In the machining industry, “machining precision” is a common term used frequently. This is mentioned several times a day, and when talking to people in the industry, machining accuracy is always discussed.
The question then arises: how can the CNC machine guarantee machining precision?
The accuracy of CNC machine tools ultimately depends on the accuracy of the machine tool itself. This accuracy includes several factors such as geometric accuracy, positioning accuracy, repeated positioning accuracy, and cutting accuracy.
Geometric accuracy:
It is also known as static accuracy, which comprehensively reflects the geometric errors of the main components of CNC machine tools after assembly.
Positioning accuracy :
This demonstrates the accuracy of the machine tool movement being measured under the control of the numerical control device. Based on the measured value of positioning accuracy, the ideal accuracy for machining the part in the automated machining process of the machine tool can be determined.
Positioning accuracy refers to the difference between the actual position of the part or tool and the standard position (theoretical or ideal position). The smaller the difference, the greater the accuracy.
Ensuring accurate part processing depends on achieving high positioning accuracy, which is a crucial prerequisite.
Repeat positioning accuracy
This refers to the consistency in position accuracy obtained by repeatedly executing the same program code on a CNC machine tool. It also includes the consistency of results obtained when processing a batch of parts under the same conditions, such as using the same CNC machine tool and operation methods, and with the same part program.
Cutting Accuracy:
This is a comprehensive inspection of the geometric and positioning accuracy of the machine tool during cutting operations.
As stated above, the precision of CNC machine tools is divided into mechanical and electrical aspects. Mechanical aspects cover the accuracy of the spindle, including offset and busbar, the accuracy of the lead screw, the accuracy of the fixture during processing, and the rigidity of the machine tool.
Electrical aspects mainly concern control methods such as semi-closed loop and fully closed loop, feedback and compensation methods, and interpolation accuracy during processing.
Thus, the precision of the machine tool does not just depend on whether it is completely closed or not.
I. Introduction to the principle
The motion chain of CNC machine tools includes the following components: CNC device → servo encoder → servo drive → motor → screw → moving parts.
Depending on the installation position of the position sensing device, the control can be classified into three types: fully closed loop control, semi-closed loop control and open loop control.
Fully closed loop control servo power system
The machine tool is equipped with position sensing devices such as grid rulers and linear induction synchronizers, which are installed on its moving parts such as workbenches. These devices provide real-time feedback on the position of moving parts.
After the CNC system processes the information, the state of the machine tool is relayed to the servo motor. The servo motor automatically compensates for any movement error through system command.
However, because it involves closed-loop control of the large inertia links of the lead screw, nut pair, and machine tool table, it can be more challenging to debug the system to a steady state.
Additionally, installing measuring devices such as grid rulers and linear induction synchronizers can be expensive and complicated, which can lead to oscillations.
Therefore, most machine tools in general do not utilize full closed-loop control.
Semi-closed loop control servo power system
A position sensing device is installed at the end of the drive motor or the end of the screw rod to detect the rotation angle of the screw or servo motor. This helps to indirectly measure the actual position of the machine tool's moving parts, which is then sent back to the control system through feedback.
With advances in mechanical manufacturing and improvements in the accuracy of speed sensing elements and screw pitches, semi-closed loop CNC machine tools have achieved a very high level of feeding accuracy.
As a result, most machine tool manufacturers have widely adopted semi-closed-loop CNC systems.
II . Practical application
Fully closed loop control system
Position sensing devices such as grid rulers and linear induction synchronizers have varying levels of accuracy, ranging from ±0.01 mm to ±0.003 mm. The level of precision affects positioning accuracy, and even with full closed-loop control, errors can occur.
Position sensing is also affected by thermal properties, specifically thermal deformation. Measuring devices are typically made of non-metallic materials and the coefficient of thermal expansion varies between various components of the machine tool.
This is a critical aspect of machine tool accuracy and must be addressed by reducing heat generation during machining to overcome thermal deformation caused by temperature. High-end machine tools utilize various methods such as hollow screw cooling, guide rail lubrication, and cutting fluid constant temperature cooling to reduce thermal deformation.
Installation of position detection device is also crucial. In theory, the closer it is to the drive shaft (pair of screws), the more accurate the measurement will be. However, due to structural space limitations, there are only two ways to install the grid ruler: close to the side of the lead screw or on the outside of the guide rail.
Although it is recommended to opt for the first installation method, it may be inconvenient for inspection and maintenance purposes. On the other hand, although a high-precision grid ruler was selected, it could not achieve the precision required for CNC machine tools.
In the first case, the installation position of the grid ruler is relatively close to the drive shaft, but still maintains a certain distance from it. This distance, together with the oscillation of the object while driving, causes problems in the detection and control of the grid ruler.
When the driven object swings towards the mounting side of the grid ruler, it mistakes the moving speed as insufficient during detection, resulting in the system outputting an acceleration signal. When the moving object swings to the other side, the grid ruler mistakes the moving speed as too fast during detection and the system outputs a slowdown signal.
These repeated operations do not improve the control of the linear coordinate axes of the CNC machine tool, but they intensify the vibration of the driving object. This leads to a peculiar phenomenon where the fully closed circuit is not as good as the semi-closed circuit.
Environmental impact of production:
Generally, machining factories have aggressive environments, where dust and vibration are common phenomena. However, grid scales and linear induction synchronizers are precision components that measure the relative position of movement through light reflection.
Dust and vibration are the two biggest factors that affect measurement accuracy. In addition, cutting oil mist and water mist are more serious during machine tool machining, greatly affecting the grid ruler and linear induction synchronizer.
Therefore, if a fully closed loop control system is used, it is essential to ensure proper installation and sealing and improve the production environment. Otherwise, the accuracy of the new machine tool, which was originally good, will decline within a year, and the machine will often sound an alarm.
Semi-closed loop control system
Because the measuring device is typically installed on top of the engine or lead screw, it is easier to seal, making environmental requirements unnecessary.
The accuracy error of the semi-closed loop control system mainly depends on the forward and reverse clearance of the screw.
Thanks to advances in mechanical processing technology, the current manufacturing level of imported lead screws is relatively high. Pairs of high-precision lead screws virtually eliminate back and forth backlash.
In addition, during the assembly process, the screw pair adopts a pair of double-row reverse ball screws, which can fully eliminate forward and reverse backlash.
Many machine tool factories use the pre-stretching method during machine tool assembly to eliminate the impact of thermal deformation on thread accuracy.
Therefore, the current semi-closed loop control system can ensure high precision of the machine tool.
Conclusion
In summary, it can be seen that, in theory, fully closed-loop control can improve the basic positioning accuracy compared to semi-closed-loop control if external factors are not considered. However, failure to address factors such as machine heat, environmental pollution, temperature rise, vibration and installation can lead to a scenario where fully closed loop control performs worse than semi-closed loop control.
While it may work well in the short term, dust and temperature changes can significantly impact the grid ruler's measurement feedback data in the long term, thereby reducing its effectiveness.
Furthermore, if there is a problem with the grid ruler, it generates an alarm, which may cause the machine tool to malfunction.
Due to cost and competitive considerations, full closed-loop control for medium and low-cost machine tools has been simplified. As a result, some aspects such as sealing and temperature rise control may not be well guaranteed.
In such cases, simply setting the grid ruler cannot improve the accuracy of the machine tool and may incur significant costs.
