Effective Solutions for Mill-Turn Composite Machining

 

Efficiency and precision are the eternal goals in the field of metalworking. With the continuous development of CNC technology, computer technology, machine tools, and machining processes, traditional machining concepts no longer meet the demands for speed, efficiency, and precision. Against this backdrop, composite machining technology has emerged. Generally speaking, composite machining refers to a set of machining techniques that can perform different processes or methods on a single piece of equipment. The current composite machining technologies are mainly of two types: one is based on different energy or motion forms, and the other, primarily mechanical, is based on the principle of process concentration. Among these, mill-turn composite machining has become one of the fastest-growing technologies in recent years.

 

▲ Mill-Turn Composite Machining

 

Aerospace parts are increasingly characterized by small batches, complex processes, and extensive use of integral thin-walled structures and hard-to-machine materials. This results in bottlenecks like long manufacturing cycles, high material removal rates, low processing efficiency, and severe machining deformations. To improve the efficiency and precision of complex aerospace products, engineers have long sought more efficient and precise machining methods. The advent of mill-turn composite machining equipment offers an effective solution to improve the precision and efficiency of aerospace parts.

 

Compared to conventional CNC machining processes, the advantages of composite machining are mainly reflected in the following areas:

 

(1) Shortening the Manufacturing Process Chain and Increasing Production Efficiency

Mill-turn composite machining allows for the completion of most or all machining steps in a single clamping, significantly shortening the manufacturing process chain. This reduces auxiliary production time due to repositioning, shortens the manufacturing cycle for fixtures, and increases production efficiency.

 

(2) Reducing the Number of Clamps and Improving Machining Precision

Fewer clamping operations reduce errors caused by reference conversions. Modern mill-turn machines often have online measurement capabilities, enabling in-place inspection and precision control of key data during the manufacturing process, thus improving part accuracy.

 

(3) Reducing Floor Space and Lowering Production Costs

While a mill-turn composite machine may have a high individual price, the shortened process chain and reduction in required equipment, fixtures, floor space, and maintenance costs can significantly reduce overall investment, production operations, and management costs.

 

 

I Key Technologies in CompositeMachining

 

Despite its advantages over conventional single-process machining, the utilization of mill-turn composite machining in the aerospace industry has not yet reached its full potential. The primary reason lies in the relatively short application time of this technology in aerospace manufacturing, as well as the ongoing exploration of mill-turn processes, CNC programming, post-processing, and simulation technologies that suit the structural process characteristics of aerospace parts. To fully exploit the capabilities of mill-turn composite equipment and enhance efficiency and precision, it is essential to overcome these challenges and achieve integrated applications.

 

1. Mill-Turn Composite Machining Process Technology

Unlike conventional machining equipment, a mill-turn composite machining center is essentially a production line. Efficient and precise machining hinges on how well the process route is planned, how clamping is done, and how tools are selected based on the part's process characteristics and the features of mill-turn machining.

 

Process concentration is the most distinctive feature of composite machining. Therefore, a scientific and rational process route is key to improving mill-turn machining efficiency and precision. When bar stock is used as a blank for impeller manufacturing, the conventional machining route involves turning the impeller's external profile on a CNC lathe, followed by precision turning for reference surfaces, five-axis CNC machining for slotting, rough machining, semi-finishing, and final machining of surfaces and hubs, and finally drilling on a five-axis machining center or drilling equipment. Using the S192F mill-turn center, the entire process can be completed in a single clamping, and when processing bar stock, the machine can even automate cutting, feeding, and impeller mass production without human intervention. The process route may be set up as follows:

 

Spindle clamping of bar stock → Rough turning of the external profile → Precision turning of the external profile → Five-axis milling for slotting → Rough machining of the flow channel → Semi-finishing of the flow channel → Precision machining of the flow channel → Drilling → Back spindle clamping → Turning of the impeller's bottom plane → Drilling.

 

As shown, a single clamping completes all impeller machining steps, greatly improving both efficiency and precision.

For mill-turn centers with dual tool towers, dual-tower equipment comes with dual-channel control systems, where the upper and lower tool towers can be independently controlled. Simultaneous machining can be achieved through synchronized commands in the code. To fully exploit the equipment's capabilities, synchronous operation of multiple processes on the part can be realized, allowing, for instance, the simultaneous rough turning of the external profile and rough boring of the inner hole, thereby improving machining efficiency. The synchronous motion of the upper and lower tool towers allows for the efficient processing of a series of holes, improving efficiency while minimizing workpiece deformation through the balancing of axial drilling forces. To achieve this functionality, thorough research into the process route sequence and synchronization is necessary during the initial process design phase.

