CNC machining technology (1)

Numerical control machining (numerical control machining) refers to a process method for processing parts on a CNC machine tool. The process regulations of CNC machine tool processing and traditional machine tool processing are generally consistent, but significant changes have also taken place. A machining method that uses digital information to control the displacement of parts and tools. It is an effective way to solve the problems of variable parts, small batches, complex shapes, and high precision, and to achieve high-efficiency and automated processing.
Interpretation:
Numerical control processing refers to the processing in which the control system issues instructions to make the tool perform various movements that meet the requirements, and expresses the shape and size of the workpiece in the form of numbers and letters, as well as processing requirements. It generally refers to the process of processing parts on CNC machine tools.
A CNC machine tool is a machine tool controlled by a computer. The computer used to control the machine tool, whether it is a special computer or a general-purpose computer, is collectively called a CNC system. The movement and auxiliary actions of the CNC machine tool are controlled by the instructions issued by the CNC system. The instructions of the numerical control system are compiled by the programmer according to the material of the workpiece, the processing requirements, the characteristics of the machine tool, and the instruction format (numerical control language or symbols) prescribed by the system. The numerical control system sends out operation or termination information to the servo device and other functional components according to the program instructions to control various movements of the machine tool. When the part processing program ends, the machine tool will automatically stop. For any kind of CNC machine tool, if there is no program command input in its CNC system, the CNC machine tool cannot work. The controlled actions of the machine tool roughly include the start and stop of the machine tool; the start and stop of the spindle, the conversion of the rotation direction and the speed; the direction, speed, and mode of the feed movement; the selection of the tool, the compensation of the length and the radius; the replacement of the tool, and the cooling The opening and closing of the liquid.
Development Background:
Numerical control technology originated from the needs of the aviation industry and was proposed by a US helicopter company in the late 1940s.
The initial idea of CNC machine tools, the Massachusetts Institute of Technology developed a three-axis CNC milling machine in 1952. This kind of CNC milling machine has been used for processing aircraft parts in the mid-1950s. In the 1960s, numerical control systems and programming work became increasingly mature and perfect. CNC machine tools have been used in various industrial sectors, but the aerospace industry has always been the largest user of CNC machine tools. Some large aviation factories are equipped with hundreds of CNC machine tools, of which cutting machines are the main ones. CNC machining parts include integral wall panels, beams, skins, bulkheads, propellers, and aircraft engine casings, shafts, disks, blades, and special cavity surfaces of liquid rocket engine combustion chambers. . The initial stage of the development of CNC machine tools is based on continuous trajectory CNC machine tools, continuous trajectory control.
Continuous trajectory control is also called contour control, which requires the tool to move on a prescribed trajectory relative to the part. Later, we will vigorously develop point-control CNC machine tools. Point control means that the tool moves from one point to another, as long as it can reach the target accurately at the end, regardless of the moving route.
Operation process:
NC programming
There are manual (manual) programming and automatic programming methods for NC machining programming. Manual programming, the entire content of the program is manually written in accordance with the instruction format specified by the CNC system. Automatic programming is computer programming, which can be divided into automatic programming methods based on language and drawing. However, no matter what kind of automatic programming method is adopted, corresponding hardware and software are required.
It can be seen that the realization of CNC machining programming is the key. But programming alone is not enough. CNC machining also includes a series of preparatory work that must be done before programming and the aftermath of programming. Generally speaking, the main contents of CNC machining process are as follows:
⑴ Select and confirm the parts and contents for CNC machining;
⑵ Process analysis of CNC machining of parts drawings;
⑶ Process design of CNC machining;
⑷ Mathematics processing of parts drawings;
⑸ Compile the processing procedure list;
⑹ Make the control medium according to the procedure list;
⑺ Check and modify the program;
⑻ Trial processing of the first piece and on-site problem handling;
⑼The finalization and filing of CNC machining process documents.
