Advancements in Aerospace Actuator Production via Hybrid Manufacturing
To tackle the lengthy production cycle of single-cylinder parts used in aerospace actuators, this paper examines the structure of the parts and the machining processes involved. It applies a combination of additive and subtractive hybrid manufacturing technology to enhance production efficiency. Additionally, the paper outlines a process flow that integrates additive manufacturing, pretreatment, and subtractive manufacturing stages, leading to the rapid production of these components.
1 Introduction
With advancements in lightweight design for aviation components, the widespread use of difficult-to-machine materials, and the accelerated pace of product upgrades, the structural complexity of single-cylinder parts (referred to as “cylinders”) in aerospace actuators is increasing. Meanwhile, the cutting performance of these materials is decreasing. This combination significantly extends product development cycles, making traditional subtractive manufacturing methods, which primarily rely on metal cutting, insufficient for meeting current development timelines.
Additive manufacturing has garnered significant attention in the industry due to its unique advantages. However, this technology comes with various limitations that hinder its widespread application and development. As a result, hybrid manufacturing technologies—combining additive and subtractive processes—have gradually become a focal point of research.
This paper provides an in-depth analysis of the cylinder processing methods for commonly used materials such as stainless steel, titanium alloy, and high-temperature alloy. It examines the hybrid manufacturing processes that integrate both additive and subtractive techniques for cylinder production. The paper outlines the process flow for each stage—additive manufacturing, pretreatment, and subtractive manufacturing (cutting processing)—to achieve efficient manufacturing.
2 Problem Analysis
The shape of the cylinder is mainly that of a body of revolution, with certain areas featuring additional components such as oil lines, oil pipe connections, and flanges. This particular type of part is classified as a complex body of revolution. The machining process for the cylinder consists primarily of cutting, heat treatment, and surface treatment. The cutting process can be divided into several steps: rough machining before heat treatment, rough machining after heat treatment, semi-finishing the hole system, and finishing the inner hole. This process is illustrated in Figure 1.
The development of new products usually relies on bar or sheet stock (including open-die forgings) as raw materials for machining. Statistics indicate that during machining, the material removal rate typically hovers around 70%, with some processes reaching up to 97%. Roughing, stock removal, and contour milling combined often account for more than 50% of the total machining time, and in some cases, this can rise to 70%. Additionally, cutting generally constitutes over 70% of the overall processing time for parts, not including auxiliary processes such as turnaround, inspection, and waiting. For hard-to-machine materials like high-temperature alloys and titanium alloys, the proportion of time spent on cutting is even greater.
From this analysis, it is clear that a key strategy for shortening manufacturing cycle times is to reduce the material removal rate during machining. While welding and precision casting can help lower metal removal rates, welding creates a large heat-affected zone, making it challenging to meet the stringent standards of the aerospace industry. Furthermore, the design and manufacturing cycles of precision casting molds tend to be too lengthy to effectively shorten overall cycle times. Although additive manufacturing can enhance machining speed, it is susceptible to step-down effects, which can diminish dimensional accuracy and surface quality. For cylindrical parts, the requirements for the hole system are extremely stringent, making it difficult to achieve the necessary level of precision through additive manufacturing alone.
Some researchers have proposed a hybrid approach that combines additive and subtractive manufacturing technologies. Existing hybrid methods can be broadly categorized into two modes: first, performing additive manufacturing followed by subtractive manufacturing, or second, alternating between additive and subtractive processes. The alternating approach typically integrates both methods on the same machine, allowing for the efficient machining of complex parts. However, this type of equipment can be quite expensive and often relies on dry machining, which results in low machining efficiency and slow heat dissipation. Consequently, it is not suitable for parts with large cutting allowances or materials that are difficult to machine.
An analysis of the structure of cylindrical parts indicates that a process involving additive manufacturing followed by subtractive manufacturing is simpler, avoids the limitations of relying on a single manufacturing method, and generally meets the machining requirements for cylindrical components. This hybrid manufacturing approach can effectively shorten the processing cycle during the part development phase, which in turn can improve the machining efficiency for challenging materials.
