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China’s Breakthrough in Aerospace Thin-Walled Component Manufacturing: Mirror Milling Technology Leading the Future
Created on 2025.08.22
Why Lighter Means Better
In aerospace engineering, weight reduction is directly linked to performance. A lighter airframe allows an aircraft to fly farther, carry more payload, and consume less fuel. For rockets, every kilogram saved in structure translates into additional payload delivered to orbit. Yet the components that make this possible—large curved aircraft skins and rocket tank bottoms—must endure extreme stresses while maintaining aerodynamic precision.
For decades, manufacturers relied on chemical milling. This process, though widely used, brought significant challenges: low accuracy, uneven wall thickness, heavy environmental pollution, and limited applicability to new materials such as aluminum-lithium alloys. As next-generation programs demanded higher strength-to-weight ratios, chemical milling became an obstacle rather than a solution.
The emergence of mirror milling technology—a dual five-axis machining method—has now redefined how thin-walled aerospace components can be produced. China’s sustained research efforts over the past decade have pushed this once import-dependent process into a field of domestic innovation and large-scale application.
Overcoming the Bottleneck of Traditional Processes
Aircraft skins must combine ultra-lightweight construction with long fatigue life, while rocket tank bottoms must withstand both propellant pressure and the tremendous forces of launch. Traditional chemical etching (chemical milling) has several inherent weaknesses:
- Low machining precision
and difficulty maintaining uniform thickness
- Environmental impact
, with high chemical waste and pollution
- Dependence on manual labor
, making scale and consistency difficult
- Incompatibility with advanced alloys
such as aluminum-lithium
- Inability to eliminate uneven wall thickness
in large-diameter parts
These issues became a severe bottleneck, limiting improvements in both aircraft and space vehicle performance.
China’s Breakthrough: Dual Five-Axis Mirror Milling
Mirror milling is widely considered the “crown jewel” of five-axis machining. The process involves the synchronized motion of two milling heads—a cutting head and a supporting head—working in mirror image. Real-time measurement of wall thickness and adaptive support prevents deformation of weak, thin-walled parts.
For decades, this technology was monopolized by European suppliers such as Dufieux (France) and M.Torres (Spain). A single 12-meter-class machine once cost over RMB 110 million (USD 15 million), typically sold under restrictive contracts that prohibited transfer of core technology.
China’s response was a decade-long national effort. A consortium including Shanghai Jiao Tong University, Topplus CNC, COMAC, AVIC Xi’an Aircraft, and Tianjin Rocket Manufacturing achieved a series of breakthroughs across the full chain—theory, technology, processes, and equipment. Key achievements included:
- Development of precision machining techniques for large flexible surfaces
- Construction of the world’s
first vertical dual five-axis mirror milling machine
- Creation of the world’s
largest horizontal dual five-axis mirror milling system
- New processes for
aircraft skins and integral rocket tank bottoms
These advances have been successfully applied to major programs such as the Y-20 transport aircraft, the C919 passenger jet, China’s crewed space program, and the 5-meter-class integral tank bottom for lunar missions.
First Deliveries: Entering the Global Top Three
In 2017, Shanghai Jiao Tong University and Shanghai Topplus CNC delivered China’s first domestically built mirror milling machine, filling a long-standing gap. This positioned China as the third country in the world to master the technology, after France and Spain.
The system overcame key technical hurdles such as damping support for ultra-thin structures, online wall-thickness measurement and compensation, and machine dynamic accuracy enhancement. When applied to rocket tank sections, wall thickness tolerance improved by five times compared with chemical milling.
The breakthrough was recognized by China’s Ministry of Industry and Information Technology as part of its “Five Years of Progress in Major Equipment Achievements.”
Expanding Capabilities: From 3 Meters to 5 Meters
Conquering the “Giant Eggshell”
Research teams next developed a 3-meter-class vertical dual five-axis machine, expanding mirror milling applications from single-curved to double-curved structures. In Tianjin, they successfully milled a 3.35-meter rocket tank bottom with a wall thickness of only 1 mm, achieving a diameter-to-thickness ratio of 3350:1.