In order to improve the degree of production automation, shorten the programming time and reduce the cost of CNC machining, a series of advanced CNC machining technologies have been developed and used in the aerospace industry. For example, computer numerical control, that is, a small or microcomputer is used to replace the controller in the numerical control system, and the software stored in the computer is used to perform calculation and control functions. This soft-connected numerical control system is gradually replacing the initial state of the numerical control system. Direct numerical control uses one computer to directly control multiple numerical control machine tools, which is very suitable for small batch and short cycle production of aircraft. The ideal control system is an adaptive control system that can continuously change processing parameters. Although the system itself is complex and expensive, it can improve processing efficiency and quality. In addition to the improvement of CNC systems and machine tools in terms of hardware, the development of CNC has another important aspect that is the development of software. Computer-aided programming (also called automatic programming) means that after a programmer writes a program in numerical control language, it is input into the computer for translation, and finally the computer automatically outputs punched tape or tape. The most widely used CNC language is the APT language. It is roughly divided into main processing program and post processing program. The former translates the program written by the programmer to calculate the tool path; the latter compiles the tool path into the part processing program of the CNC machine tool. Numerical control processing is to write programs on the computer before processing the workpiece, and then input these programs into the machine tool controlled by the computer program for instructional processing, or write instructions directly on the machine tool control panel controlled by the computer program For processing. The machining process includes: tool moving, tool change, speed change, direction change, parking, etc., all of which are completed automatically. CNC machining is an advanced method of modern mold manufacturing and processing. Of course, CNC machining methods must not only be used for mold parts processing, but also have a wide range of uses.
Process analysis
The technical issues of CNC machining of the processed parts involve a wide range of aspects. The following combines the possibility and convenience of programming to put forward some main contents that must be analyzed and reviewed.
1. The dimensioning should conform to the characteristics of CNC machining
In CNC programming, the size and position of all points, lines, and surfaces are based on the programming origin. Therefore, it is best to give the coordinate size directly on the part drawing, or try to quote the size with the same datum.
2. The conditions of geometric elements should be complete and accurate
In the programming, the programmer must fully grasp the geometric element parameters that constitute the contour of the part and the relationship between the geometric elements. Because all geometric elements of the contour of the part must be defined during automatic programming, the coordinates of each node must be calculated during manual programming. No matter which point is unclear or uncertain, programming cannot be carried out. However, due to inadequate consideration or neglect by part designers in the design process, there are often incomplete or unclear parameters, such as arc and straight line, arc and arc whether they are tangent or intersected or separated. Therefore, when reviewing and analyzing the drawings, you must be careful and contact the designer in time if you find any problems.
3. Reliable positioning reference
In CNC machining, the machining processes are often concentrated, and it is very important to locate them on the same basis. Therefore, it is often necessary to set some auxiliary datums, or add some process bosses on the blank.
4. Uniform geometry type or size
It is better to adopt a uniform geometric type or size for the shape and inner cavity of the part, so that the number of tool changes can be reduced, and it is also possible to apply a control program or a special program to shorten the length of the program. The shape of the parts is as symmetrical as possible, which is convenient for programming with the mirror machining function of the CNC machine tool to save programming time.
Part clamping
1. The basic principles of positioning and installation
When machining parts on a CNC machine tool, the basic principle of positioning and installation is to choose a reasonable positioning datum and clamping plan. Pay attention to the following points when choosing:
1. Strive to unify the benchmarks of design, process and programming calculations.
2. Minimize the number of clamping times, and process all the surfaces to be processed after positioning and clamping once as much as possible.
3. Avoid the use of machine-occupied manual adjustment processing schemes to give full play to the effectiveness of CNC machine tools.
Second, the basic principles of choosing fixtures
The characteristics of CNC machining put forward two basic requirements for the fixture: one is to ensure that the coordinate direction of the fixture is relatively fixed with the coordinate direction of the machine tool; the other is to coordinate the size relationship between the parts and the machine coordinate system. In addition, the following points should be considered:
1. When the batch of parts is not large, modular fixtures, adjustable fixtures and other general fixtures should be used as much as possible to shorten production preparation time and save production costs.
