Efficiency Enhancements in Machining Aluminum Alloy Transmission Casings
Using the processing of accessory gear casings as an example, we conduct a process analysis based on the structural characteristics of the product and the challenges involved in its production. By managing processing deformation, ensuring dimensional accuracy, and minimizing redundant materials, we can develop an effective processing plan. After conducting experimental verifications, we are able to produce qualified products.
01 Introduction
The accessory gear casing assembly is a crucial component of the aircraft engine’s transmission system. The accessory gear casing itself is the primary part of this assembly. It is created through investment casting and then refined into a finished product using CNC machine tools.
The complexity of the accessory gear casing increases with the number of parts installed on it, which also raises the difficulty of processing. This casing features multiple bearing mounting holes, accessory mounting surfaces, load-bearing seats, and pipe joint holes. Particularly important are the dimensional accuracy, shape and position accuracy, and surface roughness of the bearing mounting holes.
02 Analysis of structural characteristics and processing difficulties of accessory gear casing
The accessory gear casing blank is an aluminum alloy casting made from ZL114A aluminum alloy. It exhibits high mechanical properties and excellent casting characteristics, making it suitable for manufacturing complex, thin-walled structural components with significant load-bearing capacity.
Figure 1 illustrates the three-dimensional shape of the accessory gear case. The overall dimensions are 1553 mm in length, 438 mm in width, and 183 mm in height, giving it a slender, waist-shaped profile. The minimum wall thickness of the non-machined surface is 3 mm. Additionally, the structure is partially open, resulting in limited structural rigidity.
The accessory gear case has a total of eight bearing mounting holes. The following specifications must be met:
- The minimum diameter tolerance for the holes is ±0.01 mm, and the positional requirement between holes should be within ≤0.02 mm.
- The flatness of the front and back end faces must be ≤0.02 mm.
- The parallelism between the front and back end faces should not exceed ≤0.04 mm.
- The surface roughness value for the eight bearing mounting holes must be Ra ≤ 1.6 μm.
Overall, the requirements for dimensional tolerance, geometric tolerance, and surface roughness are quite stringent.
03 Process scheme design
3.1 Deformation control during processing
Aluminum alloy castings have low hardness and a high thermal expansion coefficient. This makes it challenging to control deformation caused by clamping, residual stress release, cutting forces, and cutting heat during processing. While it is not possible to completely eliminate product deformation in actual processing, certain measures can be implemented to minimize deformation and keep it within a reasonable range.
(1) Reasonable selection of tools. In fine processing, the use of sharp tools can significantly reduce cutting force, facilitate smooth chip removal, and minimize the generation of frictional heat. Tools can either be machine clamp tools or solid alloy tools.
While machine clamp tools offer several advantages, they have some drawbacks: the tip arc is often too large, making the tip less sharp, and the chip removal space is limited, which can hinder effective chip evacuation. As a result, cutting resistance tends to be higher during processing, leading to increased heat generation. This extra heat can easily cause deformation in the product, making machine clamp tools unsuitable for processing thin-walled aluminum alloy products.
On the other hand, solid alloy tools offer several benefits: they feature a sharp tip that requires less cutting force, possess high hardness for durability (which reduces the frequency of tool changes), maintain consistent sizes for stability in product dimensions, and have a large helix angle that facilitates effective chip removal. Coupled with the use of ample cutting fluid to swiftly dissipate heat, solid alloy tools are the preferred choice for processing accessory gear casings.
(2) Design tooling and clamping. The aluminum alloy casting has poor rigidity and consists of thin-walled parts, which means that even a small clamping force can lead to significant deformation. Therefore, it is crucial to carefully control both the point of action and the amount of clamping force used. The accessory gear casing features a slender, waist-shaped structure, making it challenging to identify an appropriate clamping position and apply the correct clamping force using traditional methods. To address this issue, a specialized tooling with a supporting function was designed (see Figure 2) to enhance the rigidity of the accessory gear casing and minimize deformation during processing.
