Key Technology and Application Prospect of Turning/Milling Machining

To improve the machining efficiency and accuracy of complex aerospace products, technicians have been seeking more efficient and precise machining process methods. The emergence of mill-turn compound machining equipment for improving aviation parts’ machining accuracy and efficiency provides an effective solution.

Processing efficiency and precision are the eternal goals of the pursuit of the metal processing field. With the continuous development of CNC technology, computer technology, machine tool technology, and machining technology, the traditional processing concept has been unable to meet the requirements of people on processing speed, efficiency, and precision.

In this context, composite machining technology came into being. Generally speaking, composite processing refers to processing equipment to complete different processes or methods of processing technology.

The current composite processing technology is mainly manifested as 2 different types, one is based on energy or movement mode of different processing methods of composite; the other is based on the principle of process concentration, based on machining process-based composite, milling and turning composite processing in recent years is one of the most rapid development of the field of processing methods.

The current aviation product parts are highlighted as a multi-species small batch, complex process, and widely used in the overall thin-walled structure and difficult to process materials, so the manufacturing process generally exists in the manufacturing cycle is long, the amount of material removal, processing efficiency is low, and the processing of deformation and other serious bottlenecks.

To improve the machining efficiency and accuracy of aviation complex products, technicians have been seeking more efficient and precise machining process methods. The emergence of mill-turn composite machining equipment to improve the machining accuracy and efficiency of aviation parts provides an effective solution.

Compared with conventional CNC machining process, composite machining has outstanding advantages mainly in the following aspects.

(1) Shorten the manufacturing process chain and improve production efficiency.

Turning and milling composite processing can realize a card to complete all or most of the machining process, thus greatly reducing the product manufacturing process chain. This on the one hand reduces the production of auxiliary time due to the change of loading card, but also reduces the manufacturing cycle of tooling fixtures and waiting time, which can significantly improve production efficiency.

(2) Reduce the number of clamping, and improve machining accuracy.

The reduction of the number of mounting to avoid the accumulation of errors caused by the transformation of the positioning reference. At the same time, most of the current mill-turn compound machining equipment has the function of online detection, which can realize the manufacturing process of key data in the detection and precision control, thus improving the machining accuracy of the product.

(3) Reduce floor space, and reduce production costs.

Although the single unit price of mill-turn composite machining equipment is relatively high, due to the shortening of the manufacturing process chain and the reduction of the equipment required for the product, as well as the number of fixtures and fittings, the workshop footprint and the reduction of equipment maintenance costs, it can effectively reduce the overall investment in fixed assets, the cost of production operations and management.

The key technology of compound machining

Although composite machining has conventional single processing can not be compared to the advantages the current utilization of mill-turn composite machining in the field of aerospace manufacturing has not been fully utilized. The key reason for this is that mill-turn composite machining in the field of aviation manufacturing application time is relatively short, applicable to the structural characteristics of aviation parts mill-turn composite machining process, CNC programming technology, post-processing, and simulation technology are still in the exploratory stage.

To give full play to the effectiveness of mill-turn machining equipment, and improve product processing efficiency and accuracy, one must comprehensively overcome and solve the above key fundamentals, and realize the integrated application.

1. mill-turn compound machining technology

Unlike conventional machining equipment, a mill-turn machining center is equivalent to a production line.

How to develop a reasonable process route, mounting methods, and selection of reasonable tools according to the process characteristics of parts and milling and turning composite machining process characteristics is the key to achieving efficient precision machining.

Process concentration is the most distinctive process characteristic of composite machining.

Therefore, a scientific and reasonable process route is the key factor in improving the efficiency and precision of mill-turn machining.

Take the S192F milling-turning composite machining center of Swiss Baomei Company shown in Fig. 1 as an example, the machine tool has five-axis milling, turning, boring, drilling, sawing, and automatic feeding, etc., and adopts the F A N U C 31i numerical control system, which has the functions of tool vector smoothing, super prospective, high-speed interpolation, etc., which is especially suitable for high-speed precision machining of shafts and rotary parts.

In aviation impeller processing, this machining center has outstanding advantages. When using the bar as the impeller blank, the conventional impeller machining process route first uses CNC lathes to turn the external profile of the impeller, and then finishes turning machining benchmarks; on this basis, the use of five-axis CNC machining centers for grooving, roughing, semi-finishing, and finishing of the profile and the hub; and finally in the five-axis machining centers or drilling equipment for hole machining.

