The selection 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 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, so that the entire process of design, process planning and programming is completed on the computer, generally do not need output Special process documentation.
Nowadays, many CAD/CAM software packages provide automatic programming functions. These software usually prompt the relevant problems of the process planning in the programming interface, such as tool selection, machining path planning, cutting amount setting, etc., the programmer only needs to set relevant information. The parameters can automatically generate the NC program and transfer it to the CNC machine for processing. Therefore, the tool selection and cutting amount determination in the numerical control machining are completed under the human-machine interaction state, which is in stark contrast with the ordinary machine tool processing. At the same time, the programmer is required to master the basic principles of tool selection and cutting amount determination. Take full account of the characteristics of CNC machining when programming. This paper discusses the choices of tool selection and cutting amount that must be faced in numerical control programming, gives some principles and recommendations, and discusses issues that should be noted.
First, the types and characteristics of commonly used CNC machining tools
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, universal connection tool holders and a small number of special tool holders. The tool holder is attached to the tool and mounted on the power head of the machine, so it has been gradually standardized and serialized. There are several ways to classify CNC tools. According to the tool structure can be divided into: 1 integral type; 2 mosaic type, using welding or clip-type connection, machine folder type can be divided into non-inverted and indexable two; 3 special type, such as compound tool, minus Seismic knives and so on. According to the materials used to manufacture the tool can be divided into: 1 high-speed steel tools; 2 carbide tools; 3 diamond tools; 4 other material tools, such as cubic boron nitride tools, ceramic tools. From the cutting process can be divided into: 1 turning tool, extra round, inner hole, thread, cutting tools and other; 2 drilling tools, including drills, reamers, taps, etc.; 3 Boring tools; 4 milling tools, etc. . In order to meet the requirements of CNC machine tools for durable, stable, easy-to-adjust, interchangeable tools, in recent years, the machine-clamp indexable tools have been widely used, reaching 30% to 40% of the total number of CNC tools in the number of metal cuts. 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. The main features are as follows:
(1) Good rigidity (especially rough machining tools), high precision, low vibration and thermal deformation;
(2) Good interchangeability for quick tool change;
(3) high life, stable and reliable cutting performance;
(4) The size of the tool is easy to adjust to reduce the time for tool change adjustment;
(5) The tool should be able to reliably break chips or roll chips in order to facilitate the removal of chips;
(6) Serialization and standardization to facilitate programming and tool management.
Second, the choice of CNC machining tools
The selection of tools is performed under the human-machine interaction of CNC programming. The tool and shank should be selected correctly based on the machine's machining capacity, the performance of the workpiece material, the machining process, the cutting amount, and other related factors. Tool selection The general principle is: easy installation and adjustment, good rigidity, high durability and high precision. In 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 dimensions of the tool must be adapted to the surface dimensions of the workpiece being machined. In production, the peripheral contours of plane parts are often machined with end mills; when milling planes, hard alloy insert milling cutters are selected; when machining bosses and grooves, high speed steel end mills are selected; surface roughing or rough machining is performed. In the case of holes, corn milling inserts with carbide inserts can be selected. For the machining of some three-dimensional profiles and variable bevel profiles, ball-end cutters, ring cutters, taper cutters, and disc-type cutters are often used.
When free-form surfaces are machined, the cutting speed of the end of the ball-end tool is zero. Therefore, in order to ensure the machining accuracy, the cutting pitch is generally very close. Therefore, the ball head is often used for surface finishing. The flat-blade cutter is superior to the ball-end cutter in terms of surface machining quality and cutting efficiency. Therefore, as long as it is not over-cut, the flat-bladed cutter should be the first choice for both rough and finish machining of the surface. In addition, the tool's durability and precision are highly related to the tool's price. It must be noticed that, in most cases, selecting a good tool increases the tool cost, but the resulting increase in machining quality and machining efficiency. , you can greatly reduce the cost of the entire process.
At the machining center, various tools are installed on the tool magazine, and the tool selection and tool change actions are performed at any time according to the procedures. Therefore, a standard tool holder must be used so that the standard tools for drilling, boring, expanding, and milling operations can be quickly and accurately mounted on the spindle or tool magazine of the machine tool. The programmer should know the structural dimensions, adjustment methods, and adjustment range of the tool holder used on the machine tool to determine the radial and axial dimensions of the tool when programming. At present, China's machining center adopts the TSG tool system. The tool holder has two kinds of straight shanks (three sizes) and taper shanks (four sizes), including a total of 16 different types of tool holders.
In the economical numerical control processing, since the sharpening, measurement and replacement of the tool are mostly performed manually, and the occupation time for the tool is longer, therefore, the arrangement order of the tools must be reasonably arranged. Generally should follow the following principles: 1 Minimize the number of tools; 2 After a tool is clamped, it should complete all the machining parts it can perform; 3 The rough finish tool should be used separately, even the same size of the tool; 4 First, after milling; 5 first finish the surface finish, after the two-dimensional contour finishing; 6 where possible, should use the automatic tool change function of CNC machine tools to improve production efficiency.
Third, the determination of CNC machining cutting amount
The principle of reasonable choice of cutting amount is that during roughing, productivity is mainly increased, but economic efficiency and processing cost should also be taken into account; when semi-finishing and finishing, the cutting efficiency should be taken into account under the premise of guaranteeing processing quality. Economics and processing costs. The specific value should be based on the machine tool specification, cutting dosage manual, and combined with experience
2, Analysis of Process for Trapezoid Thread Machining on CNC Lathe
One, trapezoidal thread processing technology analysis
1. Calculation of trapezoidal thread size Codes for trapezoidal thread Codes for trapezoidal thread are denoted by the letters “Tr” and nominal diameter×pitch, all in mm. The left-hand thread must be filled with "LH" after the dimension specification, and the right-hand thread is not marked. For example, Tr36×6, Tr44×8LH, and the like.
The national standard stipulates that the tooth angle of the metric trapezoidal thread is 30°.
Trapezoidal thread profile shown in Figure (1), the basic calculation formula for each dimension is shown in Table 1-1.