As machinery manufacturing technology continues to advance, the machine tool industry has evolved from traditional machine tools to CNC machine tools and has experienced rapid growth.
The use of CNC machine tools is increasing year by year, mainly due to the advantages of CNC technology.
CNC technology was originally developed for the production of defense industries such as aviation, shipbuilding, space flight, weapons production, etc.
It is particularly suitable for the production of small to medium quantities of machine parts with high precision, intricate geometry, multiple sizes, and frequent modifications.
Research on CNC technology began in foreign countries in the late 1940s and with the advancement of transistor integrated circuits and computing technology, it was applied in production in the late 1950s and early 1960s and has become increasingly widespread.
Given the current level of technology and economic development of China’s manufacturing industry, economically-viable CNC machine tools are more suitable for enterprises and related industries to use.
Currently, the use of these machines is high, with most of them utilizing open-loop or semi-closed-loop control systems.
The machining accuracy is greatly impacted by the mechanical accuracy of the machine tool, so it is crucial to ensure the machining quality and resolve machining errors caused by mechanical clearance.
1. CNC machine tool clearance error analysis
1.1 clearance error
The mechanical clearance error of a CNC machine tool refers to the comprehensive error caused by gaps in the machine tool chain from the head to the actuator, as shown in Figure 1.
This error originates from various sources in the machine tool’s feed chain, including the gap between the motor shaft and the tooth shaft due to key linkage, the gap between gear sub-parts, the gap between the gear and the screw due to key linkage, the gap in the coupling due to key linkage, and the gap between the screw nut, among others.
The machine opposite clearance error is caused by the existence of mechanical clearance in the machine’s drive chain.
During the movement of the machine’s actuators, when they change from forward to reverse motion, there is an error between the amount of movement of the actuators and the theoretical value (programmed value), which results in a reduction in machining accuracy on the workpiece.
When the CNC machine table is reversed in its motion direction, the existence of backlash causes the servo motor to run without actual table movement, known as “loss of motion.”
For instance, in the case of a G01 cutting motion, reverse deviation affects the accuracy of interpolation motion.
If the deviation is too great, it can result in a situation where shapes are not precise enough, such as “round not being round enough” or “square not being square enough”.
In good rapid positioning movement, reverse deviation affects the positioning accuracy of the machine tool, leading to decreased accuracy in the position between holes during drilling or boring.
If the value of the deviation is small, no action is required as it has little effect on machining accuracy.
However, if the value is larger, the stability of the system is significantly reduced, and machining accuracy is significantly impacted.
This is especially true for curve processing, as it can affect dimensional tolerance and curve consistency. In such cases, backlash measurement and compensation must be performed.
When using semi-closed-loop control CNC machine tools, backlash can affect positioning accuracy and repeat accuracy. It is important to pay attention to and study the factors that cause backlash, its influence, and compensation functions when using CNC machine tools.
In the study and practice of backlash compensation, it is important to carefully summarize and identify regular errors in the automatic compensation process and take appropriate measures to improve the machining accuracy of parts.
1.2 Measurement of clearance errors
To study the effect of backlash error on machining, we use 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 platform is a modular cross working platform that utilizes a ball screw drive to realize target trajectories and movements.
The actuating device uses a stepper motor, and the control device is made up of a PC machine, a DSP-based closed-loop motion control card, and the corresponding drive.
The motion control card receives position and trajectory instructions from the PC machine, processes and converts them into a format that the servo drive can accept, and sends them to the servo drive for amplification and output to the actuating device.
To measure the backlash error of the x-axis platform, follow these steps:
(1) Place the platform in a suitable position near the sub-section and set it to the workpiece origin through manual adjustment.
(2) Enter the distance to be tested in the movement distance input box and enter 0 in the reverse gap input box without clearance compensation.
(3) Press the forward inching button to move the screw a short distance (about 10mm) in the positive direction, then click to stop the movement.
(4) Press the test button and the system will automatically test according to the entered test distance and display the results.
(5) Repeat this process several times to measure the backlash.
(6) Use the same method, press the reverse inching button to test the backlash in the opposite direction of the x-axis.
(7) Calculate the mean of the two sets of data: The backlash for the forward x-axis motion is -0.482 and the backlash for the reverse x-axis motion is 0.480.
According to the characteristics and requirements of CNC machine tools, the general CNC system has common compensation functions, such as compensation for tool position deviation, tool radius compensation, mechanical backlash compensation, and other automatic compensation functions.
The mechanical backlash compensation method is one of the commonly used methods in open-loop and semi-closed-loop systems. The principle is based on measuring the machine backlash error and using the system parameters in the machine control system to automatically compensate for the error.
The process involves measuring the clearance error value for each motion axis and inputting it into the control unit via the control panel. When the machine is moving the tool, it will first move in the corresponding direction (e.g. longitudinal or transverse) and take the clearance value before reversing the cutter and taking the required value, compensating for the original clearance error.
This method uses a single control program to control the reverse tool travel in all programs, allowing a limited number of clearance values to be entered to compensate for all process clearance errors. It is simple and easy to use and does not affect the programming of the processing program.
The accuracy of compensation is measured by adding the calculated compensation value for backlash into the backlash input box, as illustrated in Figure 2. However, this method is dependent on the measured backlash error value for each axis of motion, which can be affected by measurement errors.
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|
The following are the limitations of this method:
(1) The gap between the screw and nut in the motion key has a non-linear relationship over the entire length of the screw, making it unreasonable to represent the total gap error with one measurement. This, combined with errors in measuring gap values, results in less accuracy in this compensation method.
(2) The integrated gap error in the feed chain is typically measured in static conditions, but the machine tool operates in a dynamic environment, leading to a significant difference between static and dynamic errors. Thus, this compensation method cannot fully compensate for the actual error.
(3) This method cannot compensate for errors caused by cutting forces.
In conclusion, compensating for backlash errors is a crucial factor in ensuring the accuracy of CNC machine tools. The system parameter compensation method is straightforward and easy to operate, but has limitations. The processing program compensation method has better results, especially for open-loop and semi-closed-loop systems without compensation functions, but requires a high level of programming skill.
Once the backlash value is entered into the numerical control system, the CNC machine will automatically compensate for it during machining. However, with continued use, the backlash may increase due to wear and tear of moving parts, so it is essential to regularly measure and compensate the backlash value of CNC machine tools. This significantly reduces or eliminates the negative impact of backlash on machine accuracy and workpiece machining accuracy.