The S192F milling and turning machining center can not only complete all the processing of the above process by one-time loading, but also realize the batch processing of the impeller by sawing, automatic feeding, and other functions when the bar material is used for processing, and the whole process can be completed automatically without human intervention.

The process route can be set up as follows: spindle loading bar material → rough turning impeller external profile → fine turning external profile → five-axis milling slotting → runner rough machining → runner semi-finishing → runner finishing → drilling → back spindle loading → turning impeller bottom plane → drilling. It can be seen that a loading card to complete all the impeller machining process, machining efficiency, and accuracy can be greatly improved.

For turning and milling machining centers with dual tool holders (such as Austria’s W F L Turning and Milling Compound Machining Center shown in Figure 2), the equipment with dual turret has a dual-channel control system, the upper and lower tool holders can be controlled individually, and synchronous machining can be achieved through synchronous statements in the code.

To give full play to the machining capacity of the equipment, it is possible to realize the simultaneous machining of multiple processes of the parts through the synchronized operation of the dual turret under the premise of the machining conditions.

Through the synchronized setting of the upper and lower tool holders, the rough boring of the bore can be completed at the same time as the rough turning of the shape, thus further improving the machining efficiency. The synchronized movement of the upper and lower tool holders to complete a series of holes not only improves the machining efficiency but also reduces the impact of workpiece deformation through the mutual offset of the axial force of drilling. To realize this function, it is necessary to carry out a systematic and in-depth study of the process program during the preliminary process design, determine the serial and parallel order of the process route, and realize the above functions through a reasonable combination of machining programs.

2. mill-turn composite machining CNC programming technology

The development of turning and milling composite machining technology also puts forward higher requirements for CNC programming technology, which also restricts the turning and milling composite machining equipment in the actual production application of a bottleneck link.

Due to the mill-turn composite machining into the actual production of the application of a shorter period, in the absence of professional solutions for composite machining, usually the use of general-purpose C A M software to plan out some of the machining programs, and then the craftspeople and then manually integrate the program to meet the composite machining machine tool on the requirements of the machining program.

This solution is very demanding on the craftsmen. Compared with the traditional CNC programming technology, the programming difficulties of mill-turn compound machining are mainly reflected in the following aspects.

(1) The variety of processes is complicated. For craftsmen, not only to be able to master CNC turning, multi-axis milling, drilling, and other processing methods of programming methods but also for the articulation between the process and the way in and out of the tool needs to be accurately defined. Therefore, in CNC programming, you need to complete the current process model and the distribution of machining allowances to have an intuitive understanding of the next process of programming and setup in and out of the tool.

(2) The programming process of string parallel order must be determined in strict accordance with the process route. Many parts in the mill-turn machining center processing can be achieved from the raw material to the finished product of the complete processing, so the results of the machining program must be consistent with the process route. At the same time, multi-channel parallel processing also needs to be in the process of CNC machining program preparation for comprehensive consideration. It can be seen, to achieve efficient composite machining, developed process – programming – simulation of the integration of process solutions.

(3) For certain functions of mill-turn complex machining, the current general-purpose CAM software does not yet support it. Compared with conventional single-device processing, mill-turn complex machining with the machine movement and processing functions is more complex, the current general-purpose C A M software is not yet sufficient to fully support the programming of these advanced functions, such as online measurement, sawing, automatic feeding, tailstock control, etc. Therefore, the program prepared by using the general-purpose C A M software still requires a lot of manual or interactive ways to be applied to the automated mill-turn compound machining.

(4) The integration of machining programs. At present, the general C A M software compiled after the completion of the N C program is independent of each other, to realize the mill-turn compound such a complex automated complete machining, the need for these independent machining program integration and integration. This integration must be guided by the process route of the part, first determine which programs are parallel, and then the machining sequence of different process methods to determine, and give the exact tool change, mounting card replacement, datum conversion, as well as in and out of the tool instruction, etc..

As can be seen, mill-turn compound machining CNC programming is very difficult, and in the current general-purpose C A M software for mill-turn compound machining, there are still many defects and deficiencies. To make up for these shortcomings, the existing general C A D / C A M software based on the development of product processes and composite machining equipment for the development of specialized programming systems is a more realistic solution.

