Measurement and Compensation of Backlash of CNC Machine Tools

With the continuous development of machinery manufacturing technology, the machine tool industry has also transitioned from traditional machine tools in the past to CNC machine tools, and has developed rapidly.

The popularity of CNC machine tools is rising year by year, mainly due to the superiority of CNC technology.

CNC technology is developed for the production of defense industries such as aviation, shipbuilding, space flight, weapons production, etc.

It is especially suitable for the production of small and medium quantities of machine parts with high machining accuracy, complex geometry, numerous sizes and frequent modifications.

It began research in foreign countries in the late forties, with the development of transistor integrated circuits and computing technology, it began to be used for production in the late fifties and early sixties, and more and more be promoted and applied.

On the current level of technology and economic development of China’s manufacturing industry and the state of economic development, the economic CNC machine tool is more suitable for our enterprises and related industries to use.

The current occupancy of such machines is high, most of which are open-loop or semi-closed-loop control systems.

The machining accuracy is greatly influenced by the machine tool’s mechanical accuracy, so it is an important link to ensure the machining quality to solve the machining error caused by mechanical clearance.

1. CNC machine tool clearance error analysis

1.1 clearance error

The mechanical clearance error of CNC machine tool refers to the comprehensive error caused by the mechanical gap from the head of the machine tool chain to the actuator, as shown in Figure 1.

The feed chain of the machine tool, the error comes from the gap between the motor shaft and the tooth shaft due to the key linkage, the gap between the gear sub-gap, the gap between the gear and the screw due to the key linkage, the gap between the key linkage in the coupling, the gap between the screw nut and so on.

The machine opposite clearance error  is due to the existence of mechanical clearance in the machine drive chain.

During the movement of the machine’s actuators, when they change from forward motion to reverse motion, there is an error between the amount of movement of the actuators and the theoretical value (programmed value), which is finally reflected as an error in the machining accuracy superimposed on the workpiece.

When the CNC machine table is reversed in the direction of its motion, the existence of backlash causes the servomotor to run without actual table movement, which is known as loss of motion.

For example, in the case of g01 cutting motion, the reverse deviation affects the accuracy of the interpolation motion.

If the deviation is too great, it will result in a situation where “round is not round enough and square is not square enough”.

In goo rapid positioning movement, reverse deviation affects the positioning accuracy of the machine tool, which reduces the accuracy of the position between the holes during drilling, boring, etc.

Such backlash does not require any measures if the value is small and has little effect on machining accuracy.

If the value is larger, the stability of the system is significantly reduced and the machining accuracy is significantly reduced,

Especially the curve processing, will affect the dimensional tolerance and curve consistency, it must be backlash measurement and compensation at this time.

Especially using semi-closed-loop control of CNC machine tools, backlash will affect the positioning accuracy and repeat accuracy, which requires us to pay attention to and study the backlash generation factors, influence and compensation function when using CNC machine tools.

In the study and practice, some regular errors in the process of automatic backlash compensation should be carefully summarized and found, appropriate processing measures should be taken to improve the machining accuracy of parts.

1.2 Measurement of clearance errors

In order to study the effect of backlash error on machining, we rely on a small 3D coordinate teaching and training platform.

This platform integrates a multi-axis motion controller, a motor and its drive, an electronic control box, and a motion platform.

The mechanism is a modular cross working platform with a ball screw drive, which is used with the realization of the target trajectory and movement.

The actuating device uses a stepper motor, and the control device consists of a pc machine, a dsp-based closed-loop motion control card, and a corresponding drive.

The motion control card accepts the position and trajectory instructions issued by the pc machine for planning and processing, converts them into an instruction format acceptable to the servo drive, sends them to the servo drive, which processes and amplifies them, and outputs them to the actuating device.

It should select the x-axis, open the control software on measuring backlash (as shown in Figure 2), and start to measure the backlash error of this axis platform in the process of motion.

(1) By manual adjustment, the platform is placed in a suitable position, generally near the sub-section is the platform, and is set to the workpiece origin.

(2) Enter the movement distance to be tested in the movement distance input box, and then enter 0 in the reverse gap input box, without clearance compensation.

(3) Press the forward inching button to move the screw in the positive direction for a short distance (about 10mm), and then click to stop the movement.

(4) Press the test button, the system will automatically test according to the test distance entered, and finally display the test results.

(5) Repeat the above action to measure the backlash several times.

Obtain the value of the backlash for the positive directional motion of the x-axis.

(6) Using the above method, press the reverse inching button to test the backlash of the x-axis moving in the opposite direction, and the measurement results are shown in Table 1.

7) Calculate the mean of the two sets of data respectively: The backlash for forward x-axis motion is -0.482, and the backlash for reverse x-axis motion is 0.480.

According to the characteristics of CNC machine tools themselves and the use of requirements, the general CNC system has common compensation function, such as compensation for tool position deviation, tool radius compensation, tool radius compensation, mechanical backlash parameter compensation and other automatic compensation function.

Among them, the mechanical backlash parameter compensation method is one of the commonly used methods in open-loop and semi-closed-loop systems.

The principle of this method is based on measuring the value of the machine backlash error, which uses the system parameters set in the machine control system to automatically compensate for clearance errors.

The process is to measure the clearance error value of each motion axis, and then enter the control unit through the control panel.

Later, when the machine is moving the tool, it will first move the tool in the corresponding direction (e.g. longitudinal or transverse movement).

When reversing the cutter, the clearance value will be taken first, and then the required value will be taken, so that the original clearance error can be compensated.

Since this method uses a single control program to control the amount of reverse tool travel in all programs, a limited number of clearance values can be entered to compensate for clearance errors in all process.

This method is simple and easy to use, and has no effect on the programming of the processing program.

Specific operation:

The compensation accuracy is measured by adding the calculated backlash compensation value into the backlash input box as shown in Figure 2.

However, the control of this method is based on the measured backlash error value of each axis of motion, so it is affected by the measurement error.

Table 1 Measurement data for backlash

Measure 1 Measure 2 Measure 3 Measure 4 Measure 5
X-axis backlash in the positive direction -0.500 -0.475 -0.475 -0.490 -0.470
X-axis backlash in the opposite direction 0.490 0.485 0.475 0.470 0.480

It has the following shortcomings:

(1) Since the gap value between the screw and nut in the motion key has a non-linear relationship over the entire length of the screw, it is unreasonable to represent the total integrated gap error with one measurement.

Combined with errors in the measurement of the gap values, this compensation method is less accurate.

2) The general feed chain integrated gap error is measured in the static conditions, but the machine tool is actually working in a dynamic environment, so there is a big difference between the static error and dynamic error.

Therefore, this compensation method cannot truly compensate for the actual error.

(3) It can not compensate for errors due to cutting forces.

2. Conclusion

In summary, backlash error compensation is an important means for CNC machine tools to ensure their machining accuracy.

The system parameter compensation method does not affect the preparation of processing procedures, which is easy to operate, simple and straightforward.

It has a certain effect in a certain range, but the limitations are greater.

The processing program compensation method has a better effect, and is suitable for open-loop and semi-closed-loop systems, especially for systems without compensation functions.

This method, however, increases the complexity of part machining programming and requires a high degree of programming skill on the part of the user.

After the backlash value is input into the numerical control system, the numerical control machine will automatically compensate for this value during machining.

However, with the long-term use of CNC machine tools, the backlash will gradually increase due to the wear and tear of the moving parts.

So it is necessary to regularly measure and compensate the backlash value of CNC machine tools, which greatly reduces or eliminates the adverse effects of backlash on machine accuracy and workpiece machining accuracy.

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