What is surface roughness

What is surface roughness?

Surface roughness refers to the roughness of the machined surface with small spacing and tiny peaks and valleys. The distance between the two peaks or troughs (wave distance) is very small (less than 1 mm), which belongs to the micro geometry error. The smaller the surface roughness is, the smoother the surface is.
The surface roughness is generally formed by the processing method and other factors, such as the friction between the cutting tool and the surface of the part, the plastic deformation of the surface layer metal when the chips are separated, and the high frequency vibration in the process system. Due to different processing methods and workpiece materials, the depth, density, shape and texture of the traces left on the machined surface are different.
Surface roughness is closely related to the matching properties, wear resistance, fatigue strength, contact stiffness, vibration and noise of mechanical parts, which has an important impact on the service life and reliability of mechanical products. Generally, RA is used.

Surface roughness, waviness and shape error

If you zoom in, the ups and downs of the surface of machining parts are actually composed of many tiny convex peaks and concave valleys. According to the different geometric features, the surface topography of machining parts is divided into surface roughness, surface waviness and shape error.

  • The wave distance is less than 1 mm, which belongs to micro geometry error surface roughness.
  • The surface waviness, which varies periodically with wave distance from 1 to 10 mm.
  • The macro geometric error is more than 10 mm and does not change periodically.

Compared with the waviness and shape error, the surface roughness is actually very fine and uneven, and the wave distance is less than 1 mm, which can not be accurately identified by the naked eye, so it must be observed clearly with the help of tools.

Surface roughness – evaluation parameters: RA, RZ, ry

The surface roughness of steel parts is a technical index to evaluate the surface quality of steel parts. The higher the surface roughness of steel parts is required (that is, the smaller the surface roughness parameter value is), the higher the processing cost is.
The unit of surface roughness is micrometer( μm) 001 mm. Generally, there are three evaluation parameters as follows:
The arithmetic mean deviation Ra of contour refers to the average absolute value of the distance from each point on the contour to the center line of the contour within a certain sampling length.
RA can fully reflect the height characteristics of the surface micro geometry, but due to the limitation of the function of the measuring instrument, it is not used as the evaluation parameter for the surface which is too rough or too smooth.
201707131499912663718150 - What is surface roughness
The ten point average height RZ of micro roughness refers to the sum of the average height of the five largest contour peaks and the average depth of the five largest contour valleys within the sampling length.
RZ can only reflect the peak height of the contour, but can not reflect the sharp or flat geometric characteristics of the peak. At the same time, if different points are taken, the RZ values are different, so the subjective influence of the measurer is greater.
The maximum height of the profile, ry, is the distance between the peak line and the valley bottom line of the profile within the sampling length.
Ry is the sum of the vertical distances between the highest point and the lowest point of the ten micro unevenness points and the central line, so it is not as accurate as the geometric characteristics reflected by RZ value. It is of practical significance for some surfaces that are not allowed to have deep machining marks and the surface quality of small parts.

Measurement method of surface roughness

Comparison method: directly compare the surface to be measured with the surface roughness template, which is mostly used in workshop to evaluate the workpiece with larger surface roughness value.
Light cut method: use light cut principle, measure with double tube microscope. It is often used to measure RZ from 0.5 to 60 μ m.
Interferometry: Based on the principle of light wave interference, measure with interference microscope. RZ and ry can be measured.
Needle tracing method: it is a method to measure the surface roughness Ra value by using a diamond needle to scratch gently on the surface to be measured.

