When machining deep-cavity, thin-walled parts, are you often troubled by these problems: severe tool vibration, substandard surface quality, and dimensional inconsistencies due to deformation? These seemingly intractable problems are not unsolvable. This article will guide you through overcoming these difficulties one by one by optimizing CNC programs, innovating tooling solutions, and cleverly suppressing vibration.
PART.01
Introduction
Currently, people increasingly pursue the ultimate experience of ultra-thin and aesthetically pleasing products. Most products on the market strive for ultra-thinness and superior surface quality, such as ultra-thin mobile phones and ultra-thin laptops. While ensuring safety, aerospace components are also designed to be as lightweight as possible. On the one hand, this reduces the overall weight of the product, allowing for increased flight time with the same fuel consumption; on the other hand, with a fixed overall weight, lighter components allow for more fuel to be carried or more passengers to be transported, thus improving efficiency.
PART.02
Machining Challenges
A typical deep-cavity, thin-walled part is shown in Figure 1. The machining challenges are as follows.
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Figure 1 Typical Deep-Cavity Thin-Walled Parts
(1) Poor Tool Rigidity: The deeper the cavity of the machined part, the worse the rigidity of the part; the longer the tool extends from the machine tool, the worse the tool rigidity. The tool clamping length is shown in Figure 2. Generally, the tool extension length L should not exceed three times the tool diameter D. For example, if the tool diameter is 10mm, the extension length should ideally be controlled within 30mm. This principle is mainly based on tool rigidity [1, 2] and cutting stability. Excessive extension length reduces tool rigidity, increases the possibility of vibration and offset, thus affecting machining accuracy and surface quality.
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Figure 2 Tool Clamping Length
(2) Cutting Vibration: The thinner the part, the lower its rigidity, and the more prone it is to vibration, tool breakage, etc., during machining. The tool edge is easily chipped, significantly shortening its service life and increasing machining costs.
PART.03
Technical Requirements
For rotating deep cavity thin-walled parts, taking impellers as an example, dynamic balance must be ensured after machining. Generally, the dynamic balance accuracy of the impeller is selected as G6.3 or G2.5 grade. The profile tolerance of the blade is generally required to be ±0.07mm, the thickness tolerance of the blade is required to be ±0.15mm, and the surface roughness value of the blade is Ra=0.8μm.
PART.04
Process Analysis
(1) Blank Preparation: Forging diameter 1000mm, material is titanium alloy, which is lightweight and strong. The forging process[3] of the blank can effectively improve the relative density of the part and enhance the structural strength of the part.
(2) Rough Machining: Rough turning of the outer and inner circles on a CNC lathe, leaving a 1.5mm allowance on each side. (3) Semi-finishing: The outer diameter and inner hole are semi-finished on a CNC lathe, leaving a 1mm allowance on each side. The process reference positioning pin hole is then finished for use in subsequent machining on the five-axis CNC machine. A two-pin positioning method is used on one side, restricting the part's six degrees of freedom.
(4) Roughing: The cavity is rough-machined on a five-axis CNC machine (DMU MONOBLOCK 80P), leaving a 1.5-2mm machining allowance on each side.
(5) Heat Treatment: Annealing is performed to remove residual internal stress. The purpose is to completely release the internal stress generated during roughing and reduce the part's machining hardness.
(6) Semi-finishing: On a five-axis CNC machine, a ball end mill is used to semi-finish the cavity. A 0.3-0.5mm machining allowance is left on each side of the cavity blades for finish milling and subsequent polishing.
(7) Heat Treatment: Normalizing the parts increases their hardness, ensuring sufficient strength during later operation; it also further releases residual internal stress.
(8) Finishing: The inner diameter is precision turned to the finished size on a CNC lathe. Threaded holes and pin holes are machined on a CNC milling machine. M6 threaded holes are milled to ensure perpendicularity and compliance with go/no-go gauge requirements; pin holes are bored. The cavity blades are finished on a five-axis CNC machine, with a 0.08mm machining allowance on each side. A small allowance is left for manual polishing; the tool mark depth is generally 0.05mm.
(9) Polishing: The cavity blades are first manually rough polished until tool marks are removed, then manually fine polished until the surface quality requirements are met.
PART.05
Product Processing Solution
During rough machining, the impeller has good overall rigidity, and with a 1mm allowance on each side, the rough machining is not difficult. During semi-finishing and finishing, the blades are exposed, resulting in poor blade rigidity. Furthermore, the blades are deep, requiring a sufficiently long tool clamping length [4] to ensure machining to the blade root, leading to a severe lack of tool rigidity. To address these machining challenges, the following aspects are considered:
1) Use an extended, thin-tapered HSK tool holder (see Figure 3) to minimize tool extension and enhance tool rigidity.
Image Figure 3 Extended, thin-tapered HSK tool holder
2) When finishing the blades, apply modeling clay (see Figure 4). The clay has a vibration-absorbing effect, allowing the ball end mill to absorb vibrations generated during blade cutting.
Image Figure 4 Applying modeling clay
3) Selection of ball end mills. Use a 10mm four-flute ball end mill with a transverse cutting edge, made of carbide, with a cutting edge length of 15mm and a rake angle of approximately 16° [5]. The tool is relatively sharp, with low cutting resistance and relatively low vibration.