 

2. CNC Programming Technology for Mill-Turn Machining

The advancement of mill-turn machining technology demands higher standards for CNC programming, which has become a bottleneck in applying mill-turn equipment in actual production. Without specialized composite machining solutions, general CAM software is typically used to plan part of the machining program, which is then manually integrated by process engineers to meet the demands of composite machining. This method places high demands on process engineers. Compared to traditional CNC programming, mill-turn programming poses several challenges:

 

 

(1)Diverse processes

For process engineers, it is essential not only to master the programming methods for various machining methods like CNC turning, multi-axis milling, and drilling, but also to accurately define the connection between operations and the approach and retract paths. Therefore, during CNC programming, engineers need to have a clear understanding of the process model and the distribution of machining allowances after each operation to facilitate the programming and tool path settings for the subsequent operation.

 

(2) Determining the sequence of parallel and serial operations must strictly follow the process route

Many parts can be fully machined from raw material to finished product on a mill-turn composite machining center. Thus, the final CNC program must align with the process route. Additionally, multi-channel parallel machining needs to be thoroughly considered during the CNC programming process. Therefore, to achieve efficient composite machining, integrated solutions combining process, programming, and simulation need to be developed.

 

(3) Certain functions of mill-turn composite machining are not yet supported by current general CAM software

Compared to conventional single-machine machining, mill-turn composite machining has more complex machine movements and processing functions. Current general CAM software is still insufficient to fully support advanced features like online measurement, cutting, automatic feeding, and tailstock control. As a result, the programs generated by general CAM software often require substantial manual or interactive intervention before they can be applied to automated mill-turn composite machining.

 

(4) Program integration

NC programs generated by general CAM software are independent of each other. For automated and complex mill-turn composite machining, these independent programs must be integrated. This integration should be guided by the part's process route, first determining which programs are parallel and then defining the machining sequence for different processes. Accurate instructions for tool changes, clamping replacements, reference conversions, and approach/retract movements must also be provided.

 

It is evident that CNC programming for mill-turn composite machining is very challenging, and there are still many shortcomings and deficiencies in using general CAM software for this process. To address these issues, a more practical solution is to develop specialized programming systems based on existing general CAD/CAM software that cater to product processes and composite machining equipment. This approach not only reduces redundant software investments but also avoids problems like the inability to reuse process knowledge and the complexity of staffing caused by non-unified programming platforms.

 

 

3. Post-Processing Technology for Mill-Turn Machining

 

Corresponding to CNC programming technology, mill-turn composite machining, due to its complex processes and numerous moving parts, imposes higher requirements on current post-processing software and technologies. Compared to conventional CNC equipment, the challenges of post-processing for mill-turn composite machining are mainly reflected in the following aspects:

 

(1) Accurate and strict motion transitions between processes

Given the variety of processes on mill-turn composite equipment, after completing the current operation, the machine must automatically and accurately switch machining methods, tools, and moving components in a timely manner to ensure correctness and safety. To achieve this, it is necessary to set up appropriate approach and retract tool paths, as well as the timing for automatic tool changes and the on/off of coolant. More importantly, the positions of non-moving components during the current operation must be specified to prevent collisions between moving and non-moving parts during tool changes and machining, ensuring a safe and stable process.

 

(2) Automated determination of process sequences and CNC programs

In composite machining, the process route is relatively long, and manually organizing and integrating NC code after post-processing is not only inefficient but also prone to errors. An ideal solution is for the post-processing system to automatically determine the machining sequence and the process methods embedded in the toolpath files, ensuring that these are retained in the NC code after post-processing. Therefore, the toolpath file generated after CNC programming must not only contain process methods and tool position data but also include machining sequences, tool types, and tool numbers. This allows for automated determination of process sequences, methods, and tools during post-processing.

 

(3) Post-processing for different machining methods

The post-processing program for mill-turn composite machining must handle multi-axis CNC milling, turning, and drilling, as well as functions like cutting, automatic feeding, tailstock control, and program loop calls. The post-processing algorithm for mill-turn composite machining must encompass all existing CNC processing methods and seamlessly integrate and coordinate between different machining methods and movements.

 

(4) Maximizing advanced features of control systems

The CNC systems used in mill-turn composite machining centers are highly advanced, such as the FANUC 31i system used in Bumotec S192FT and the SINUMERIK 840D system used in WFL 150. These advanced control systems offer features like automatic feed optimization, tool vector smoothing, superior look-ahead, and high-speed, high-precision interpolation. Therefore, it is crucial to reflect these advanced CNC system functions in the appropriate sections of the machining code generated during post-processing to fully utilize the capabilities of mill-turn composite equipment.

 

(5) Handling and calling non-cutting functions

In addition to turning, milling, drilling, and boring functions, composite machining centers have non-cutting functions needed for process transitions, such as automatic feeding, unloading, spindle docking, and tailstock control. These functions need to be treated as common modules in the post-processing phase, callable by the program. The sequence and timing of these calls must be determined according to the process route. Currently, post-processing software does not fully support these features.

 

 

4. Simulation Technology for Mill-Turn Machining

 

Due to the numerous moving parts and complex functionalities of mill-turn composite machining, post-programming simulation becomes especially critical. Since the adoption of mill-turn composite machining in China's aerospace manufacturing industry is relatively recent, there are currently no mature simulation application technologies. Most manufacturers rely on trial cutting to verify and optimize programs, leading to long process preparation cycles, high development risks, and increased machining costs.

