In the machining industry, machining accuracy is a constant buzzword, repeated several times daily. Whenever you chat with someone in the industry, machining accuracy is almost always mentioned. So, what exactly ensures the machining accuracy of machine tools? This video explains the electrical precision control of machine tools very clearly. Let's take a look.
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The machining accuracy of CNC machine tools ultimately depends on the accuracy of the machine tool itself. CNC machine tool accuracy includes geometric accuracy, positioning accuracy, repeatability, and cutting accuracy.
Geometric accuracy, also known as static accuracy, comprehensively reflects the geometric shape errors of key components of a CNC machine tool after assembly.
Positioning accuracy: This indicates the achievable accuracy of the movement of each moving part of the machine tool under the control of the CNC device. The measured positioning accuracy value can be used to determine the optimal workpiece machining accuracy achievable during the machine tool's automated machining process. This value refers to the difference between the actual position of a part or tool and the standard position (theoretical position, ideal position). The smaller the difference, the higher the accuracy. It is the prerequisite for ensuring part machining accuracy.
Repeatability refers to the consistency of positioning accuracy achieved by repeatedly running the same program code on a CNC machine tool. It refers to the consistency of successive results obtained when machining a batch of parts under identical conditions (on the same CNC machine tool, with different operating methods and the same part program).
Cutting accuracy is a comprehensive examination of the machine tool's geometric and positioning accuracy under cutting conditions.
As can be seen above, the accuracy of CNC machine tools is divided into two aspects: mechanical and electrical. The mechanical aspect includes spindle accuracy, such as runout and busbar; leadscrew accuracy; fixture accuracy during machining; and machine tool rigidity. The electrical aspect primarily involves control methods, such as semi-closed loop and fully closed loop, as well as feedback and compensation methods, and interpolation accuracy during machining. Therefore, machine tool accuracy is not determined by whether the machine tool is fully closed loop.
I. Principle Introduction
The kinematic chain of a CNC machine tool consists of the CNC unit → servo encoder → servo drive → motor → leadscrew → moving element. Depending on the installation location of the position detection device, it is categorized as fully closed loop, semi-closed loop, or open loop control.
1. Fully Closed-Loop Feed Servo System
A position sensing device (such as a linear encoder or linear induction synchronizer) is installed on a machine tool's moving component (such as a worktable) to provide real-time feedback on the moving component's position. This information is processed by the CNC system and communicated to the servo motor, which then automatically compensates for motion errors based on system commands. However, since these components, such as the lead screw, nut assembly, and machine table, are contained within the closed loop, debugging the system to ensure stable operation is complex. Furthermore, measuring devices like linear encoders and linear induction synchronizers are expensive, complex to install, and can cause oscillation. Therefore, fully closed-loop control is generally not used on machine tools.
2. Semi-Closed-Loop Feed Servo System
A position sensing device is installed at the end of the drive motor or the lead screw to detect the rotational angle of the lead screw or servo motor, indirectly measuring the actual position of the machine tool's moving component and providing feedback to the control system. Due to advances in mechanical manufacturing and improvements in the accuracy of speed sensing elements and lead screw pitch, semi-closed-loop CNC machine tools can now achieve remarkably high feed accuracy. Most machine tool manufacturers widely adopt semi-closed-loop CNC systems.
II. Practical Applications
1. Fully Closed-Loop Control Systems
Position detection devices (such as linear encoders and linear induction synchronizers) have varying accuracy levels (±0.01mm, ±0.005mm, ±0.003mm, and ±0.02mm). Therefore, fully closed-loop control will also have errors, and positioning accuracy is affected by the accuracy level.
The thermal performance (thermal deformation) of the position detection device. The measuring device is generally made of non-metallic materials, and its thermal expansion coefficient is inconsistent with that of other machine tool components. It is a key factor in machine tool operating accuracy. Therefore, it is necessary to address the heat generation during machining to overcome temperature-induced thermal deformation. High-end machine tools use various methods, such as hollow screw cooling, guideway lubrication, and constant-temperature cooling of cutting fluid, to reduce thermal deformation during machining.
The installation of the position detection device is also very important. In theory, the closer it is to the drive axis (screw assembly), the more accurate the measurement. Due to space limitations, there are only two ways to install a scale: near the leadscrew assembly or outside the guide rail. The first installation method is recommended whenever possible, but it's inconvenient to inspect and maintain. Conversely, even if a high-precision scale is selected, it may not actually meet the accuracy requirements of the CNC machine tool. Even in the first case, the scale is mounted relatively close to the drive axis, but there is still a certain distance from the axis. This distance, combined with the oscillation of the object during driving, creates significant challenges for scale detection and control. When the object swings toward the scale's mounting position, the scale mistakenly interprets the movement as insufficient, and the system generates an acceleration signal. However, the object immediately swings to the other side, and the scale mistakenly interprets the movement as excessive, resulting in a deceleration signal. This repetitive cycle fails to improve the control of the CNC machine's linear axes and instead exacerbates the vibration of the object, leading to the unusual phenomenon of a fully closed loop system being inferior to a semi-closed loop.
Impact of the production environment: Mechanical processing plants generally operate in harsh environments, with dust and vibration being common. However, scales and linear induction synchronizers are precision components that rely on light reflection to measure relative position. Dust and vibration are the biggest factors affecting measurement accuracy. Furthermore, during machining, cutting oil and water mist are quite severe, significantly impacting scales and linear induction synchronizers. Therefore, a fully closed-loop control system is essential. Besides ensuring proper installation and sealing, the production environment must be improved. Otherwise, this phenomenon can occur. A newly arrived machine tool may have excellent accuracy, but within a year, it not only degrades but also experiences frequent alarms.
2. Semi-closed-loop control system
Since the measuring device is mounted on the motor or lead screw, it is easier to seal and therefore has less environmental requirements. The accuracy error of a semi-closed-loop control system is primarily determined by the backlash of the lead screw. With advancements in machining technology, the manufacturing process of imported lead screws is now more sophisticated, and the high-precision lead screw pair virtually eliminates backlash. Furthermore, during assembly, the lead screw pair utilizes a double-row counter-rotating ball screw pair, which completely eliminates backlash. In addition, many machine tool manufacturers pre-stretch the lead screw during assembly to eliminate the impact of thermal deformation on the screw drive accuracy. Therefore, currently, semi-closed-loop control systems are capable of ensuring very high machine tool accuracy.
III. 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 heat generation, environmental pollution, temperature rise, vibration, and installation are not effectively addressed, full-closed-loop control may be inferior to semi-closed-loop control. This may be effective in the short term, but over time, the effects of dust and temperature fluctuations on the scale will seriously affect the measurement feedback data, rendering it ineffective. Furthermore, scale failures will generate alarms, causing the machine tool to stop operating.
For low- and mid-range machine tools, due to cost and competitiveness considerations, the full-closed-loop control system is often simplified, with inadequate sealing and temperature rise control measures. Under these circumstances, simply adding a scale, at a high cost, will not improve machine tool accuracy.