Different surface roughness
Surface features
Surface roughness (RA) value
Examples of processing methods
Knife marks are visible
Ra100、Ra50、Ra25、
Rough turning, rough planing, rough milling and drilling
Slight knife marks
Ra12.5、Ra6.3、Ra3.2、
Finish turning, finish planing, finish milling, rough hinge and rough grinding
Processing trace can’t be seen, micro processing direction
Ra1.6、Ra0.8、Ra0.4、
Fine turning, fine grinding, fine hinge and grinding
Dark and glossy
Ra0.2、Ra0.1、Ra0.05、
Grinding, honing, superfinishing and polishing

Effect of surface roughness on properties

The influence of surface roughness on parts is mainly shown in the following aspects:

Influence on wear resistance

If the surface roughness is too large or too small, it will not wear.
The rougher the surface is, the smaller the effective contact area is, the greater the pressure is, the greater the friction resistance is, and the faster the wear is.
If the surface roughness is too small, the wear will be intensified. Because the surface is too smooth to store lubricating oil, it is not easy to form oil film between the contact surfaces, and it is easy to cause molecular adhesion and increase wear.

Affect the stability of coordination

For the clearance fit, the rougher the surface is, the easier it is to wear, which makes the clearance gradually increase during the working process; For the interference fit, the actual effective interference is reduced and the connection strength is reduced due to the flattening of micro convex peaks during assembly.

Influence on fatigue strength

There are large wave troughs on the surface of rough parts, which are very sensitive to stress concentration like sharp notch and crack, thus affecting the fatigue strength of steel parts.

Affect corrosion resistance

Rough parts surface, easy to make corrosive gas or liquid through the surface of the micro concave valley into the metal inner layer, resulting in surface corrosion.

Affect sealing

The rough surfaces can not be tightly bonded, and the gas or liquid leaks through the gap between the contact surfaces.

Influence on contact stiffness

Contact stiffness is the ability of the joint surface of steel parts to resist contact deformation under the action of external force. The stiffness of the machine largely depends on the contact stiffness between the parts.

Affect the measurement accuracy

The surface roughness of steel parts and measuring tools will directly affect the measurement accuracy, especially in precision measurement.
In addition, the surface roughness has different effects on the coating, thermal conductivity and contact resistance, reflection ability and radiation performance, resistance of liquid and gas flow, and current flow on the conductor surface.

The relationship among dimension tolerance, form and position tolerance and surface roughness

The relationship among dimension tolerance, form and position tolerance and surface roughness is as follows:

1. Numerical relationship between shape tolerance and dimension tolerance
When the dimensional tolerance accuracy is determined, there is an appropriate value corresponding to the shape tolerance, that is, about 50% of the dimensional tolerance value is generally taken as the shape tolerance value; About 20% of the dimension tolerance value of instrument industry is taken as the shape tolerance value; In the heavy industry, about 70% of the dimensional tolerance value is taken as the form tolerance value. It can be seen that the higher the dimensional tolerance accuracy is, the smaller the proportion of the shape tolerance in the dimensional tolerance is. Therefore, in the design of dimensioning and shape tolerance requirements, except for special cases, when the dimensional accuracy is determined, 50% of the dimensional tolerance value is generally taken as the shape tolerance value, which is not only conducive to manufacturing but also to ensuring the quality.
2. Numerical relationship between form tolerance and position tolerance
There is also a certain relationship between form tolerance and position tolerance. From the angle of error formation, shape error is caused by machine tool vibration, tool vibration and spindle runout; The position error is caused by the non parallel of the machine tool guide rail, the non parallel or non vertical of the tool clamping, and the clamping force. From the definition of the tolerance zone, the position error contains the shape error of the measured surface. For example, the parallelism error contains the flatness error, so the position error is much larger than the shape error. Therefore, in general, when there is no further requirement, if the position tolerance is given, the shape tolerance is no longer given. When there are special requirements, the shape tolerance and position tolerance can be marked at the same time, but the marked shape tolerance value should be less than the marked position tolerance value, otherwise, the parts can not be manufactured according to the design requirements.
3. Relationship between form tolerance and surface roughness
Although there is no direct relationship between shape error and surface roughness in numerical value and measurement, there is a certain proportional relationship between them under certain processing conditions. According to experimental research, surface roughness accounts for 1 / 5 ~ 1 / 4 of shape tolerance in general accuracy. Therefore, in order to ensure the shape tolerance, the maximum allowable value of the corresponding surface roughness height parameter should be appropriately limited.
In general, the tolerance values among dimension tolerance, shape tolerance, position tolerance and surface roughness have the following relation: dimension tolerance > position tolerance > shape tolerance > surface roughness height parameter
It is not difficult to see from the numerical relationship between dimension, form and position and surface roughness that the numerical relationship among them should be well coordinated in design. When marking the tolerance value on the drawing, the following rules should be followed: the given roughness value of the same surface should be less than its shape tolerance value; The shape tolerance should be less than the position tolerance; The difference between positions should be less than the tolerance value. Otherwise, it will bring all kinds of troubles to the manufacturing industry. However, in the design work, how to deal with the relationship between dimensional tolerance and surface roughness and the relationship between various fitting accuracy and surface roughness are the most important issues.
Generally, it is determined according to the following relationship:

