Precision and Consistency Assurance in Digital Fabrication of Major Structural Assemblies
The integration of digital technology in design, manufacturing, and inspection has successfully addressed bottlenecks in the rapid measurement and accurate inspection of large structural components. During the development phase, a thorough dimensional feature inspection method known as “first article inspection” is used to verify key factors such as product manufacturability, process operability, equipment accuracy, and production capacity. In the mass production phase, a “first, middle, and tail” measurement approach is applied to assess typical dimensional features, ensuring comprehensive quality control. This method enhances inspection efficiency while guaranteeing the delivery of high-quality products.
01 Introduction
As digital technology continues to expand in applications related to aircraft design, manufacturing, and inspection, the requirements for precision in manufacturing and assembly of aircraft products are rising. Large structural components, such as frames, beams, panels, and joints, serve as the primary load-bearing elements. These components typically involve long processing cycles, intricate manufacturing processes, numerous inspection criteria, and a need for high precision in inspections.
Currently, routine inspections still rely on traditional methods, which significantly limit efficiency and do not meet production schedule demands. To tackle the challenges of improving product inspection accuracy and efficiency, new inspection methods are being developed within a 3D design environment and under digital manufacturing conditions, aiming to enhance both accuracy and efficiency in the inspection process.
02 Analysis of Quality Control Factors for Large Structural Components
Large structural components are essential for the aircraft’s airframe and aerodynamic shape. Their manufacturing involves complex assembly and coordination, requiring high machining precision. These components are characterized by their large dimensions, intricate shapes, and the use of various materials, making technology and quality control in the manufacturing process challenging.
To streamline production organization during aircraft manufacturing, large structural components are typically categorized based on their structural characteristics. This categorization includes panels, frames, beams, ribs, flanges, stringers, and joints. Panels play a significant role in determining the aircraft’s aerodynamic shape and are the primary load-bearing components.
The structure of panels, as illustrated in Figure 1, generally includes features such as grooves, ribs, lower limits, through-holes, and bosses. Although the overall dimensions of the parts are relatively large, the ribs and webs are typically thin. The final part usually results in a curved surface or is partially aligned with the theoretical outer edge. With the exception of mating and connection holes, the tolerances for these parts are relatively high.
Panels are made from pre-stretched aluminum alloy sheet stock and are typically machined using a CNC milling machine to shape the structural elements. This is followed by plastic forming to achieve the final design.
(1) Process characteristics Large structural parts have the characteristics of large-scale and complex structures, diversified materials and precise manufacturing due to their special requirements for use. Their process characteristics are as follows.
1) The structural forms of the parts are complex, primarily due to their aerodynamic shapes that are integral to aircraft design. The peripheral contours involve intricate assembly relationships with other components, making clamping challenging and complicating the processing technology.
2) The raw materials for these parts can be produced through die forging or by using pre-stretched thick plates. To minimize mold costs and shorten production cycles, plate blanks are typically preferred. However, this method results in larger cutting allowances and can lead to stress build-up during processing.
3) The reinforcement ribs have a complicated structure, with wall thicknesses as thin as 1mm. This reduced thickness results in weak rigidity, making deformation likely during processing.
4) The precision assembly requirements demand high standards for the size and geometric tolerances of the parts. For instance, the size tolerance for the web is ±0.1mm, indicating the need for high processing accuracy.
5) As the performance of new-generation aircraft continues to improve, there is an increasing use of high-performance materials. The proportion of challenging materials, such as titanium alloys, ultra-high-strength steels, and low-density lightweight materials, is rising. This shift makes cutting tasks more difficult and leads to significant tool wear.
(2) Dimensional elements mainly include the following aspects.
1) Internal and external shape control elements of components primarily relate to the constraints on the overall fitting relationships between parts. These elements usually consist of single curved surfaces or double curved surfaces. The main method of detection involves contact measurement using three-dimensional coordinate measuring machines.
2) Dimensional control elements of part ribs, flanges, and web thickness pertain to the constraints on the structural dimensions within the component body. These dimensions typically consist of parallel dimensions or transition dimensions. The detection methods generally rely on conventional measuring tools, such as vernier calipers and micrometers, assessing each point individually.
3) Dimensional control elements related to part shape and position refer to the constraints on the positional dimensions of ribs, depressions, and reference holes in the component structure. Each position generally involves multiple dimensional constraints. The detection methods are based on design, manufacturing, and inspection datums, and can be executed through either direct or indirect measurement.
4) Dimensional control elements concerning part fitting areas, datum surfaces, and intersection holes relate to the structural dimensions that represent the assembly relationships of components. These dimensions are crucial as they directly influence the assembly quality. Contact measurement is frequently carried out using coordinate measuring machines.
5) Surface quality control factors mainly encompass surface roughness, residual stress, and the flatness of machining tool offsets. Inspections are primarily conducted visually and by touch.
(3) Inspection difficulties mainly include the following three aspects.
