Release time:

2021-02-21

【Summary】The selection of tools and the determination of cutting parameters in CNC machining are completed in a human-machine interaction state, which contrasts sharply with ordinary machine tool processing. This also requires programmers to master the basic principles of tool selection and cutting parameter determination, fully considering the characteristics of CNC machining during programming, and being able to correctly select cutting tools and cutting parameters. CNC machining tools must adapt to the high speed, high efficiency, and high degree of automation of CNC machine tools, and generally should include general-purpose tools, general-purpose tool holders, and a small number of special tool holders. Tool holders connect the tools and are mounted on the machine tool's power head, thus they have gradually been standardized and serialized. There are various methods for classifying CNC tools. 1. Structure of CNC machine tools: 1.1 Integral type; 1.2 Insert type, using welding or clamping connection, with clamping type further divided into non-rotating and rotating types; 1.3 Special types, such as composite tools, vibration-damping tools, etc. 2. Materials used for manufacturing tools: 2.1 High-speed steel tools; 2.2 Carbide tools; 2.3 Diamond tools; 2.4 Tools made from other materials, such as cubic boron nitride tools, ceramic tools, etc. 3. Classification of cutting processes: 3.1 Turning tools, divided into various types such as external round, internal hole, thread, cutting tools, etc.; 3.2 Drilling tools, including drill bits, reamers, taps, etc.; 3.3 Boring tools; 3.4 Milling tools, etc. To meet the requirements of CNC machine tools for tool durability, stability, ease of adjustment, and interchangeability, in recent years, clamping type rotating tools have been widely used, accounting for 30% to 40% of all CNC tools, with metal removal volume accounting for 80% to 90% of the total. 4. Characteristics of CNC tools: 4.1 Good rigidity (especially for rough machining tools), high precision, low vibration, and thermal deformation; 4.2 Good interchangeability, facilitating quick tool changes; 4.3 High lifespan, stable and reliable cutting performance; 4.4 Tool dimensions are easy to adjust to reduce tool change adjustment time; 4.5 Tools should reliably break or curl chips to facilitate chip removal; 4.6 Standardization and serialization to facilitate programming and tool management. 5. Tool selection: Tool selection is carried out in a human-machine interaction state during CNC programming. Tools and tool holders should be correctly selected based on the machine tool's processing capacity, workpiece material properties, processing procedures, cutting parameters, and other related factors. The general principle for tool selection is: easy installation and adjustment, good rigidity, high durability, and high precision. Under the premise of meeting processing requirements, shorter tool holders should be selected as much as possible to improve the rigidity of tool processing. When selecting tools, the dimensions of the tools should match the surface dimensions of the workpiece being processed. In production, for machining the peripheral profile of flat parts, end mills are often used; for milling flat surfaces, carbide insert milling cutters should be selected; for processing bosses and grooves, high-speed steel end mills should be chosen; for machining rough surfaces or rough holes, corn milling cutters with carbide inserts can be selected; for processing some three-dimensional profiles and variable angle contours, ball end mills, ring mills, conical mills, and disc mills are commonly used. When machining free-form surfaces (molds), since the end cutting speed of ball end tools is zero, to ensure processing accuracy, the cutting pitch is generally set to be very close, thus ball ends are often used for finishing curved surfaces. However, flat end tools outperform ball end tools in terms of surface processing quality and cutting efficiency, so as long as it is ensured that there is no overcutting, flat end tools should be prioritized for both rough and finish machining of curved surfaces. Additionally, the durability and precision of tools are greatly related to tool prices. It must be noted that in most cases, while selecting good tools increases tool costs, the resulting improvement in processing quality and efficiency can significantly reduce overall processing costs. In machining centers, various tools are mounted on the tool library, allowing for tool selection and replacement actions at any time as specified by the program. Therefore, standard tool holders must be used to enable standard tools for drilling, boring, reaming, milling, etc., to be quickly and accurately mounted onto the machine tool spindle or tool library. Programmers should understand the structural dimensions, adjustment methods, and adjustment ranges of the tool holders used on the machine tool to determine the radial and axial dimensions of the tools during programming. 6. Principles for arranging tools: In the processing of economical CNC machine tools, since tool grinding, measurement, and replacement are mostly done manually, occupying a long auxiliary time, it is necessary to reasonably arrange the order of tool arrangement. Generally, the following principles should be followed: 6.1 Minimize the number of tools as much as possible; 6.2 Once a tool is clamped, it should complete all processing steps it can perform; 6.3 Rough and finish machining tools should be used separately, even if they are of the same size and specification; 6.4 Mill before drilling; 6.5 Perform finishing on curved surfaces before finishing on two-dimensional profiles; 6.6 Whenever possible, utilize the automatic tool change function of CNC machine tools to improve production efficiency, etc. 7. Principles for reasonably selecting cutting parameters: During rough machining, the main focus is on improving productivity, but economic factors and processing costs should also be considered; during semi-finish and finish machining, cutting efficiency, economy, and processing costs should be balanced while ensuring processing quality. Specific values should be determined based on the machine tool manual, cutting parameter handbook, and combined with experience. The following factors should be considered: 7.1 Cutting depth t. Under the conditions allowed by the rigidity of the machine tool, workpiece, and tool, t equals the machining allowance, which is an effective measure to improve productivity. To ensure the machining accuracy and surface roughness of the parts, a certain allowance should generally be reserved for finish machining. The finish machining allowance for CNC machine tools can be slightly less than that for ordinary machine tools (Welcome to follow the WeChat public account: Industrial Manufacturing). 7.2 Cutting width L. Generally, L is proportional to the tool diameter d and inversely proportional to the cutting depth. In the processing of economical CNC machine tools, the range of values for L is generally: L = (0.6 to 0.9)d. 7.3 Cutting speed v. Increasing v is also a measure to improve productivity, but v is closely related to tool durability. As v increases, tool durability decreases sharply, so the selection of v mainly depends on tool durability. Additionally, cutting speed is also greatly related to the processing materials. For example, when milling alloy steel 30CrNi2MoVA with an end mill, v can be around 8m/min; while milling aluminum alloy with the same end mill, v can be selected to be above 200m/min. 7.4 Spindle speed n (r/min). The spindle speed is generally selected based on the cutting speed v. The calculation formula is: v = ∏nd/1000. CNC machine tools generally have a spindle speed adjustment (multiplication) switch on the control panel, which can adjust the spindle speed in whole multiples during processing. 7.5 Feed speed vF. vF should be selected based on the processing accuracy and surface roughness requirements of the parts, as well as the materials of the tools and workpieces. Increasing vF can also improve production efficiency. When the surface roughness requirement is low, vF can be selected to be larger. During processing, vF can also be manually adjusted through the adjustment switch on the machine tool control panel, but the maximum feed speed is limited by the rigidity of the equipment and the performance of the feed system.

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