Tool selection and cutting amount determination in NC machining

The selection of the tool and the determination of the cutting amount are important contents in the CNC Machining process. It not only affects the machining efficiency of the CNC machine tool, 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. In particular, the connection between the microcomputer and the CNC machine makes it possible to complete the entire process of design, process planning and programming on the computer.

Nowadays, many CAD/CAM software includes automatic programming functions. These softwares generally prompt the process planning related problems in the programming interface, such as tool selection, machining path, cutting usage setting, etc., programmers only need to set relevant parameters. The NC program can be automatically generated and transferred to the CNC machine for processing. Therefore, the tool selection and cutting amount determination in NC machining is completed under the human-computer interaction state, which is in sharp contrast with the ordinary machine tool processing. At the same time, the programmer must also master the basic principles of tool selection and cutting amount determination. Fully consider the characteristics of CNC machining when programming. This paper analyzes the problem of tool selection and cutting amount determination that must be faced in NC programming.

First, the types and performance 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, general-purpose tools and universal joint holders should be included. The shank is to be connected to the tool and mounted on the power head of the machine, so it has been standardized and serialized. There are several ways to classify CNC tools. According to the tool structure can be divided into: 1 integral; 2 mosaic. According to the materials used in the manufacture of tools can be divided into: 1 high-speed steel tools; 2 carbide tools; 3 diamond tools; 4 ceramic tools. From the cutting process can be divided into: 1 milling tool; 2 drilling tools; 3 boring tools; 4 turning tools.

The performance of the tool material:

(1) The hardness of the high hardness tool material should be higher than the hardness of the workpiece

(2) sufficient toughness to withstand cutting forces, vibrations and shocks;

(3) High wear resistance and wear resistance are the ability of materials to resist wear;

(4) The ability of high heat resistant tool materials to maintain hardness, wear resistance, strength and toughness at high temperatures;

(5) Good processability

Second, the choice of CNC machining tools

The selection of the tool should be based on the machining capability of the machine tool, the performance of the workpiece material, the machining process, the amount of cutting and other relevant factors. The general principle of tool selection is: easy installation and adjustment, good rigidity, durability and high precision. Under the premise of meeting the processing requirements, try to choose a shorter tool holder to improve the rigidity of the tool processing.

When selecting a tool, the size of the tool should be adapted to the surface size of the workpiece being machined. In production, the plane profile is often processed by an end mill; when milling the plane, the carbide insert face milling cutter should be selected; when machining the blank surface or roughing the hole, the milling cutter with the carbide insert can be selected; For the machining of some three-dimensional and variable-angle contours, ball-end milling cutters, ring milling cutters, tapered milling cutters and trapezoidal milling cutters are often used. When performing surface machining, the ball cutter should be used, and the radius of the ball cutter should be smaller than the minimum radius of curvature of the surface. Since the cutting speed of the end of the ball head tool is zero, in order to ensure the machining accuracy, the cutting line spacing is generally very dense, and the flat head tool is superior to the ball end knife in terms of surface processing quality and cutting efficiency, so as long as the accuracy is ensured Under the premise, whether it is roughing or finishing of the surface, the flat knife should be preferred.

In CNC machining, since the sharpening, measurement and replacement of the tool are mostly manual and manual, the occupation time is long. Therefore, the arrangement order of the tools must be arranged reasonably. Generally, the following principles should be followed: 1 Minimize the number of tools; 2 After a tool is clamped, all the machining parts that can be carried out should be completed; 3 The rough-finished tools should be used separately, even for tools of the same size; 4 First face back hole; 5 first surface finishing, then 2D 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 the amount of CNC machining cutting

The principle of reasonable choice of cutting amount is that when roughing, it is generally based on improving productivity. In the case of semi-finishing and finishing, the cutting efficiency, economy and processing cost should be taken into account on the premise of ensuring the processing quality. The specific values ​​should be based on the machine tool performance, the cutting amount manual, and the experience. At the same time, the spindle speed, depth of cut and feed rate are adapted to each other to form the optimum cutting amount. For the selection of cutting amount, please refer to Table 1.

Table 1 Selection of cutting amount (high speed steel end mill)

(1) When the amount of back-feeding knife is allowed in the machine tool, the rigidity of the workpiece and the tool is allowed, the amount of back-feeding knife should be equal to the machining allowance as much as possible, which can reduce the number of passes and improve production efficiency. In order to ensure the machining accuracy and surface roughness of the parts, a small amount of finishing allowance should be left, generally 0.2 - 0.5mm.

(2) The cutting width L is generally proportional to the tool diameter d and inversely proportional to the depth of cut. In CNC machining, the range of general L is: L = (0.6- 0.9)d.

(3) Cutting speed Cutting speed is also a measure to improve productivity, but the cutting speed is closely related to tool durability. As the cutting speed increases, the tool durability drops sharply, so the choice of cutting speed depends mainly on the tool durability. In addition, the cutting speed is also very much related to the processing material. For example, when milling 45 steel with an end mill, the cutting speed can be about 26m/min: when milling the aluminum alloy with the same end mill, the cutting speed can be 129m/min. the above.

(4) Spindle speed n (r/min) The spindle speed is generally selected according to the cutting speed. The calculation formula is: n = 1000 / d, where d is the tool diameter (mm). The control panel of the CNC machine is generally equipped with a spindle speed trimming (magnification) switch, which can adjust the spindle speed during the machining process.

(5) Feed rate F The feed rate should be selected according to the machining accuracy and surface roughness requirements of the part and the tool and workpiece material.

The principle of determining the feed rate:

First, when the quality requirements of the workpiece can be guaranteed, in order to improve production efficiency, a higher feed rate can be selected. Usually selected in the range of 100 - 200mm / min.

Second, when cutting, machining deep holes or machining with high-speed steel tools, it is advisable to choose a lower feed rate, generally in the range of 20-50 mm/min.

Third, when the processing accuracy and surface roughness are high, the feed rate should be selected to be smaller, generally in the range of 20-5Omm/min.

In the CNC machining process, the feedrate can also be manually adjusted by the feedrate override switch on the machine control panel, but the maximum feedrate is limited by the stiffness of the machine and the performance of the feed system.

With the wide application of CNC machine tools in production practice, CNC programming has become one of the key issues in CNC machining. In the process of programming the NC program, it is necessary to select the tool and determine the amount of cutting in the human-computer interaction state. Therefore, the programmer must be familiar with the selection method of the tool and the principle of determining the amount of cutting, so as to ensure the processing quality and processing efficiency of the part, and give full play to the advantages of the CNC machine tool.

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Jiangsu Dingtian Stainless Steel Products Co., Ltd. , https://www.dingtiancasting.com