The accessory gear case is attached to an adapter plate. When aligning the product or changing the equipment for processing, there is no need to loosen the small pressure plate. Both the adapter plate and the accessory gear case can be moved and transported together, which effectively minimizes deformation of the accessory gear case under stress, reduces the number of clamping instances, and saves working hours.
Fifteen small pressure plates are evenly arranged for clamping, ensuring that the accessory gear case experiences stable stress during processing, which helps prevent deformation.
The auxiliary support features an adjustable structure. When supporting the weak inner cavity of the accessory gear case, a dial indicator is positioned around the support area to prevent deformation caused by excessive support. This setup effectively increases rigidity, which helps minimize vibration and reduce deformation.
(3) Plane processing strategy
When finishing the plane and approaching the final size, first loosen the small pressure plate to relieve the compressive stress. Next, return the accessory gear case to a free state before lightly re-tightening it. The clamping force should be set just enough to prevent loosening. Experimental tests have shown that controlling the tightening torque of the small clamping plate screw between 78 and 83 N·m results in minimal deformation of the processed product, thereby meeting the design requirements.
3.2 Machining accuracy control
Machining accuracy refers to how closely the actual geometric parameters—such as size, shape, and the relative position of surfaces—match the ideal geometric specifications after precision turning parts have been processed. The greater the degree of conformity, the higher the machining accuracy.
(1) Aperture accuracy control
When using multi-axis linkage for hole processing, machine tool linkage errors may occur. This can make it challenging to maintain the required diameter, roundness, and coaxiality tolerances for the bearing mounting holes and pin holes in the accessory gear case. To address this, precision boring is performed on the bearing mounting holes with non-standard tolerances, utilizing the X, Y, and Z axes of a fixed machine tool. For pin holes that have standard tolerance values (H7 tolerance), reaming is carried out using the same axes. This approach helps eliminate the risk of machine tool linkage errors and ensures the diameter accuracy of each hole.
(2) Hole position accuracy control
The accessory gear case’s structural characteristics require processing to be performed repeatedly on both vertical and horizontal machining centers. However, positioning errors can occur during the transition between processes and equipment, which can negatively impact the positional accuracy between holes. To address this, the accessory gear case employs the classic “one side and two pins” positioning method, secured with tooling (see Figure 2).
This approach offers several advantages: it ensures accurate and reliable positioning, effectively eliminates the six degrees of freedom of the product, and allows multiple surfaces to be processed either simultaneously or separately. This flexibility enhances processing accuracy and cutting efficiency. Additionally, this method serves as a positioning reference throughout all stages of production, from roughing to finishing, maintaining consistent semi-finishing and finishing allowances, minimizing reflection errors, and facilitating easy clamping.
3.3 Control of excess material
The accessory gear case features a multi-pipeline structure with a minimum pipe diameter of 6mm. Excess material can easily accumulate inside each pipe and at the bottom of each bolt hole. If excess material builds up inside a pipe, it can block the flow and prevent the normal supply of oil. This blockage may lead to wear on engine accessories and could potentially cause serious issues such as shaft seizure and engine burnout. Therefore, it is necessary to implement specific measures to control the presence of excess material.
1) Before processing, use a water stopper to seal the pipe mouth of the casting blank (see Figure 3) to prevent excess material from entering the pipe.
2) After processing each threaded hole and light hole, begin by using compressed air to blow away any chips. Next, wipe the surfaces with a white silk cloth dipped in anhydrous alcohol. Finally, use dry compressed air to ensure there are no residues remaining.
3) During product inspection, an endoscope is used to examine the interior of each pipeline to confirm that it is free of any excess material.
4) Once the product CNC processing and inspection are completed, each pipe opening is sealed with a process plug to prevent any foreign materials from entering.
5) When storing the product in the warehouse, it should be wrapped in a plastic bag and placed in a designated wooden box for protection.
04 Processing verification
Processing verification is carried out on two products. Some test results are shown in Table 1. The test shows that the product is qualified.
05 Conclusion
By examining the structural features and manufacturing challenges of the accessory gear case, we conducted thorough research on processing deformation, dimensional accuracy, and control of excess material. As a result of implementing appropriate control measures, we were able to produce qualified products. The findings from this research have been widely applied in subsequent mass production.
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