The existence of transmission backlash will affect the positioning accuracy and repeatability of the machine tool positioning accuracy, resulting in the machining error of the part, the need for regular determination and compensation of the machine tool of the reverse deviation of the coordinate axes.

With the continuous development of machinery manufacturing technology, the popularity of CNC machine tools is increasing year by year due to the superiority of CNC technology. CNC technology is particularly suitable for the production of small and medium batches of mechanical parts with high machining accuracy, complex geometry, numerous sizes and frequent re-modeling. The market share of economical CNC machine tools is high. But most of these CNC machine tools belong to the open-loop or semi-closed-loop servo system, both of which can not be controlled as a linear displacement detection and feedback device with a full closed-loop servo system for the machine tool mechanical transmission part of the effective compensation. Thus, the existence of backlash affects the positioning accuracy and repeatability of the machine tool, reflected in the workpiece will cause machining errors. Therefore, to solve the problem of machining errors due to mechanical clearance is an important part of ensuring the quality of machining.



1, the adverse effects of transmission backlash

CNC machine tool mechanical clearance error, refers to the first end of the chain of movement from the machine tool to the implementation of the whole process due to mechanical clearance caused by the integrated error, the error comes from the motor shaft and tooth axis due to the key linkage caused by the gap, gear gap, gears and screw by the key coupling caused by the gap, key coupling coupling caused by the gap in the coupling, the gap in the screw nut gap, and so on. Machine tool backlash error, refers to the presence of mechanical clearance in the machine tool transmission chain, machine tool actuators in the process of movement from forward motion into reverse motion, the amount of movement of the actuator and the theoretical value (programmed value) there is an error, and finally reflected in the superimposed to the workpiece on the machining accuracy of the error.

Such as in the G01 cutting movement, the reverse deviation will affect the accuracy of interpolation movement, if the deviation is too large will result in “round is not round enough, square is not square enough” situation; and in the G00 rapid positioning movement, the reverse deviation affects the positioning accuracy of the machine tool, so that drilling, boring and other hole machining hole between the positional accuracy of the reduction. Such a backlash if the value is small, the impact on machining accuracy is not significant, there is no need to take any measures; if the value is large, the stability of the system is significantly reduced, the machining accuracy is significantly reduced, especially curve machining, which affects the dimensional tolerances and the consistency of the curve, at this time, it is necessary to carry out the determination of the backlash and compensation. At the same time, as the equipment is put into operation for a longer and longer time, the reverse deviation will also increase with the gradual increase in the gap caused by wear and tear of the movement vice, so the need for regular determination of the machine tool axes of the reverse deviation and compensation.

2, the measurement of transmission backlash

In order to reduce the transmission backlash on the accuracy of the machine tool, you can take the adjustment and pre-tensioning method to reduce the gap, but the mechanical transmission gap can not be completely eliminated. In order to better eliminate errors, modern CNC systems provide transmission backlash soft compensation to minimize the transmission backlash on the impact of parts machining accuracy, that is, through the measurement of the transmission backlash value, the value will be stored in the corresponding parameters of the CNC system, and thereafter whenever the CNC machine tool reverse movement, the CNC system will control the servomotor to go a distance, this distance is equal to input the transmission backlash value to the system. This distance is equal to the transmission backlash value input into the system, thus approximating the compensation of the transmission backlash error.

1. Manual Measurement

The following is an example of measuring the Z-direction backlash of a machine tool:

(1) Operate the machine manually to move the Z-axis one distance in the positive direction.

(2) will have a lever with a percentage of the table mounted on the machine guideway, the table head pointing to the side of the skateboard, the needle disk set to zero (that is, this point as a reference).

(3) Using the point feed method or handwheel method, operate the machine to move the Z-axis in the same direction for a certain distance (i.e., in the same direction as the direction of movement just now).

(4) Then move the same distance in the negative direction of the Z-axis.

(5) Observe the difference between the stop position and the reference position. There are two cases: First, the pointer has not changed, then the backlash is positive, use the point feed mode or handwheel mode to operate the machine tool to move in the negative direction of the Z-axis, and record the distance moved until the percentage meter indicator changes, at which time the backlash is the difference between the distance moved by the lathe and the percentage of the indicated value; second, the percentage meter pointer changes, then the backlash is negative, and the backlash is the percentage of the indicated value.

(6) X-drive backlash value measurement and Z-drive backlash value measurement is the same.

(7) In order to avoid the chance error of a single measurement, near the midpoint of the stroke and the two ends of the three positions were measured several times (generally 7 times), the average value of each position, the average value of the maximum value for the measured value of the backlash. It should be noted that the measurement must be moved a distance first, otherwise the correct value of the reverse deviation can not be obtained.

2. Programmed measurement

If the measurement is programmed, the measurement process can be made easier and more precise.

For example, to measure the backlash of the Z-axis on a SINUMERIK 802C machine tool, you can first install the percentage gauge on the machine guideway and then run the following program to measure:

N10 G91 G01 Z50 F0.5; table shift right

N20 Z-50; table left

N30 G04 F5; pause for observation

3, the precautions

1, in the measurement must first move a distance, otherwise you can not get the correct reverse deviation value.

2, the measurement of linear motion axis of the backlash, the measurement tool is usually used micrometer or percent gauge, if the conditions allow, you can use a dual-frequency laser interferometer for measurement. When using a micrometer or a percentage table for measurement, it should be noted that the table base and the table rod should not be extended too high and too long, because if the cantilever is long during measurement, the table base is easy to be moved by the force, resulting in inaccurate counting, and the compensation value will not be accurate.

3、The results measured under different running speeds of the table will be different. In general, the measured value at low speeds is larger than that at high speeds, especially when the machine axis load and resistance to movement are large. Low-speed movement of the table speed is low, not easy to occur overshoot overtravel (relative “backlash”), so the measured value is larger; at high speeds, due to the high speed of the table, easy to occur overshoot overtravel, the measured value is small. Therefore, we should measure as low as possible.

4, in the CNC system for the compensation value of the input

For the characteristics of the CNC machine itself and the use of requirements, the general CNC system has a commonly used compensation functions, such as tool point position deviation compensation, tool radius compensation, tool radius compensation, mechanical backlash parameter compensation and other automatic compensation functions. Among them, the mechanical backlash parameter compensation method is one of the commonly used methods for open-loop and semi-closed-loop systems. The principle of this method is through the actual measurement of the machine tool backlash error value, the use of machine tool control system set up in the system parameters to realize the automatic compensation of the gap error. Therefore, as long as the input of a limited number of clearance values can be compensated for all the clearance errors in the machining process, this method is simple and easy to implement, and has no impact on the preparation of the machining program.

5. Conclusion

Before the machine tool factory, the manufacturer has measured the clearance value of the feed system, and compensation, but the machine tool in long-term use, due to wear and tear and other reasons, the amount of compensation will not be appropriate, will inevitably have an impact on the machining accuracy of the workpiece. Therefore, it is necessary to measure and compensate the backlash of the machine tool, which is of great significance to improve the machining accuracy of the workpiece. When an axis of the machine tool is instructed to change the direction of movement, the CNC device will automatically read the reverse clearance value of the axis, the coordinate displacement instruction value compensation, correction, so that the machine tool is accurately positioned in the instruction position to eliminate or reduce the adverse effects of reverse deviation on the accuracy of the machine tool.