The selection of tools and the determination of cutting amount are important contents in 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 microcomputers and CNC machine tools. The entire process of design, process planning and programming is completed on the computer, and generally no special process files need to be output.
owadays, many CAD/CAM software packages provide automatic programming functions. These software generally prompt relevant issues of process planning in the programming interface. For example, tool selection, machining path planning, cutting amount setting, etc., as long as the programmer sets the relevant parameters, the NC program can be automatically generated and transmitted to the CNC machine tool to complete the processing. Therefore, the tool selection and cutting amount determination in CNC machining are completed in the state of human-computer interaction, which is in sharp contrast to ordinary machine tool machining. At the same time, 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 article explores the issues of tool selection and cutting amount determination that must be faced in CNC programming, gives several principles and suggestions, and discusses the issues that should be paid attention to.
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 tools, general connection tool holders and a small number of special tool holders. The tool holder should be connected to the tool and installed 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:
- Integral type.
- Inlaid type, connected by welding or machine clamping, the machine clamping type can be divided into non-indexable and indexable types.
- Special types, such as composite tools, shock-absorbing tools, etc.
According to the materials used to manufacture the tools, it can be divided into:
- High-speed steel tools.
- Carbide tools.
- Diamond tools.
- Other material tools, such as cubic boron nitride tools, ceramic tools, etc.
From the cutting process, it can be divided into:
- Turning tools, divided into external circles, internal holes, threads, cutting tools, etc.
- Drilling tools, including drill bits, reamers, taps, etc.
- Boring tools.
- Milling tools, etc.
In order to meet the requirements of CNC machine tools for durable, stable, easy to adjust and replaceable tools, machine-clamped indexable tools have been widely used in recent years, accounting for 30% to 40% of the total CNC tools, and the amount of metal removal 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:
- Good rigidity (especially roughing tools), high precision, low vibration resistance and thermal deformation.
- Good interchangeability, easy to change tools quickly.
- Long life, stable and reliable cutting performance.
- The size of the tool is easy to adjust to reduce the tool change adjustment time.
- The tool should be able to reliably break or roll chips to facilitate chip removal.
- Serialization and standardization to facilitate programming and tool management.
Selection of CNC Machining Tools
The selection of tools is carried out under the human-machine interaction state of CNC programming. The tools and tool holders should be correctly selected according to the processing capacity of the machine tool, the performance of the workpiece material, the processing procedures, 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 tools, the size of the tool should be adapted to the surface size of the workpiece being processed. In production, end mills are often used for processing the peripheral contours of planar parts. When milling planes, carbide blade milling cutters should be selected. When processing bosses and grooves, high-speed steel end mills should be selected. When processing blank surfaces or rough machining holes, corn milling cutters with carbide blades can be selected. For the processing of some three-dimensional surfaces and variable bevel contour shapes, ball head milling cutters, annular milling cutters, conical milling cutters and disc milling cutters are often used.
When processing free-form surfaces, the cutting speed of the end of the ball head tool is zero. Therefore, in order to ensure machining accuracy, the cutting line spacing is generally very close, so the ball head is often used for finishing of curved surfaces. Flat-head tools are superior to ball-head tools in terms of surface machining quality and cutting efficiency. Therefore, as long as overcutting is guaranteed, flat-head tools should be preferred for both rough machining and finishing of curved surfaces. In addition, the durability and accuracy of the tool are closely related to the price of the tool. It must be noted that in most cases, although the selection of good tools increases the tool cost. However, the improvement in machining quality and machining efficiency brought about by this can greatly reduce the overall machining cost.
In the machining center, various tools are installed on the tool magazine respectively, and tool selection and tool change actions are performed at any time according to the program. Therefore, standard tool holders must be used so that standard tools used for drilling, boring, expansion, milling and other processes can be quickly and accurately installed on the machine tool spindle or tool magazine. Programmers should understand the structural dimensions, adjustment methods and adjustment range of the tool holders 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 my country adopts TSG tool system, and its tool holders are straight (three specifications) and taper (four specifications), including 16 tool holders for different purposes.
In economical CNC machining, since the sharpening, measurement and replacement of tools are mostly done manually, which takes a long time, the arrangement order of tools must be arranged reasonably. Generally, the following principles should be followed:
- Minimize the number of tools.
- After a tool is clamped, it should complete all the processing parts it can perform.
- Tools for rough and fine machining should be used separately, even if they are tools of the same size.
- Mill first and then drill.
- Perform surface finishing first, then two-dimensional contour finishing.
- Where possible, the automatic tool change function of CNC machine tools should be used as much as possible to improve production efficiency, etc.
The Exact Amount of Cutting in CNC Machining
The principle of rationally selecting cutting parameters is that, in rough machining, productivity is generally the main focus, but economy and machining costs should also be considered. In semi-finishing and finishing, cutting efficiency, economy and machining costs should be taken into account while ensuring machining quality. The specific value should be determined based on the machine tool manual, cutting parameter manual, and experience.
Cutting 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 parts, a certain allowance should generally be left for finishing. The finishing allowance of CNC machine tools can be slightly smaller than that of ordinary machine tools.
