Classification of surface roughness of parts!

Roughness grade classification

The steel needs to be derusted before corrosion, and only after reaching a certain level of derusting and surface roughness can it be preserved. Rust removal grade and surface roughness grade are two different concepts.

The corrosion of steel surface is divided into four grades according to GB8923:

A The steel surface is completely covered with oxide scale and almost no rust;

B. The surface of the steel that has been rusted and part of the oxide scale has fallen off;

C. The oxide scale has peeled off due to rust, or it can be scraped off the surface of steel with a small amount of pitting corrosion;

D oxide scale has been completely peeled off due to corrosion, and pitting corrosion has generally occurred on the surface of the steel.

Rust removal level, sometimes referred to as surface cleanliness

The derusting grade of the steel surface is represented by the letters Sa, St or FI according to the derusting method used. If there are Arabic numerals after the letter, it indicates the degree of removal of oxide scale, rust and especially coatings and other attachments.

Spray or projectile descaling is represented by the letter sa. There are 4 levels. 

Sa1, mild spray or projectile rust removal; 

Sa2, side bottom spray or projectile rust removal; Sa2.5, very side bottom spray or projectile rust removal; Sa3, spray or projectile rust removal to make the steel surface clean.

Derusting of hand tools is denoted by St, and there are two grades. St2 is a thorough hand or power tool rust removal; St3 is a very thorough hand or power tool rust removal.

Flame descaling is represented by FI.

Sa1 level-equivalent to the US SSPC-SP7 level. Generally simple manual brushing and emery cloth polishing methods are used. This is the lowest level of the four cleanliness, and the protection of the coating is only slightly better than that of the untreated workpiece. The technical standard of Sa1 level treatment: oil stains, grease, residual oxide scale, rust spots, and residual paint should not be visible on the surface of the workpiece. Sa1 level is also called manual scrubbing and cleaning level. (Or cleaning level)

Sa2 level-equivalent to the US SSPC-SP6 level. The sandblasting cleaning method is adopted, which is the lowest level in the sandblasting treatment, that is, the general requirement, but the protection of the coating is much higher than that of manual brushing. The technical standard of Sa2 level treatment: the surface of the workpiece should not be visible greasy, dirt, oxide scale, scale, paint, oxide, corrosive, and other foreign substances (except for defects), but the defects are limited to no more than per square meter of surface 33%, may include slight shading; a small amount of slight discoloration caused by defects and rust; oxide skin and paint defects. If the original surface of the workpiece has dents, slight rust and paint will remain at the bottom of the dents. Sa2 level is also called commodity cleaning level (or industrial level).

Sa2.5 level-is commonly used in the industry and can be used as the level of acceptance technical requirements and standards. Sa2.5 level is also called near-white cleaning level (near-white level or out-of-white level). The technical standard of Sa2.5 level treatment: the same as the first half of Sa2 requirements, but the defects are limited to no more than 5% of the surface per square meter, which may include slight shadows; a small amount of slight discoloration caused by defects and rust; oxide scale and paint defects .

Sa3-level is equivalent to the US SSPC-SP5 level, which is the highest processing level in the industry, also known as the white cleaning level (or white level). The technical standard of Sa3 level processing: Same as Sa2.5 level, but 5% of shadows, defects, rust, etc. have to exist.

Ra is the surface roughness after rust removal. The old standard is called smoothness.Now the new standard is defined as roughness, and the expression method is different.The smoothness is expressed by the code name, and the roughness is expressed by the value Ra, which is more scientific.

Finish and surface roughness are two different things

Geometric shape errors are divided into three types: macro geometric shape errors (shape tolerance), surface waviness (waviness) and micro geometric shape errors (surface roughness). At present, they are usually divided by wave distance. Those with wave distance less than 1mm belong to the surface. Roughness, the wave distance within 1~10mm belongs to the surface waviness, and the one greater than 10mm belongs to the shape error.

The commonly used surface roughness is Ra, as well as Rz and Ry. Ra is the arithmetic mean deviation of the contour, Rz is the ten-point height of the microscopic unevenness, and Ry is the maximum height of the contour.

1. The machining accuracy of a general lathe can reach IT8～IT7, and the surface roughness is Ra25～Ra1.6.

2. The drilling machine is used for drilling processing accuracy up to IT13～IT11, surface roughness Ra80～Ra20; used for reaming accuracy up to IT10, surface roughness Ra10～Ra5.; used for reaming accuracy up to IT7, surface roughness Ra5 ~ ra1.25.

3. The processing accuracy of the milling machine is generally IT9～IT8, and the surface roughness is Ra6.3～Ra1.6.

4. The machining accuracy of the planer is IT9～IT8, and the surface roughness is Ra25～Ra1.6.

5. The processing accuracy of the grinder is generally IT6～IT5, and the surface roughness is Ra0.8～Ra0.1.

0.012—Flower△13 (the most accurate)

0.025-Flower △ 12

0.05-flower △ 11

0.1-flower △ 10

0.2—Flower△9

0.4-flower △ 8

0.8—Flower△7

1.6—Flower△6

3.2—Flower△5

6.3—Flower△4

12.5—Flower△3

25—Flower△2

50—Flower△1

100—Flower△0 (the roughest)

Comparison table of surface roughness (finishness) between China and the United States

The surface roughness is divided into Ra, Rz, Ry, according to GB 3505 excerpt:

The three commonly used Surface Roughness Parameters and their Values are:

Arithmetic mean deviation of contour (Ra)--arithmetical mean deviation of the profile;

Ten-point height of microscopic unevenness (Rz)--the point height of irregularities;

The maximum height of the profile (Ry)--maximum height of the profile.

Ra--the arithmetic average of the absolute value of the contour offset within the sampling length L.

Rz--the sum of the average value of the 5 largest profile peak heights and the average of the 5 largest profile valley depths within the sampling length.

Ry--the distance between the top line of the contour peak and the bottom line of the contour valley within the sampling length L.

If the roughness is not marked on the drawing, select Ra /Rz /Ry, and the default is Ra.

Sino-U.S. surface roughness (finishness) comparison table

Old Chinese standard (finishness) New Chinese standard (roughness) Ra American standard (micron) Ra American standard

(Micro inches) Ra

▽ 4 6.3 8.00 320

6.30 250

▽ 5 3.2 5.00 200

4.00 160

3.20 125

▽ 6 1.6 2.50 100

2.00 80

1.60 63

▽ 7 0.8 1.25 50

1.00 40

0.80 32

▽ 8 0.4 0.63 25

0.50 20

0.40 16

Surface roughness refers to the small spacing and small peak and valley unevenness of the processed surface. The distance (wave distance) between the two wave crests or two wave troughs is very small (below 1mm), which is difficult to distinguish with the naked eye, so it belongs to the microscopic geometric shape error. The smaller the surface roughness, the smoother the surface. The surface roughness has a great influence on the performance of mechanical parts, mainly in the following aspects:

① Surface roughness affects the wear resistance of parts. The rougher the surface, the smaller the effective contact area between the mating surfaces, the greater the pressure, and the faster the wear.

② The surface roughness affects the stability of the mating properties. For clearance fits, the rougher the surface, the easier it is to wear, which will gradually increase the clearance during work. For interference fits, the microscopic peaks are flattened during assembly, which reduces the actual effective interference. The strength of the connection.

③ Surface roughness affects the fatigue strength of parts. There are large wave troughs on the surface of rough parts. Like sharp notches and cracks, they are very sensitive to stress concentration, thereby affecting the fatigue strength of the parts.