With the development of science and technology, people have higher and higher requirements for the processing quality of parts. At the same time, products are frequently modified, and in general machining, the proportion of single products and small and medium-sized batch products is increasing. In order to ensure product quality, improve productivity and reduce costs, it is required that machine tools not only have good versatility and flexibility, but also have a high degree of automation in the machining process. NC machining technology is an efficient and flexible automatic machining technology developed in this environment, which is controlled by the digital information of NC machine tools and is suitable for single piece and small and medium batch production with high precision and complex parts shape.
CNC machine tool is an efficient and flexible mechatronics equipment that integrates computer technology, modern control technology, sensor detection technology, information processing technology, network communication technology, hydraulic pneumatic technology, optical electromechanical technology and so on, and is the basis of modern manufacturing technology.
1 composition and NC machining process of NC machine tool
NC machine tool refers to a machine tool that uses digital information recorded on the media to control the machine tool through a NC device, so that it can automatically perform the specified machining process.
1) Composition of CNC machine tools
CNC machine tools automatically process workpieces according to pre-programmed processing programs. CNC machine tools are usually composed of machine tool body, servo system, CNC device and control medium, as shown in Figure 2-61.
Figure 2-61 Composition of CNC machine tools
(1) Control medium. The control medium, also known as the information carrier, is an intermediate medium that conveys people's operation intention to CNC machine tools, and it contains all the information necessary for machining a part. Commonly used control media include punched tape, magnetic tape, magnetic disk and other carriers that can store instruction information. For a short NC machining program, you can directly input the NC device through the keyboard on the NC operation panel.
(2) numerical control device. The numerical control device is the center of the numerical control machine tool, which receives the information sent by the control medium, transforms it and processes it, and then converts it into pulse signals to control the machine tool action.
(3) servo system. Servo system is a device that connects the moving parts of machine tool with numerical control system, including servo drive mechanism and movable parts of machine tool. It is the executive part of machine tool numerical control system, which converts the pulse signal sent by numerical control device into the movement of machine tool parts, so that the workbench or tool rest can be accurately positioned or make strict relative movement according to the expected trajectory, and the parts that meet the requirements of the drawings can be processed.
(4) machine tool body. Machine tool body is the mechanical part of CNC machine tools. Besides the general components such as main transmission device, feed transmission device, lathe bed, workbench and auxiliary parts, there are also special components, such as tool magazine for storing tools, automatic tool changer and rotary workbench.
2) CNC machine tool parts processing process
CNC machine tool parts processing process is that the operator first makes it into the manufacturing process according to the processing pattern, compiles the processing program on the basis of being familiar with the processing technology, and transforms people's will into information acceptable to CNC machine tools with the specified code and program format. Record the information on the control media (such as information input media such as punched tape, magnetic tape or disk) to make it an instruction to control the machine tool. After processing the input information, the numerical control device sends command pulses to the servo system of each coordinate axis of the machine tool, drives the corresponding moving parts of the machine tool, and controls other actions such as speed change, tool change, start and stop of the machine tool, so as to automatically process the workpiece meeting the requirements of the pattern.
2 classification of CNC machine tools
CNC machine tools can be classified according to process use, motion trajectory, control mode, etc. Only the classification according to process use is introduced here. CNC machine tools are classified according to their technological uses, that is, processing characteristics or main processing procedures, mainly including CNC lathes (including turning centers), CNC milling machines (including milling centers), CNC boring machines, CNC grinding machines (including grinding centers), CNC drilling machines (including drilling centers), CNC broaching machines, CNC planers, CNC cutting machines, CNC gear machining machines and CNC electric spark machining machines (including electric machining centers, etc.) Figure 2-62 shows the appearance of CK5 CNC lathe, and its processing range is the same as that of ordinary lathe, mainly processing rotary parts such as shafts and disks. Figure 2-63 shows the appearance of XJK125 CNC milling machine, which is especially suitable for processing complex parts, and can finish milling, drilling and boring.
figure 2-62 CK5 appearance of CNC lathe
figure 2-63 XJK125 appearance of CNC milling machine
machining center (MC) is a CNC machine tool for multi-process machining of workpieces. It has a tool magazine and an automatic tool changer (ATC). After the workpiece is clamped once, the machining center can automatically select and replace tools according to different processes under the control of the numerical control system; Automatically change the spindle speed, feed rate, tool trajectory relative to the workpiece and other auxiliary functions; Finish the machining of multiple processes on several surfaces of the workpiece in turn. Therefore, it reduces the time of workpiece clamping, measuring and machine tool adjustment, shortens the time of workpiece storage and handling, and improves the productivity and machine tool utilization, which is an important development direction of CNC machine tools. Figure 2-64 shows the layout of JCS-18 machining center. Its shape is similar to that of a vertical milling machine. The lathe bed 1 is provided with a sliding seat 2 for transverse movement (Y-axis direction). The workbench 3 moves longitudinally (X-axis direction) on the slide seat. A frame column 5 is arranged at the back of the lathe bed. The headstock 9 moves vertically (Z-axis direction) on the column guide rail. The left and right parts of the column are numerical control devices 6, the left front part is equipped with a tool magazine 7 and an automatic tool changing manipulator 8, and the lower left part is equipped with a lubricating device 4. There are 16 tools in the tool magazine, which can complete all kinds of hole processing and milling. The numerical control operation panel 1 is hung in front of the operator's right for easy operation. The working states of the machine tool are displayed on the panel. Machining centers are usually divided into horizontal, vertical and universal machining centers according to the spatial position of the spindle during machining.