  • 1. When the shape tolerance is 60% of the dimension tolerance (medium relative geometric accuracy), RA ≤ 0.05it;
  • 2. When the shape tolerance is 40% of the dimension tolerance (higher relative geometric accuracy), RA ≤ 0.025it;
  • 3. When the shape tolerance is 25% of the dimension tolerance (high relative geometric accuracy), RA ≤ 0.012it;
  • 4. When the shape tolerance is less than 25% of the dimension tolerance (ultra-high relative geometric accuracy), RA ≤ 0.15tf (shape tolerance value).

The simplest reference value: the dimension tolerance is 3-4 times of the roughness, which is the most economical.

Selection of geometrical tolerance

1. Selection of geometric tolerance items
It is necessary to give full play to the function of comprehensive control items, so as to reduce the geometric tolerance items given in the drawing and the corresponding geometric error detection items.
On the premise of meeting the functional requirements, simple measurement items should be selected. For example, coaxiality tolerance is often replaced by radial run out tolerance or radial run out tolerance. However, it should be noted that the radial circular runout is the combination of coaxiality error and cylindrical surface shape error, so the given runout tolerance value should be slightly greater than the coaxiality tolerance value, otherwise it will be too strict.
2. Selection of tolerance principle
According to the functional requirements of the measured elements, the function of tolerance and the feasibility and economy of adopting the principle of tolerance should be brought into full play.
The principle of independence is used in the situation that the requirements of dimensional accuracy and form and position accuracy are quite different and need to meet the requirements respectively, or there is no connection between the two, so as to ensure the movement accuracy, sealing, and no tolerance.
The inclusion requirement is mainly used in the situation where the nature of cooperation needs to be strictly guaranteed.
The maximum entity requirement is used for the central element, and is generally used when the accessory requirement is assemblability (no matching property requirement).
Minimum solid requirement is mainly used to ensure the strength and minimum wall thickness of steel parts.
The combination of the reversible requirement and the maximum (minimum) entity requirement can make full use of the tolerance zone, expand the range of the actual dimensions of the measured elements, and improve the efficiency. It can be selected on the premise of not affecting the performance.
3. Selection of datum elements
1) Selection of datum position

  • (1) Select the joint surface of steel parts in the machine as the reference position. For example, the bottom plane and side of the box, the axis of disk parts, the supporting journal or supporting hole of rotary parts, etc.
  • (2) Datum elements should have enough size and rigidity to ensure stable and reliable positioning. For example, it is more stable to combine two or more distant axes into a common datum axis than one datum axis.
  • (3) The surface with more accurate machining is selected as the reference part.
  • (4) Try to make the assembly, processing and testing benchmark unified. In this way, the error caused by the disunity of datum can be eliminated; It can also simplify the design and manufacture of fixtures and measuring tools, which is convenient for measurement.