To overcome the extreme weak rigidity—likened to machining a “giant eggshell”—engineers introduced laser scanning, automated thickness compensation, and digital deformation control. This enabled precise, green manufacturing with dramatically shorter cycle times.
From First Article to Mass Production
At Shanghai Aerospace 149 Factory, mirror milling processes for rocket tank bottoms were refined over three years. Accuracy improved to ±0.1 mm, while processing time dropped from 20 days to just 6–7 days. In October 2023, the factory celebrated the rollout of its 100th 3-meter-class integral tank bottom, marking the technology’s maturity for serial production.
Scaling Up to 5 Meters
In 2024, China achieved another world first—mirror milling a 5-meter-diameter rocket tank bottom. The challenge was immense: wall thickness uniformity became far harder to maintain, and deformation risks multiplied. Engineers designed a 6-meter-span dual-head vertical machine with 110° synchronized support, enhanced real-time thickness scanning, and upgraded clamping systems.
This achievement marked a full transition from polluting, low-precision chemical milling and welding to clean, high-precision mirror milling for large rocket structures.
Applications in Aviation: From the C919 to the Y-20
Ensuring the C919 Supply Chain
The C919 passenger jet requires fuselage skins with a fatigue life exceeding Boeing and Airbus standards, many of them made from aluminum-lithium alloy. Traditional chemical milling could not meet these demands.
At one point, the C919 supply chain depended on imported mirror milling machines. But when foreign equipment failed, China faced potential supply disruptions. Domestic teams quickly stepped in, delivering horizontal dual five-axis mirror milling systems with higher precision, efficiency, and range than imported alternatives.
Today, five automated production lines based on domestic mirror milling have been installed, supplying skins for over 60 aircraft sets, ensuring secure and reliable production for the C919 program.
Supporting the Y-20 Transport Aircraft
The Y-20 large transport aircraft features massive curved fuselage skins, described by engineers as “metallic films.” These ultra-large, ultra-thin structures are extremely prone to deformation during machining. Traditional CNC equipment could not adapt.
To solve this, AVIC Xi’an Aircraft and its partners developed the world’s largest mirror milling machine, a 12-meter horizontal dual five-axis system. It enabled automated, precise milling of large skins, transforming what was once a critical bottleneck into a robust process.
The equipment not only met but exceeded performance expectations, establishing a complete process and equipment system for large curved skins. It now underpins both the Y-20 and C919 programs, and was recognized by People’s Daily as a landmark in high-end machine tool innovation.
Broader Impact on China’s Machine Tool Industry
Mirror milling’s success has extended beyond aerospace into the wider high-end machine tool sector. Its influence includes:
- Development of
series-configured horizontal five-axis panel milling machines for aluminum and titanium alloys
- Establishment of China’s first automated panel milling production line
- Efficiency improvements up to
four times higher than conventional gantry mills
- Advances in
large-span, ultra-high precision five-axis gantry machining for civil aircraft components
- Transition of assembly processes from manual fitting to
high-efficiency interchangeable assembly
These achievements not only secured independence in a once-monopolized technology, but also raised China’s global competitiveness in precision manufacturing.
Toward a New Era in Aerospace Manufacturing
From 3-meter to 5-meter tank bottoms, from aluminum-lithium alloys to ultra-thin metallic skins, China’s mirror milling programs have achieved a series of world-firsts. Over more than a decade of effort, Chinese teams have delivered breakthroughs in theory, processes, equipment, and industrial application.
The technology now supports key national projects including the C919, Y-20, Long March rockets, crewed spaceflight, and lunar exploration. By replacing chemical milling with high-precision, green mirror milling, China has not only removed long-standing bottlenecks but also positioned itself at the forefront of next-generation aerospace manufacturing.
As applications expand and technology continues to evolve, mirror milling will play an even greater role—helping China’s aerospace sector reach new heights while reshaping the future of global aerospace manufacturing.
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