CNC machine tools and correct selection

The selection of CNC cutting tools and the determination of cutting parameters are important contents in CNC machining technology. They not only affect the machining efficiency of CNC machine tools, but also directly affect the machining quality. The development of CAD/CAM technology has made it possible to directly utilize CAD design data in CNC machining, especially the connection between DNC system microcomputers and CNC machine tools, which enables the entire process of design, process planning, and programming to be completed on a computer, generally without the need to output specialized process files.
At present, many CAD/CAM software packages provide automatic programming functions. These software generally prompt process planning related issues in the programming interface, such as tool selection, machining path planning, cutting amount setting, etc. Programmers can automatically generate NC programs and transmit them to CNC machine tools to complete processing as long as they set relevant parameters.
Therefore, the selection of cutting tools and the determination of cutting parameters in CNC machining are completed in a human-machine interaction state, which is in stark contrast to ordinary machine tool machining. At the same time, it also requires programmers to master the basic principles of tool selection and cutting parameter determination, fully consider the characteristics of CNC machining during programming, and be able to correctly select cutting tools and cutting parameters.
1、 Types and characteristics of commonly used cutting tools in CNC machining Case Studies
CNC machining tools must adapt to the characteristics of high-speed, efficient, and highly automated CNC machine tools, generally including universal tools, universal connecting tool handles, and a small number of specialized tool handles. The tool handle needs to be connected to the tool and installed on the power head of the machine tool, so it has gradually been standardized and serialized.
1. Classification of CNC cutting tools
According to the tool structure, it can be divided into:
Integral; Embedded type, using welding or machine clamp connection, machine clamp type can be divided into two types: non indexable and indexable; Special types, such as composite cutting tools, shock absorbing cutting tools, etc.
According to the materials used for manufacturing cutting tools, they can be divided into:
High speed steel cutting tools; Hard alloy cutting tools; Diamond cutting tools; Other material tools, such as cubic boron nitride tools, ceramic tools, etc.
From the perspective of cutting technology, it can be divided into:
Turning tools, including outer circles, inner holes, threads, cutting tools, etc; Drilling tools, including drills, reamers, taps, etc; Boring tools; Milling tools, etc.
In order to meet the requirements of CNC machine tools for tool durability, stability, easy adjustment, and interchangeability, machine clamp type indexable tools have been widely used in recent years, reaching 30% to 40% of the total number of CNC tools, and metal removal accounts for 80% to 90% of the total.
2 Characteristics of CNC cutting tools
Compared with the tools used in ordinary machine tools, CNC tools have many different requirements, mainly including the following characteristics:
Good rigidity (especially for rough machining tools), high accuracy, low vibration resistance and thermal deformation; Good interchangeability, easy to quickly change tools; Long service life, stable and reliable cutting performance; The size of the cutting tool is easy to adjust to reduce tool change adjustment time; The cutting tool should be able to reliably break or roll chips to facilitate the removal of chips; Serialization and standardization to facilitate programming and tool management.
2、 Selection of CNC machining tools
The selection of cutting tools is carried out in the human-machine interaction state of CNC programming. The correct selection of cutting tools and tool handles should be based on the processing capacity of the machine tool, the performance of the workpiece material, the processing process, cutting amount, and other relevant factors. The overall principle of tool selection is: easy installation and adjustment, good rigidity, high durability and accuracy. On the premise of meeting the processing requirements, try to choose a shorter tool handle to improve the rigidity of tool processing.
When selecting a tool, it is important to ensure that the size of the tool matches the surface size of the workpiece being processed. In production, the processing of the contour around flat parts often uses end mills; When milling flat surfaces, a hard alloy blade milling cutter should be selected; When processing bosses and grooves, choose high-speed steel end mills; When processing rough surfaces or rough holes, corn milling cutters with carbide blades can be selected; Ball end milling cutters, circular milling cutters, conical milling cutters, and disc milling cutters are often used for the processing of some three-dimensional surfaces and variable bevel contour shapes.
