Precision Improvement Strategies in Multi-Tasking Turn-Mill Centers
This article analyzes methods for optimizing machining accuracy, using a 9-axis milling-turning machining center as an example. It discusses the current application status of the center, presents results from accuracy inspections, outlines strategies for accuracy optimization, and identifies key development directions. This analysis serves as a reference for enhancing milling-turning machining centers and improving the quality of automated machining.
01 Introduction
Maintaining precise machining accuracy is essential during milling and turning operations with a 9-axis milling-turning machining center. The complex structure of these machines can lead to protective movements when they are subjected to external forces or impacts, which can reduce geometric accuracy and ultimately affect the quality of milling and turning operations.
To effectively tackle these issues and enhance the accuracy of machining centers, technicians must perform a thorough inspection to identify any problems. Based on the inspection results, they can then implement suitable strategies to optimize accuracy. This proactive approach can significantly improve the overall performance of the machines and ensure they meet the demands of actual milling and turning tasks.
02 Turning and milling compound 9-axis machining center and its application status
2.1 Basic overview
The 9-axis turning and milling compound machining center is a high-efficiency CNC equipment that combines several advanced technologies. This machine builds on the traditional CNC lathe by adding a milling function, allowing for the seamless integration of various processing methods, including turning and milling. In specific applications, the machining center can securely hold workpieces and perform multiple operations simultaneously, such as turning, milling, drilling, boring, and tapping. With advantages like high production efficiency, excellent product quality, and low production costs, this type of composite machining center has become widely adopted in modern mechanical automation production and processing.
2.2 Main components
The main components of the 9-axis turning and milling machining center include the machining spindle, three linear guides for the X, Y, and Z axes, three rotary guides for the A, B, and C axes, and three additional linear guides for the U, V, and W axes. The machining spindle is responsible for cutting; the X, Y, and Z axes control the direction of material feed; the A, B, and C axes manage the angle of the material; and the U, V, and W axes handle the position of the material. In specific applications, the coordinated operation of these nine axes allows the entire CNC machining process to be more precise and efficient, effectively meeting the practical CNC machining needs of modern mechanical products.
2.3 Current Application Status
Currently, 9-axis milling and turning machining centers are widely utilized in high-precision machining applications, including the manufacturing of electronic components, automotive parts, aerospace components, and molds. The effective use of these machining centers allows for the production of finely crafted mechanical parts that meet the specific requirements of various industries, while also significantly enhancing machining efficiency.
However, practical applications have shown that these machining centers can experience precision deviations during operation due to the influence of certain external factors. This can adversely affect machining accuracy. To address this issue, machinery manufacturing companies, researchers, and technicians should intensify their precision inspection efforts and implement appropriate technical measures to mitigate precision errors based on actual conditions. This approach can help ensure that machining precision is effectively optimized.
03 Accuracy Inspection Results of a 9-Axis Mill-Turning Machining Center
To effectively identify accuracy issues in the practical applications of 9-axis mill-turning machining centers, this study focused on an M40-G 9-axis mill-turning machining center (refer to Figure 1) from a machinery manufacturer for a comprehensive geometric accuracy inspection. This complex machining center features a dual-turret, dual-spindle configuration with a Y-axis. The upper turret has been developed into a milling head, capable of high-speed and efficient rotation in practical applications, with a tool magazine located on the B-axis. Both the main spindle and the counter spindle can control the C-axis, and both are designed as high-performance electric spindles. The upper milling spindle can be linked across the X, Y, Z, and B axes, while the lower turret can be linked across the X and Y axes.
The overall structure of the machining center is complex and requires high precision, making adjustments particularly challenging. During practical operation, an operator error caused the milling spindle housing of the upper turret to collide with the spindle housing, severely compromising the geometric accuracy of the upper turret, spindle, and various guideways. Concurrently, since the sub-spindle, lower turret, and main spindle of the machining center have a certain relative precision, after adjusting the main spindle, maintenance technicians are required to readjust the sub-spindle and lower turret as well.
3.1 Main and Sub-spindle Accuracy Inspection
During the inspection of the geometric accuracy of the main and sub-spindles of this multi-tasking machining center, the maintenance technician began by removing the spindle chuck. They then thoroughly cleaned the reference end face and the inner bore of the spindle. Following the inspection sequence outlined in the maintenance manual, the technician assessed the spindle for radial runout, axial play, and axial runout. The results showed that the spindle’s geometric accuracy was fully acceptable.