2) Determination of benchmark quantity
Generally speaking, the number of datum should be determined according to the orientation and positioning geometric function requirements of tolerance items. Most directional tolerances only need one datum, while positioning tolerances need one or more datum. For example, for the tolerance items of parallelism, perpendicularity and coaxiality, generally only one plane or one axis is used as the datum feature; For the positional tolerance project, two or three datum elements may be used to determine the positional accuracy of the hole system.
3) Arrangement of benchmark sequence
When more than two datum elements are selected, the order of datum elements should be clear and written in the tolerance frame in the order of the first, second and third. The first datum element is the main one and the second one is the second.
4. Selection of geometric tolerance value
General principle: select the most economical tolerance value on the premise of meeting the part function.
According to the functional requirements of steel parts, considering the economy of processing and the structure and rigidity of steel parts, the tolerance value of elements is determined according to the table. And consider the following factors:

  • The shape tolerance given by the same element should be less than the position tolerance;
  • The shape tolerance value of cylindrical parts (except the straightness of axis) should be less than its dimension tolerance value; On the same plane, the flatness tolerance value should be less than the parallelism tolerance value of the plane to the datum.

The parallelism tolerance value should be less than the corresponding distance tolerance value.
General proportional relationship between surface roughness and shape tolerance: generally, Ra value of surface roughness can be taken as 20% ~ 25% of shape tolerance value.

  • For the following cases, considering the difficulty of machining and the influence of other factors except the main parameters, under the condition of meeting the functional requirements of the parts, it is appropriate to reduce the selection of grade 1-2
  • The hole is relative to the shaft;
  • Slender and large shafts and holes; Shaft and hole with larger distance;
  • Parts surface with larger width (more than 1 / 2 length);
  • Tolerance of parallelism and perpendicularity of line to line and line to line relative to face.

5. Specification of unmarked tolerance for form and position
In order to simplify the drawing, it is not necessary to note the form and position tolerance on the drawing for the form and position accuracy that can be guaranteed by the general machine tool processing. The unmarked form and position tolerance shall be implemented according to the provisions of GB / t1184-1996. The general contents are as follows:

  • (1) Three tolerance grades of H, K and L are specified for straightness, flatness, perpendicularity, symmetry and circular runout
  • (2) The unmarked roundness tolerance value is equal to the diameter tolerance value, but cannot be greater than the unmarked tolerance value of radial circle runout.
  • (3) The tolerance value of cylindricity without indication is not specified, but controlled by the tolerance of roundness, straightness and parallelism of element.
  • (4) The unmarked parallelism tolerance value is equal to the larger of the dimension tolerance between the measured feature and the datum feature and the unmarked tolerance value of the shape tolerance (straightness or flatness) of the measured feature, and the longer of the two features is taken as the datum.
  • (5) The coaxiality tolerance value is not specified. If necessary, the unmarked tolerance of coaxiality can be equal to the unmarked tolerance of circular runout.
  • (6) The tolerance values of undeclared line profile, surface profile, inclination and position are controlled by the linear dimension tolerance or angle tolerance of each element.
  • (7) The tolerance value of full run out is not specified.

6. The drawing representation of the tolerance value without indication in shape and position
If the tolerance value not indicated in GB / t1184-1996 is adopted, the standard and grade code shall be indicated in the title block or technical requirements“ GB/T1184—K”。
If the working tolerance of “tolerance principle according to GB / t4249” is not marked on the drawing, the requirements of “GB / T1800.2-1998” shall be followed.

What is the difference between “surface finish” and “surface roughness” of steel and metal?

