As a machine, you are certainly no stranger to surface roughness, but why is the roughness expressed as 0.8, 1.6, 3.2, 6.3, 12.5, do you know? Let's take a look:

The concept of surface roughness

During the machining of the parts, due to the plastic deformation of the metal surface during cutting, the vibration of the machine tool and the tool marks left on the surface by the tool, the various surfaces of the parts, no matter how smooth the machining is, as for observation under a microscope, You can see the uneven peaks and valleys, as shown in the figure:

 

 

 

The micro-geometric shape feature composed of peaks and valleys with small spacing on the machined surface is called surface roughness. Generally speaking, the surface roughness formed by different processing methods is also different.

The effect of surface roughness on mechanical products

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 and reduces The connection strength.

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.

4) Surface roughness affects the corrosion resistance of parts. Rough surface makes it easy for corrosive gas or liquid to penetrate into the inner metal layer through microscopic valleys on the surface, causing surface corrosion.

5) Surface roughness affects the sealing performance of parts. Rough surfaces cannot be closely attached, and gas or liquid leaks through the gaps between the contact surfaces.

6) Surface roughness affects the contact stiffness of parts. Contact stiffness is the ability of the joint surface of the part to resist contact deformation under the action of external force. The stiffness of the machine largely depends on the contact stiffness between the parts.

7) Affect the measurement accuracy of parts. The surface roughness of the measured surface of the part and the measuring surface of the measuring tool will directly affect the accuracy of the measurement, especially during precision measurement.

In addition, the surface roughness has varying degrees of influence on the coating, thermal conductivity and contact resistance, reflectivity and radiation performance of the parts, the resistance of liquid and gas flow, and the flow of current on the conductor surface.

Common processing methods and achievable roughness values

 

 

 

 

Is the surface roughness the same as the surface finish?

Surface finish is another term for surface roughness. The surface finish is put forward according to the human visual point of view, and the surface roughness is put forward according to the actual surface micro-geometry. Because of its conformity with international standards (ISO), China adopted surface roughness after the 1980s and abolished surface finish. After the promulgation of the national standards for surface roughness GB3505-83 and GB1031-83, the surface finish is no longer used.

There is a corresponding comparison table for surface finish and surface roughness (see the figure below). Roughness has a calculation formula for measurement, while smoothness can only be compared with a model gauge. Therefore, the roughness is more scientific and rigorous than the finish.

 

 

 

 

 

Why is the surface roughness value expressed as 0.8, 1.6, 3.2, etc.?

Everything comes from the great priority number system!

The French engineer Renault saw that there are many specifications of the wire rope on the hot air balloon, so he thought of a way to open 10 to the 5th power to get a number 1.6, and then multiply it to get 5 priority numbers as follows:

1.0

1.6

2.5

4.0

6.3

This is a geometric sequence, the latter number is 1.6 times the former number, then there are only 5 kinds of steel ropes below 10, and there are only 5 kinds of steel ropes from 10 to 100, namely 10, 16, 25, 40, 63.

However, this division method is too sparse, so Mr. Lei will continue his efforts and open 10 to the power of 10 to get the R10 priority number system as follows:

1.0

1.25

1.6

2.0

2.5

3.15

4.0

5.0

6.3

8.0

The common ratio is 1.25, so there are only 10 kinds of wire ropes within 10, and there are only 10 kinds of wire ropes from 10 to 100, which is more reasonable. At this time, someone must say that the front numbers of this series seem to be similar. For example, 1.0 and 1.25 are almost the same. I usually round them, but the interval between 6.3 and 8.0 is large. Is this reasonable?

Reasonable and unreasonable, let's make an analogy. For example, the natural numbers 1, 2, 3, 4, 5, 6, 7, 8, and 9 seem to be very smooth. We use this sequence to pay wages. We send 1000 to Zhang San and 2000 to Li Si. Both are convinced. Suddenly inflation, 8000 was issued to Zhang San and 9000 to Li Si. In the past, Li Si's salary was twice that of Zhang San, but now it has become 1.12 times. Do you think Li Si can be willing? He is the supervisor, and sending him 16,000 is almost the same. Zhang San will not complain that the supervisor has 8,000 more than him.

There are two ways to compare things in nature: "relative" and "absolute"! The priority number system is relative.

