In the machining industry, machining accuracy is a pet phrase.
It is said several times every day, and when chatting with people in the industry, the machining accuracy will definitely mention without saying three sentences.
So what is the guarantee of the machining accuracy of the CNC machine?
The machining accuracy of CNC machine tools ultimately depends on the accuracy of the machine tool itself.
The accuracy of CNC machine tools includes geometric accuracy, positioning accuracy, repeat positioning accuracy and cutting accuracy.
It is also known as static accuracy, which comprehensively reflects the comprehensive geometric error of the key parts of CNC machine tools after assembly.
It shows the accuracy of the movement of the measured machine tool under the control of the numerical control device.
According to the measured positioning accuracy value, the best workpiece machining accuracy that can be achieved in the automatic machining process of the machine tool can be judged.
It refers to the gap between the actual position of the part or tool and the standard position (theoretical position, ideal position),
The smaller the gap, the higher the accuracy.
It is the prerequisite to ensure the accuracy of parts processing.
Repeat positioning accuracy
It refers to the content of consistency of position accuracy obtained by repeatedly running the same program code on a CNC machine tool.
It is also the content of consistency of the continuous results obtained by processing a batch of parts under the same conditions (on the same CNC machine tool, with different operating methods and the same part program).
It is a comprehensive inspection of the geometric accuracy and positioning accuracy of the machine tool under cutting conditions.
It can be seen from the above that CNC machine tools is divided into mechanical and electrical aspects according to the accuracy.
Mechanical aspects are the accuracy of the spindle, such as run-out and bus bar etc.; the accuracy of the lead screw; the accuracy of the fixture during processing, the rigidity of the machine tool and so on.
Electrical aspects are mainly control methods such as semi-closed loop and fully closed loop etc., as well as feedback and compensation methods, and interpolation accuracy during processing.
Therefore, the accuracy of the machine tool does not depend on whether the machine tool is fully closed loop.
I. Principle introduction
The motion chain of CNC machine tools includes CNC device → servo encoder → servo drive → motor → screw → moving parts.
According to the different installation positions of the position detection device, it is divided into full closed loop control, semi-closed loop control, and open loop control.
Fully closed loop control feed servo system
It installs position detection devices (such as grating rulers, linear induction synchronizers, etc.) on the moving parts of the machine tool (such as workbenches), and provides real-time feedback on the position of the moving parts.
After processing by the CNC system, the state of the machine tool is notified to the servo motor, and the servo motor automatically compensates for the motion error through the system command.
But because it puts the large inertia links of the lead screw, the nut pair and the machine tool table in a closed loop, it is more troublesome to debug the system in a stable state when debugging.
In addition, measuring devices such as grating rulers and linear induction synchronizers are expensive and complicated to install, which may cause oscillations.
Therefore, general machine tools do not use full closed loop control.
Semi-closed loop control feed servo system
It installs the position detection device 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, indirectly measure the actual position of the machine tool moving parts, and send it back to the control system via feedback.
Due to the improvement of the mechanical manufacturing level and the improvement of the accuracy of the speed detection element and the screw pitch, the semi-closed-loop CNC machine tool has been able to achieve a fairly high feed accuracy.
Most machine tool manufacturers have widely adopted semi-closed-loop CNC systems.
II. Practical application
Fully closed loop control system
Position detection devices (such as grating rulers, linear induction synchronizers, etc.) have different accuracy levels (±0.01mm, ±0.005mm, ±0.003mm, ±0.02mm).
Therefore, the full closed loop control will also have errors, and the positioning accuracy is affected by the accuracy level.
Position detection has thermal properties (thermal deformation) and measuring devices are generally non-metallic materials.
The coefficient of thermal expansion is not consistent with the various components of the machine tool, which is a critical aspect of the accuracy of the machine tool.
So it is important to address the issue of heat generation during machining to overcome the thermal deformation caused by temperature.
High-end machine tools will use various methods, such as screw hollow cooling, guide rail lubrication, cutting fluid constant temperature cooling, etc., to reduce thermal deformation during the machining process.
The installation of the position detection device is also very important,
In theory, the closer to the drive axis (screw pair), the more accurate the measurement.
Due to the limitation of structural space, there are only two ways to install the grating ruler, one is to install it near the side of the lead screw, and the other is to install it on the outside of the guide rail.
It is recommended to choose the first installation method as much as possible, but it is inconvenient for inspection and maintenance.
On the contrary, a high-precision grating ruler was selected, but it did not actually achieve the accuracy required by CNC machine tools.
Even in the first case, the installation position of the grating ruler is relatively close to the drive axis, but the installation position is still a certain distance from the drive axis.
The combination of this distance and the swing of the object during driving brings a lot of trouble to the detection and control of the grating ruler.
When the driven object swings to the mounting side of the grating ruler, the grating ruler mistakenly believes that the moving speed is insufficient during detection, and the system will give an acceleration signal.
The driving object immediately swings to the other side, and the grating ruler mistakenly believes that the moving speed is too fast during detection, and the system gives a deceleration signal.
Such repeated operation did not improve the control of the linear coordinate axes of the CNC machine tool, but intensified the vibration of the driving object, resulting in a peculiar phenomenon that the fully closed loop is not as good as the semi-closed loop.
Production environmental impact:
Generally, the environment of machining factories is harsh, and dust and vibration are common phenomena, but grating scales and linear induction synchronizers are precision components.
The working principle is to measure the relative movement position by the reflection of light.
Dust and vibration are precisely the biggest factors affecting measurement accuracy.
In addition, the cutting oil mist and water mist are more serious during the machining of the machine tool, which greatly affects the grating ruler and linear induction synchronizer.
Therefore, if using a fully closed loop control system, it not only needs to install and seal well, but also to improve the production environment.
Otherwise, this phenomenon will occur:
The accuracy of the new machine tool that just arrived is good, but after less than a year, not only the accuracy has dropped, the machine also often alarms.
Semi-closed loop control system
Since the measuring device is installed on the top of the motor or the lead screw, it is easier to seal, so there is no requirement for the environment.
The accuracy error of the semi-closed loop control system mainly depends on the forward and reverse clearance of the screw.
With the improvement of mechanical processing technology, the current manufacturing technology level of imported lead screws is relatively high, and the high-precision lead screw pair basically eliminates the forward and reverse clearance.
In addition, in the assembly process, the screw pair adopts a double-row reverse ball screw pair, which can completely eliminate the forward and reverse clearance.
In addition, many machine tool factories use the pre-stretching method when assembling the machine tool, which eliminates the influence of the thermal deformation of the machine tool on the accuracy of the screw drive.
Therefore, the current semi-closed loop control system has been able to ensure that the machine tool achieves high accuracy.
In summary, it can be seen that: in theory, if external factors are not taken into account, the full-closed-loop control may improve the basic positioning accuracy than the semi-closed-loop control.
However, if factors such as machine heat, environmental pollution, temperature rise, vibration, and installation are not properly addressed, there will be a situation where full closed-loop is worse than semi-closed loop.
It may be effective in a short time, but in a long term, the influence of dust and temperature changes on the grating ruler will seriously affect the measurement feedback data, and thus lose its effect.
At the same time, if there is a problem with the grating ruler, an alarm will be generated, causing the machine tool to fail to work.
For medium and low-end machine tools, due to the consideration of production costs and competitiveness, the full-closed-loop control has been simplified.
For example, sealing and temperature rise control is not well guaranteed.
Under this condition, it takes a lot of costs, and simple configuring the grating ruler cannot improve the accuracy of the machine tool.