Different molds have different processing requirements for precision. When purchasing materials from mold steel suppliers, mold factories will determine the surface roughness or finish of the workpiece. So, what is the surface finish? What is surface roughness? What is the difference between surface finish and surface roughness?
First of all, surface finish and surface roughness are the same concept. Surface finish is another term of surface roughness. Surface finish is put forward according to human vision, while surface roughness is put forward according to the actual surface micro geometry. Because it is in line with the international standard (ISO), China adopted the surface roughness and abolished the surface finish after 1980s. After the promulgation of the national standards of surface roughness GB3505-83 and gb1031-83, the standard of surface finish is no longer used.
There is a corresponding comparison table between surface finish and surface roughness. There is a formula to calculate the roughness, but the finish can only be compared with the template gauge. So roughness is more scientific and rigorous than finish.
The surface glossiness is the intensity of diffuse reflection of light on the surface of an object. If the surface diffuse reflection is strong, it is closer to the mirror effect and the glossiness is high. Otherwise, if the surface diffuse reflection is weak, the glossiness is low. Therefore, the glossiness is also called mirror glossiness. The influencing factors of surface gloss are related to the physical properties of the surface and the chemical properties of the materials used on the surface. The method of measuring the surface glossiness of an object needs to use a surface glossometer.
Surface roughness refers to the roughness of the machined surface with small spacing and small peaks and valleys. The distance between the two peaks or troughs (wave distance) is very small (less than 1 mm), which belongs to the micro geometry error. The smaller the surface roughness is, the smoother the surface is.
20210614234017 29251 - What is surface roughness

The surface roughness is generally formed by the processing method and other factors, such as the friction between the cutting tool and the surface of the part, the plastic deformation of the surface layer metal when the chips are separated, and the high frequency vibration in the process system. Due to different processing methods and workpiece materials, the depth, density, shape and texture of the traces left on the machined surface are different.
Surface roughness is closely related to the matching properties, wear resistance, fatigue strength, contact stiffness, vibration and noise of mechanical parts, which has an important impact on the service life and reliability of mechanical products. Generally, RA is used.
The influence of surface roughness on parts is mainly shown in the following aspects:
The rougher the surface is, the smaller the effective contact area is, the greater the pressure is, the greater the friction resistance is, and the faster the wear is.
For the clearance fit, the rougher the surface is, the easier it is to wear, which makes the clearance increase gradually; For the interference fit, the actual effective interference is reduced and the connection strength is reduced due to the flattening of micro convex peaks during assembly.
There are big wave troughs on the surface of rough parts which affect the fatigue strength. They are very sensitive to stress concentration like sharp corner notch and crack, thus affecting the fatigue strength of steel parts.
The rough surface of steel parts that affect the corrosion resistance is easy to make corrosive gas or liquid penetrate into the inner metal layer through the micro concave valley of the surface, resulting in surface corrosion.
The rough surfaces that affect the sealing performance can not be tightly bonded, and the gas or liquid leaks through the gap between the contact surfaces.
Influence on contact stiffness contact stiffness is the ability of the joint surface of steel parts to resist contact deformation under the action of external force. The stiffness of the machine largely depends on the contact stiffness between the parts.
The surface roughness of steel parts and measuring tools will directly affect the measurement accuracy, especially in precision measurement.
In addition, the surface roughness has different effects on the coating, thermal conductivity and contact resistance, reflection ability and radiation performance, resistance of liquid and gas flow, and current flow on the conductor surface.

Comparison table of vdi3400, RA and Rmax

RA index is often used in domestic production; Rmax index is commonly used in Japan, which is equivalent to RZ index; The vdi3400 standard is commonly used in European and American countries to mark the surface roughness. Factories making European mold orders often use the VDI index. For example, they often hear customers say that “the surface of this product is made according to vdi30”.

20210613164039 64019 - What is surface roughness

Vdi3400 surface has a corresponding relationship with the commonly used standard RA, many people often have to look up the data to find the corresponding value, the following table is very complete, recommended collection.

Comparison table of vdi3400 standard and RA

20210613164039 88096 - What is surface roughness

Comparison table of RA and Rmax

20210613164039 34469 - What is surface roughness

Comparison table of surface finish and surface roughness
Finish
level
(old bid)
Roughness
Ra
( μ m)

1) Surface appearance;

2) Processing method;

3) Application examples.

▽1
400~800
▽2
200~400

1) Obvious tool marks;

2) Rough turning, boring, planing and drilling;

3) Rough machined surface;

4) Welding seam before welding, thick drilling wall, etc.

