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Types and selection of commonly used tools for CNC machining

Posted by: Chris Na 2021-05-26 Comments Off on Types and selection 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.

2.1 There are many ways to classify CNC tools
A. According to the tool structure can be divided into
(1)Integral type;
(2) Mosaic type, welding or machine clamp type connection, machine clamp type can be divided into two types: non-indexable and indexable;
(3) Special types, such as compound cutters, shock-absorbing cutters, etc.
B. According to the materials used to manufacture the tool, it can be divided into:
(1) High-speed steel cutting tools;
(2) Cemented carbide tools;
(3) Diamond tools;
(4) Cutting tools of other materials, such as cubic boron nitride cutting tools, ceramic cutting tools, etc.
C. 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 tool;
(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, and the number has reached 30%-40% of the entire CNC tools. Metal removal The amount accounts for 80% to 90% of the total.
2.2 Compared with the tools used on ordinary machine tools, CNC tools have many different requirements, mainly as follows:
(1) Good rigidity (especially rough machining tools), high precision, vibration resistance and small thermal deformation; good interchangeability, easy to change tools quickly;
(2) High life, stable and reliable cutting performance;
(3) The size of the tool is easy to adjust to reduce the tool change adjustment time;
(4) The tool should be able to reliably break or roll chips to facilitate the removal of chips;
(5) Serialization and standardization to facilitate programming and tool management.
2. Selection of CNC machining tools
The tool selection is carried out under the human-computer interaction state of CNC programming. It should be based on the processing capacity of the machine tool, the performance of the workpiece material, and the
The cutting amount and other related factors in the work process should correctly select the tool and the tool holder. 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.
(1) When selecting a tool, the size of the tool should 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 flat parts; when milling planes, you should choose carbide blade milling cutters, when processing bosses and grooves, choose high-speed steel end mills; processing rough surfaces 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.
(2) When processing free-form surfaces (molds), since the end cutting speed of the ball-end tool is zero, in order to ensure the machining accuracy, the cutting line spacing generally adopts the top-end dense pitch, 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.
(3) On the machining center, various tools are installed in the tool magazine, and the tool selection and tool pressing 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 tapered shanks (4 specifications), including a total of 16 tool holders for different purposes.
(4) 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 carry out should be completed; the rough and finishing tools should be used separately, even if they are tools of the same size; ④First Drilling after milling; ⑤ Surface finishing first, and 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.
3. 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 cutting parameters manual of the machine tool manual and combined with experience.
The following factors should be considered specifically:
(1) Depth of cut ap. When the rigidity of the machine tool, workpiece and tool permits, 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. During the processing of economical CNC machine tools, the general value range of L is L=(0.6~0.9)d.
(2) 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.
(3) 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, programmers 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 parts, give full play to the advantages of CNC machine tools, and improve the economic efficiency and production level of the enterprise.