Solving Complex Grooving Challenges in Split Receivers with CNC Milling
This paper addresses the machining challenges associated with the multi-boss annular groove in a split casing. It presents a technical innovation along with a comprehensive CNC milling process plan. Process optimization was carried out in two key areas: the selection of CNC tools and cutting parameters, as well as the planning of cutting paths. These improvements allowed the tool path for the annular groove to automatically navigate around the bosses, significantly enhancing machining efficiency while maintaining high quality.
1 Introduction
Split casings are essential load-bearing components of large aircraft engines. They experience aerodynamic loads, mass inertia, and thermal stresses due to temperature variations during engine operation. These components are commonly made from difficult-to-machine materials such as titanium alloys, stainless steel, and high-temperature alloys.
The structure of split casings consists of two half-rings that are connected to form a complete ring using bolts and locating pins along their longitudinal mounting edges. Once the complete ring is formed, strict requirements must be met regarding the roundness of the front and rear mounting edges, the positional accuracy of the hole system, and the precision of the mounting seat plane.
The manufacturing challenges associated with aircraft engine casings arise from their complex shape and structure, the use of difficult materials, thin walls with low rigidity, and high removal rates due to design and manufacturing processes. These challenges lead to lengthy process flows, low machining efficiency, and significant deformation errors. Additionally, the machining process requires multiple rotations and repeated clamping, which further complicates production. Low adaptability of process parameters also hampers production efficiency. Factors such as residual stress and cutting forces can result in machining deformation, negatively affecting the quality of the casing.
2 Analysis of Machining Difficulties
A model of a split-frame engine casing is shown in Figure 1.
Analysis reveals the main difficulties in machining split-receiver casings in the following areas.
1) The design tolerances for the casing are quite strict, with many requirements regarding relative positioning. Once the split-receiver casing is assembled into a complete ring, the tolerances for roundness and coaxiality at the front and rear mounting edges are also narrow. The split-receiver design necessitates repeated assembly and disassembly during machining, which makes it challenging to maintain consistency between the two halves. This process can easily lead to stress release deformation, resulting in deviations in roundness and positioning.
2) The casing has various external features and a complex shape. Its outer surface is designed with annular reinforcement ribs and longitudinal mounting edges, creating multiple annular grooves. Within these grooves, you’ll find several bosses, including linkage ring stop bosses and borescope bosses. These bosses are numerous, closely spaced, and irregularly arranged.
3) The removal of material in split-receiver machining is substantial, making the process challenging. Typically, split-receiver blanks are full-ring open-die forgings. The presence of longitudinal mounting edges means that all material within the annular grooves on the outer surface of the casing must be removed using CNC milling, resulting in a considerable amount of material being taken away. This complexity in manufacturing makes split-receiver machining particularly intricate.
The outer surface of a split-receiver features annular grooves and various bosses, which can only be machined using CNC milling. However, machining the annular grooves on the receiver’s outer surface requires the removal of a significant amount of material, which not only complicates the process but also makes it inefficient. In fact, CNC milling constitutes over 50% of the entire machining cycle in split-receiver manufacturing.
Current split-receiver machining processes are overly fragmented, leading to inadequate allocation of stock allowances between steps and a heavy reliance on specialized tools and measuring instruments. This results in lengthy machining cycles and low overall efficiency.
This paper presents an efficient CNC milling method specifically designed for split-receiver machining. The method enhances machining accuracy and improves efficiency by integrating a comprehensive custom CNC milling process plan with effective toolpath planning. This approach has been successfully implemented in a specific split-receiver machining process, reducing the machining cycle to just two months and significantly increasing machining efficiency.
3 Overall plan of CNC milling process
The split casing features a divided structure. To ensure that the two halves of the casing fit together properly and maintain design accuracy, the manufacturing process is divided into three stages: rough machining, semi-finishing, and finishing. Between these processing stages, we incorporate stress relief heat treatment as well as corrections for both the end face datum and the longitudinal joint surface datum.
CNC milling is carried out in three steps: rough milling of the outer shape, finishing milling of the outer shape, and finishing milling of the boss plane.
(1) Rough Milling of the Outer Shape
After the rough turning process, the outer shape of the casing undergoes rough milling to remove the large excess material from the annular groove on its outer surface. This step is crucial for ensuring that the internal stress within the material is fully released, significantly reducing deformation of the part during the finishing stage. While performing rough milling, a 1mm allowance is left on the end face of the boss, and a 0.5mm allowance is preserved on the side face of the boss, the bottom face of the annular groove, and the side face.
Following the rough milling, the casing is split into two halves, and the joint surface is reprocessed. During the rough milling of the outer shape, the two halves of the casing are positioned eccentrically to account for the machining allowance of the joint surface. This careful arrangement ensures that, after machining the joint surface, the two halves can be reassembled into a complete ring with a uniform and appropriate outer shape allowance.
(2) Complete the milling of the boss’s outer shape.
The side of the boss, the bottom surface of the annular groove, and the side surface are all machined in place. Allowances are left for the linkage device support table plane, the boss’s end face, and other surfaces that have high requirements for size and positioning.
