CNC turning technology has been widely used in the machinery manufacturing industry. How to process workpieces efficiently, reasonably and qualitatively, each engineering and technical personnel engaged in the industry has more or less experience. The author has been engaged in numerical control teaching, training and processing for many years, and has accumulated certain experience and skills. Now take the GSK980T series machine tool produced by Guangzhou CNC Equipment Factory as an example to introduce several CNC turning machining techniques.
At present, we are exposed to textbooks and technical books on CNC turning. The first sentence of the program is to establish the workpiece coordinate system, that is, G50 Xα Zβ is used as the first sentence of the program. According to the instruction, a coordinate system can be set so that the coordinate value of a certain point of the tool in this coordinate system is (Xα Zβ) (the origin of the workpiece coordinate system is set to the right end surface of the workpiece). Using this method to write the program, after the knife is set, the knife must be moved to the predetermined position set by G50 to process. The process of finding the position is as follows.
In the above steps, step 6 is that the tool positioning is critical at XαZβ. Otherwise, the workpiece coordinate system will be modified and the workpiece cannot be processed normally. Anyone who has experience in machining knows that the above process of positioning the tool to XαZβ is cumbersome. Once an accident occurs, there is no servo on the X or Z axis, tracking error, power failure, etc., the system can only be restarted, and the system loses after the restart. The memory of the workpiece coordinate value set by G50, "Reset, zero return operation" no longer works, you need to re-run the tool to the XαZβ position and reset G50. If it is mass production, after processing one piece, return to the G50 starting point to continue processing the next piece, and if there is a slight mistake in the operation, it is possible to modify the workpiece coordinate system. In view of the various drawbacks of using the G50 to establish the workpiece coordinate system in the first sentence of the above program, the author tries to fix the workpiece coordinate system on the machine tool, and after changing the first sentence G50 XαZβ to G00 Xα Zβ, the problem is solved. The operation process only needs to adopt the first five steps of the above-mentioned G50 process, that is, after completing steps 1, 2, 3, 4, 5, the tool is moved to a safe position, the program is called up, and the automatic operation can be performed. Even if there is an unexpected situation such as power failure, after restarting the system, move the cursor to the block that can be safely processed without affecting the machining process of the workpiece in the edit mode, and continue machining according to the automatic operation mode. The essence of the above program using G00 instead of G50 is to fix the workpiece coordinate system on the machine tool. It is no longer limited by the origin of the G50 Xα Zβ program. It does not change the workpiece coordinate system. It is easy to operate and reliable. It has received unexpected results. effect. China Metal Processing Online
First, the first sentence of the program uses the skill of G00
At present, we are exposed to textbooks and technical books on CNC turning. The first sentence of the program is to establish the workpiece coordinate system, that is, G50 Xα Zβ is used as the first sentence of the program. According to the instruction, a coordinate system can be set so that the coordinate value of a certain point of the tool in this coordinate system is (Xα Zβ) (the origin of the workpiece coordinate system is set to the right end surface of the workpiece). Using this method to write the program, after the knife is set, the knife must be moved to the predetermined position set by G50 to process. The process of finding the position is as follows.
- After the knife is set, the workpiece blank is clamped;
- The spindle rotates forward, and the handwheel reference cutter flattens the right end face A of the workpiece;
- The Z axis does not move, release the tool along the X axis to point C, input G50 Z0, and the computer memorizes the point;
- In the program entry mode, input G01 W-8 F50 to turn the workpiece to a step;
- The X axis does not move, the tool is released along the Z axis to point C, and the workpiece step diameter γ of the turned workpiece is stopped and measured, and G50 Xγ is input, and the computer memorizes the point;
- In the program entry mode, enter G00 Xα Zβ, the tool runs to the programmed program origin, and then enter G50 Xα Zβ, the computer memorizes the program origin.
