Advanced Machining Strategies for Deep-Recess Thin-Wall Structures
This paper examines the machining challenges associated with deep-cavity thin-walled parts. It addresses issues related to machining and deformation by optimizing CNC machining programs and processes, enhancing tool rigidity, and minimizing cutting vibrations. This approach offers a new solution for machining similar components.
1. Introduction
People are increasingly seeking ultra-thin and visually appealing products. Many items on the market aim for both ultra-thinness and high surface quality, such as slim mobile phones and laptops. In the aerospace industry, components are also designed to be as lightweight as possible while ensuring safety. This focus on lightweight designs reduces the overall weight of the product, allowing for longer flight times with the same fuel consumption. Additionally, when the total weight remains constant, lighter components enable the transport of more fuel or additional passengers, thereby improving efficiency.
2. Machining Challenges
A typical deep-cavity thin-walled part is shown in Figure 1. Its machining challenges are as follows.
(1) Poor Tool Rigidity
The rigidity of a machined part decreases as the depth of its cavity increases. Similarly, the rigidity of a tool diminishes as its extension from the machine tool increases. The tool clamping length is depicted in Figure 2. Generally, the tool extension length (L) should not exceed three times the tool diameter (D). For instance, if the tool diameter is 10 mm, the extension length should ideally be limited to 30 mm. This guideline is primarily based on considerations of tool rigidity and cutting stability. Excessive extension length can lead to reduced tool rigidity, increasing the risk of vibration and misalignment, which in turn affects machining accuracy and surface quality.
(2) Cutting Vibration
The thinner the part, the less rigid it becomes, making it more susceptible to vibration and tool breakage during machining. The cutting edge of the tool can easily chip, which significantly shortens its service life and raises machining costs.
3 Technical Requirements
When machining rotating, deep-cavity, thin-walled parts, such as impellers, it is essential to ensure dynamic balance. The typical dynamic balance accuracy for impellers is classified as either G6.3 or G2.5 grade. Additionally, the tolerance for the blade profile is generally set at ±0.07 mm, while the tolerance for blade thickness is ±0.15 mm. The required surface roughness for the blades is Ra = 0.8 μm.
4. Process Analysis
(1) Blank Preparation
The forging diameter is 1000 mm, and the material used is titanium alloy, which is both lightweight and strong. The forging process effectively improves the relative density of the parts and enhances their structural strength.
(2) Rough Machining
The outer and inner diameters are rough machined on a CNC lathe, with a 1.5mm allowance on each side.
(3) Semi-finishing
The outer diameter and inner hole are semi-finished on a CNC lathe, with a 1mm allowance on each side. For the finishing process, a reference positioning pin hole is used to guide the machining positioning of the five-axis CNC machine tool in the subsequent operations. A one-sided two-pin positioning system restricts the part’s six degrees of freedom in space.
(4) Rough Machining
The cavity is rough machined on a five-axis CNC machine tool (DM UMONOBLOCK 80P), with a 1.5-2mm machining allowance on each side.
(5) Heat Treatment
Annealing alleviates residual internal stress, aiming to completely release the stress generated during rough machining and reduce the hardness of the part.
(6) Semi-finishing
On a five-axis CNC machine tool, a ball end mill is utilized for semi-finishing the cavity. A machining allowance of 0.3 to 0.5 mm is left on each side of the cavity blades for finish milling and subsequent polishing.
(7) Heat Treatment
Normalizing the part increases its hardness, ensuring sufficient strength during later operation; it also further releases residual internal stress.
(8) Finishing
The inner diameter is precisely turned to the finished size using a CNC lathe. Threaded holes and pin holes are machined with a CNC milling machine. The M6 threaded holes are milled to ensure they are perpendicular and meet go/no-go gauge requirements, while the pin holes are bored. The cavity blades are finished on a five-axis CNC machine tool, with a machining allowance of 0.08 mm on each side. Additionally, allow some space for manual polishing, as the depth of the tool marks is typically 0.05 mm.
(9) Polishing
First, manually rough polish the cavity blades until there are no tool marks, then manually fine polish the cavity blades until the surface quality requirements are met.
5. Product Machining Solutions
During rough custom aluminum CNC machining, the impeller exhibits good overall rigidity, and with a 1mm allowance on each side, the machining difficulty is manageable. However, during the semi-finishing and finishing stages, the blades become exposed, which results in reduced rigidity. Additionally, the blades are deep, necessitating a sufficiently long tool clamping length to reach the blade root. This requirement leads to a significant reduction in tool rigidity. To solve these machining challenges, the following aspects are considered:
1) Use an extended, thin-tapered HSK tool holder (see Figure 3) to minimize the tool extension length and enhance tool rigidity.
2) When finishing the blades, apply modeling clay (see Figure 4). Modeling clay has a vibration-absorbing effect, and the vibration generated by the ball end mill when cutting the blades can be absorbed by the modeling clay.
3) Ball end mill selection. A φ10mm four-flute ball end mill with a chisel edge was utilized. The material of the tool was cemented carbide, featuring a cutting edge length of 15mm and a rake angle of approximately 16°. The tool was relatively sharp, which led to low cutting resistance and minimal vibration during operation.
6. Conclusion
To tackle the machining challenges associated with deep-cavity, thin-walled parts, we optimized both the toolpath and cutting methods. We implemented several physical adjustments: we increased the rake angle and the number of cutting edges to reduce cutting resistance. Additionally, we wrapped the front section of the blade with clay to absorb vibrations and minimize blade movement. These modifications successfully resolved the machining issues and provided us with valuable experience.
If you want to know more or inquiry, please feel free to contact info@anebon.com
CE Certificate China custom CNC aluminum parts, custom CNC milling Parts, and metal fabrication parts. All the employees in the factory, store, and office of Anebon are struggling with one common goal: to provide better quality and service.