When machining free form surfaces (molds), due to the zero cutting speed at the end of the ball head tool, in order to ensure cnc machining LED heatsink accuracy, the cutting line spacing generally adopts the top dense distance, so the ball head is often used for precision machining of surfaces. However, flat end cutting tools are superior to ball end cutting tools in terms of surface processing quality and cutting efficiency. Therefore, as long as it is ensured that the surface cannot be cut, flat end cutting tools should be given priority in both rough and precision machining of curved surfaces. In addition, the durability and accuracy of cutting tools are greatly related to tool prices. It must be noted that in most cases, selecting a good tool increases tool costs, but the resulting improvement in processing quality and efficiency can greatly reduce the entire processing cost.
On the machining center, various cutting tools are installed on the tool library and can be selected and changed at any time according to program regulations. Therefore, it is necessary to use standard tool handles in order to quickly and accurately install the standard tools used in drilling, boring, expanding, milling and other processes onto the machine tool spindle or tool library. Programmers should understand the structural dimensions, adjustment methods, and adjustment range of the tool handle used on the machine tool, in order to determine the radial and axial dimensions of the tool during programming. At present, China’s machining centers use the TSG tool system, which has two types of tool handles: straight shank (3 specifications) and conical shank (4 specifications), including a total of 16 tool handles for different purposes.
In the machining process of economical CNC machine zinc tools, due to the fact that the grinding, measurement, and replacement of cutting tools are mostly manually carried out, which takes up a long auxiliary time, it is necessary to arrange the order of tool arrangement reasonably. Generally, the following principles should be followed: ① minimize the number of cutting tools; ② After a cutting tool is clamped, all machining steps that it can perform should be completed; ③ Tools for rough and fine machining should be used separately, even if they are of the same size and specification; ④ Milling before drilling; ⑤ Perform surface finishing first, followed by 2D contour finishing; ⑥ Wherever possible, the automatic tool change function of CNC machine tools should be utilized as much as possible to improve production efficiency.
3、 Determination of cutting amount during the machining process
The principle of reasonable selection of cutting amount is: during rough machining, the main focus is on improving productivity, but economic efficiency and processing costs should also be considered; During semi precision machining and precision machining, consideration should be given to cutting efficiency, economy, and machining cost while ensuring machining quality. The specific values should be determined based on the machine tool manual, cutting quantity manual, and experience. Specifically, the following factors should be considered:
Cutting depth ap. When the stiffness of the machine tool, workpiece, and tool allows, ap is equal to the machining allowance, which is an effective measure to improve productivity. In order to ensure the machining accuracy and surface roughness of the parts, a certain margin should generally be left for precision machining. The precision machining allowance of CNC machine tools can be slightly smaller than that of ordinary machine tools.
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 general range of L values is: L=(0.6~0.9) d.
Cutting speed V. Improving V is also a measure to improve productivity, but it is closely related to tool durability. As v increases, the tool durability decreases sharply, so the choice of v mainly depends on the tool durability. In addition, the cutting speed is also closely related to the processed material. For example, when milling alloy steel 30CrNi2MoVA with an end milling cutter, v can be used at around 8m/min; When using the same end milling cutter to mill aluminum alloy, V can be selected at least 200m/min.
Spindle speed n (r/min). The spindle speed is generally selected based on the cutting speed v. The calculation formula is: V=pnd/1000. The control panel of CNC machine tools is generally equipped with a spindle speed adjustment (multiplication) switch, which can adjust the spindle speed in integer multiples during the machining process.
Feed rate Vf. VF should be selected based on the machining accuracy and surface roughness requirements of the parts, as well as the tool and workpiece materials. The increase in Vf can also improve production efficiency. When the surface roughness requirement for machining is low, Vf can be chosen larger. During the machining process, Vf can also be manually adjusted through the adjustment switch on the machine tool control panel, but the maximum feed speed is limited by equipment stiffness and feed system performance.
With the widespread application of CNC machine tools in production practice and the formation of quantitative production lines, CNC programming has become one of the key issues in CNC machining marine. In the process of programming CNC programs, it is necessary to select cutting tools and determine cutting parameters in real-time under human-machine interaction. Therefore, programmers must be familiar with the selection method of cutting tools and the principle of determining cutting parameters, in order to ensure the processing quality and efficiency of parts, fully utilize the advantages of CNC machine tools, and improve the economic benefits and production level of enterprises.