CNC Knowledge: Use the machining program to clear the clearance error

01

Preface

The machining precision of CNC lathes is not only related to its own precision, but also closely related to the CNC machining program. Poorly written machining programs will not only cause unexpected errors, but also falsely believe that there is a problem with the accuracy of the machine tool. When repairing the machine tool, the problem cannot be found and there is no way to start. A similar case is analyzed below.

02

Description of the fault

The part shown in Figure 1 was processed with a CK6140 CNC lathe. It was found that there was no problem with the accuracy of the first part. The outer diameter and inner hole of the second piece were two threads larger than the first piece (1). wire = 10 μm, same as below). The outer diameter and inner hole of the third piece were two threads larger than the second piece. Such problems still occurred when treatment continued. The operator thought there was an accuracy problem. machine tool and maintenance required.

Figure 1 Part structure

After checking that there is no problem with the accuracy of the machine tool, the routine machining procedures for this inner hole are as follows.

N40G0X150Z100;

N50 T0202;

N60G0X25Z1;

N70G1Z-105F0.25;

N80 G0Z100;

N90 G0X150;

There is no retraction command in the X direction after processing the N70 program. The operator stated that there is no surface roughness requirement for the inner hole. Such programming can improve treatment efficiency. the machine tool. Other technicians also agreed with the operator’s point of view, so he found himself in a situation where he didn’t know what to do.

03

Failure analysis

According to the analysis of the fault phenomenon, each part has an outer diameter and an inner hole two wires larger than the previous part, and the problem may lie in the X direction. After checking, there is no screw movement problem in the X direction, and the clearance in the X direction is exactly two threads, and the clearance compensation amount is also two threads, which indicates that there is no problem with the precision of the machine. tool. When I had no choice but to change programs, I inserted the next program segment between N60 and N70.

N63G0X24;

N66G0X25;

Keep other procedures unchanged, retest and the fault is resolved, which means the problem lies in backlash compensation. The following analyzes two situations.

The trajectory of the conventional program for machining the inner hole is shown in Figure 2, and the trajectory of the modified program for machining the inner hole is shown in Figure 3. Point A in Figures 2 and 3 is the starting point of the inner hole tool, and its coordinates are X=150, Z=100; the coordinates of point B are X=25, Z=1, the coordinates of point C are X; =25, Z=-105; the coordinates of point H are X=25, Z=0; the coordinates of point G are X=24, Z=1 and F are the two ends of the ball screw in the X direction; nut.

Figure 2 Trajectory of conventional interior hole machining program

As shown in Figure 2, when the tool moves from A to B, the actual gap between the X-axis ball screw and the nut is at point F. At this point, there is no no gap between the screw and the nut at point E. When the tool moves from B to C, the ball screw of the X axis does not move. When the tool moves to point H and starts cutting, the tool is subjected to a compressive force from the workpiece in the negative direction of the X axis. Under the action of this force, the ball screw nut moves slightly toward the negative end of the X axis when F When all the spaces at the end are transferred to the E end, the micro- movement stops. The CNC system cannot know this process, so point C is equivalent to a ball screw reverse set shifted in the negative direction of the female is well adjusted 2, and after running N80 GOZ100, it will not affect the change direction X clearance of ball screws. Run N90 G0X150, the X axis is reversed, and the CNC system automatically performs ball screw backlash compensation. Due to the action of external force, the reverse clearance of the X axis has been transferred from end F to end E, and there is no gap when point A is shifted by a point A instead of the original position of point A. reverse clearance amount in the positive direction of the X axis. In this way, each time the inner hole turning tool cycles, as long as the extrusion force in the negative direction of the X axis is compensated by an amount of play in the positive direction of. the X axis, causing the above failure.

In Figure 3, program instructions N63 and N66 are added. After the N66 program is executed, the actual gap between the ball screw and the nut is in F, and there is no gap in E at this time. From H to C, although the tool is subjected to the negative extrusion force of the workpiece towards the X axis, there is no gap at E and the ball screw is positioned by the servo motor, so there will be no errors in the Direction X, and the problems mentioned above will not occur in the event of a continuous processing fault.

Figure 3 Program path for machining the inner hole after modification

04

Conclusion

The above examples illustrate that eliminating backlash is one of the important ways to ensure machining accuracy. The system parameter compensation method does not affect the writing of processing programs. Although the system parameter compensation method is easy to use, simple and clear, it has great limitations. Using machining programs to eliminate backlash is suitable for open and semi-closed loop systems, especially systems without compensation. It has a great practical effect. However, this method of programming requires a high degree of process knowledge and understanding of the structure of the machine tool from the programmer.