2. Only consider the use of special fixtures during mass production, and strive to have a simple structure.
3. The loading and unloading of parts should be fast, convenient and reliable to shorten the machine stop time.
4. The parts on the fixture should not hinder the machining of the surface of the parts by the machine tool, that is, the fixture should be open, and its positioning and clamping mechanism components should not affect the knife during processing (such as collisions, etc.).
Machining error
Numerical control machining error △ addition is formed by programming error △ editing, machine tool error △ machine, positioning error △ fixed, tool setting error △ tool and other errors.
Namely: △number plus=f (△editing+△machine+△fixed+△tool)
among them:
1. Programming error △ is composed of approximation error δ and rounding error. The approximation error δ is produced in the process of approximating a non-circular curve with a straight line segment or a circular arc segment, as shown in Figure 1.43. The rounding error is the error produced by rounding the coordinate value to an integer pulse equivalent value during data processing. Pulse equivalent refers to the displacement of each unit pulse corresponding to the coordinate axis. Normal-precision CNC machine tools generally have a pulse equivalent value of 0.01mm; more precise CNC machine tools have a pulse equivalent value of 0.005mm or 0.001mm, etc.
2. The error of the machine tool is caused by the error of the CNC system and the feed system.
3. The positioning error △ is always caused when the workpiece is positioned on the fixture and the fixture is positioned on the machine tool.
4. Tool setting error △ tool is generated when determining the relative position of the tool and the workpiece.
main feature:
CNC machine tools select aircraft parts with complex profiles as the processing objects from the very beginning, which is the key to solving the difficulty of ordinary processing methods. The biggest feature of CNC machining is the use of punched tape (or tape) to control the machine tool for automatic processing. Because airplanes, rockets, and engine parts have different characteristics: airplanes and rockets have zero parts, large component sizes, and complex shapes; engine zero, small component sizes, and high precision. Therefore, the CNC machine tools selected by the aircraft and rocket manufacturing departments and the engine manufacturing departments are different. In aircraft and rocket manufacturing, large-scale CNC milling machines with continuous control are mainly used, while in engine manufacturing, both continuous-control CNC machine tools and point-control CNC machine tools (such as CNC drilling machines, CNC boring machines, machining centers, etc.) are used. .
Process concentration
CNC machine tools generally have tool holders and tool magazines that can automatically change tools. The tool change process is automatically carried out under program control. Therefore, the process is relatively concentrated. Process concentration brings huge economic benefits:
⑴ Reduce the floor space of the machine tool and save the workshop.
⑵Reduce or no intermediate links (such as intermediate inspection of semi-finished products, temporary storage and handling, etc.), which saves time and manpower.
automation
No need to manually control the cutting tools during CNC machine processing, and the degree of automation is high. The benefits are obvious.
⑴ Reduced requirements for operating workers:
A senior worker of an ordinary machine tool cannot be trained in a short time, and a CNC worker who does not need programming has a very short training time (for example, a CNC lathe worker needs one week, and he will write simple processing programs). In addition, the parts processed by CNC workers on CNC machine tools have higher precision than those processed by ordinary workers on traditional machine tools, and it takes less time. ⑵Reduce the labor intensity of workers: CNC workers are excluded from the processing most of the time during the processing, which is very labor-saving.
⑶ Stable product quality: The processing automation of CNC machine tools eliminates the fatigue, carelessness, estimation and other human errors of workers on ordinary machine tools, and improves the consistency of products.
⑷High processing efficiency: The automatic tool change of the CNC machine tool makes the processing process compact and improves labor productivity.
High flexibility
Although the traditional general-purpose machine tool is flexible, it is inefficient; while the traditional special machine is highly efficient, but has poor adaptability to parts, high rigidity and poor flexibility, and it is difficult to adapt to the fierce competition in the market economy. Products are frequently remodeled. As long as the program is changed, new parts can be processed on the CNC machine tool, and it can be automated, with good flexibility and high efficiency. Therefore, the CNC machine tool can well adapt to market competition.
strong ability
The machine tool can accurately process various contours, and some contours cannot be machined on ordinary machine tools. CNC machine tools are especially suitable for the following occasions:
1. Parts that are not allowed to be scrapped.
2. Development of new products.
3. Processing of urgently needed parts.