and chamfered, rounded surfaces for simplified labeling. As shown in Figure 3.
3) Marking method when tooth (tooth) shape is not drawn on the working surface of gear, involute spline, thread, etc. As shown in Figure 4.
4) When the same surface has different surface roughness requirements, the dividing line must be drawn with a thin solid line, and the corresponding surface roughness symbol and size should be noted. As shown in Figure 5.
5) When local heat treatment or partial coating needs to be indicated, the range shall be drawn with a thick dotted line and the corresponding size shall be marked, and the requirements may also be written in the surface roughness symbol. As shown in Figure 6.
6) The continuous surface of the part and the surface of repeated elements (holes, grooves, teeth, etc.) and a discontinuous surface connected by a thin solid line, the code (symbol) is marked only once. As shown in Figure 7.
7) When most of the surface requirements of the parts are the same, they shall be uniformly marked in the upper right corner, and the word “rest” shall be added. To simplify the labeling, or when the position is limited, the simplified codes can be marked, and the method of omitting notes can also be used. Still, the meaning of these simplified codes (symbols) must be explained near the title column. When unified and simplified marking is adopted, the code and text description shall be 1.4 times the code and text marked on other surfaces on the graph. As shown in Figure 8.
Q30: How to choose the surface roughness?
A: The choice of surface roughness must not only meet the functional requirements of the surface of the part but also consider the economy of processing.
Q31: When using the analogy method to determine the surface roughness, what principles are generally used to select the height parameter?
A: On the same component, the surface roughness value of the working surface should be smaller than that of the non-working surface. The surface roughness value of friction surfaces should be smaller than that of non-friction surfaces. The surface roughness value for surfaces experiencing rolling friction should be smaller than that of sliding friction surfaces. Surfaces with higher motion speed and higher unit pressure should have smaller surface roughness values. Surface roughness values should be selected smaller for surfaces subjected to cyclic loads or susceptible to stress concentration, such as fillets and grooves. High-precision mating surfaces, surfaces with small clearances in mating fits, and surfaces requiring reliable connections and subjected to heavy loads should have smaller surface roughness values. For mating surfaces with similar requirements, the smaller the component size, the smaller the surface roughness value should be. For the same accuracy grade, the surface roughness value of small-sized components should be smaller than that of large-sized components, or, specifically, shafts should have smaller surface roughness values than holes. For mating surfaces, their dimensional tolerances, form tolerances, and surface roughness should be coordinated, and there is generally a specific corresponding relationship between them.
Q32: When the surface roughness Ra is 50~100μm, what are the characteristics of the surface shape, and how to apply it?
A: The surface shape is characterized by obvious tool marks, which are rarely used when applied to rough processing surfaces. Casting, forging, and gas-cutting blanks can meet this requirement.
Q33: When the surface roughness Ra is 25μm, what are the characteristics of the surface shape and how to apply it?
A: The surface shape is characterized by visible tool marks, which are rarely used when applied to rough processing surfaces. Casting, forging, and gas-cutting blanks can meet this requirement.
Q34: When the surface roughness Ra is 12.5μm, what are the characteristics of the surface shape, and how to apply it?
A: The surface shape is characterized by micro-knives, which is applied to a relatively precise level of rough machining surface, and has a wide range of applications, such as shaft-end faces, chamfers, surfaces of screw holes and rivet holes, and contact surfaces of gaskets.
Q35: When the surface roughness Ra is 6.3μm, what are the characteristics of the surface shape, and how to apply it?
A: The surface shape is characterized by visible machining traces. It is applied to semi-rough machining surfaces, non-contact-free surfaces such as brackets, boxes, clutches, pulley sides, and cam sides, surfaces in contact with bolt heads and rivet heads, and all shafts and holes. Undercut groove, joint surface of the general shutter, etc.
Q36: When the surface roughness Ra is 3.2μm, what are the characteristics of the surface shape, and how to apply it?
A: The surface shape is characterized by slight processing traces, and it is applied to semi-finishing surfaces, surfaces such as boxes, brackets, covers, sleeves, etc., that are connected to other parts without matching requirements, surfaces that require bluing, and pre-processing that require knurling surface, all outer surfaces where the spindle does not contact, etc. It is the surface roughness value achieved economically by basic cutting methods such as turning.
Q37: When the surface roughness Ra is 1.6μm, what are the characteristics of the surface shape, and how to apply it?
A: The surface shape feature is that the processing traces cannot be seen clearly. It is applied to the surface with high surface quality requirements, the working surface of medium-sized machine tools (regular precision), the joint surface of the spindle box and cover surface of combined machine tools, and the working surface of medium-sized flat pulleys and V-belt pulleys, The press-in hole of the bushing sliding bearing, generally the journal of low-speed rotation—non-matching surfaces of some important parts of aviation and aerospace products.