In the above steps, step 6 is that the tool positioning is critical at XαZβ. Otherwise, the workpiece coordinate system will be modified and the workpiece cannot be processed normally. Anyone who has experience in machining knows that the above process of positioning the tool to XαZβ is cumbersome. Once an accident occurs, there is no servo on the X or Z axis, tracking error, power failure, etc., the system can only be restarted, and the system loses after the restart. The memory of the workpiece coordinate value set by G50, "Reset, zero return operation" no longer works, you need to re-run the tool to the XαZβ position and reset G50. If it is mass production, after processing one piece, return to the G50 starting point to continue processing the next piece, and if there is a slight mistake in the operation, it is possible to modify the workpiece coordinate system. In view of the various drawbacks of using the G50 to establish the workpiece coordinate system in the first sentence of the above program, the author tries to fix the workpiece coordinate system on the machine tool, and after changing the first sentence G50 XαZβ to G00 Xα Zβ, the problem is solved. The operation process only needs to adopt the first five steps of the above-mentioned G50 process, that is, after completing steps 1, 2, 3, 4, 5, the tool is moved to a safe position, the program is called up, and the automatic operation can be performed. Even if there is an unexpected situation such as power failure, after restarting the system, move the cursor to the block that can be safely processed without affecting the machining process of the workpiece in the edit mode, and continue machining according to the automatic operation mode. The essence of the above program using G00 instead of G50 is to fix the workpiece coordinate system on the machine tool. It is no longer limited by the origin of the G50 Xα Zβ program. It does not change the workpiece coordinate system. It is easy to operate and reliable. It has received unexpected results. effect. China Metal Processing Online
Second, the skills to control dimensional accuracy
- Modify the tool compensation value to ensure the dimensional accuracy. Because the workpiece error exceeds the workpiece tolerance due to the first tool error or other reasons, and the machining requirements cannot be met, the workpiece can be resized to the required size by modifying the tool compensation. The radial dimension is as follows:
- The absolute coordinate input method is modified according to the principle of “large reduction, small increase†at the tool compensation 001~004. If the workpiece size is 0.1mm when cutting with a No. 2 cutting knife, and the tool compensation display at X002 is X3.8, you can input X3.7 to reduce the No. 2 tool compensation.
- Relative coordinate method As above, the same effect can be obtained by inputting U-0.1 at the 002 tool.
For the same reason, the control of the axial dimension is also analogous. If you use a No. 1 external circular cutter to machine a certain shaft section, the length is 0.1mm, you can input W0.1 at the 001 knife compensation.
- Semi-finishing eliminates the influence of screw clearance to ensure dimensional accuracy. For most CNC lathes, after a long time of use, the size of the machined workpiece often appears unstable due to the influence of screw clearance. At this time, we can perform a semi-finishing after the roughing to eliminate the influence of the screw gap. If the outer circle is roughed by the No. 1 knife G71, you can input U0.3 at the 001 tool offset, call the G70 fine car once, stop the measurement, then input U-0.3 at the 001 tool compensation, and call the G70 fine car again. . After this semi-finished car, the influence of the screw clearance is eliminated, and the dimensional accuracy is ensured.
- Programming to ensure dimensional accuracy
- Absolute programming guarantees that dimensional accuracy programming has absolute programming and relative programming. Relative programming refers to the coordinate system determined by the end point of each line segment on the machining contour curve with the starting point of the line segment as the coordinate origin. That is to say, the relative programmed coordinate origin is often in the transformation, and the cumulative error is inevitable when the continuous displacement. Absolute programming is a relatively uniform reference point in the whole process of machining, that is, the coordinate origin, so the cumulative error is smaller than the relative programming. . When CNC turning a workpiece, the accuracy of the radial dimension of the workpiece is generally higher than that of the axial dimension. Therefore, when programming, the radial dimension is preferably absolutely programmed. Considering the convenience of processing and programming, the axial dimension is often relative programming. However, for important axial dimensions, absolute programming is preferred.
- Numerical conversion guarantees dimensional accuracy In many cases, the dimensional reference on the pattern is inconsistent with the dimensional reference required for programming. Therefore, the reference size on the pattern should first be converted to the size in the programmed coordinate system. As shown in Fig. 2b, except for the size of 13.06 mm, the other are the programmed sizes obtained by directly converting the dimensions according to Fig. 2a. Among them, the three dimensions of φ29.95mm, φ16mm and 60.07mm are the programmed sizes obtained by taking the average of the two extreme dimensions respectively.
- Modifying the program and tool compensation control dimensions In CNC machining, we often encounter such a phenomenon: after the program is automatically run, stop the measurement, and find that the workpiece size does not meet the requirements, and the dimensional changes are irregular. If the workpiece shown in Fig. 3 is machined with an external cylindrical cutter No. 1, after the rough machining and semi-finishing, the radial dimensions of each shaft segment are as follows: φ30.06mm, φ23.03mm and φ16.02mm. In this regard, the author uses the modified procedure and the knife compensation method to remedy the method as follows:
- Modify the X30 in the original program unchanged, X23 is changed to X23.03, X16 is changed to X16.04, so that each shaft segment has a uniform tolerance of 0.06mm beyond the nominal size;
- Change the knife to enter U-0.06 at No. 1 knife and 001.
After the above-mentioned program and the correction of the two-pronged tool, the fine car program is called, and the workpiece size can be effectively guaranteed.
CNC turning machining is an automatic machining method based on NC program. In actual machining, the operator can only produce high-quality machining programs and produce high-quality workpieces only with strong program commanding ability and rich practical skills.
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