AERO milling cutter and Safe-LockTM system enable ultra-high-speed machining of large aluminum aircraft parts
Aluminum in general is a material with good machinability and is often processed under more aggressive cutting conditions than steel. In recent years, there is a growing demand for ultra-high-speed machining of large-sized aluminum parts using high-power milling machines exceeding 80 kW especially in the aerospace industry. In order to meet the demand of heavy duty machining that maximizes the capabilities of advanced machining centers, however, a number of challenges with regard to cutting tools and tool holders must be overcome.
Tool Damage and Breakage
Especially notable under aggressive cutting conditions, as the machine’s spindle speed increases, the centrifugal force on the tool mounted at the end of the spindle will also increase. Therefore, even if the runout accuracy of the tool cutting edge is initially controlled, runout may occur during rotation. This would result in a shorter tool life and may cause chipping or sudden tool breakage in some cases.
Reduced Machining Accuracy
Ultra-high-speed machining requires a larger cutting load than usual. If the rigidity of the tool or tool holder is insufficient, deflection or chattering may occur during machining. In addition, runout of the cutting edge will increase due to the centrifugal force, which may result in undesirable machined surface quality and dimensional accuracy.
Tool Pull Out
When the cutting load increases, a large force is applied to the tool in the direction away from the spindle. The axial and rotational forces can cause the cutting tool to rotate in the tool holder. In the worst case, the cutter is pulled out of the chuck. Under such a scenario, not only is the tool damaged, the machined workpiece may also suffer defects. Conventional side lock mechanism for tool pull out may deteriorate runout accuracy.
Reduced Spindle Life
When the tool holder and cutter receive a large centrifugal force due to high-speed rotation, a large load is also applied to the spindle. This reduces the life of the spindle and necessitates frequent replacement.
In order to solve the above problems, a tooling system with high rigidity and ability to minimize centrifugal force is necessary. In addition, heat and a large amount of cutting chips are generated during machining, so the cutter must be equipped with welding resistance, lubricity, and excellent chip evacuation capabilities.
Mitsubishi Heavy Industries, Ltd.
Founded in 1884, Mitsubishi Heavy Industries, Ltd. (MHI) is a multinational company headquartered in Tokyo, Japan. MHI has a proven track record in a wide range of industries, from shipbuilding, transportation systems, civil aircraft, infrastructure such as power generation systems, to space systems.
In the aircraft field, MHI’s Oe Factory in Nagoya and Eba Factory in Hiroshima manufacture large parts such as the main wings of the Boeing 787 and sections of the fuselage of the Boeing 777. The Oe Factory and Eba Factory have a site area of 387,700-square-meter and 669,908-square-meter respectively.
A section ofMHI’s Eba Factory in Hiroshima that is currently designated for the production of the Boeing passenger aircraft 777X.
Currently, MHI is actively involved in the manufacturing of the Boeing passenger aircraft 777X. MHI is responsible for producing sections of the rear and tail fuselage as well as the passenger entry doors. Among them, OSG cutting tools are used for the roughing and finishing of large aluminum structural parts (about 4 meters in size) of the rear fuselage panels.
MHI’s 777X project team and two applications engineers from Makino pose for a photograph with a prototype part at the 777X 1st Part Completion Ceremony at the Eba Factory in Hiroshima on October 31, 2016. Photo courtesy of MHI.
“For the 777X, there are approximately 100 new large components required to be made,” said Hirokazu Unno, an MHI lead engineer with more than 20 years of experience at the Manufacturing Engineering Department’s Tool Design Team.
“Traditional method would have been to start with a large number of machines and suppliers,” Unno explains. “However, to reduce costs, we shifted focus to increasing productivity per machine and start up with as few facilities as possible.”
Like Unno, Jun Eto is also a key member of the 777X project from the start and belongs to the same department with 10 years of experience in aircraft parts machining.
MHI 777X Project Member Spotlight
From left, Jun Eto and Hirokazu Unno.
Eto is an MHI engineer with 10 years of experience in aircraft parts machining. He belongs to the Manufacturing Engineering Department’s Tool Design Team in Nagoya and is a key member of the 777X project. After launching the 777X project, Eto joins the Mitsubishi SpaceJet development team and has been in charge of technical development of parts machining since July 2020.
Unno is an MHI engineer with more than 20 years of experience in aircraft parts machining, cutting tools and jig design. He belongs to the Manufacturing Engineering Department’s Tool Design Team in Nagoya and is a key member of the 777X project. Even now, Unno visits the Eba Factory in Hiroshima regularly to continue to support machining process improvement of the 777X project.
“Speed is an important factor when we are required to produce large numbers of new parts,” Eto adds. “We opted for a new machining process in order to further improve precision and reduce machining time.”
