CNC machine tools are a highly practical course. The CNC machine tool experiment class has a very good auxiliary role for students' learning. Through the experiment students can not only deepen the understanding of classroom knowledge, but also help students find out in the course study. Knowing the problem, it can improve the ability to do it. In many years of teaching practice, the author realized that it is necessary to lead students to do numerical control programming experiments. It is necessary for the experimental teachers to give key explanations to the students in the following questions, so that students can have a comprehensive understanding of NC programming through experiments and master basic manual programming. method.
1 machine coordinate system and programming coordinate system in CNC machine tools, machine tool linear motion coordinate axes X, Y, Z according to ISO and China's JB3051! The command standard of 82 CNC machine tool coordinates and motion direction is defined as a right-handed Cartesian coordinate system. The tool movement coordinate parallel to the main shaft (transfer cutting power) is defined as the Z axis (indicated by the middle finger), and the direction in which the tool is away from the workpiece is the positive direction. When the machine has several spindles, select a spindle perpendicular to the mounting face of the workpiece as the Z axis. The X-axis is horizontal and perpendicular to the Z-axis and parallel to the mounting surface of the workpiece (indicated by the thumb). The direction in which the tool is away from the workpiece is the positive X-axis. The Y-axis coordinate axis is perpendicular to the X-axis and the Z-coordinate. When + X, + Z are determined, press the right hand rule to determine the Y axis and direction. Due to the relative movement of the workpiece and the tool during machining of the CNC machine, for the convenience of programming, it is assumed that the workpiece is fixed, and all are programmed with the coordinate system of the tool movement. The coordinate system is called the programming coordinate system or the workpiece coordinate system. In the numerical control machine tool experiment, the NIM-9702 CNC lathe is adopted by the Japanese FANUC company standard. It is different from the JB3051-82 standard in China in determining the programming origin. In order to prevent confusion during student programming, in addition to emphasizing in the experimental instruction book This must be emphasized again in the experiment. For example, the origin of the machine coordinate system is set at the O point of the root of the chuck, XOZ is the machine coordinate system, the Z axis is parallel to the machine tool spindle, the X axis is horizontal, and the positive direction is away from the workpiece direction. After starting the machine, the machine tool is determined by returning to the machine zero point A. Coordinate system XOZ. On the lathe, the origin of the coordinate system for turning parts is generally set on the end face of the workpiece. In the figure, X p O p Z p is the programmed coordinate system, which has a fixed conversion relationship with the machine coordinate system, where X = X p, Z = chuck thickness + workpiece elongation + Z p, its programming origin can not be determined by G92, it must be set with its special command zero offset G59 to determine the conversion between the programmed coordinate system and the machine coordinate system. relationship. The G92 command is used to set the tool change point when the programmed coordinate system has been established.
At the beginning of programming, when using absolute size programming, you must first establish a programming coordinate system to determine the programming origin. In the CNC milling machine experimental system, there are three ways to establish a programming coordinate system: (1) In the manual mode function, press X directly. (or Y, Z), then enter the coordinate value, indicating the coordinate value of the current tool in the workpiece coordinate system, thus establishing the workpiece coordinate system, which is a simple and effective method to establish the workpiece coordinate system.
(2) Set the workpiece coordinate system in the program using the G92 command. For example: G92 X10 Y10 indicates that the current tool position is X10 Y10 in the workpiece coordinate system, thus determining the programmed coordinate system. It is also independent of the machine coordinate system.
(3) Use the zero offset function to use G53, G54, G56, G57, G58, and G59 to indicate different workpiece origins. It is independent of the current position of the tool. The set workpiece origin is actually a position in the machine coordinate system. The machine tool of this method must make each axis find the machine reference point to establish the machine coordinate system after starting or resetting.
There are two methods in the NIM-9702 CNC system: one is to set the programming origin in the parameter function, and the other is to set in the first section of the machining program with the G59 command. Where X = 0, Z = chuck thickness + work outside elongation. In the G59 X0 Z100, the programming origin is at the center point of the right end face of the workpiece, and the elongation outside the workpiece is 80mm. When using absolute coordinate programming, the center point of the right end face is taken as the origin. Both methods have a certain conversion relationship with the machine coordinate system.
2 Using tool radius compensation programming In contour machining, since the tool always has a certain radius (such as a milling cutter), the tool center motion track is not equal to the actual path of the required machined part, and the tool center motion track or the part is used during programming. Actual trajectory For machine tools with tool radius compensation, you can directly program part contours, eliminating the hassle of calculating trajectories in the tool. AMC! 01 CNC milling machine has tool radius compensation function, it uses C function tool compensation, this method can effectively avoid tool interference, has a good sharp angle processing technology, and has high processing efficiency. In the actual contour machining process, the tool radius compensation execution process can generally be divided into three steps: tool compensation establishment, tool compensation, and tool compensation cancellation. Tool radius compensation is only performed in the specified two-dimensional coordinate plane, and the plane designation is indicated by G codes G17, G18, G19 (default is G17). The tool radius value is specified by the tool number.
Pay attention to the following points when programming: (1) The program segment G31, G42, G40 must have the G00 or G01 command.
(2) There shall be no transfer processing instructions such as mirroring, transfer, calling subroutine, etc. between the G41, G42 and G40 blocks.
C-clamping automatically inserts an arc at a sharp corner (3). In some cases, to avoid mis-cutting, an auxiliary or auxiliary straight section must be introduced. For example, to mill a square perimeter, use the right knife compensation program to add the tool compensation route: ABP 2 adds the tool compensation in the AB segment, and passes the P 1 point in the BP 2 segment to make the joint cutting, which ensures the processing quality and cancels the knife. The supplementary route is P 4 P 1 C, and the tool compensation is canceled in the P 1 C section. The procedure is as follows: N10 G92 X- 5 YO Z1 T1 N20 S1000 M03 F150 N30 G91 Z- 3 N40 G42 G01 X10 N50 X25
N70 X- 20 N80 Y- 20 N90 G40 G01 Y- 10 N100 Z3 N110 M02 3 Process division to get a part drawing should first guide students to carefully analyze how the part is processed, do not rush to programming, carefully analyze the parts selected process. The process can be concentrated. In a single loading, all the processes should be completed as much as possible. The general principle of the process division is as follows: (1) According to the tool division process, the number of tool changes is reduced, the idle time is compressed, and the unnecessary time is reduced. Positioning error, the tool is concentrated in the process of machining parts.
(2) According to the roughing and finishing process, the roughing and the finishing are carried out as much as possible. If the roughing parts are deformed, the finishing can not be arranged immediately.
(3) According to the principle of the first hole and the back hole, to improve the machining accuracy of the hole.
4 Machining allowance Machining allowance relates to the accuracy of parts and machine efficiency, which is also a crucial issue. When starting programming, you should first determine the machining allowance for each process. The machining allowance of CNC machine tools is equal to the sum of each intermediate machining allowance. The selection should be based on the following conditions: (1) Try to use the smallest total machining allowance to minimize the machining time and reduce the machining cost of parts.
(2) There should be sufficient machining allowance, especially for finishing, and the machining allowance should be able to meet the drawing requirements.
(3) The deformation caused by heat treatment of the parts should be taken into consideration.
(4) The machining allowance is determined by the size of the machined part.
(5) According to the equipment to determine the machining allowance, the teaching equipment requires a small machining allowance.
The students have a deep understanding of the above problems through the experiments of machining plane parts on the milling machine and turning the shaft parts on the lathe. Basically, they can be independently programmed and processed. The processed parts basically meet the accuracy requirements. Students mastered the basic programming methods through experiments.
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