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The preparation process of the CNC program, the selection method of the tool and the cutting amount

Posted by: Chris Na 2021-05-26 Comments Off on The preparation process of the CNC program, the selection method of the tool and the cutting amount

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 the DNC system microcomputer and the CNC machine tool, so that the entire process of design, process planning and programming is completed on the computer, generally not Need to output a special process file.

At present, 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 holder selection. For tool selection, machining path planning, cutting amount setting, etc., programmers can automatically generate NC programs and transfer them to CNC machine tools to complete processing as long as they set relevant parameters.

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. When programming, fully consider the characteristics of CNC machining, and be able to correctly select the cutting tools and cutting parameters.

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:

Inlaid type, using welding or machine clamp type connection, machine clamp type can be divided into two types: non-indexable and indexable;
Special types, such as compound cutters, shock-absorbing cutters, etc.

According to the materials used to manufacture the tool, it can be divided into:

High-speed steel tools;
Tungsten carbide tools;
Diamond tool
Tools of other materials, such as cubic boron nitride tools, ceramic tools, etc.

From the cutting process, it can be divided into:

Turning tools, including outer circle, inner hole, thread, cutting tool, etc.;
Drilling tools, including drills, reamers, taps, etc.;
Boring tool;
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:

Good rigidity (especially rough machining tools), high precision, vibration resistance and small thermal deformation; good interchangeability, easy to change tools quickly; high life, stable cutting performance, reliable tool size is easy to adjust, to reduce the time of tool change adjustment ; The 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.

Selection 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: convenient installation and adjustment, good rigidity, high durability and high 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.

In the free-form surface (mold) processing, since the end cutting speed of the ball-end tool is zero, to ensure the machining accuracy, the cutting line spacing generally adopts the top-end close pitch, so the ball end is often used for the finishing of the curved surface. 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 of straight shanks (3 specifications) and taper shanks (4 specifications), including a total of 16 tool holders for different purposes.

In the machining process of economical CNC machine tools, 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 the processing steps that it can perform 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.

Determination of cutting parameters during processing

The principle of reasonable selection of cutting parameters is: during rough machining, productivity is generally improved, but economy and processing costs should also be considered; during semi-finishing and finishing, cutting efficiency and cutting efficiency should be taken into account 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. Specifically, the following factors should be considered:

Cutting depth ap. When the rigidity 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 finishing. The finishing 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 depth of cut. In the processing of economical CNC machine tools, the general value range of L is: L=(0.6~0.9)d.

Cutting speed V. Increasing V is also a measure to improve productivity, but v has a close relationship with tool durability. With the increase of v, the tool durability declines sharply, so the choice of v mainly depends on the tool durability. In addition, the cutting speed is also closely related to the processing material. For example, when milling alloy steel 30CrNi2MoVA with an end mill, v can be about 8m/min; when milling aluminum alloy with the same end mill, V can be more than 200m/min. .

Spindle speed n (r/min). The spindle speed is generally selected according to the cutting speed v. The calculation formula is: V=pnd/1000. The control panel of the CNC machine tool is generally equipped with a spindle speed adjustment (magnification) switch, which can adjust the spindle speed in integer multiples during the machining process.

Feed speed Vf. vF should be selected according to the machining accuracy and surface roughness requirements of the parts, as well as the tool and workpiece material. The increase in Vf can also improve production efficiency. When the roughness of the processed surface is low, Vf can be selected to be larger. During processing, Vf can also be adjusted manually through the trim switch on the machine control panel, but the maximum feed rate is limited by the rigidity of the equipment and the performance of the feed system.

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