Q38: When the surface roughness Ra is 0.8μm, what are the characteristics of the surface shape, and how to apply it?
A: The surface shape feature is that the direction of the processing marks can be distinguished. It is applied to the surface of the sliding guideway of medium-sized machine tools (ordinary precision), the guide rail pressure plate, the surface of cylindrical pins and conical pins, the dial of general precision, the outer surface that needs to be chrome-plated and polished, and rotates at a medium speed. The journal, the positioning pin is pressed into the hole, etc. It is a commonly used value for mating surfaces, important mating places for medium and heavy equipment, and economical for grinding.
Q39: When the surface roughness Ra is 0.4μm, what are the characteristics of the surface shape, and how to apply it?
A: The surface shape is characterized by micro-discrimination of the direction of the machining traces, applied to medium-sized machine tools (improved precision) sliding guideway surfaces, working surfaces of sliding bearings, main surfaces of fixture positioning elements and drill sleeves, working journals of crankshafts and camshafts, indexing The surface of the disc, the working surface of the journal and the bushing under high-speed work, etc.
Q40: When the surface roughness Ra is 0.2μm, what are the characteristics of the surface shape, and how to apply it?
A: The surface shape is characterized by the direction of indistinguishable machining traces, which is applied to the taper hole of the spindle of the precision machine tool and the conical surface of the tip; the joint surface of the precision spindle and the rotating shaft with a small diameter, the piston pin hole of the piston, and the airtight surface and supporting surface are required. The leaf pot and the back of the blade of the aero-engine.
Q41: When the surface roughness Ra is 0.1 μm, what are the characteristics of the surface shape, and how to apply it?
A: The surface shape is characterized by a dark glossy surface, which is applied to the hole where the spindle box of the precision machine tool matches the sleeve, the surface that the instrument is subject to friction during use, such as guide rails, groove surfaces, etc., the surface of the hole for hydraulic transmission, and the working surface of the valve. , the inner surface of the cylinder, the surface of the piston pin, etc. General mechanical design limit value. Grinding is very uneconomical.
Q42: When the surface roughness Ra is 0.05μm, what are the characteristics of the surface shape, and how to apply it?
A: The surface shape is characterized by a bright glossy surface, which is applied to the raceway of the rolling bearing ring, the surface of the ball and the roller, the working surface of the medium-precision clearance fit parts in the measuring instrument, the measuring surface of the working gauge, etc.
Q43: When the surface roughness Ra is 0.025μm, what are the characteristics of the surface shape, and how to apply it?
A: The surface shape is characterized by a mirror-like glossy surface, which is applied to the surface of the raceway, ball, and roller of the particularly precise rolling bearing ring, the mating surface of the plunger and the plunger sleeve in the high-pressure oil pump, and ensures a highly airtight bonding surface, etc.
Q44: When the surface roughness Ra is 0.012μm, what are the characteristics of the surface shape, and how to apply it?
A: The surface shape is characterized by a foggy mirror surface, which is applied to the measuring surface of the instrument, the working surface of the high-precision clearance fit parts in the measuring instrument, the working surface of the gauge block with a size exceeding 100mm, etc.
Q45: When the surface roughness Ra is 0.008μm, what are the characteristics of the surface shape, and how to apply it?
A: The surface shape is characterized by a mirror surface, which is applied to the working surface of the gauge block, the measuring surface of the high-precision measuring instrument, and the metal mirror surface of the optical measuring instrument.
Q46: How to choose the value Ra of the thread surface roughness parameter?
A: When the precision grade of ordinary coarse thread is grade 4, Ra is 0.4~0.8μm.
When the precision grade of ordinary coarse thread is grade 5, Ra is 0.8μm.
When the precision grade of ordinary coarse thread is grade 6, Ra is 1.6~3.2μm.
When the precision grade of ordinary thread with fine teeth is grade 4, Ra is 0.2~0.4μm.
When the precision grade of ordinary thread with fine teeth is grade 5, Ra is 0.8μm.
When the precision grade of ordinary thread with fine teeth is grade 6, Ra is 1.6~3.2μm.
Q47: How to choose the value Ra of bonded surface roughness parameter?
A: The combination form is a bond, and Ra is 0.2~0.5μm at the moving position along the hub groove.
The combination form is a bond, and Ra is 0.2~0.4μm at the position moving along the shaft groove.
The binding form is a bond, and the fixed part, Ra is 1.6μm.
