6 interpretations of metal processing surface roughness

1. The concept of roughness

After the parts are processed, large or small crests and troughs are caused on the surface of the workpiece due to tools, built-up tumors and scales. The height of these peaks and valleys is very small, and they can usually be seen only by zooming in. This feature of microscopic geometry is called surface roughness.

2. Evaluation parameters of roughness

Expressed by the three codes of Ra\Rz\Ry plus a number, there will be corresponding surface quality requirements in the mechanical drawings, generally the surface roughness of the workpiece Ra<>

Contour arithmetic mean deviation Ra: the arithmetic mean of the absolute value of the contour deviation within the sampling length L

Ten-point height of microscopic unevenness Rz: the sum of the average value of the 5 largest contour peak heights and the average of the 5 largest contour valley depths within the sampling length l

Maximum contour height Ry: the distance between the top line of the contour peak and the bottom line of the contour valley within the sampling length L

3. Roughness measurement and marking

The surface roughness can be quantitatively evaluated by measuring the values of Ra, Rz and Ry with electronic or optical instruments. In actual production, people’s vision and touch are often used to compare the roughness of the sample with the processed surface.

Marking method: Mark the features of the machined surface with symbols on the part drawing. It is a basic symbol. It is meaningless to use this symbol alone. When adding parameter values, it means that the surface can be obtained by any method.

4. Various machining processes obtain roughness grades

For surface roughness values and surface characteristics, obtaining methods, and application examples, please refer to the table below

5. The influence of surface roughness on the performance of mechanical parts

Surface roughness has a great influence on the quality of parts, mainly focusing on the wear resistance, mating properties, fatigue strength, workpiece accuracy and corrosion resistance of the parts.

5.1. Impact on friction and wear. The influence of surface roughness on the wear of parts is mainly reflected in the peaks and peaks. The two parts are in contact with each other, which is actually part of the peak contact. The pressure at the contact is very high, which can cause the material to produce plastic flow. The rougher the surface, the more severe the wear.

5.2 The impact on the nature of the fit. The two-component fit is nothing more than two forms, interference fit and clearance fit. For interference fits, as the peaks of the surface are flattened during assembly, the amount of interference is reduced and the connection strength of the components is reduced; for clearance fits, as the peaks are continuously ground, the degree of clearance will change. Big. Therefore, the surface roughness affects the stability of the mating properties.

5.3 The effect of resisting fatigue strength. The rougher the surface of the part, the deeper the dent, the smaller the radius of curvature of the wave trough, and the more sensitive it is to stress concentration. Therefore, the greater the surface roughness of the part, the more sensitive it is to stress concentration, and the lower its fatigue resistance.

5.4 Resistance to corrosive effects. The rougher the surface of the part, the deeper the wave trough. In this way, dust, deteriorated lubricating oil, acidic and alkaline corrosive substances are easy to accumulate in these valleys, and penetrate into the inner layer of the material, aggravating the corrosion of parts. Therefore, reducing the surface roughness can enhance the corrosion resistance of the parts.

6. Ways to improve the surface finish

Mainly divided into two categories: increase the corresponding process and improve the original process

Increase the corresponding process: adding polishing, grinding, scraping, rolling and other processes can not only improve the finish but also improve the accuracy; in addition, the ultrasonic rolling technology at home and abroad combines the plastic fluidity of the metal, which is different from the traditional rolling Cold work hardening can increase the roughness by 2-3 grades and improve the overall performance characteristics of the material.

Improvements in the original process:

6.1 Choose the cutting speed reasonably. Cutting speed V is an important factor affecting surface roughness. When processing plastic materials, such as medium and low-carbon steels, low cutting speeds are prone to scaly, and medium speeds are prone to forming built-up tumors, which will increase the roughness. Avoid this speed zone, the surface roughness value will decrease. Therefore, constantly creating conditions to increase the cutting speed has always been an important direction to improve the technological level.

6.2. Reasonably choose the feed rate. The size of the feed directly affects the surface roughness of the workpiece. Generally, the smaller the feed, the smaller the surface roughness and the smoother the surface of the workpiece.

6.3. Reasonably choose the geometric parameters of the tool. Front and rear corners. Increasing the rake angle can reduce the extrusion deformation and friction when the material is cut, and also reduce the total cutting resistance, which is conducive to chip removal. When the front angle is constant, the larger the clearance angle, the smaller the blunt radius of the cutting edge, and the sharper the cutting edge; in addition, it can also reduce the friction and extrusion between the flank surface and the machined surface and the transition surface, which is beneficial to reduce the surface roughness Degree value. Increasing the arc radius r of the tool tip can reduce the surface roughness value; reducing the auxiliary deflection angle Kr of the tool can also reduce the surface roughness value.

6.4. Choose a suitable tool material. The tool with good thermal conductivity should be selected to transfer the cutting heat in time and reduce the plastic deformation of the cutting zone. In addition, the tool should have good chemical properties to prevent the tool from having an affinity with the material to be processed. When the affinity is too large, it is easy to produce built-up tumors and scales, resulting in excessive surface roughness. If the surface layer is coated with cemented carbide or ceramic material, when cutting, an oxide protective film is formed on the knife surface, which can reduce the coefficient of friction with the machined surface, so it is beneficial to improve the surface finish.

6.5. Improve the performance of the workpiece material. The toughness of the material determines its plasticity. Good toughness has a greater possibility of plastic deformation. During machining, the surface roughness of the part is greater.

6.6. Choose a suitable cutting fluid. The correct selection of cutting fluid can significantly reduce the surface roughness. The cutting fluid has the functions of cooling, lubrication, chip removal and cleaning. It can reduce the friction between the workpiece, the tool and the chips, take away a large amount of cutting heat, reduce the temperature of the cutting zone, and remove small chips in time.