The selection of the tool 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. The development of CAD/CAM technology makes it possible to directly use CAD design data in CNC machining, especially the connection between microcomputer and CNC machine tool, so that the whole process of design, process planning and programming is completed on the computer, generally no output is required. Specialized process documentation.
Nowadays, many CAD/CAM software packages provide automatic programming functions. These softwares generally prompt the process planning related topics in the programming interface, such as tool selection, machining path planning, cutting usage setting, etc. The parameters can be automatically generated by the NC program and transferred to the CNC machine for processing.
Therefore, the tool selection and cutting amount determination in NC machining is completed under the human-computer interaction state, which is in sharp contrast with the ordinary machine tool processing. At the same time, the programmer must also grasp the basic principles of tool selection and cutting amount determination. Fully consider the characteristics of CNC machining when programming. In this paper, the tool selection and cutting quantity determination problems that must be faced in NC programming are discussed. Several principles and suggestions are given, and the topics that should be paid attention to are discussed.
1. Types and characteristics of commonly used tools for CNC machining
CNC machining tools must adapt to the high speed, high efficiency and high degree of automation of CNC machine tools. Generally, they should include general-purpose tools, universal connecting shanks and a small number of special shanks. The shank is to be connected to the tool and mounted on the power head of the machine, so it has been standardized and serialized. There are several ways to classify CNC tools.
According to the tool structure can be divided into:
(1) monolithic;
(2) Inlaid type, using welding or machine clamp type connection, the machine clamp type can be divided into two types: non-indexing and indexable;
(3) Special types, such as composite tools, shock-absorbing tools, etc.
According to the materials used to manufacture the tool, it can be divided into:
(1) high speed steel cutters;
(2) Carbide tools;
(3) diamond cutters;
(4) Other material tools, such as cubic boron nitride tools, ceramic tools, etc.
From the cutting process can be divided into:
(1) Turning tools, which are divided into outer circle, inner hole, thread, cutting tool, etc.;
(2) Drilling tools, including drill bits, 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, changeable, etc., in recent years, machine-clampable indexable tools have been widely used, reaching 30% to 40% of the entire CNC tool in number, 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 features:
(1) Good rigidity (especially rough machining tools), high precision, low vibration resistance and thermal deformation;
(2) Good interchangeability, easy to change the knife quickly;
(3) High life, stable and reliable cutting performance;
(4) The size of the tool is easy to adjust to reduce the tool change time;
(5) The tool should be able to reliably break the chip or the chip to facilitate the removal of the chip;
(6) Serialization, standardization, to facilitate programming and tool management.
2, the choice of CNC machining tools
The selection of the tool is carried out in the human-computer interaction state of the NC programming. The tool and tool holder should be selected correctly according to the machining capacity of the machine tool, the performance of the workpiece material, the machining process, the amount of cutting and other relevant factors. The general principle of tool selection is: easy installation and adjustment, good rigidity, durability and high precision. Under the conditions that meet 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 being machined. In the production, the peripheral contour of the plane parts is often processed by an end mill; when milling the plane, the cemented carbide insert milling cutter should be selected; when the bosses and grooves are machined, the high-speed steel end mill is selected; the surface of the blank is machined or roughed. For the hole, the corn milling cutter with the carbide insert can be selected; for the machining of some three-dimensional and variable bevel contours, ball-end milling cutters, ring milling cutters, conical milling cutters and disc milling cutters are often used.
In the free-form surface machining, since the cutting speed of the end of the ball-end tool is zero, in order to ensure the machining accuracy, the cutting line spacing is generally very dense, so the ball head is often used for the finishing of the surface. The flat-blade tool is superior to the ball-end knives in terms of surface quality and cutting efficiency. Therefore, the flat-blade knives should be preferred if the surface is roughed or finished under the conditions of no-cutting. In addition, the durability and accuracy of the tool are highly dependent on the price of the tool. It must be noted that in most cases, the choice of a good tool increases the cost of the tool, but the resulting improvement in processing quality and processing efficiency. , can make the entire processing cost greatly reduced.
In the machining center, various tools are mounted on the tool magazine, and the tool selection and tool change operations are performed at any time according to the program. Therefore, a standard tool holder must be used in order to quickly and correctly mount the standard tool for drilling, boring, expanding, milling, etc. on the machine tool spindle or tool magazine. The programmer should be aware of the structural dimensions, adjustment methods, and adjustment range of the tool holder used on the machine to determine the radial and axial dimensions of the tool during programming. At present, China's machining center adopts TSG tool system. Its tool holder has two kinds of handles: straight handle (three specifications) and taper shank (four specifications), including 16 different handles.
In the economical NC machining, since the sharpening, measuring and replacing of the tool are mostly manual and manual, the occupation time is long. Therefore, the arrangement order of the tools must be arranged fairly. Generally follow the following principles:
(1) Minimize the number of tools;
(2) After a tool is clamped, all the processing parts that can be carried out should be completed;
(3) The rough-finished tools should be used separately, even for tools of the same size;
(4) First milling and drilling;
(5) Perform surface finishing first, followed by 2D contour finishing.
Whenever possible, the automatic tool change function of CNC machine tools should be utilized as much as possible to improve production efficiency.