Next, the technician inspected the proximal and distal ends of the spindle for runout, which also met the required standards. However, further examination revealed that the parallelism between the spindle axis and the Z-axis guideway measured 0.01 mm over a distance of 300 mm or better, indicating poor overall geometric accuracy. Additionally, the coaxiality between the main and sub-spindle measured φ0.01 mm or better, confirming further issues with geometric accuracy.
As a result, maintenance technicians will need to adjust the parallelism between the spindle axis and the Z-axis guideway, as well as the coaxiality between the main and sub-spindles during subsequent operation and maintenance tasks.
3.2 Upper Turret Milling Axis Accuracy Inspection
Due to a collision between the upper turret of this multi-tasking machining center and the spindle, maintenance technicians must conduct a thorough inspection of the milling axis accuracy.
When the milling axis was positioned at the machine’s 0° point, the technicians assessed the parallelism between the milling axis and the X-axis. The inspection revealed that the parallelism was measured at 0.01 mm per 100 mm or worse, indicating poor geometric accuracy.
To improve this geometric accuracy during subsequent repairs, the maintenance technicians will need to make appropriate adjustments to the parallelism between the upper turret milling axis and the X-axis.
3.3 Results of the Spindle and Lower Turret Accuracy Inspection
The maintenance technicians conducted a thorough inspection of the geometric accuracy between the spindle and the lower turret. The inspection method involved attaching a gauge holder to the spindle, rotating the cutterhead to the first tool position, and installing a special inspection tool onto the tool holder. They then set the X-axis coordinate to zero, used a micrometer indicator on the inspection tool, and manually rotated the spindle to check for any accuracy changes between the spindle and the lower turret.
During the inspection, it was found that the accuracy change between the two CNC components was 0.015 mm or more, indicating poor geometric accuracy. As a result, the maintenance technicians will need to readjust the cutterhead center in future operation and maintenance work to optimize the geometric accuracy between the spindle and the lower turret.
04 Precision Optimization Strategy for a 9-Axis Milling/Turning Machining Center
4.1 Precision Optimization Strategy for the Main and Sub-spindles
When adjusting the parallelism between the main spindle axis and the Z-axis guideway of a multi-axis machining center, it is essential to maintain high precision in the coaxiality between the sub-spindle and the main spindle. After adjusting the main spindle’s accuracy, maintenance technicians must also readjust the sub-spindle to ensure both axes meet the specified design requirements.
To facilitate this process, the maintenance team designed a spindle adjustment plate. By loosening the upper and lower fastening screws on the spindle seat, technicians can adjust the parallelism between the main spindle axis and the Z-axis guideway, as well as the coaxiality between the main spindle and the sub-spindle.
To perform the adjustments, a test mandrel is installed on the spindle, and a micrometer is fixed to the lower turret. The X-axis is then moved slowly, bringing the micrometer close to the mandrel and setting its reading to zero. Next, the Z-axis is moved gradually from the near end of the mandrel to the far end, checking the parallelism between the main spindle axis and the Z-axis guide rail to ensure it stays within 0.01 mm over a distance of 300 mm. A dial indicator is secured to the sub-spindle, and it is slowly rotated to verify the coaxiality with the main spindle, which should also be within 0.01 mm.
By repeatedly adjusting the screws, the design accuracy can be achieved. Once qualified, this adjustment is complete, and the maintenance technician will tighten the screws on the spindle seat. This method optimizes the geometric accuracy between the main spindle and sub-spindle of the multi-functional machining center, ensuring operational performance and machining accuracy that meet the actual application requirements of the machining center.
4.2 Precision optimization strategy for the upper turret milling axis
To address the issue of poor parallelism between the upper turret milling axis and the X-axis of the composite machining center, the maintenance technician should follow these steps for proper adjustments:
First, install the inspection core rod on the upper turret milling axis and secure the micrometer on the spindle.
Slowly move the Z-axis until the micrometer approaches the side of the core rod. Adjust the micrometer to zero.
Loosen the fastening screw on the upper turret milling axis. Use a wooden hammer to gently adjust the upper turret milling axis to correct the parallelism with the X-axis.