This on the one hand reduces the duplication of investment in the acquisition of software but also avoids the process knowledge that can not be reused due to the programming platform is not uniform, complex staffing, and other defects.

3. mill-turn composite processing post-processing technology

Corresponding to the CNC programming technology, mill-turn complex machining due to the complexity of the process method, moving parts, and other reasons, so the current post-processing software and technology puts forward higher requirements. Compared with conventional CNC equipment, the difficulties of its post-processing are mainly reflected in the following aspects.

(1) The articulation movement between different processes requires strict accuracy. As a result of a variety of machining processes on the mill-turn equipment, so after the completion of the current process must be timely and accurately completed the automatic switching of the processing mode, tools, and moving parts to ensure that the machining process is correct and safe.

To achieve this purpose, on the one hand, the requirements of setting a reasonable way into and out of the tool and automatic tool change, coolant on and off timing, and more importantly, in the current process need to be set in the current process of processing the position of other non-moving parts. To avoid the machine tool in the tool change and processing of moving parts and non-moving parts of the collision between the process to ensure the safety and stability of the machining process.

(2) The need for automatic determination of the process sequence and CNC program. Due to the relatively long process route in composite machining, relying on manual completion of the N C code after the organization and integration is not only inefficient but also prone to errors. The ideal solution is to automatically determine the machining sequence and the process method embedded in the tool position file during post-processing and to automatically maintain it in the NC code after post-processing is completed.

For this reason, CNC programming is completed after the tool position file information needs to contain not only the corresponding process methods, and tool position information but also needs to contain the corresponding machining sequence, the type and number of tools used so that in the post-processing process to achieve the process sequence, process methods and tools automatically determine.

(3) Post-processing technology for different machining methods. The post-processing program of mill-turn compound processing not only requires the realization of multi-axis CNC milling, turning, and drilling post-processing but also the realization of sawing, automatic feeding, tailstock control, program cycle calling, etc. The post-processing algorithms of mill-turn compound processing include the post-processing of all existing CNC machining process types, and also the realization of seamless integration and motion articulation between different machining modes. Integration and motion articulation between different machining methods.

(4) The full utilization of the advanced functions of the control system. Currently used for mill-turn machining center CNC systems are very advanced control systems, such as the Baomei S192F T FANUC 31i system, and the WFL 150 SINU M E R I K840D system. Most of these advanced control systems are equipped with advanced features such as automatic feed optimization, tool vector smoothing, super foresight, and high-speed, high-precision interpolation. Therefore, it is necessary to reflect the functions of these advanced CNC systems in the appropriate position in the machining code completed in the post-processing stage to realize the full utilization of the effectiveness of the mill-turn machining equipment.
(5) non-cutting function processing and call.

Composite processing machine tools in addition to having a car, milling, drilling, boring, and other cutting functions, but also in the transition between the various processes require non-cutting functions, such as automatic feeding, unloading, spindle docking, tailstock control, etc. In the post-processing needs to be these functions as a common module for the program to call, call the order and timing need to be determined according to the process route. These functions can not be provided by the current post-processing software.

4. Simulation technology for mill-turn machining

Turn-mill complex machining due to more moving parts, complex functions, and programming being completed after the processing simulation is particularly important. As China’s aviation manufacturers mill and turnning composite machining into actual production in a relatively short period of time, there is no mature simulation application technology, so most manufacturers go through the trial cut processing to verify and optimize the program, which leads to a long process preparation cycle, development risk, and high processing costs.

In order to improve the application level and programming efficiency of mill-turn machining, the application of simulation technology must be vigorously promoted. At present, the software used for mill-turn machining simulation is mainly T o p S o l i d , Gibbs, etc., but this software are generally expensive, and fewer of them are introduced in the field of aviation manufacturing in China.

In fact, the realization of mill-turn machining simulation can also be achieved with the help of the current general-purpose CNC machining simulation software (such as Vericut, NCSimul, etc.), according to the structure of the mill-turn machining equipment, movement characteristics, special features, and CNC system, through customization and macro function development to achieve the machining process of motion simulation.

To realize the simulation of mill-turn machining by using general CNC machining simulation software, we need to first build a relatively real machine environment in the simulation system, focusing on the establishment of relative motion and geometric positional relationships between the moving parts of the machine tool. On this basis, the establishment of the tool library used in the machining process and the corresponding tool number.

