What Guarantees the Machining Accuracy of Machine Tools?
In the machining industry, people often mention machining accuracy. They repeat it several times every day. When talking with others in the industry, they usually bring up machining accuracy within three sentences.So what guarantees the machining accuracy of machine tools? This video explains the electrical accuracy control of machine tools very clearly. Let’s take a look.
The accuracy of the machine tools themselves ultimately guarantees the machining accuracy of CNC machine tools. The accuracy of CNC machine tools includes geometric accuracy, positioning accuracy, repeated positioning accuracy and cutting accuracy.
Four Types of Precision of CNC Machining Tools
Geometric Accuracy:
Also known as static accuracy, it is a comprehensive reflection of the comprehensive geometric shape error of the key parts of the CNC machine tool after assembly.
Positioning Accuracy:
It indicates the accuracy that the moving parts of the measured machine tool can achieve under the control of the CNC device. Based on the measured positioning accuracy value, we can determine the best workpiece processing accuracy that the machine tool can achieve during automatic processing. 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 premise for ensuring the processing accuracy of the part.
Repeated Positioning Accuracy:
It refers to the consistency of the position accuracy obtained by repeatedly running the same program code on the CNC machine tool. It describes how consistently a CNC machine tool processes a batch of parts under the same conditions (same machine, different operators, and same part program).
Cutting Accuracy:
It is a comprehensive inspection of the geometric accuracy and positioning accuracy of the machine tool under cutting processing conditions.
From the above, we can see that the accuracy of CNC machine tools includes both mechanical and electrical aspects. The mechanical aspect includes spindle accuracy, such as runout, busbar, etc.; screw accuracy; fixture accuracy during processing, machine tool rigidity, etc. The electrical aspect mainly includes control methods such as semi-closed loop, full closed loop, etc., as well as feedback and compensation methods, interpolation accuracy during processing, etc. Therefore, we should not determine the accuracy of machine tools solely based on whether they use full closed-loop control.
1. Principle Introduction
The motion chain of CNC machine tools includes CNC device → servo encoder → servo drive → motor → lead 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.
1. Full Closed-loop Control Feed Servo System
The system installs the position detection device (such as a grating ruler or linear induction synchronizer) on the moving parts of the machine tool (such as the workbench) and feeds back their position in real time. The CNC system processes this information, informs the servo motor of the machine tool status, and the servo motor automatically compensates for the motion error according to the system command.
However, since it puts the lead screw, nut pair and machine tool workbench, which have large inertia links, in the closed loop, it is more troublesome to debug the stable state of the system during debugging. In addition, measuring devices such as grating rulers and linear induction synchronizers are expensive, complicated to install, and may cause oscillation, so general machine tools do not use full closed-loop control.
2. Semi-closed Loop Control Feed Servo System
The system installs the position detection device at the end of the drive motor or the lead screw to detect the rotation angle of the lead screw or servo motor. It then indirectly measures the actual position of the moving parts of the machine tool and sends this information back to the control system through feedback. Due to the improvement of mechanical manufacturing level and the improvement of speed detection elements and lead screw pitch accuracy, semi-closed loop CNC machine tools can achieve a fairly high feed accuracy. Most machine tool manufacturers widely use semi-closed loop CNC systems.
2.Practical Application
2.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), so there will be errors in the fully closed-loop control, and the positioning accuracy is affected by the accuracy level.
The thermal performance (thermal deformation) of the position detection device plays a key role in the working accuracy of the machine tool. Since the manufacturer generally uses non-metallic materials to make the measuring device, its thermal expansion coefficient differs from that of various machine tool components. Therefore, engineers must solve the heating problem during machine tool processing to overcome the thermal deformation caused by temperature. High-end machine tools will use various methods, such as screw hollow cooling, guide rail lubrication, and cutting fluid constant temperature cooling to reduce thermal deformation during machine tool processing.
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 structural space limitations, there are only two ways to install the grating ruler, one is to install it near the screw pair side, and the other is to install it on the outside of the guide rail. We recommend choosing the first installation method whenever possible, although it makes inspection and maintenance inconvenient. On the contrary, some users select a high-precision grating ruler but fail to achieve the actual accuracy required by the CNC machine tool. Even in the first case, the installation position of the grating ruler is relatively close to the drive axis, but the installation position is a certain distance from the drive axis after all.
This distance combined with the swing of the object during driving brings great trouble to the detection and control of the grating ruler. When the driven object swings toward the installation side of the grating ruler, the grating ruler mistakenly thinks that the moving speed is insufficient during detection, and the system gives an acceleration signal.
The driven object immediately swings to the other side, and the grating ruler mistakenly thinks that the moving speed is too fast during detection, and the system gives a deceleration signal. This repeated operation did not improve the control of the linear coordinate axes of the CNC machine tool, but aggravated the vibration of the driven object, resulting in the strange phenomenon that the full closed loop is not as good as the semi-closed loop. Influence of production environment: Generally, the environment of mechanical processing factories is relatively bad, and dust and vibration are common phenomena. However, grating rulers 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 the measurement accuracy. In addition, when the machine tool is processing, the cutting oil mist and water mist are relatively serious, which has a great impact on the grating ruler and linear induction synchronizer. Therefore, a fully closed-loop control system should be used. In addition to doing a good job of installation and sealing, the production environment must be improved. Otherwise, this phenomenon will occur. The accuracy of the new machine tool that has just arrived is good, but after less than a year, not only the accuracy has decreased, but the machine tool also often alarms.
2.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 positive and reverse clearance of the lead screw. With the improvement of mechanical processing technology, the current manufacturing process level of imported lead screws is relatively high, and the high-precision lead screw pair basically eliminates the positive and reverse clearance.
In addition, in the assembly link, the lead screw pair uses a double-row reverse ball screw pair, which can completely eliminate the positive and reverse clearance. In addition, many machine tool factories use pre-stretching method to assemble the machine tools, which eliminates the influence of thermal deformation of the machine tools on the transmission accuracy of the screws. Therefore, the current semi-closed loop control system can ensure that the machine tools can achieve very high accuracy.
3. Conclusion
In summary, it can be seen that, in theory, if external factors are not considered, full closed-loop control may improve basic positioning accuracy compared to semi-closed-loop control. However, if factors such as machine tool heating, environmental pollution, temperature rise, vibration, and installation cannot be well resolved, the full closed loop will be inferior to the semi-closed loop. It may be effective in a short period of time, but over time, the impact of dust and temperature changes on the grating ruler will seriously affect the measurement feedback data, thus losing its effect. At the same time, when there is a problem with the grating ruler, an alarm will be generated, causing the machine tool to fail to work.
For low- and medium-end machine tools, due to production costs and competitiveness, the supporting facilities of full closed-loop control have been simplified, such as sealing and temperature rise control. Under such conditions, it costs a lot to simply configure a grating ruler and cannot improve the accuracy of the machine tool.