According to Eto, when working with new parts and machining processes, troubles relating to the workpiece often occur at startup. “So, we have to eliminate all potential troubles on the tool side,” said Eto.
In the past, MHI had introduced a horizontal machining center with up to 30,000 min-1 spindle speed for heavy duty machining. However, full benefits of high-speed machining could not be obtained with the machine, conventional tool and tool holder at the time. Thus, the company mainly used vertical machining centers with less than 20,000 min-1 spindle speed for the machining of aluminum parts.
The Makino A6.
With the launch of the Boeing passenger aircraft 777X project, MHI decided to introduce a 120 kW high-speed and high power 5-axis horizontal machining center, the Makino A6, with a maximum speed of 33,000 min-1 to improve productivity substantially.
A photograph of the Boeing 777X. Photo courtesy of The Boeing Company.
“We felt that the 777X project was a great opportunity to introduce some of the best equipment in the world,” said Eto.
Based on preliminary research in Japan and overseas, the project team knew for a fact that the machining center is highly reliable. The challenge was how to operate it.
“High-speed milling machines offer high performance, but it is essential to pay close attention to the operating cost including the spindle unit,” said Eto. “In order to increase productivity and reduce operating costs, it is necessary to control not only the machine, but the total performance from the spindle to the tool tip.”
For the tooling, MHI conducted in-house research and cutting trials in advance to select the most appropriate cutter and tool holder. In order to achieve stable production at ultra-high-speed machining of over 30,000 min-1, it is utmost critical to minimize the variation in performance of the cutting tool and tool holder. Specifically, it is crucial to control the variation when the tool is preset in the tool holder.
The start of production was under a tight schedule at the time. It was important that the tooling used have minimal difference between the process simulation and the actual performance. The more consistent the performance, the sooner MHI would be able to start up the actual machine without the need to conduct more tests for cutting condition settings. Among the tools that MHI have tested was OSG’s AERO end mill series, which the company has been using for a long period of time. The tools from this series have also demonstrated good productivity and minimal difference from the simulation in field trials by machine tool manufacturers. Based on the results of the cutting trials, MHI selected OSG’s AERO end mill series for the cutter.
OSG’s AERO end mill series is designed to maximize the full potential of high-performance equipment over 80 kW. The AERO series is most ideal for high-efficiency machining of large aluminum aircraft components. The AERO-O-ETS 3-flute short type carbide end mill with internal coolant hole can be used for roughing up to finishing. It features a sharp cutting edge specification to enable excellent cutting, and an optimal flute geometry to facilitate trouble-free chip evacuation. Furthermore, its DLC coating provides a shiny surface with optimized end mill performance particularly in aluminum alloys that require welding resistance and lubricity. Its thin coating layer enables a sharp cutting edge, which contributes to smooth and superb surface finish, even in high-feed milling.
The AERO-O-ETS end mill is a part of OSG’s AERO end mill series, which is designed to maximize the full potential of high-performance equipment over 80 kW. The AERO series is most ideal for high-efficiency machining of large aluminum aircraft components.
For the tool holder, MHI selected a high rigidity and precision shrink fit holder developed by German manufacturer HAIMER, as well as their Safe-LockTM system for pull out protection of shank tools, which is a unique innovation of the company. The system’s special drive keys in the chuck and grooves in the tool shank prevent the milling cutter from spinning during extreme machining to enable greater cutting speeds, longer tool life and a safer machining environment.
Developed by German manufacturer HAIMER, the Safe-LockTM system’s special drive keys in the chuck and grooves in the tool shank prevent the milling cutter from spinning during extreme machining. The end result – greater cutting speeds and longer tool life. OSG also provides shrink fit holders and shrink fit devices compatible with the Safe-LockTM system.
“Although it is rare for a cutter to completely come off during the machining of aluminum, the tool has slipped at the corner during roughing,” said Eto.
“If slippage occurs during high-speed milling at 120 kW, the tool will chip. In the worst case scenario, the cutter will eventually become a round bar that cannot perform any cutting and will collide with the workpiece, which damages the spindle,” Eto explains. “This scenario must be avoided at all costs, so we decided to install the Safe-LockTM system.”
MHI installed the HAIMER Tool Management system at its Eba Factory – a working area specifically designed for tool storage, assembly, presetting and administration. Preset areas have an image of being dark and are typically situated at the corner of a factory. To eliminate this common perception, MHI purposely installed the Tool Management system as a bright, clean and open area at the factory entrance to symbolize the company’s attitude toward tool management and commitment to quality.
Furthermore, in order to suppress variations in the overall tooling, HAIMER’s Tool Dynamic Comfort Plus balancing machine is used to better control the influence of centrifugal force under high rotation. In order to manage preset errors, an automatic tool measuring device is also deployed.
