Surface roughness is a core indicator for measuring the machining quality of large CNC precision machined parts, directly affecting their wear resistance, fatigue strength, and sealing performance. In CNC machining, surface roughness arises from the interaction between the tool and workpiece, the matching of cutting parameters, and the dynamic characteristics of the machining system. Its control requires a comprehensive approach considering multiple dimensions, including cutting parameters, tool characteristics, workpiece material, machining system stability, and cutting conditions.
Cutting parameters are the direct determinants of surface roughness. The combination of cutting speed, feed rate, and depth of cut directly affects chip morphology and cutting force distribution. At low cutting speeds, ductile materials are prone to built-up edge formation due to insufficient cutting temperature, leading to surface tearing. Medium- and high-speed cutting can reduce built-up edge, but in ultra-high-speed cutting, insufficient cooling can exacerbate tool wear and worsen surface quality. Feed rate is positively correlated with surface roughness; theoretically, surface roughness is proportional to the square of the feed rate. However, excessively small feed rates can cause the cutting edge to press against the workpiece surface, increasing plastic deformation. The depth of cut has a relatively small impact on surface roughness during roughing, but if it is too small during finishing, it may cause surface chatter marks due to the compression of the cutting edge radius.
The influence of tool characteristics on surface roughness is reflected in three aspects: material, geometric parameters, and wear condition. Tool material must be selected based on the workpiece material and cutting conditions: high-speed steel tools are suitable for low-speed machining, carbide tools can withstand medium-to-high-speed cutting, and ceramic tools are suitable for high-speed finishing. Among tool geometric parameters, increasing the tip radius can smooth cutting marks, but excessive radius can easily lead to overcutting; decreasing the principal cutting edge angle can evenly distribute cutting force and reduce workpiece vibration; optimizing the secondary cutting edge angle can reduce the height of the residual area. Tool wear condition is equally critical; when the flank wear exceeds a threshold, the blunting cutting edge will compress the workpiece surface, forming a cracked layer and significantly increasing roughness.
Differences in the physical and mechanical properties of workpiece materials impose differentiated requirements on surface roughness control. When cutting ductile materials (such as steel and aluminum alloys), built-up edge and chip adhesion are easily generated, which needs to be improved by increasing the cutting speed or using tools with anti-adhesion coatings. Brittle materials (such as cast iron and ceramics) are prone to forming fragmented chips during cutting, leading to surface pitting, requiring small depths of cut, high feed rates, and sharp cutting edges. Difficult-to-machine materials (such as titanium alloys and high-temperature alloys) have poor thermal conductivity and high hardness, requiring cubic boron nitride (CBN) tools and high-pressure cooling systems to prevent tool overheating and wear.
Machining system stability is fundamental to ensuring surface accuracy. Spindle radial runout, excessive guideway clearance, or insufficient chuck clamping force can cause fluctuations in cutting depth or workpiece movement, forming periodic chatter marks. When machining slender shaft parts, without follow rest or center rest support, bending deformation can easily cause surface ripples. Vibration in the machining system (such as chatter) directly deteriorates surface quality and needs to be suppressed by adjusting cutting parameters, increasing system damping, or using anti-vibration pads.
The impact of cutting conditions on surface roughness is reflected in both cooling and lubrication, and the machining environment. Appropriate selection of cutting fluid can reduce cutting temperature, decrease tool wear, and flush away chips, preventing scratches on the machined surface. For example, aluminum alloy machining requires specialized cutting fluids to prevent oxidation, while difficult-to-machine materials require high-pressure cooling systems to control cutting temperature. Fluctuations in ambient temperature and humidity can cause thermal deformation of machine tool components, affecting machining accuracy; therefore, temperature-controlled workshops and humidity control are necessary to ensure stability.
Surface roughness control of large CNC precision machined parts requires a coordinated effort from multiple aspects, including process system design, cutting parameter optimization, tool management, workpiece material compatibility, and environmental control. By scientifically matching cutting parameters, selecting high-performance tools, enhancing process system stability, and optimizing cutting conditions, surface quality can be significantly improved, meeting the stringent precision requirements of high-end equipment.
