Principles for selecting cutting parameters
Cutting parameters refer to the three parameters of cutting speed, feed rate, and depth of cut during the cutting process. They directly affect machining quality, production efficiency, and tool life, and are the core content of cutting process development. The selection of cutting parameters must follow certain principles. While ensuring machining quality, it is necessary to maximize production efficiency and reduce machining costs. At the same time, the durability of the tool and the load capacity of the machine tool must also be considered. The rational selection of cutting parameters is a key technical step in machining, requiring comprehensive consideration of multiple factors such as the workpiece material, tool material, machine tool performance, and machining requirements.
Ensuring machining quality is the primary principle for selecting cutting parameters. Machining quality includes dimensional accuracy, form and position accuracy, and surface quality, and the size of the cutting parameters directly affects these indicators. Cutting speed has the most significant impact on surface quality. Excessively high cutting speeds will increase cutting temperatures, causing defects such as burns and oxidation on the workpiece surface. Especially when processing plastic materials, built-up edge is prone to occur, resulting in increased surface roughness. Excessively low cutting speeds will increase cutting forces, causing workpiece vibration, and affecting dimensional accuracy and form accuracy. Feed rate mainly affects surface roughness. The larger the feed rate, the greater the residual area height and the higher the surface roughness value. Therefore, a smaller feed rate (such as 0.05-0.1mm/r) is required for fine machining to obtain a lower surface roughness (Ra≤1.6μm); a larger feed rate (such as 0.2-0.5mm/r) can be used for rough machining to improve efficiency. The impact of cutting depth on processing quality is relatively small, but excessive cutting depth will increase cutting force and cutting heat, resulting in workpiece deformation and increased tool wear, affecting dimensional accuracy; excessively small cutting depth may cut in the hardened layer on the workpiece surface, increasing tool wear and also affecting processing quality.
Improving production efficiency is a key goal in selecting cutting parameters. While ensuring machining quality, the largest possible cutting parameter should be selected to shorten machining time. Of the three cutting parameters, depth of cut has the greatest impact on production efficiency, as increasing the cutting depth allows for more machining stock to be removed in a single pass, reducing the number of passes. Therefore, during roughing, a larger cutting depth (e.g., 2-10mm) should be preferred to remove the majority of the stock in a single pass. Feed rate has the second-largest impact on production efficiency. If tool strength and machine power permit, feed rate should be appropriately increased to improve material removal rate per unit time. Cutting speed has a relatively small impact on production efficiency, but during finishing, increasing it can shorten machining time. Especially when high-speed cutting technology is employed, cutting speeds can be increased to several or even dozens of times that of conventional cutting, significantly improving production efficiency. For example, when processing 45 steel, the rough machining cutting speed of the carbide tool is 80-120m/min, the feed rate is 0.2-0.4mm/r, and the cutting depth is 3-5mm; the finishing cutting speed is 150-200m/min, the feed rate is 0.1-0.2mm/r, and the cutting depth is 0.5-1mm.
Extending tool life is a key consideration when selecting cutting parameters. Tool life and cutting parameters are closely related. Excessively high cutting parameters lead to accelerated tool wear, shortened tool life, increased tool changes, and increased tool costs. According to cutting principles, cutting speed has the greatest impact on tool life. Generally speaking, a 20% increase in cutting speed reduces tool life by more than half. Feed rate has the second-largest impact on tool life, while depth of cut has a relatively minor influence. Therefore, when selecting cutting parameters, a balance must be struck between productivity and tool life. For expensive tools (such as cubic boron nitride and diamond tools), cutting speeds should be appropriately reduced to extend their life. For low-cost tools such as high-speed steel, cutting speeds can be appropriately increased to improve productivity. Furthermore, the selection of cutting parameters must consider the tool material and geometry. Carbide tools have better heat resistance and can be used at higher cutting speeds; high-speed steel tools have poorer heat resistance and should be used at lower cutting speeds. Larger rake and clearance angles reduce cutting forces, allowing for increased feed and depth of cut.
Considering the load capacity of the machine tool and fixture is essential for selecting cutting parameters. The size of the cutting parameters directly determines the magnitude of the cutting force and cutting torque. Exceeding the load capacity of the machine tool or fixture can lead to machine overload, fixture deformation, or workpiece loosening, affecting machining quality and production safety. Before selecting cutting parameters, it is necessary to understand parameters such as the machine tool’s rated power, spindle speed range, maximum feed rate of the feed system, and clamping force of the fixture to ensure that the cutting force and cutting torque are within the allowable range. For example, the spindle power of a conventional lathe is generally 3-10kW, and the maximum cutting force is 5-10kN. Therefore, when machining large-diameter, high-strength materials, it is necessary to appropriately reduce the cutting depth and feed rate to avoid overloading the machine tool. For precision CNC machine tools, due to their high rigidity and precision, larger cutting parameters can be used to fully utilize the machine tool’s performance. In addition, the rigidity of the workpiece also affects the selection of cutting parameters. For workpieces with poor rigidity, such as slender shafts and thin-walled parts, a smaller cutting depth and feed rate should be used, and auxiliary supports should be used to prevent workpiece deformation.
The selection of cutting parameters is a dynamic optimization process that requires adjustment based on actual machining conditions. During the trial cutting phase, a lower cutting parameter should be used to conduct trial machining. Observe whether the cutting process is stable, tool wear is normal, and the machining quality meets the requirements. The cutting parameters can then be gradually adjusted to the optimal state. During mass production, tool wear and machining quality should be regularly checked. Depending on the degree of tool wear, the cutting speed or feed rate should be appropriately reduced to ensure consistent machining quality. Furthermore, the use of cutting fluid must be considered. Effective cooling and lubrication measures can lower cutting temperatures and minimize tool wear, allowing for higher cutting parameters. For example, when using extreme pressure emulsion, cutting speeds can be increased by 10%-20%, and feed rates by 5%-10%. By comprehensively considering various factors and continuously optimizing cutting parameters, high-quality, efficient, and low-cost machining can be achieved.
