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The choice of cutting tools and the determination of cutting amount

Advance CNC Machining

The choice of cutting tools and the determination of cutting amount in CNC machining

The choice of cutting tools 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. Development of CAD / CAM technology

The exhibition makes it possible to directly use CAD design data in CNC machining, especially the connection between the microcomputer and the CNC machine tool, so that the entire process of design, process planning and programming is completed on the computer, and there is generally no need to output special process files.

At present, many CAD / CAM software packages provide automatic programming functions. These software generally prompt related problems of process planning in the programming interface, such as tool selection, machining path planning, cutting amount setting, etc., as long as the programmer sets the relevant The parameters can be automatically generated NC program and transmitted to the CNC machine tool to complete the processing. Therefore, tool selection and cutting in CNC machining

The amount of cutting is determined in the state of human-machine interaction, which is in sharp contrast to the ordinary machine tool processing. It also requires programmers to master the basic principles of tool selection and cutting amount determination, and fully consider the characteristics of CNC machining when programming. This paper discusses the problems of tool selection and cutting amount determination that must be faced in CNC programming, gives some principles and suggestions, and discusses the issues that should be paid attention to.

1.The types and characteristics of commonly used CNC machining tools

CNC machining tools must be adapted to the characteristics of high speed, high efficiency and high degree of automation of CNC machine tools. Generally, they should include universal tools, universal connection tool holders and a small number of special tool holders. Knife handle to connect knife

The tool is installed on the power head of the machine tool, so it has been gradually standardized and serialized.

There are many ways to classify CNC tools. According to the cutter structure, it can be divided into:

  • ① Integral;
  • ②Mosaic type, using welding or machine clip connection, machine clip type can be divided into two types of non-indexable and indexable;
  • ③Special types, such as compound cutters, shock-absorbing cutters, etc.

According to the materials used to make tools, they can be divided into:

  • ① High-speed steel cutter;
  • ② Carbide cutting tools;
  • ③ Diamond cutter;
  • ④Tools of other materials, such as cubic boron nitride tools, ceramic tools, etc.

It can be divided into:

  • ①Turning tools are divided into outer circles, inner holes, threads, cutting tools, etc .;
  • ②Drilling tools, including drill bits, reamers, taps, etc .;
  • ③ Boring tool;
  • ④Milling tools, etc.

In order to meet the requirements of CNC machine tools for tool durability, stability, easy adjustment, and replacement, machine-clamped indexable tools have been widely used in recent years, and the number has reached 30% to 40% of the entire CNC tool. The volume accounts for 80% to 90% of the total.

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

  • (1) Good rigidity (especially rough machining tools), high precision, low vibration resistance and thermal deformation;
  • ⑵ Good interchangeability, convenient for quick tool change;
  • ⑶ Long life, stable and reliable cutting performance;
  • ⑷The size of the tool is easy to adjust to reduce the time of tool change adjustment;
  • ⑸ The tool should be able to break or curl chips reliably to facilitate the removal of chips;
  • ⑹Serialization and standardization to facilitate programming and tool management.

2.The choice of CNC machining tools

Tool selection is carried out in the state of human-computer interaction of NC programming. It should be based on the processing capacity of the machine tool, the performance of the workpiece material, the processing procedure, the cutting amount and other related factors.

Choose the tool and the handle. The general principle of tool selection is: easy installation and adjustment, good rigidity, high durability and 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 to be processed. In production, end mills are often used to process the contours of flat parts; when milling planes, carbide insert mills should be selected; when machining 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 varying bevel contour shapes, ball head milling cutters, ring milling cutters, cone milling cutters and disc milling cutters are often used.

In free-form surface machining, because 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 obtained very densely, so the ball head is often used for the finishing of the curved surface. The flat-end cutter is superior to the ball-end cutter in terms of surface processing quality and cutting efficiency. Therefore, as long as the surface is not cut, whether it is rough or precision machining of the curved surface, the flat-head cutter should be preferred. In addition, the durability and accuracy of the tool have a great relationship with the price of the tool. It must be noted that in most cases, although the choice of a good tool increases the cost of the tool, the resulting improvement in processing quality and processing efficiency , You can greatly reduce the entire processing cost.

On the machining center, various tools are installed in the tool magazine, and the tool selection and tool change operations can be performed at any time according to the program. Therefore, a standard tool holder must be used so that the standard tools for drilling, boring, expanding, milling and other processes can be quickly and accurately installed on the machine tool spindle or tool magazine. The programmer should understand the structure size, adjustment method and adjustment range of the tool holder used on the machine tool, so as to determine the radial and axial dimensions of the tool during programming. At present, the machining center in China adopts the TSG tool system. Its tool handles are of two types: straight shank (three specifications) and tapered shank (four specifications), which includes 16 different types of tool shanks.

In economical CNC machining, because 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 reasonably. Average

The following principles should be followed:

  • ① Try to reduce the number of tools;
  • ②After a tool is clamped, all the processing parts that can be carried out should be completed;
  • ③The tools for rough finishing should be used separately, even if they are the same size specifications;
  • ④Mill first and then drill;
  • ⑤ Finish the curved surface first, then finish the 2D contour finishing;
  • ⑥When possible, the automatic tool change function of the CNC machine tool should be used as much as possible to improve production efficiency.

3. Determination of cutting amount for CNC machining

The principle of reasonable choice of cutting amount is that when roughing, productivity is generally the main factor, but economic and processing costs should also be considered; when semi-finishing and finishing, the cutting efficiency should be taken into account while ensuring the quality of processing. Economical and processing costs. The specific value should be based on the machine tool manual, cutting dosage manual, and combined with experience.

  • (1) Cutting depth t. When the rigidity of the machine tool, workpiece and tool is allowed, t is equal to the machining allowance, which is an effective measure to improve productivity. In order to ensure the processing accuracy and Surface roughness should generally be left with a certain margin for finishing. The finishing allowance of CNC machine tools can be slightly smaller than ordinary machine tools.
  • ⑵ Cutting width L. Generally, L is directly proportional to the tool diameter d and inversely proportional to the cutting depth. In economical numerical control machining, the general value range of L is: L = (0.6 ～ 0.9) d.
  • (Cutting speed v). Increasing v is also a measure to increase productivity, but v has a close relationship with tool durability. With the increase of v, the durability of the tool drops sharply, so the choice of v mainly depends on the durability of the tool. In addition, the cutting speed is also closely related to the processing material. For example, when using an end mill to mill an alloy just 30CrNi2MoVA, v can be about 8m / min; when using the same end mill to mill aluminum alloy, v can be more than 200m / min .
  • ⑷ Spindle speed n / (r / min /). The spindle speed is generally selected according to the cutting speed v. The calculation formula is: where, d is the diameter of the tool or workpiece (mm). 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 whole multiples during the processing.
  • ⑸Feeding speed vF. vF should be selected according to the machining accuracy and surface roughness requirements of the parts and the tool and workpiece materials. The increase in vF can also improve production efficiency. When the processing surface roughness requirement is low, vF can be selected to be larger. During the machining process, vF can also be manually adjusted through the trim switch on the machine control panel, but the maximum feed rate is limited by the equipment stiffness and feed system performance. With the widespread application of CNC machine tools in production, CNC programming has become one of the key issues in CNC machining. In the process of CNC programming, it is necessary to select the tool and determine the cutting amount in real time in the state of human-computer interaction. Therefore, programmers must be familiar with the selection method of cutting tools and the principle of determining the amount of cutting, so as to ensure the processing quality and processing efficiency of parts, give full play to the advantages of CNC machine tools, and improve the economic efficiency and production level of enterprises
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