figure 2-64 layout 2-64 JCS-18 machining center
1-lathe bed; 2-slide seat; 3-workbench; 4-Lubrication device; 5— upright post; 6-numerical control device; 7-tool magazine; 8— Tool changing manipulator; 9— Spindle box; 1— Programming of operation panel
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The so-called programming is to compile the technological process, technological parameters, tool movement amount and direction of parts and other auxiliary actions (tool changing, cooling, clamping, etc.) into a certain table according to the movement order and the instruction code and program format given by the CNC machine tool used. This table is called "Parts Processing Program Sheet" or "Program Sheet" for short, and then, This whole process from analyzing the drawings of parts to making the control media needed by CNC machine tools is called programming.
generally speaking, the steps of NC programming are: process design → numerical calculation → preparation of parts processing program sheet → preparation of control medium or program input → program verification and trial cutting.
1) process design
on the basis of a comprehensive analysis of the part drawing, determine the process parameters such as the clamping and positioning method, processing route (such as tool setting point, tool changing point and feed route), cutter and cutting parameters (such as feed speed, spindle speed, cutting width and cutting depth, etc.).
2) Numerical calculation
According to the part drawing and the determined machining route, the tool trajectory is calculated.
general numerical control devices have the functions of linear interpolation and circular interpolation. For machining simple parts composed of arcs and straight lines, it is only necessary to calculate the coordinate values of the intersection or tangent point (base point) of adjacent geometric elements on the outline of the part, and get the coordinate values of the starting point and end point of the straight line, the starting point and end point of the arc and the center of the circle. When the shape of the part is complex and inconsistent with the interpolation function of the numerical control device, it is necessary to make more complicated calculations.
3) Write a part processing program sheet
Write a part processing program sheet according to the calculated coordinate values of the tool trajectory, the determined cutting parameters and auxiliary actions, combined with the instruction codes and program segment formats specified by the CNC system.
4) preparation of control medium or program input
after the program list is written, the operator or programmer must input the machining information into the numerical control device, or move the program to a certain control medium according to the different input and output devices of the numerical control system. Commonly used control media include USB flash drive, magnetic disk, magnetic tape, etc.
5) program verification and trial cutting
The compiled program can only be used formally after verification and trial cutting. The verification method is to directly input the contents of the control medium into the numerical control device to check whether the tool trajectory is correct. On the CNC machine tool with CRT graphic display screen, it can be verified by simulating the cutting process of the workpiece. Otherwise, the pen can replace the knife, and the workpiece can be replaced by graph paper, so that the machine tool can run idle and draw the machining trajectory.
these methods can only check whether the tool trajectory is correct, but can't check the machining accuracy. Therefore, the trial cutting of parts should also be carried out. If it is found that the accuracy of the parts can not meet the requirements through trial cutting, the program sheet and control medium should be modified and the error compensation method should be adopted until the qualified parts are processed.
4 features and applications of CNC machine tools
1) features of CNC machine tools
(1) high machining accuracy and stable machining quality. Because of the high manufacturing precision of the CNC machine tool itself and the automatic processing according to the predetermined procedure, the human operation error is avoided, so that the consistency of the same batch of parts is good and the product quality is stable.
(2) high productivity. Because many parts of the parts can be processed in one clamping, many intermediate processes (such as marking, etc.) are omitted, and generally only the first piece inspection is needed, which greatly shortens the production preparation time, so the productivity is high.
(3) The degree of automation is high, which reduces the labor intensity and improves the working conditions. Except for manual clamping of blanks, all machining processes are automatically completed by machine tools, which reduces the labor intensity of operators and improves working conditions.
(4) Strong adaptability and good economic benefits. When machining with CNC machine tools, when the machining object changes, it is only necessary to re-program the CNC program, and generally it is not necessary to redesign tools, fixtures and molds to realize the machining of parts. This greatly shortens the product development cycle and provides a shortcut for new product development. At the same time, the machining accuracy is high, the quality is stable, the rejection rate is reduced, the production cost is reduced, and the productivity is high, so good economic benefits can be obtained.
(5) It is beneficial to the modernization of production management. Using CNC machine tools to process can accurately calculate the processing hours of parts, effectively simplify the inspection and the management of fixtures and semi-finished products, which is conducive to the modernization of production management. Because of the use of digital information, it is easy to form an integrated system that combines computer-aided design and manufacturing closely.
However, CNC machine tools have high cost, complex technology and difficult maintenance, which requires high quality management and operation personnel.
2) Application of CNC machine tools
CNC machine tools are widely used, especially suitable for processing parts with the following characteristics: multi-variety and small batch production parts; Parts with complex structure and high precision requirements; Processing frequently modified parts, because CNC machine tools can save a lot of tooling costs, so that the overall cost is reduced; Key parts that are expensive and are not allowed to be scrapped; Urgent parts requiring the shortest production cycle.