 

To improve the application of mill-turn composite machining and enhance programming efficiency, the adoption of simulation technology must be significantly promoted. Currently, the main software used for mill-turn composite machining simulation includes TopSolid and Gibbs, but these are generally expensive and less commonly introduced in China's aerospace manufacturing field. In fact, mill-turn composite machining simulation can also be achieved using general CNC simulation software such as Vericut or NCSimul. By customizing and developing macro functions based on the machine structure, movement characteristics, special functions, and CNC systems, it is possible to simulate the machining process.

 

To achieve mill-turn composite machining simulation using general CNC simulation software, it is first necessary to construct a relatively realistic machine environment within the simulation system. The focus should be on establishing the relative motion relationships and geometric positions of the machine's various moving parts. Based on this foundation, a tool library and corresponding tool numbers used in the machining process must be set up. Then, configure the CNC system and machining references of the machine, and load the NC code generated by the post-processing stage into the simulation system to execute the machining process simulation. Unlike conventional CNC machining, some functions (such as multi-channel machining or tailstock control) may require macro function development and customization to be fully implemented.

 

 

II Application Prospects and Development Suggestions for Mill-Turn Machining Technology

 

 

In recent years, mill-turn composite machining centers have been introduced to China's aerospace manufacturing industry, including aircraft, aero-engine, and accessory plants. The equipment mainly includes products from Austria's WFL series and Switzerland's Bumotec milling-turning centers. However, since their application in production is relatively recent, there is a general lack of mature machining processes, programming techniques, and post-processing technologies that align with the product and equipment characteristics. As a result, the introduced mill-turn composite machining equipment is currently operating at a relatively low level of efficiency.

 

The main challenges in aerospace product manufacturing are long process routes, complex procedures, low machining efficiency, significant deformation, and high costs. Both aircraft and engine manufacturing fields have vast potential for the application of mill-turn composite machining.

 

For instance, the milling of a fuselage frame involves dozens of steps: material preparation, rough and fine machining of inner and outer shapes, hole drilling, manual finishing, and inspection, requiring multiple re-clampings. Similarly, integral blade disk machining in aerospace engines starts with forging blanks and involves turning, milling, polishing, surface treatment, and flaw detection. These parts have long production cycles, often occupying machines for hundreds of hours, and require different types of CNC machines, along with numerous fixtures, tools, and measurement instruments. The frequent re-clamping not only prolongs waiting times but also accumulates positioning errors, impacting part accuracy and final quality.

 

Mill-turn composite machining, with its ability to complete most or all operations in a single setup, offers a new path for the efficient and precise machining of complex aerospace components. The advantages are mainly reflected in:

 

• Significant reduction in clamping times, improving efficiency while eliminating errors caused by changing machines or clamping methods.
• Process concentration, shortening the machining process chain and reducing waiting times and non-operating machine periods.
• Versatility in machining operations such as turning, milling, and drilling without altering positioning, reducing the number of required fixtures and ensuring consistent dimensional accuracy.
• On-machine measurement capabilities, allowing in-process and inter-process measurement to control accuracy throughout the entire machining cycle.

 

These advantages can effectively address the current shortcomings in aerospace part manufacturing, significantly enhancing product accuracy and efficiency.

 

To fully harness the potential of advanced composite machining equipment and further improve manufacturing efficiency and quality for aerospace products, several key areas need attention:

 

• Research into composite machining processes that align with the characteristics of aerospace parts, including determining process routes, clamping methods, tools, cooling strategies, and cutting parameters.
• Development and customization of CNC programming, post-processing, and simulation systems that match the equipment's structural and process characteristics, creating an integrated solution for process-programming-post-processing-simulation, thereby reducing the reliance on highly skilled personnel.
• Establishment of process standards through the accumulation of experience from simulation, trial cutting, and actual production, which can serve as a guide for subsequent part manufacturing.
• Talent cultivation, since composite machining represents cutting-edge technology in the machining field. Both process programming and operational maintenance are more complex than conventional equipment, making a high-level R&D team essential for ensuring efficient and healthy equipment operation.

 

 

 

III Conclusion

 

 

Currently, composite machining equipment is evolving towards broader process capabilities, higher efficiency, larger-scale operations, and modularization. The aerospace manufacturing sector has always been a crucial stage for advanced manufacturing technologies, and with the accelerating pace of aerospace product upgrades, dispersed process equipment will gradually be replaced by flexible, automated equipment with concentrated processes. This shift offers composite machining technology a broader space for development and application.

 

The trend toward more integrated, flexible manufacturing systems aligns well with the demands of modern aerospace manufacturing. As aircraft and aerospace components become more sophisticated and production timelines shorten, manufacturers will increasingly rely on advanced machining technologies like mill-turn composite systems to streamline processes, improve precision, and reduce costs. The ongoing development in automation and modularization of equipment will further enhance flexibility, allowing manufacturers to quickly adapt to new designs and production requirements, ensuring that the aerospace industry remains at the cutting edge of technological innovation.