(3) Complete the milling of the boss end face and the plane of the linkage device support table.
Finish milling is the final step in processing the outer shape. This process ensures that the size, position, and surface roughness of all grooves, seats, and bosses on the outer shape meet the required specifications.
4. Efficient CNC Milling Method for Multi-Boss Annular Grooves
Due to the height of the longitudinal mounting edge of the receiver, the depth of the annular groove in the receiver is large, and the receiver diameter is relatively large, resulting in significant material removal during aluminium CNC milling. The numerous bosses in the annular groove, with small distances between the bosses and between the bosses and the groove sides, restrict the CNC tool diameter and tool path. Tool selection and milling path selection require rapid material removal while effectively avoiding the various bosses. This makes CNC programming very complex and labor-intensive. This paper addresses process improvements in CNC tool and cutting parameter selection, tool entry optimization, and cutting path planning to improve rough milling efficiency.
4.1 CNC Tool and Cutting Parameter Selection
When rough milling, choose tools with larger diameters whenever possible. Process selection should proceed in the order of large cutters first, smaller cutters second, and wide areas first, narrow areas later. For roughing and removing large stock, a 20mm diameter machine-clamped cutter is selected. This square shoulder milling cutter is suitable for side milling with large cutting depths and fast feed rates. The tool can cut stainless steel materials with a cutting depth of up to 8mm per layer and a feed rate of up to 500mm/min, with high material removal efficiency. The R of the root of the housing boss is 1.5mm. If a 3mm diameter ball milling cutter is used directly to clean the root, the feed rate is relatively slow and 3 to 4 passes are required, resulting in low processing efficiency. Therefore, a rod milling cutter with a turning angle of R1.5mm is used to remove most of the root allowance, and then a 3mm diameter ball milling cutter is used to clean the root in one pass, which can improve processing efficiency.
4.2 Cutting tool path optimization
(1) Efficient rough machining of annular groove surface In the depth direction of the annular groove, the tool path is reasonably layered according to the boss height, and large diameter tools are used as much as possible to give priority to removing the upper layer of the boss material. Reasonable stratification of the tool path, cutting areas from large to small, and giving priority to processing large cutting areas, is conducive to continuous and stable removal of large allowances, reducing the number of programs, reducing non-cutting movements, reducing the number of feeds and retractions, and improving processing efficiency. The annular groove needs to be divided into multiple machining zones along its circumference, based on the distance between the boss side and adjacent surfaces, as well as the tool diameter. For bosses with ample tool clearance, programming can be used to automatically avoid the bosses. Zone milling and conformal milling, as shown in Figure 2, allow for flexible toolpath arrangement, simplifying programming while reducing program count and improving machining efficiency.
(2) High-efficiency finishing of annular groove surface To improve machining efficiency and surface quality, machining is carried out in the order of “annular groove side surface → boss side surface → annular groove bottom surface”.
When machining the sides of the casing’s annular groove, all four sides are treated as a single unit and processed using five-axis side milling with a spiral milling method. This approach differs from the traditional method of reciprocating milling each surface individually. By utilizing a fully down-cut milling toolpath, the cutting conditions for the tool are improved, leading to reduced cutting forces and less tool chatter. Additionally, this toolpath requires only a single feed and retraction, which saves time and eliminates the need for indirect milling that can occur with zone-by-zone milling, ultimately enhancing surface quality.
For finishing the bottom surface of the annular groove, the presence of the boss requires the bottom surface to be divided into multiple areas, each of which needs individual programming. This area milling method involves complex programming, with many connected cuts between areas. Consequently, the non-cutting movement time of the machine tool increases, resulting in lower tool path efficiency and multiple cuts on the surface, which negatively impacts surface quality.
To enhance the machining sequence, the process is optimized. First, the boss shape is finely milled, as illustrated in Figure 3. Next, an end mill is used to machine the bottom surface of the annular groove within a specified range at the root of the boss, ensuring both the boss shape and the bottom surface of the adjacent annular groove are finely machined. Finally, the remaining bottom surface of the casing’s annular groove is processed. When the tool encounters the boss, it is lifted, allowing the tool to pass over the top of the boss before being lowered again to continue milling. Since the material around the boss has already been removed, the tool can feed directly into the cut. This strategy avoids the boss, resulting in higher cutting efficiency, fewer programs needed, reduced non-cutting movements of the machine tool, and less indirect cutting in the milling area.
5 Machining Examples
This machining method was applied to a split engine casing with a maximum outer diameter of 600 mm, featuring 78 different types of bosses. The finished casing, as shown in Figure 4, demonstrates excellent surface quality. The dimensional and positional accuracy of each component meets the specified drawing requirements, resulting in a significant improvement in both machining efficiency and quality.
6 Conclusion
The CNC milling process for split housings was effectively planned by rationally arranging the machining sequence and inter-process allowances, which ensured high machining accuracy. A new method was proposed for efficiently rough milling multi-boss annular grooves and effectively finishing the surfaces of annular grooves. This improved CNC milling efficiency, reduced tool wear, and enhanced both machining quality and stability.
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