MHI’s new 5-axis horizontal machining center and the corresponding equipment are installed at the Eba Factory, where the 777X fuselage assembly plant is located, in order to streamline the production process.
Members of the 777X project pose for a photograph in front of the Makino A6 high-speed and high power 5-axis horizontal machining center at MHI’s Eba Factory in Hiroshima, Japan. From left, Kentaro Masutani from Primetals Technologies Japan, Masaru Fujinaga, Osamu Nakano, Yusuke Kamio, Ayakatsu Eki, Takanori Terasako, Hirokazu Unno, Noriaki Sugihara and Masaki Murakami from MHI.
The project is being tackled by a team of young technicians including Takanori Terasako, who are enthusiastic about the opportunity to manufacture aircraft parts using a new machining center, tooling system, and machining method. Since the schedule for the new model development project is relatively tight, lead engineers Eto and Unno from the Oe Factory joined the Eba project team at the time of startup. In addition, Kentaro Masutani, a technician who is highly experienced in machining large-scale machine parts also participated as a supporting member from steel-plant builder Primetals Technologies Japan Co., Ltd., a subsidiary of MHI.
“In the simulation, we set the cutting conditions while also looking at the load on the bearings,” said Eto. “In order to reduce the difference between the actual cutting and the simulation, high quality of the tool cutting edge, preset accuracy, and vibration characteristics from the machine to the tool tip must be achieved,” Eto elaborates. So, the machine operators were requested to realize very high accuracy in preset operations.
“Unlike before, it was difficult at first to satisfy the automatically measured balance value and preset accuracy,” said Terasako. “However, OSG’s AERO end mill has a high shank accuracy and we are able to preset relatively smoothly.”
Takanori Terasako, MHI technician and a member of the 777X project, uses the HAIMER Tool Dynamic balancing machine at the Eba Factory in Hiroshima.
“In terms of cutting conditions, steel and aluminum are very different,” said Masutani. “But I think that the technology introduced for high-speed machining of aluminum can be applied to the machining of steel as well.”
From left,Primetals Technologies Japan Technician Kentaro Masutani and MHI technician Takanori Terasako discuss details of the application for the 777X project.
According to Unno, various problems arose during startup due to the adoption of a new machining process. “But thanks to the stable performance of the tooling, we were able to concentrate resources to trouble-shoot quickly,” said Unno.
Eto adds that the AERO end mill is able to operate smoothly under identical conditions set during the simulation. “It is the stable quality of the tooling side that makes this replication possible,” said Eto.
Although it was the first time for MHI to introduce a 33,000 min-1 spindle speed machine, its project team was able to overcome various challenges. Despite having to create a newly designed part using a new machining method, the team successfully completed the setup of the equipment in a short period of three weeks to complete the first product.
With the perfect combination of machine tool, cutter and holder, stable slotting and pocketing at a speed of 33,000 min-1 and a feed rate of 25,000 mm/min can be achieved. With the OSG AERO end mill, metal removal rate reaches 6,200 cc/min at its maximum. Machining time is reduced by approximately 75 percent versus conventional machines.
MHI is currently producing several dozens of these aluminum parts every month. The cutter and tool holder from OSG and HAIMER are continuing to demonstrate consistent performance. Also, machine spindles in general are replaced in roughly a year. In MHI’s case, its machine spindle is still in good shape after more than three years and can be used under identical cutting condition.
From left, HAIMER Area Sales Manager Tom Yamashita explains features of the HAIMER Power Clamp shrink fit machine to MHI’s Osamu Nakano.
Osamu Nakano, who is in charge of introducing and managing milling machines and their associated equipment adds that “compared to previous machines, I am surprised that there are few failures around the spindle and made me recognize the importance of tooling quality.”
Unno joins in and adds that by selecting the most optimum tool and tool holder, the required machining time has been reduced to 25 percent versus conventional method, and the life of the machine spindle has more than tripled. “In addition, by utilizing the Safe-LockTM system, work defects caused by tool pull out can be prevented and results have demonstrated its effectiveness,” said Unno.
From left, Jun Eto and Hirokazu Unno, MHI lead engineers from the Manufacturing Engineering Department’s Tool Design Team. Since the schedule for the new model development of the 777X project was relatively tight, the pair joined the Eba project team in Hiroshima at the time of startup to provide support.
According to Eto, mass production of the 777X began a couple of years ago and production rates are planned to increase. It is necessary to further enhance machining efficiency. At MHI, the company always strives to channel new ideas into solutions that move the world forward. “We are not satisfied with the status quo as growth does not stop,” said Eto. “We will continue to gather information from all over the world and pursue the latest technology to move aircraft manufacturing forward.”