1) The part’s structure is complex, requiring multiple inspection methods to meet its inspection requirements. This demands a high level of skill from the inspectors.
2) There are numerous dimensional factors involved in the part, most of which are standard sizes. Inspectors must perform repetitive tasks frequently, which can lead to visual or perceptual fatigue, increasing the likelihood of mistakes and reducing work efficiency.
3) The size accuracy of the part is critical, necessitating a high standard for the inspection environment, measurement equipment, and methods. Inspectors must take the measurement environment into account, develop optimized measurement techniques, and be proficient in using high-precision inspection equipment for dimensional assessments.
03 Digital Manufacturing Processing Plan
Digital manufacturing is a technology that uses digital simulation to describe the manufacturing process. As computer digital technology, network information technology, and manufacturing technology continue to advance and integrate, manufacturing companies and systems are increasingly achieving intelligent control and process management. This technology has found widespread applications in industries such as aviation, aerospace, and automobile manufacturing.
(1) Application of simulation software
In a three-dimensional design environment, computer programming software helps create processing plans. The entire part processing procedure is simulated using simulation software, which effectively prevents quality issues with the parts during manufacturing.
(2) CNC program identification and control
The CNC program used for processing parts is evaluated and verified based on seven key aspects:
1. Determine the programming basis.
2. Establish a process model.
3. Define the processing operations and generate the tool paths.
4. Use post-processing software.
5. Simulate and verify the CNC processing program.
6. Proofread and verify the CNC processing program.
7. Conduct on-site trial processing of the CNC program and finalize it to create an effective processing program.
This systematic approach solidifies the CNC processing process and ensures the stability of the operations.
(3) Digital inspection
The widespread use of digital measuring equipment has greatly enhanced the inspection process for large structural components. By fully utilizing the accuracy and versatility of measuring machines, we can achieve superior inspection quality. In a controlled measurement environment with a single measurement station, it is beneficial to use digital measuring devices—such as coordinate measuring machines, image measuring instruments, and three-dimensional scanners—to inspect parts. This approach minimizes potential measurement errors associated with manual techniques, thereby improving overall inspection quality.
(4) Advantages of digital manufacturing
Compared to traditional manufacturing, digital manufacturing integrates a digital virtual control system, which includes components such as computers, networks, software, models, reports, and graphics, into the existing framework of “people, machines, materials, methods, and environment.” This approach transforms information from the production process into numerical data, establishes mathematical models, and enables intelligent management.
Digital manufacturing assists manufacturing companies in enhancing their production management capabilities, both in planning and in the production processes. Its key features include the following:
1) High-performance machining equipment, capable of program control and tool changeover, allows multiple machining steps to be completed in a single clamping and positioning process. This results in a streamlined forming process, reduced tooling needs, and lower manufacturing costs.
2) Digital manufacturing management minimizes the need for manual control and interference, leading to stable machining quality and high repeatability. This effectively meets the high-precision machining requirements of the aerospace industry.
3) The system can quickly adapt to high-mix, small-batch production processes, reducing part preparation time and process turnaround time. This enhances production efficiency and shortens development cycles.
4) By utilizing digital machining simulation technology, CNC machining processes can be simulated and verified. This ensures the accuracy of CNC machining programs and the appropriateness of cutting parameters. Additionally, it allows for predictions regarding tool life and supports online precision compensation. As a result, CNC program errors can be corrected, and process parameters optimized before actual machining takes place, improving machining accuracy and reducing waste.
04 Development Process Quality Control Plan
The first article inspection is a thorough process that involves both process control and finished product inspection of the initial batch of parts or assemblies produced. Its primary purpose is to ensure that the product aligns with the design requirements. This method is widely used in quality management to evaluate the effectiveness of trial production, re-production, process control, and planning.
For large structural components with complex shapes, the high precision of digital processing allows for the implementation of first article inspection to maintain product quality. By overseeing the quality throughout the entire production process of the first batch, we can accurately assess the results and determine whether the parts conform to the design specifications.
(1) First-article inspection implementation process quality control record mainly includes the following aspects.
1) Process record.
When issues are identified in the first-article manufacturing process, the operator or inspector who discovers the problem will complete the first-article production process problem record form. They will detail the existing issues and provide improvement suggestions concerning process documents, tooling, and equipment. Following this, the process technicians will outline their recommended solutions, and the inspection personnel will verify both the identified problems and the proposed solutions.
2) Dimension inspection record.
According to the part model and technical requirements, the inspection personnel will collaborate with process technicians to identify the quality factors for inspection. They will measure the dimensional factors generated during the part processing one by one. While process dimensions can be measured, they need to be recorded only if they impact subsequent processing. The actual measured values will be documented.
The first-article inspection report will include the inspection factors identified based on the technical requirements in the drawing, the measuring instruments and other equipment used, the measurement results, the names of the personnel conducting the measurements, and any characteristic identifications, among other details. This process will continue until all processing steps for the part are completed.
