The formation, influencing factors and improvement measures of surface roughness
Surface roughness refers to the unevenness of a machined surface, characterized by small peaks and valleys with small spacing between them. It is a key indicator of part surface quality and directly affects performance characteristics such as wear resistance, sealing, and fatigue strength. The formation of surface roughness is primarily influenced by various factors during the cutting process. During cutting, friction, compression, and tearing between the tool and the workpiece leave cutting marks on the workpiece surface, creating surface roughness. Specifically, as the tool cuts the workpiece, the chips rub and compress against the workpiece surface during formation, causing plastic deformation and forming tiny bumps and depressions. Simultaneously, the tool’s cutting edge radius and flank face rub against the workpiece surface, further exacerbating surface unevenness. Furthermore, vibration, built-up edge (BUE), and burrs generated during the cutting process can significantly affect surface roughness. For example, vibration can leave periodic ripples on the workpiece surface, while BUE and burrs can create irregular protrusions.
Numerous factors influence surface roughness, primarily including tooling, workpiece material, cutting parameters, and machining equipment. Regarding tooling, the geometric parameters of the tool significantly influence surface roughness. A too small rake angle increases cutting forces and heat, leading to increased plastic deformation on the workpiece surface and higher surface roughness. A too small clearance angle increases friction between the tool’s flank and the workpiece surface, also increasing surface roughness. A larger tool edge radius exerts greater compression on the workpiece surface during cutting, resulting in higher surface roughness. Tool wear also affects surface roughness. When tool wear reaches a certain level, its cutting edge becomes blunt, increasing cutting forces and significantly increasing surface roughness. Regarding workpiece material, properties such as hardness, plasticity, and toughness significantly influence surface roughness. Materials with high plasticity are more susceptible to significant plastic deformation during cutting, resulting in higher surface roughness. Materials with higher hardness, on the other hand, are more susceptible to lower surface roughness.
Cutting parameters are one of the most important factors affecting surface roughness. Cutting speed, feed rate, and back-cut depth all have varying degrees of influence on surface roughness. The effect of cutting speed on surface roughness is more complex. At low cutting speeds, built-up edge (BUE) and scale are more likely to form, resulting in increased surface roughness. When the cutting speed reaches a certain level, BUE and scale disappear, and surface roughness decreases. However, when the cutting speed is too high, the cutting temperature rises sharply, exacerbating tool wear and increasing surface roughness. Feed rate has the most significant impact on surface roughness. A higher feed rate results in wider and deeper cut marks on the workpiece surface, resulting in higher surface roughness. Therefore, a lower feed rate is typically used during finish turning to achieve better surface quality. Back-cut depth has a relatively minor effect on surface roughness. Within a certain range, increasing back-cut depth only increases cutting forces and heat but has little effect on surface roughness. However, excessive back-cut depth can cause significant plastic deformation in the workpiece, increasing surface roughness .
The precision and stability of machining equipment also affect surface roughness. The spindle rotation accuracy and guideway straightness of machining equipment like lathes and milling machines can affect the machining accuracy of the workpiece, and thus the surface roughness. For example, spindle rotation accuracy errors can cause radial runout of the tool during cutting, creating irregular ripples on the workpiece surface and increasing the surface roughness. Guideway straightness errors can cause tool offset during feed, affecting the workpiece’s dimensional accuracy and surface quality. Furthermore, vibration of machining equipment can also adversely affect surface roughness. Vibration can shift the relative position between the tool and the workpiece, causing instability in the cutting process and increasing the surface roughness.
A series of effective measures can be taken to improve the surface roughness of parts. When selecting cutting tools, appropriate tool materials and geometric parameters should be selected based on the workpiece material and processing requirements. For example, when machining plastic materials, larger rake and relief angles can be used to reduce cutting forces and friction. When machining hard materials, tool materials with better hardness and wear resistance can be selected. Furthermore, tools should be regularly sharpened and replaced to maintain sharpness and minimize the impact of tool wear on surface roughness. When selecting cutting parameters, cutting speed, feed rate, and back-cut should be appropriately determined. For fine turning, higher cutting speeds and lower feed rates can be used to achieve better surface quality. Furthermore, cooling lubricants can be used. Cooling lubricants not only reduce cutting temperatures and tool wear, but also reduce friction between the tool and the workpiece, improving surface roughness. Regarding machining equipment, regular maintenance and servicing are essential to improve accuracy and stability and minimize the impact of vibration on surface roughness. For parts requiring higher precision, subsequent processing techniques such as grinding and polishing can be used to further reduce surface roughness and improve surface quality.
