Following on from the previous installment's purely practical content: the critical role of rotary axis positioning accuracy in five-axis machining, this will be discussed from the following aspects:
1. Discuss the full closed-loop and semi-closed-loop control modes of the rotary axis of the machine tool rotary table;
2. Discuss the role of two high-precision positioning rotary axes in 5- axis machining on machine tools;
Based on the experimental scenario described in the previous article, the editor provides the following experimental conclusions:
Although the rotary table under test typically operates under full closed-loop control, adjusting the machine's parameters can easily position the table in semi-closed-loop control mode. Therefore, the positional errors of the rotary table under the two control modes can be directly compared (see Figures 7 and 8 ). For comparison, 720 measurement points are evenly distributed along the circumference of the table, and bidirectional approach is performed first: clockwise ( CW ) and counterclockwise ( CCW ). The high-resolution positioning characteristics are manifested in position consistency, position independence, and the absence of systematic influences in rotary axis positioning. In full closed-loop control, the rotary axis achieves high positioning accuracy across the entire angle measurement range, making it almost impossible to distinguish the approach direction between forward and reverse approaches. This measurement curve primarily reflects the operating characteristics of the angle encoder under position control. Under semi-closed-loop control, there is a significant reverse error between the two directions, with a significantly larger range of variation than single-direction approach. The main reason is position-related errors in the mechanical transmission components (e.g., backlash, friction, and gear meshing factors). The high-frequency amplitude during clockwise measurement is greater than that during counterclockwise measurement, indicating that the tooth surface is worn. This phenomenon is consistent with the commonly used rotation direction.
Use a reference encoder to determine the positional accuracy of the rotary table.
All measurements described here were performed on the aforementioned machine tool. The control system allows switching between full closed-loop and semi-closed-loop modes. In full closed-loop mode, position feedback is provided using an RCN 8310 angle encoder. In semi-closed-loop mode, the position of the rotary table is calculated using the motor encoder signal and the worm gear ratio. Since the measurements were taken on the same machine tool using the same feed axis drive system, direct comparisons are possible. The positioning accuracy of the rotary table was determined using the measurement procedures outlined in ISO 230-2 and ISO 230-3 standards.
In production, increasing the flexibility of product specifications necessitates the use of 5- axis machining technology. A versatile tooling system allows for multi-faceted and complete machining, enhancing automation, flexibility, and machine tool utilization. Because 5- axis technology allows for the extensive use of standard cutting tools and enables changes in tool orientation along the milling path, it is suitable for machining complex geometries.
Determine static positioning accuracy using ISO 230-2 standard
First, the static positioning accuracy of the rotary table was determined using the ISO 230-2 standard. To do this, the 360° measurement range was evenly divided into 12 parts, spaced 30° apart . This angular step size corresponds to the typical number of measurement points using a collimator with a multi-faceted reflector. The measurement points were sequentially approached at a feed rate of 1000 °/min . Then, the final position was measured statically using a reference encoder on the rotary table. To obtain statistically significant measurement results, the operation was repeated five times in both clockwise and counterclockwise rotations. For comparative measurements, approximate initial conditions were used, employing not only the same machine tool but also omitting the C- axis compensation tables stored in the control system for both control modes .
In full closed-loop control mode (Figure 9 ), the measurement accuracy stabilized within ±1.3" , which is in line with expectations for using an angle encoder. Comparison revealed that the measurement results under semi-closed-loop control (Figure 10 ) showed lower positioning accuracy of the rotary table in any rotational direction, only ±5" . Furthermore, a noticeable 31" reverse error was observed when changing the approach direction . In the second measurement, clockwise and counterclockwise rotations were performed, and the positioning accuracy values at 12 sampling points were saved in the compensation value table, which was then activated.
After setting and activating nonlinear error compensation on the machine tool CNC system, excellent measurement results are now achieved in both control modes, reaching an acceptable level of accuracy (see Figures 11 and 12 ). Under full closed-loop control, the accuracy is improved to ±0.35" . In measurements under semi-closed-loop control, the position error across the entire rotation range is low, only ±1.4" . However, a small backlash of 1.0" is still noticeable. It must be noted that the compensation values represent the discrete state of the machine tool and are only applicable to the first measurement; the measured values in the table are static values. However, during operation, the machine tool's state and position do not change consistently due to thermal and mechanical loads, as well as wear of mechanical components. Therefore, after a period of time, using static table-compensated position errors will be difficult to achieve the high quality shown in Figures 11 and 12. (Figure 13 : Position error under semi-closed-loop control according to ISO 230-2 standard ( 60 measurement points, with compensation))
Measurements were repeated in semi-closed-loop control mode, using 12 sampling points and 60 measurement points to illustrate the positioning accuracy characteristics between the sampling points used for compensation. This is a typical number of sampling points for measurements using a collimator and multi-faceted mirrors. Figure 13 shows the results of multiple measurements. The figure shows a large positional error of ±4.5" and a reverse error of 4.0" . Furthermore, high-order nonlinearity is clearly visible between the selected sampling points. Similar to the small-range error in Figure 8 , it cannot be modeled using the compensation values in Figure 10 , and therefore the CNC system cannot handle it. Using a compensation table in semi-closed-loop control cannot guarantee the positioning accuracy between sampling points, and the final result is significantly different from Figure 12. Therefore, this compensation method is only suitable for rotary tables approaching a known position. For example, this applies to 3+2 axis machining.