3) Appraisal record.
Once all CNC machined parts processing steps are completed, the following documents and materials must be fully prepared and submitted to the unit’s first-article appraisal team for evaluation and review: the first-article production process problem record sheet, the first-article inspection report, product drawings, technical specifications, and any other relevant documents.
The production process will be inspected based on six criteria:
1. Whether the production process is being operated in accordance with regulations.
2. Whether special processes have been confirmed in advance.
3. Whether the equipment used is qualified.
4. Whether the production conditions are under control.
5. Whether discrepancies between the production process and actual conditions have been addressed.
6. Whether the product quality characteristics align with the digital model drawings.
The unit’s first-article appraisal team, which may be led by the technical person in charge, should consist of relevant personnel from design, technology, inspection, production, heat treatment, and surface treatment. Customer representatives may also be invited to participate when necessary.
(2) Key Points for Inspection and Quality Review Control during the Implementation of First Article Inspection
In the final stage of implementing the first article inspection, it is essential to conduct a comprehensive review of the formation process of product quality characteristics. This review should ensure that the inspection results align with the planned outcomes based on the design model, drawings, and quality control records from each stage of the production process. These stages include considerations related to “people, machines, materials, methods, environment, and measurement.”
1. People: This refers to the operators and inspectors involved in product manufacturing. They should have the necessary qualifications and adhere strictly to the operational procedures as outlined in the process documents. Additionally, they must record production information and actual measured values of quality factors. Compliance with self-inspection, mutual inspection, and special inspection processes is crucial to meet quality traceability requirements.
2. Machines: This includes all equipment, tools, and auxiliary production tools. All machinery must be functional and well-maintained, following the prescribed standards during their operational use and routine upkeep.
3. Materials: This encompasses raw materials, semi-finished products, and components used in production. It is important to verify that all material brands and conditions are correct. If necessary, re-inspection processes—such as spectral analysis or chemical composition analysis—should be implemented.
4. Methods: This relates to the requirements set forth in design documents, process documents, quality documents, and basic management documents utilized in the production process. These documents must be thorough, clear, and current. Any discrepancies between the production process and documented requirements must be corrected per regulations, documented, and verified for compliance.
5. Environment: This pertains to the environmental controls at the production site, including the adequacy of supporting infrastructure and the working conditions, such as temperature and humidity, which must meet specified requirements.
6. Measurement: This refers to the quality elements involved in process testing. Appropriate measuring instruments must be used, ensuring they have the necessary accuracy and range. These instruments should be verified and validated by a metrology department or measurement agency and must comply with their validity period. Additionally, personnel responsible for measurement must possess relevant qualifications, perform testing according to established methods, and record measured values in a standardized manner.
05. Quality Control Plan for Mass Production
During the mass production phase, since design finalization has been completed, design changes are relatively rare, and the production line is relatively stable, after the first-article inspection of a CNC turned part passes and the NC machining program is finalized, the qualified internal and external dimensions tested during the first-article inspection can be used as a representative basis for spot-checking and inspecting typical and significant internal and external dimensions of subsequent parts. Given the high stability of digital machining, for the same part, provided that the manufacturing conditions (man, machine, material, method, environment, and measurement) remain unchanged, it is possible to inspect large structural parts by only verifying the dimensional elements of the “first, middle, and last” machining phases of the NC program. This approach significantly improves machining inspection efficiency and reduces repetitive work for inspectors.
Figure 2 shows an example of segmented control of quality elements for a frame product. The product’s machining process consists of five NC program segments, each of which generates 100 dimensional elements, for a total of 500 dimensional elements for the entire frame. Based on the principle of unchanged manufacturing conditions, combined with the detection accuracy and application range of digital measuring equipment, typical dimensions are selected only from the dimensions formed by the first, middle and last segments in each program segment to verify product quality. This not only meets quality control requirements, but also greatly speeds up the progress of parts production.
06 Conclusion
By validating inspection methods for multiple large structural components and comparing the results with traditional inspection techniques, we found that quality control of large structural parts in digital manufacturing can achieve consistent product quality. This validation confirms that the production process can meet design requirements effectively. It ensures that factors such as process design, tooling equipment, operators, and the production environment consistently facilitate the creation of products that adhere to these design standards.
Additionally, this approach enables targeted monitoring of critical points and weaknesses in the production process. By promptly addressing issues identified during production, we can avoid significant technical risks and economic losses in subsequent production phases. This ultimately enhances both production and inspection efficiency while ensuring stable and reliable product quality.
Furthermore, by optimizing traditional inspection methods according to product manufacturing characteristics within a 3D design environment, we can ensure consistent product quality, improve inspection efficiency, standardize processing environment standards, and meet the evolving needs of digital product manufacturing and inspection technologies. This progression paves the way for future advancements toward intelligent, high-speed, precise, and high-quality development.
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