The combination form is the shaft groove, and Ra is 1.6μm at the position moving along the hub groove.
The combination form is shaft groove, and Ra is 0.4~0.8μm when moving along the shaft groove.
The joint form is the shaft groove, and the fixed part, Ra is 1.6μm.
The combination form is the hub groove, and Ra is 0.4~0.8μm at the moving position along the hub groove.
The combination form is a hub groove, and Ra is 1.0μm at the position moving along the shaft groove.
The joint form is the hub groove, the fixed part, Ra is 1.6~3.2μm.
Note: All non-working surfaces Ra are 6.3μm.
Q48: How to choose the value Ra of the surface roughness parameter of the rectangular spline?
A: Ra is 6.3 μm at the outer diameter for internal splines.
For internal splines, Ra is 0.8 μm at the inner diameter.
Internal spline, at the key side, Ra is 3.2 μm.
For external splines, Ra is 3.2 μm at the outer diameter.
For external splines, Ra is 0.8 μm at the inner diameter.
For external splines, Ra is 0.8 μm at the key side.
Q49: How to choose the gear surface roughness parameter value Ra?
A: When the part is tooth surface precision grade is 5, Ra is 0.2~0.4μm.
When the tooth surface accuracy grade is grade 6, Ra is 0.4μm.
When the part is tooth surface precision grade is 7, Ra is 0.4~0.8μm.
When the tooth surface accuracy grade is grade 8, Ra is 1.6μm.
When the part is tooth surface precision grade 9, Ra is 3.2μm.
When the part is tooth surface precision grade is 10, Ra is 6.3μm.
When the position is an outer circle with a precision grade of 5, Ra is 0.8~1.6μm.
When the position is an outer circle with a precision grade of 6, Ra is 1.6~3.2μm.
When the position is an outer circle with a precision grade of 7, Ra is 1.6~3.2μm.
When the position is an outer circle with an accuracy grade of 8, Ra is 1.6~3.2μm.
When the position is an outer circle with a precision grade of 9, Ra is 3.2~6.3μm.
When the position is an outer circle with a precision grade of 10, Ra is 3.2~6.3μm.
When the position is end face accuracy class 5, Ra is 0.4~0.8μm.
When the position is end face accuracy class 6, Ra is 0.4~0.8μm.
When the position is the end face accuracy grade is 7, Ra is 0.8~3.2μm.
When the position is the end face precision grade is 8, Ra is 0.8~3.2μm.
When the position is the end face precision grade is 9, Ra is 3.2~6.3μm.
When the position is the end face precision grade is 10, Ra is 3.2~6.3μm.
Q50: How to choose the value Ra of the surface roughness parameter of the worm gear?
A: When the precision grade of the tooth surface of the worm is grade 5, Ra is 0.2μm.
When the worm part is tooth surface precision grade 6, Ra is 0.4μm.
When the tooth surface accuracy grade of the worm part is grade 7, Ra is 0.4μm.
When the precision grade of the tooth surface of the worm is grade 8, Ra is 0.8μm.
When the tooth surface precision grade of the worm part is grade 9, Ra is 1.6μm.
When the accuracy grade of the tooth top of the worm is grade 5, Ra is 0.2μm.
When the accuracy grade of the tooth top of the worm is grade 6, Ra is 0.4μm.
When the accuracy grade of the tooth top of the worm is grade 7, Ra is 0.4μm.
When the accuracy grade of the tooth top of the worm is grade 8, Ra is 0.8μm.
When the precision grade of the tooth top of the worm is grade 9, Ra is 1.6 μm.
Note: The part of the worm is the tooth root, and Ra is 6.3μm.
When the worm wheel part is tooth surface precision grade 5, Ra is 0.4μm.
When the worm gear part is tooth surface precision grade 6, Ra is 0.4μm.
When the worm gear part is tooth surface precision grade 7, Ra is 0.8μm.
When the worm gear part is tooth surface precision grade 8, Ra is 1.6μm.
When the worm gear part is tooth surface precision grade 9, Ra is 3.2μm.
Note: The part of the worm wheel is the tooth root, and Ra is all 3.2μm.
Q51: How to choose the value Ra of the sprocket surface roughness parameter?
When the surface accuracy of sprockets is normal, Ra is 1.6~3.2μm.
When the working surface of the sprocket is of high precision, Ra is 0.8~1.6μm.
When the part is tooth bottom precision, Ra is 3.2μm.
When the part is tooth bottom, the precision is high, and Ra is 1.6μm.
When the part is tooth top accuracy, Ra is 1.6~3.2μm.
When the position is the tooth top, the precision is high, and Ra is 1.6~6.3μm.