Some people say that his product specifications are 10 tons, 20 tons, 30 tons, and 40 tons. Now it seems unreasonable, right? If you take twice, it should be 10 tons, 20 tons, 40 tons, 80 tons, or keep the head and tail, it should be 10 tons, 16 tons, 25 tons, 40 tons, and the ratio of 1.6 is reasonable.

This is "standardization". People often see "standardization" on the forums, but they are actually talking about "standard parts". What they do is to sort out the standard parts of the whole machine, which is called standardization. Actually it is not like this. . For true standardization, you have to serialize all the parameters of your product according to the priority number system, and then serialize the function parameters and dimensions of all parts using the priority number system.

Natural numbers are infinite, but in the eyes of mechanical designers, there are only 10 numbers in the world, which is the R10 priority number. Moreover, multiplying, dividing, exposing, and extracting these 10 numbers, the result is still in these 10 numbers, how amazing! When you design, when you don’t know what size you should choose, just choose from these 10 numbers. How convenient is it?

1.0 N0

1.12 N2

1.25 N4

1.4 N6

1.6 N8

1.8 N10

2.0 N12

2.24 N14

2.5 N16

2.8 N18

3.15 N20

3.55 N22

4.0 N24

4.5 N26

5.0 N28

5.6 N30

6.3 N32

7.1 N34

8.0 N36

9.0 N38

Two priority numbers, such as 4 and 2, whose serial numbers are N24 and N12 respectively, multiply them and add their serial numbers together, and the result is equal to N36, which is 8; divide and subtract serial numbers, which is equal to N12, which is 2 ; The cube of 2, multiply its serial number N12 by 3 to get N36, which is 8. For the square root of 4, divide its serial number N24 by 2 to get N12, which is 2. What if you find the fourth power of 2? N12*4=N48, there is no here, what should I do? In the above list, there is no number written in it, which is 10, and its serial number is N40. If the serial number is greater than 40, only look at the part greater than 40. For example, N48 looks at N8, which is 1.6, and then multiplies by 10 to get 16. . If the serial number is N88, look at N8 to get 1.6, and then multiply it by 100 to get 160, because the serial number of 100 is N80, and the serial number of 1000 is N120, and so on for mechanical design, using these 20 numbers for a lifetime is enough. But sometimes you need to use the R40 number system, there are 40 numbers, it is more complete, if not enough, there is the R80 series. I have memorized the R40 number system backwards, and I don’t need a calculator for general calculations. In simple terms, calculate the torsion resistance of 45 steel with 40 diameters. Its torsion coefficient is 0.5*π*R^3. The torsion stress is half of the yield point of 360, which is 180MPa. The pi ratio is 3.15. Come out in a while. Some people say you don't add safety factor? Let's just say, is it 1.25, 1.5, or 2? Ha ha.

The golden ratio is 0.618, which is 1.618. There is also 1.6 here.

The square root sequence is the root number 1, the root number 2, and the root number 3. Is it easy to find? (The serial number of 3 is N19)

What is the square of π? Equal to 10. Is it convenient when you calculate the pressure bar to be stable?

The torsion coefficient of the round rod is about 0.1*D^3. Now you can calculate the torsion coefficient verbally, right?

Why does the big screw jump directly from M36 to M40?

Why is the gear ratio of 6.3 or 7.1?

Why does channel steel have a No. 12.6 which is rare in the market?

Why does the outsourcing factory call and say that there is no 140 square tube, but there are 120 and 160? Because the R5 number system has priority over the R20 number system.

Why does the parameter of the standard part have a first sequence and a second sequence? Generally speaking, the first sequence is the R5 sequence.

Why does Inventor's screw hole list have M11.2? Now you know it’s not a nonsense number, right?

There are also steel plate thickness, section steel model, gear modulus, all standard parts, functional parameters on all industrial product samples, dimensional parameters, standard tolerance tables, etc., and their sources are slowly becoming clear in our minds. . It can be said that we have understood half of the mechanical design manual and those industrial products that have not yet been made.

Then, when we design a product, we can design a series of products at the same time, instead of performing the so-called "standardization" after the design is finished; furthermore, if the product is destined to be serialized, then we can even compare the actual working conditions Design the product without knowing it well, because the priority number system has included all models.

Priority number system applications, listed above