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How to choose CNC machining tool and feed rate?

Posted by: Chris Na 2021-05-26 Comments Off on How to choose CNC machining tool and feed rate?

The selection of cnc tools and the determination of cutting parameters are important contents in the CNC machining process. It not only affects the machining efficiency of CNC machine tools, but also directly affects the machining quality. The development of CAD/CAM technology makes it possible to directly use CAD design data in CNC machining, especially the connection between a microcomputer and a CNC machine tool, so that the entire process of design, process planning and programming is completed on the computer, generally without output Special process documents.

Nowadays, many CAD/CAM software packages provide automatic programming functions. These softwares generally prompt related issues of process planning in the programming interface, such as tool selection, machining path planning, cutting amount setting, etc. The programmer only needs to set the relevant The parameters can be automatically generated and transferred to the CNC machine tool to complete processing. Therefore, the selection of tools and the determination of cutting parameters in CNC machining are completed under the state of human-computer interaction, which is in sharp contrast with ordinary machine tool processing. At the same time, programmers must master the basic principles of tool selection and cutting parameters. Fully consider the characteristics of CNC machining when programming. This article discusses the problem of tool selection and cutting parameters that must be faced in NC programming, gives some principles and suggestions, and discusses the problems that should be paid attention to.

1. Types and characteristics of commonly used tools for CNC machining

CNC machining tools must adapt to the characteristics of high-speed, high-efficiency, and high degree of automation of CNC machine tools. Generally, they should include general-purpose tools, general-purpose connection tool holders and a small number of special tool holders. The tool holder must be connected to the tool and mounted on the power head of the machine tool, so it has gradually been standardized and serialized. There are many ways to classify CNC tools. According to the tool structure, it can be divided into: ①Integral type; ②Inlaid type, which adopts welding or machine clamp connection, and the machine clamp type can be divided into two types: non-indexable and indexable; ③Special types, such as composite tools, reduce Vibrating knives, etc. According to the materials used to manufacture the tools, it can be divided into: ① high-speed steel tools; ② cemented carbide tools; ③ diamond tools; ④ tools of other materials, such as cubic boron nitride tools, ceramic tools, etc. From the cutting process, it can be divided into: ① turning tools, including outer circle, inner hole, thread, cutting tools, etc.; ② drilling tools, including drills, reamers, taps, etc.; ③ boring tools; ④ milling tools, etc. . In order to meet the requirements of CNC machine tools for tool durability, stability, easy adjustment, and exchangeability, in recent years, machine-clamped indexable tools have been widely used, reaching 30% to 40% of the entire CNC tools. Metal removal The amount accounts for 80% to 90% of the total.

Compared with the tools used on ordinary machine tools, CNC tools have many different requirements, mainly including the following characteristics:

⑴ Good rigidity (especially for rough machining tools), high precision, low vibration resistance and thermal deformation;

⑵Good interchangeability, convenient for quick tool change;

⑶High life, stable and reliable cutting performance;

⑷The size of the tool is easy to adjust to reduce the tool change adjustment time;

⑸ The tool should be able to reliably break or roll chips to facilitate the removal of chips;

⑹Serialization and standardization to facilitate programming and tool management.

Second, the choice of CNC machining tools

The selection of the tool is carried out under the man-machine interaction state of CNC programming. The correct selection of tools and tool holders should be based on the processing capacity of the machine tool, the properties of the workpiece material, the processing procedure, the cutting amount and other related factors. The general principle of tool selection is: easy installation and adjustment, good rigidity, high durability and precision. On the premise of meeting the processing requirements, try to choose a shorter tool holder to improve the rigidity of tool processing.

When selecting a tool, the size of the tool must be adapted to the surface size of the workpiece to be processed. In production, end mills are often used to process the peripheral contours of plane parts; when milling planes, carbide blade milling cutters should be selected; when processing bosses and grooves, high-speed steel end mills should be selected; rough surface or rough machining When drilling holes, you can choose corn milling cutters with carbide inserts; for the processing of some three-dimensional profiles and variable bevel contours, ball-end milling cutters, ring milling cutters, tapered milling cutters and disc milling cutters are often used.

When performing free-form surface machining, since the end cutting speed of the ball-end tool is zero, in order to ensure the machining accuracy, the cutting line spacing is generally very dense, so the ball end is often used for surface finishing. The flat-end tool is superior to the ball-end tool in terms of surface processing quality and cutting efficiency. Therefore, as long as it is guaranteed not to cut, whether it is roughing or finishing of curved surfaces, flat-end tools should be preferred. In addition, the durability and accuracy of the tool has a great relationship with the price of the tool. It must be noted that in most cases, the selected tool increases the tool cost, but the resulting increase in processing quality and efficiency , You can greatly reduce the overall processing cost.

On the machining center, various tools are installed in the tool magazine, and the tool selection and tool change actions are carried out at any time according to the program regulations. Therefore, standard tool holders must be used so that standard tools used in drilling, boring, expanding, milling and other processes can be quickly and accurately installed on the spindle or tool magazine of the machine tool. The programmer should understand the structure size, adjustment method and adjustment range of the tool holder used on the machine tool in order to determine the radial and axial dimensions of the tool during programming. At present, my country’s machining center adopts the TSG tool system, and its tool holders have two types: straight shank (three specifications) and tapered shank (four specifications), including a total of 16 tool holders for different purposes.

In economical CNC machining, since the sharpening, measurement and replacement of tools are mostly performed manually, which takes a long time to assist, it is necessary to arrange the order of the tools in a reasonable manner. Generally, the following principles should be followed: ①Minimize the number of tools; ②After a tool is clamped, all processing parts that can be completed should be completed; ③The rough and finishing tools should be used separately, even if they are tools of the same size; ④ Milling first, then drilling; ⑤ Surface finishing first, then two-dimensional contour finishing; ⑥ When possible, the automatic tool change function of CNC machine tools should be used as much as possible to improve production efficiency.

Third, the determination of cutting parameters for CNC machining

The principle of reasonable selection of cutting parameters is that during rough machining, productivity is generally improved, but economy and processing costs should also be considered; semi-finishing and finishing should take into account cutting efficiency, while ensuring the quality of machining. Economy and processing cost. The specific value should be determined according to the machine tool manual, cutting parameter manual, and combined with experience.

⑴Cutting depth t. When the rigidity of the machine tool, workpiece and tool allows, t 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 finishing. The finishing allowance of CNC machine tools can be slightly smaller than that of ordinary machine tools.