Next, slowly move the X-axis from the near end of the core rod to the far end, checking the parallelism between the upper turret milling axis and the X-axis. Ensure that it is within the tolerance of 0.01 mm per 100 mm.
If the parallelism does not meet the standard, repeat the above steps until it is corrected. If the required parallelism is achieved, tighten the fastening screw on the upper turret milling axis.
By following these steps, the technician ensures proper parallelism between the spindle and the X-axis of the upper turret milling spindle, which optimizes the geometric accuracy of the spindle and meets the machining center’s requirements for subsequent production operations.
4.3 Precision Optimization Strategy for the Spindle and Lower Turret
To optimize the geometric accuracy between the spindle and lower turret of the composite machining center, maintenance technicians begin by adjusting the X-axis position. They then rotate the spindle and measure the micrometer deviation in the X-axis direction. If the deviation is within 0.015 mm, this confirms compliance, allowing them to proceed with further optimization.
Next, the technicians focus on the Y-axis direction. They start by loosening the cutterhead screws and tightening only one screw diagonally. Using a wooden hammer, they gently tap the cutterhead in the Y direction while simultaneously checking the geometric accuracy deviation in the Y-axis. Again, if the deviation is within 0.015 mm, this confirms compliance, and they can conclude the adjustments for the Y-axis.
Afterward, the technicians need to reset the X-axis to zero and ensure all screws on the toolholder are tightened.
This method successfully optimizes the geometric accuracy of the spindle and lower turret of the multi-tasking machining center. As a result, it significantly improves machining efficiency and quality, enhancing the overall performance of the machining center and meeting the high-precision machining requirements of production.
05 Main Development Direction of Milling-Turn Composite 9-Axis Machining Center
Through the analysis and summary of previous practical applications, it has been observed that while the milling-turn composite 9-axis machining center offers significant advantages in CNC machining, such as high machining efficiency and excellent accuracy, its complex overall structure presents challenges. If negatively impacted during actual operation, it can lead to a series of geometric accuracy errors that may hinder production efficiency.
To minimize the likelihood of these issues and maintain precise geometric accuracy in these machining centers, researchers and technicians must leverage advances in modern science and technology. In-depth studies on the future development of these systems are essential for improving their reliability and effectiveness.
At present, the main development directions of this type of machining center include the following aspects.
(1) Automation
Integrate advanced programmable logic controllers into the machining center’s operations and monitor the system in real-time through effective programming. This will allow for the prompt detection of any operational anomalies. Additionally, support maintenance personnel in conducting various inspections to identify geometric accuracy deviations in the corresponding CNC lathe machining parts, enabling timely and effective optimization and processing.
(2) Intelligence
Advanced intelligent technologies, such as neural networks and machine learning, are being effectively integrated into machining centers. This allows these machines to simulate human thinking and operate autonomously. When there is a geometric accuracy deviation caused by external factors, intelligent technology can scientifically analyze the location and main causes of the deviation. Based on this analysis, it offers tailored operation and maintenance recommendations to the maintenance staff. This support simplifies their work processes, reduces their workload, and significantly enhances both the precision optimization efficiency and the quality of these machining centers.
(3) Artificial Intelligence
The advanced artificial intelligence robot is now integrated with the machining center, replacing manual operations. This integration effectively reduces the risk of human errors that can affect the machining center’s performance and helps maintain precise geometric accuracy. This advancement will significantly enhance the control of geometric accuracy and improve the quality of production processing for the 9-axis turning and milling composite machining center.
06 Conclusion
In summary, the 9-axis mill-turn machining center is an advanced system in today’s CNC machining field, offering significant advantages in the production and processing of high-precision mechanical parts. However, due to its complex structure, inspecting and adjusting geometric accuracy can be challenging. Additionally, these machining centers are prone to geometric accuracy deviations, influenced by various external factors, which can negatively impact production and normal operation.
To effectively tackle this issue, researchers and maintenance technicians should prioritize the inspection and optimization of accuracy in 9-axis mill-turn machining centers. It is essential to conduct in-depth research on their future development based on practical applications. This focus will improve the overall geometric accuracy and production quality of these machining centers, providing essential support for their continued operation and growth.
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