1. The concept of roughness
After the parts are processed, large or small peaks and troughs are caused on the surface of the workpiece due to tools, built-up edge and scales. The heights and valleys of these peaks and valleys are so small that they are usually only visible with magnification. This microscopic geometric shape feature is called surface roughness.
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2. Roughness evaluation parameters
Indicated by the three codes of RaRzRy plus numbers, there will be corresponding surface quality requirements in the mechanical drawings. Generally, the surface of the workpiece surface roughness Ra<0.8um is called: mirror surface.
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Contour arithmetic mean deviation Ra: the arithmetic mean of the absolute value of the contour deviation within the sampling length L
Ten-point height Rz of microscopic roughness: the sum of the average value of the five largest profile peak heights and the average value of the five largest profile valley depths within the sampling length l
The maximum height Ry of the profile: the distance between the peak line of the profile and the bottom line of the profile valley within the sampling length L
3. Measurement and labeling of roughness
The surface roughness can be quantitatively evaluated by measuring the values of Ra, Rz and Ry with electronic or optical instruments. In actual production, the roughness is often identified by comparing the sample block with the processed surface by human vision and touch.
Marking method: Mark the features of the machined surface with symbols on the part drawing. It is a basic symbol, and it is meaningless to use this symbol alone. When adding parameter values, it means that the surface can be obtained by any method.
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4. Roughness grade obtained by various machining processes
Please refer to the table below for the numerical value and surface characteristics of surface roughness, obtaining methods, and application examples
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5. The effect 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, fit properties, fatigue resistance, workpiece precision and corrosion resistance of parts.
5.1. Effect on friction and wear. The influence of surface roughness on the wear of parts is mainly reflected in the top of the peak. The two parts are in contact with each other. In fact, they are part of the top of the peak. The pressure at the contact point is very high, which can make the material flow in shape. The rougher the surface, the more severe the wear.
5 .2 Effects on fit properties. The fit of two components is nothing more than two forms, interference fit and clearance fit. For interference fit, because the peak of the surface is squeezed flat during assembly, the amount of interference is reduced and the connection strength of the components is reduced; for clearance fit, as the peak is continuously ground, the degree of clearance will change. big. Therefore, surface roughness affects the stability of fit properties.
5.3 Effect on fatigue strength. The rougher the surface of the part, the deeper the dent, and the smaller the radius of curvature of the trough, the more sensitive it is to stress concentration. Therefore, the greater the surface roughness of a part, the more sensitive it is to stress concentration, and the lower its fatigue resistance will be.
5.4 Resistance to corrosive effects. The greater the surface roughness of a part, the deeper its troughs. 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. Methods to improve surface finish
Mainly divided into two types: increasing the corresponding process and improving 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 precision; in addition, the ultrasonic rolling technology at home and abroad combined with the plastic fluidity of metal is different from the traditional rolling Cold work hardening can improve the roughness by 2-3 levels, and improve the overall performance characteristics of the material.
Improvements on 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 steel, scales are easy to occur at low cutting speeds, and built-up edge is easy to form at medium speeds, which will increase the roughness. By avoiding this velocity region, the surface roughness value decreases. Therefore, constantly creating conditions to increase the cutting speed has always been an important direction to improve the technological level.
6.2 Choose the feed rate reasonably. The feed rate directly affects the surface roughness of the workpiece. Generally, the smaller the feed rate, the smaller the surface roughness and the smoother the workpiece surface.
6.3 Reasonable selection of tool geometry parameters. front and back 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 beneficial to chip removal. When the rake angle is constant, the larger the rake angle, the smaller the radius of the blunt circle 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 processed surface and transition surface, which is beneficial to reduce surface roughness degree value. Increasing the radius r of the tool nose arc can reduce the surface roughness value; reducing the secondary deflection angle Kr of the tool can also reduce the surface roughness value.
6.4 Select the appropriate tool material. A tool with good thermal conductivity should be selected to transfer cutting heat in time and reduce plastic deformation in the cutting area. 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, built-up edge and scales are easily generated, resulting in excessive surface roughness. If the surface is coated with cemented carbide or ceramic materials, an oxide protective film will be formed on the knife surface during cutting, which can reduce the friction coefficient 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, the higher the toughness, the greater the possibility of plastic deformation, and the greater the surface roughness of the part during machining.
6.6 Select the appropriate cutting fluid. Proper selection of cutting fluid can significantly reduce surface roughness. Cutting fluid has the functions of cooling, lubricating, chip removal and cleaning. It can reduce the friction between the workpiece, the tool and the chip, take away a large amount of cutting heat, reduce the temperature of the cutting zone, and discharge the fine chips in time.