In theory, a static compensation table for a CNC system can use a large number of sampling points, but this requires an impractical amount of measurement. Furthermore, the thermal state of the machine tool is not constant during machining and measurement. This will be discussed below.
According to ISO 230-230 standard, thermal drift, or the effect of temperature on machining, is determined.
According to ISO 230-3 standard, measurements performed under each control mode showed that heat generation in the rotary shaft drive train and other mechanical components of the rotary shaft caused position drift. In this measurement, compensation was activated in the machine tool CNC system based on the sampling points described earlier. Furthermore, the effect of position drift on the accuracy of the rotary shaft was determined according to ISO 230-3 standard. For this purpose, two positions ( 0° and 180° ) were defined and approached from two directions (clockwise and counterclockwise). Between measurements, five cyclic movements were performed at a feed rate of 3000 °/min to allow the temperature rise within the measurement range ( 0° to 180° ) to reach the required level. Measurements were continuously recorded until the position of the rotary table stopped thermally drifting.
The measurement results are shown in Figure 14. Using an angle encoder in the fully closed-loop control maintains stable positioning accuracy even with periodic rotation of the rotating shaft and temperature increases in the transmission chain components. In this configuration, the position is affected by temperature and measured by the angle encoder, with the measurement results fed back to the position control loop. The maximum value in this measurement is 0.5" .
Compared to the full closed-loop mode, the positioning accuracy in the semi-closed-loop mode varies significantly over time (Figure 15 ). This variation involves two parameters: the amplitude, which reaches a maximum of 8" , and a shorter time constant of approximately 2 minutes. Furthermore, position drift factors overlap at the 0° measurement position. This factor, resulting from the heating of the machining center's structural components, has a significantly longer time constant. Consequently, the distance between the two measurement points also changes continuously.
Furthermore, the reverse error in the two approach directions increases to 3" . In some applications, regardless of how large the error is, a short time constant is a problem in many machining applications. For example, in small-batch machining applications or periodic variations, the rotary axis performs positioning movements, then comes to a standstill (machining in the reverse direction), and the axis moves continuously. Changing broken tools also falls under this time constant problem.
Using a compensation table is not helpful for the rotary table being tested. The positional error appears to stabilize after approximately 25 minutes, but its state changes each time the machine tool or feed axis stops moving, such as during a second clamping or clamping of a new workpiece. This results in significant uncertainty in the achievable accuracy and directly affects the workpiece accuracy in 5- axis simultaneous machining and even 3+2 axis machining.
in conclusion
As can be seen from the standard rotary table of a high-end machining center: in semi-closed-loop control mode, the position error can reach 8" within 10 minutes . This is equivalent to a deviation of 20 µm on a radius of 0.5 m . The rotary axis has a relatively complex structural design, including servo motors and mechanical transmission systems. Environmental factors also make it difficult to measure various errors, making online compensation for the position error of the rotary axis almost impossible.
The positioning accuracy of the machine tool rotary table was measured in both full-closed-loop and semi-closed-loop control modes using a suitable reference encoder for comparative measurements. The measurements determined the static positioning accuracy of the rotary axis and its stability under cyclic load-induced heating. The effectiveness of compensation at 12 sampling points was also determined and compared. In all these measurements, the full-closed-loop control mode exhibited stable results with high positioning accuracy and low backlash. In the semi-closed-loop control mode, compensation significantly improved the initial positioning error, but under cyclic loads, the transmission system components could not maintain stable accuracy. The measurement results showed that the main characteristic of a shorter time constant was the temperature change of the mechanical transmission system over time, effectively preventing the machine tool CNC system from compensating. It was also observed that compensation could not offset the higher-order, nonlinear effects between sampling points.
In fully closed-loop control mode, Heidenhain angle encoders directly measure the motion of the rotary shaft. This allows most influencing factors and time-dependent changes in the mechanical system to be accounted for in the position control loop. These factors include mechanical transmission errors, temperature effects, and wear. The only exception is the error measurable by the angle encoder, but its dynamic performance exceeds the dynamic performance range of the position control loop. The RCN series of absolute angle encoders with built-in bearings and stator couplings are ideal for high-precision rotary shafts with mechanical transmissions and for direct-drive technology. These encoders feature a fully enclosed design, resulting in high system accuracy, simple installation, and excellent resistance to contamination. If encoder installation is not possible due to structural limitations, modular angle encoders with optical scanning can be used. The ERA 4000 and ECA 4000 series offer a considerable number of signal cycles and high positioning accuracy. For the rotary table applications discussed here, special attention must be paid to the rigidity and performance of the rotary table bearings, as their performance directly affects the actual measurement accuracy achievable by the angle encoder, and thus the positioning accuracy of the rotary table. The design of rotary axes needs to provide ideal working conditions for 5- axis machining applications with high process reliability and high throughput .