Then configure the numerical control system of the machine tool and the machining reference of the numerical control program, and load the post-completed N C code into the simulation system, you can execute the simulation of the machining process. Unlike conventional CNC machining, some functions (such as multi-channel machining, tailstock control, etc.) need to be developed and customized through macro functions.

Application prospects and development proposals

In recent years, mill-turn machining centers have been introduced in China’s aircraft, aircraft engines, and accessories factories and other aviation manufacturers. The type of equipment is mainly concentrated in the Austrian W F L company’s mill-turn composite series of products and the Swiss Baomei company’s mill-turn composite machining centers and so on.

However, due to the practical application of the time being not long, the general lack of product process characteristics and equipment process characteristics compatible with the mature machining process, programming means post-processing, and other technical means. Therefore, the current introduction of mill-turn machining equipment is basically at a relatively low level of operation.

The aviation products manufacturing process faces the main problems highlighted as long process routes, process complexity, low processing efficiency, processing deformation is serious, high processing costs, milling and turning composite machining both in aircraft manufacturing or in the field of engine manufacturing has a very broad space for development.

Such as aircraft fuselage overall frame milling usually through the material / blank preparation, reference processing, roughing inside the shape, roughing shape, finishing reference, semi-finishing and finishing inside shape, semi-finishing and finishing the shape, hole machining, pincer trimming, testing and other dozens of processes, many times to complete the flip clamping.

The current aviation engine field of the whole leaf disk processing is also used as a whole forging blank, after turning and milling, milling, polishing, surface treatment and strengthening, testing, and flaw detection dozens of processes to complete.

These parts often have long manufacturing cycles, occupying the machine time usually hundreds of hours, and the machining process requires the use of many different types of CNC machine tools and a large number of fixtures, cutting tools, measuring tools, and so on. In addition, the repeated replacement of the mounting card not only causes a long waiting time in the parts manufacturing process, affecting the production cycle but also causes the accumulation of mounting errors, thus affecting the dimensional accuracy of the parts and machining results.

Turn-mill composite machining can realize the processing of all or most of the above typical aerospace parts through one-time loading, thus providing a new way for efficient and precise processing of complex aerospace parts. Its application advantages are mainly reflected in the following aspects:

(1) The number of card loading is significantly reduced, improving machining efficiency while eliminating errors caused by changes in machine tools and card loading methods.

(2) The process is more centralized, can significantly shorten the machining process chain, and reduce waiting time and machine non-working time.

(3) Without changing the positioning state under the premise of turning, milling, drilling, and other machining methods to realize the machining process, reduce the number of fixtures to ensure the consistency of dimensional accuracy.

(4) Most of the current mill-turn machining has the function of online measurement, which can be used to measure the machining results in the process and between the processes in situ, to achieve the accuracy control of the entire machining process.

It can be seen, that milling and turning composite machining equipment has these advantages can effectively make up for the current aviation complex parts manufacturing process deficiencies, and can significantly improve the processing accuracy and efficiency of the product.

To give full play to the processing efficiency of advanced composite machining equipment, and further improve the manufacturing efficiency and quality of aviation products, there is an urgent need to carry out the following aspects of work.

(1) Combined with the process characteristics of aviation product parts, an in-depth study of the composite machining process, including the development of process routes, loading mode, tooling, cooling, and cutting parameters, such as the reasonable selection.

(2) According to the motion structure of the composite machining equipment and the process characteristics of the product, develop and customize the corresponding CNC programming, post-processing, cutting simulation, and other systems, to form an integrated solution of process – programming – post-simulation, and to reduce the requirements of composite machining on the craftsmen.

(3) Form process specifications. Combined with the simulation, test cutting, and actual production process experience accumulated, the formation of solidified process specifications for mill-turn compound machining, to guide the subsequent processing of other parts.

(4) Focus on the cultivation of talents. Composite machining equipment is currently a representative of the cutting-edge technology in the field of machining, both process preparation and operation and maintenance are more complex than conventional equipment, a high-level R & D team is to realize the health of the equipment, the key to efficient operation.

Conclusion

The current composite machining equipment is moving towards a greater range of processes, higher efficiency, and large-scale and modular direction. The aviation product manufacturing field has been an important stage for advanced manufacturing technology, with the increasing speed of aviation product replacement, the process of dispersed processing equipment will be gradually replaced by the process of centralized flexible automation equipment, which provides a broader development and application space for composite machining technology.