Common reasons for machining out-of-tolerance caused by long-term use of machine tools

After the new CNC machining center is installed and debugged, the accuracy will not be out of tolerance for a period of time. However, with the relationship between the processing intensity and the service cycle, it is easy to cause the accuracy of the workpiece to be out of tolerance. The reasons for this problem are nothing more than two main reasons: the machine tool itself and the foundation change. Let's take a look at the details:

The excess caused by the foundation

We know that machine tool installation is required before new machine tools are used. This is what we commonly call "leveling". Generally speaking, the machine tool will not be in direct contact with the ground, and a shock-proof pad is installed in the middle. However, with the long-term use of the machine tool, the foundation under a certain anti-vibration pad has sag or virtual points. All of them will cause changes in the set accuracy of the machine tool, resulting in the out-of-tolerance of the entire machining accuracy. At this time, it is necessary to readjust the foundation to restore the initial accuracy of the machine tool.

The tolerance caused by the machine tool itself

The problem of the machine tool itself is the main reason for the machining over-tolerance. We know that CNC machining centers are highly mechatronics equipment. In terms of machinery, the most prone to accuracy problems is the transmission accuracy of each shaft drive chain, including screw clearance, pitch changes, and axial movement. System aspects include both hardware and settings. The hardware aspects include whether the detection components are in good condition, whether the cable plug-ins are in good contact, etc.; the setting aspects include whether the servo motor speed is too high, whether the position deviation value is set correctly, and whether the gain of the position loop is set reasonably.

Measures to remedy machining out-of-tolerance through NC programming

We know that all actions of CNC machining center are completed under the control of CNC system. Therefore, the unsatisfactory machining phenomenon can be remedied by programming. It includes several measures such as eliminating the influence of machine tool clearance, reducing the influence of the cumulative error of the numerical control system, and eliminating the influence of the position of the workpiece tolerance zone.

Eliminate the influence of machine tool clearance

After the machine tool is used for a long time, the ball screw pair, guide rail wear and thread pitch will change to varying degrees, and the CNC system has a gap compensation function. The measured backlash value of the machine tool can be entered into the numerical control system to offset the amount of clearance and improve the machining accuracy.

Eliminate the influence of the position of the tolerance zone

Many dimensions of the workpiece have tolerances, and the position of the tolerance zone cannot be the same. When we compile the processing program, we generally compile it according to the contour of the workpiece, that is, according to the basic size of the part, ignoring the influence of the position of the tolerance zone. By adjusting the basic size of the workpiece and the position of the tolerance zone, and using the same tool compensation value, the machining accuracy of the CNC machining center can be effectively guaranteed.

Reduce the influence of the cumulative error of the numerical control system

We know that the CNC system will generate a certain cumulative error during the operation of rapid movement and interpolation. When it reaches a certain value, it will cause the CNC machining center to generate displacement and positioning errors, which will affect the machining accuracy. Generally, the accumulated error of the system can be eliminated by using the mode programming and inserting the reference point return instruction.

Conclusion

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