Q52: How to choose the value Ra of the pulley surface roughness parameter?
A: When the part is the working surface of the pulley, when the diameter of the pulley is ≤120mm, Ra is 0.8μm.
When the part is the working surface of the pulley, when the diameter of the pulley is ≤300mm, Ra is 1.6μm.
When the part is the pulley’s working surface, when the pulley’s diameter is > 300mm, Ra is 3.2μm.
Q53: How to choose the value Ra of the surface roughness parameter of hydraulic components?
A: The location is the crank of the piston pump; at the piston, Ra is 1.6~0.8μm.
The parts connect rod journals, bearing bushes, and central journals, and Ra is 0.4μm.
The position is the outer cylinder of the piston, and at the side surface, Ra is 0.8 μm.
The parts are the piston pump connecting the rod hole, cylinder barrel, slide valve bushing, plunger, and piston, and Ra is 0.8~0.4μm.
The position is the slide valve, the plunger valve of the high-pressure pump, and the valve seat, Ra is 0.2~0.1μm.
Q54: How to choose the surface roughness parameter value Ra of the mating surface of the sliding bearing?
A: The position is at the shaft tolerance grade IT7-IT9, and Ra is 0.2~3.2μm.
The position is at the shaft tolerance grade IT11-IT12, and Ra is 1.6~3.2μm.
The position is at the hole tolerance grade IT7-IT9, and Ra is 0.4~1.6μm.
The part is at the hole tolerance grade IT11-IT12, and Ra is 1.6~3.2μm.
Q55: How to choose the value of the surface roughness parameter Ra of the conical joint?
A: The part is the sealing junction of the outer conical surface, and Ra is ≤0.1μm.
The part is the centering junction of the outer conical surface, and Ra is ≤0.2μm.
The location is other joints on the surface of the outer cone, and Ra is ≤1.6~3.2μm.
The part is the sealing junction of the inner cone surface, and Ra is ≤0.2μm.
The part is the centering junction of the inner conical surface, and Ra is ≤0.8μm.
The parts are other joints on the surface of the inner cone, and Ra is ≤1.6~3.2μm.
Q56: How does the selection of surface roughness affect fit properties?
A: It affects the reliability and stability of the mating performance. Initial wear will quickly remove the peaks for clearance fit, resulting in increased clearance. During assembly, the peaks will be flattened for an interference fit, reducing the actual effective interference, especially for small-size fits. Therefore, mating surfaces requiring high stability, small clearance for dynamic fits, and firm and reliable connection for static fits should have lower Ra values. For the same tolerance grade, smaller-sized components should have lower Ra values compared to larger-sized ones, especially for 1~3 grade tolerance, and the Ra value should be smaller for shafts compared to holes with the same tolerance grade. Additionally, for components with the same mating properties, smaller-sized parts tend to have smaller Ra values.
Q57: How does the selection of surface roughness affect wear resistance?
A: After machining, the component’s surface contains:
- Peaks and valleys.
- Resulting in only peak-to-peak contact, which reduces the contact area and increases the unit pressure.
- Leading to accelerated wear.
Therefore, frictional surfaces generally have lower Ra values than non-frictional surfaces, and rolling friction surfaces tend to have lower Ra values than sliding friction surfaces, especially when the movement speed is high, and the unit pressure is significant.
Q58: How does the choice of surface roughness affect fatigue strength?
A: The rougher the part’s surface, the more sensitive it is to stress concentration, leading to fatigue damage. Therefore, the Ra value should be lower in areas subjected to cyclic loads and prone to stress concentration, such as corners and grooves. The extent to which surface roughness affects the fatigue strength of a part varies depending on the material. It has less noticeable effects on cast iron components but has a more significant impact on stronger steel components.
Q59: How does the selection of surface roughness affect sealing performance?
A: For statically sealed surfaces with no relative sliding, excessive depth of micro-roughness valleys prevents the complete filling of the sealing material after preloading, leaving gaps that cause leakage. The rougher the surface, the more severe the leakage. For dynamically sealed surfaces with relative sliding, the micro-roughness is generally around 4-5μm, which is beneficial for lubricant retention. If the surface is too smooth, it hinders lubricant retention and may lead to frictional wear. The direction of the machining texture also influences the effectiveness of the seal.
Q60: How does the selection of surface roughness affect vibration and noise?
The surface roughness of mechanical equipment’s moving components affects vibration and noise during operation, especially in high-speed rotating parts such as rolling bearings, gears, engine crankshafts, and camshafts. This phenomenon becomes more pronounced in such components. Therefore, the smoother and quieter the moving components, the smaller the Ra value of the surface roughness.
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