▽ 3
100~200

1) Visible tool marks;

2) Rough turning, planing, milling and drilling;

3) General non combined surfaces, such as shaft end face, chamfering, gear and pulley side, non working surface of keyway, weight reducing hole surface.

▽4
50~100

1) Visible processing traces;

2) Turning, boring, planing, drilling, milling, filing, grinding, rough reaming, milling;

3) Mating surfaces of unimportant parts, such as the end faces of pillars, supports, shells, bushings, shafts, covers, etc. The free surface of the fastener, the through-hole surface of the fastener, the non centering surface of the internal and external spline, and the circular surface of the gear top ring which is not used as the measurement reference.

▽5
25~50
1) Slight processing trace;

2) Turning, boring, planing, milling, scraping 1-2 points / cm ^ 2, pulling, grinding, filing, rolling and milling;

3) It is connected with other parts and does not form a matching surface, such as the end face of box, shell, end cover and other parts. Fixed bearing surfaces with centering and matching characteristics are required, such as centering between shafts, working surfaces of keys and keyways. Unimportant fastening thread surface. Surfaces requiring knurling or oxidation treatment.
▽6
12.5~25

1) Can’t see the processing trace clearly;

2) Turning, boring, planing, milling, reaming, pulling, grinding, rolling and scraping 1-2 points / cm ^ 2 for gear milling;

3) shell hole of G-class bearing with diameter over 80mm, tooth surface of ordinary precision gear, locating pin hole, surface of V-belt pulley, outer diameter of internal spline centering with outer diameter, and centering shoulder surface of bearing cover.

▽7
6.3~12.5

1) Direction of discernible processing trace;

2) Turning, boring, pulling, grinding, end milling, scraping 3-10 points / cm ^ 2, rolling;

3) the surfaces required to ensure the centering and matching characteristics, such as the surface of taper pin and cylindrical pin, the shaft diameter and shell hole matched with grade g precision rolling bearing, the shaft diameter of medium speed rotation, the shaft diameter and shell hole matched with Grade E and D rolling bearing with diameter more than 80mm, and the centering inner diameter of internal and external spline, External spline key side and centering outer diameter, interference fit it7 class hole (H7), clearance fit it8 ~ it9 class hole (H8, H9), grinding gear surface, etc.

▽8
3.2~6.3
1) The direction of micro processing trace;

2) Reaming, grinding, boring, pulling, scraping 3-10 points / cm ^ 2, rolling;

3) It is required to keep the matching surface stable for a long time, the matching surface of shaft and hole of it7 grade, the surface of gear with high precision, the important parts under the action of variable stress, the surface of shaft diameter matching with E and D grade bearing with diameter less than 80mm, the surface of shaft contacting with rubber seal, the measuring surface of it13-it16 grade hole and shaft gauge with size greater than 120mm.
▽9
1.6~3.2
1) The direction of processing trace cannot be identified;

2) Cloth wheel grinding, grinding, grinding, super processing;

3) The surface of important parts under the action of variable stress. Ensure the fatigue strength, corrosion resistance and durability of the parts, and do not damage the surface with matching properties, such as shaft diameter surface, air tight surface and supporting surface, cone centering surface, etc. It 5, it 6 matching surface, high-precision gear surface, shaft diameter surface matched with G-grade rolling bearing, it 7-it 9 hole with size larger than 315 mm, it 10-it 12 hole with size larger than 120-315 mm and measuring surface of shaft gauge.
▽10
0.8~1.6
1) Dark light and glossy surface;

2) Super processing; 3) The surface of the important parts which bear the action of large strain stress. Ensure the cone surface with accurate centering. Hole surface for hydraulic transmission. The inner surface of the cylinder liner, the outer surface of the piston pin, the guide surface of the instrument and the working surface of the valve. It10-it12 class hole with size less than 120mm and measuring surface of shaft gauge.

▽11
0.4~0.8
▽12
0.2~0.4
▽13
0.1~0.2
▽14
Source: China Flanges Manufcturer – Yaang Pipe Industry (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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