3 Problem Solving
Based on the structural characteristics and machining techniques of cylindrical parts, the hybrid manufacturing process is divided into three stages: additive manufacturing, pretreatment, and subtractive manufacturing. The detailed process is illustrated in Figure 3.
3.1 Additive Manufacturing Stage
The purpose of the additive manufacturing stage is to effectively allocate cutting allowances and process additions to meet the clamping requirements during the cutting process. This stage aims to lower the material removal rate while maintaining the quality of the cutting process.
(1) Process addition design
Process additions refer to additional structures that are incorporated during the cutting process to enhance or meet the clamping requirements of parts. These structures are typically removed before the final processing of the parts. For cylindrical parts, the process additions are usually positioned at both ends of the cylindrical axis to accommodate specific process requirements.
Based on established practices, if the total length of the part is less than 150 mm, only one process addition is needed at one end of the part. During subsequent processing, this additional structure can be clamped using a self-centering chuck.
If the cylinder’s length is 150 mm or more, process additions are required at both ends. In this case, a self-centering chuck and a center frame are utilized to secure the additional structures at both ends during processing, as illustrated in Figure 4.
When the end faces of the part consist entirely of rotary structures within 30 mm—or if the self-centering chuck does not interfere—these rotary structures can serve as process additions without requiring extra design. However, if there are no suitable rotary structures at both ends of the part for direct clamping, additional process additions must be designed.
(2) Design of the remaining hole system allowances
The outer wall of the cylinder includes various bosses, such as oil pipes, interfaces, and flanges. These bosses are equipped with different hole systems, including threaded holes, fuse holes, oil interface holes, and mechanical interface holes. Each of these hole systems generally has geometric tolerance requirements and is categorized as precision holes.
To ensure machining accuracy, a machining allowance of 2mm must be maintained in the diameter direction and above the end face of these holes. For normal hole system diameters less than 10mm, the inner hole is typically filled directly with solid material. However, for normal hole system diameters of 10mm or more, a layered filling method is commonly used to enhance cutting quality when boring the axial hole system. This approach helps to reduce surface roughness and improve dimensional accuracy while minimizing the machining allowance.
Specifically, solid filling is performed only within a 2mm range outside the wall of the axial hole system, while the normal hole system is left reserved as indicated in Figure 5. The shape design is based on the mean difference model shown in the drawing.
(3) Axial hole allowance design
When the diameter of the axial hole is less than 10 mm, the inner hole is typically machined as a solid and filled hole. For axial hole diameters of 10 mm or greater, a machining allowance of 2 mm is generally provided on one side. The axial hole allowance is illustrated in Figure 6.
(4) Blank printing requirements
To maintain the vertical alignment of the cylinder’s axial direction and ensure consistent axial fiber direction in the part, it is recommended to tilt the 3D printer slightly if it is small. However, it is strictly prohibited to position the cylinder horizontally along the axial direction.
3.2 Pretreatment stage
(1) Blank surface treatment
Residual stress tends to accumulate between the layers of metal parts during the deposition process. Additionally, the surface can become rough due to the step effect. If these stresses are not eliminated promptly, the subsequent subtractive manufacturing process may lead to warping and deformation of the part, which can negatively affect processing quality. Therefore, it is essential to use 80-120 mesh gravel for sandblasting treatment. The part after sandblasting is shown in Figure 7. This step not only removes the residual stress created during the deposition process but also improves the surface quality of the part.
(2) Benchmark preprocessing
Under normal circumstances, the finishing work includes machining the outer circle and chamfering both ends, as well as finishing the inner hole based on specific requirements. When processing additional features at one end, there are two scenarios:
1. The first involves trimming the additional outer circle at one end while chamfering the inner hole at the other end.
2. The second involves both trimming the additional outer circle and chamfering the inner hole at the other end simultaneously.
After trimming the outer circles at both ends, a clamp and a frame are utilized. The trimmed outer circle serves as the reference point, and the inner hole is then adjusted to ensure it aligns with the trimmed outer circle.
3.3 Subtractive manufacturing stage
The clamping method for a cylinder, which is a complex rotating body, typically involves either a clamping with a top or a clamping with a frame. The process flow for the subtractive manufacturing stage is illustrated in Figure 8.
The first step is to 3D print the blank. Next, a horizontal lathe is used to fine-machine the inner hole according to a reference line (see Figure 9). It is important to maintain the continuity of the milling process to avoid interruptions.
In the third step, a four-axis vertical machining center machines the flanges and holes on the part’s diameter using a clamping and lifting method. The fourth step involves removing unnecessary attachments based on the part’s structure.
The fifth step requires a three-axis vertical machining center to alternately remove attachments and machine the end flange structure, depending on whether the end faces of the part have flange attachments. In the sixth step, the internal bore is ground according to the part’s specifications. Bench work is carried out to remove burrs from both the internal and external surfaces, eliminating tool marks on the machined surface to complete the entire machining process.
Finally, inspections and the application of dry film lubrication are conducted as needed.
4 Application Promotion
(1) Analysis of Parts and Blank Design
A single cylinder made of GH3625 measures 100 mm in length and has flange structures on both ends. While there are no bosses, interfaces, or holes along the main body of the part, a hinge interface needs to be milled using a four-axis vertical machining center. The precision requirements for the inner hole of the part are not stringent, which means a lathe can achieve the necessary machining accuracy.
In designing the blank, an additional length of 30 mm was added at one end, and a 2 mm margin was set aside for the flange surface. The diameters of the non-concentric axial holes on the flange surface are 3 mm, 5 mm, 6 mm, and 6.2 mm, all of which are smaller than the standard diameter of φ10 mm. Therefore, these holes need to be filled. The final blank model is depicted in Figure 10.
(2) Pre-processing stage
After processing the blank, 80-120 mesh sand is utilized for sandblasting to eliminate residual stress from the surface of the part. Given that the part is short and the inner hole is shallow, only the outer circle and the inner hole are trimmed for reference. The opposite end face is then chamfered.
(3) Subtractive manufacturing stage
During the processing, the additional outer circle is first clamped, using the polished end face as the reference point for the process. Based on this reference, the right end face is machined to ensure it meets the appropriate length. Then, the inner hole is bored, the inner groove is cut, and other necessary operations are performed. According to the drawing specifications, a chamfer is machined at the left inner hole to complete the inner hole processing.
Next, a four-axis vertical machining center is employed to clamp the additional outer circle and tighten it for processing the hinge interface. Subsequently, a three-axis vertical machining center is used to clamp the additional outer circle with a self-centering chuck to align the position of the inner hole. After this, the hole system on the right flange surface is machined.
Continuing with the three-axis vertical machining center, the CNC turned component is secured in a modular fixture. Using the holes on the left flange as a positioning reference, the additional internal holes are aligned, allowing for the machining of the flange on the right end face of the part. While ensuring the overall length of the part is maintained, the holes on the right end face of the flange are also machined.
Finally, benchwork is performed to remove any burrs from the exterior of the part and eliminate any tool marks on the machined surface, thus completing the part. The finished part is illustrated in Figure 11.
5 Conclusion
This paper examines the structure and process of a single cylinder during the development phase, utilizing both additive and subtractive hybrid manufacturing technologies for cylinder production. It outlines the process flow and methodologies for the three stages: additive manufacturing, pretreatment, and subtractive manufacturing. These approaches were successfully implemented in a cylinder machining project, resulting in a reduced machining cycle during the development phase and enhanced machining efficiency for difficult-to-machine materials. This process is not only relevant for machining cylinder parts but can also be adapted for various other components and industries, providing a wide range of applications and valuable insights.
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