Unified Approach to Overcoming Deep-Cavity Thin-Wall Machining Difficulties

Do machining professionals often face challenges when processing deep-cavity, thin-walled parts, such as severe tool vibration, poor surface quality, and dimensional inconsistencies caused by deformation? These seemingly difficult issues can be resolved. This article will guide you through overcoming these challenges by optimizing CNC programs, innovating tooling solutions, and effectively suppressing vibration.

 

PART.01 Introduction

People are increasingly seeking ultra-thin and aesthetically pleasing products. Many items on the market, such as mobile phones and laptops, aim for both ultra-thinness and superior surface quality. In the aerospace industry, components are also designed to be as lightweight as possible, all while ensuring safety. This lightweight design not only reduces the overall weight of the product, which can lead to increased flight time without additional fuel consumption, but it also allows for the possibility of carrying more fuel or transporting more passengers if the overall weight remains the same. Ultimately, this approach enhances efficiency in aerospace operations.

 

PART.02 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 it extends further out from the machine. The proper tool clamping length is illustrated in Figure 2. Generally, the tool’s extension length (L) should not exceed three times its diameter (D). For example, if the tool diameter is 10 mm, the extension length should ideally be limited to 30 mm. This guideline is based on the tool’s rigidity and cutting stability. An excessively long extension can reduce the tool’s rigidity and increase the likelihood of vibration and displacement, which in turn can negatively affect machining accuracy and surface quality.

 

(2) Cutting Vibration
The thinner the part, the lower its rigidity. This makes it more prone to vibration and tool breakage during machining. The cutting edge of the tool can become easily chipped, significantly shortening its service life and increasing machining costs.

 

PART.03 Technical Requirements

When machining rotating, deep-cavity, thin-walled prototype CNC parts, such as impellers, it is essential to ensure dynamic balance. Typically, the dynamic balance accuracy for impellers is specified to be at G6.3 or G2.5 grade. The tolerance for the blade profile is generally set at ±0.07 mm, while the tolerance for blade thickness is ±0.15 mm. Additionally, the required surface roughness for the blade should be Ra = 0.8 μm.

 

PART.04 Process Analysis

(1) Blank Preparation
The forging diameter is 1000 mm, and the material is a titanium alloy, known for being lightweight and strong. The forging process effectively improves the parts’ relative density 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 1 mm allowance on each side. The finishing process utilizes a reference positioning pin hole, which serves as a guide for positioning on the five-axis CNC machine in subsequent steps. One-sided two-pin positioning restricts the six degrees of freedom of the part in space.

 

(4) Rough Machining
The cavity is rough machined on a five-axis CNC machine tool (DMU MONOBLOCK 80P), with a 1.5-2mm machining allowance on each side.

 

(5) Heat Treatment
Annealing alleviates residual internal stress, aiming to fully release the internal tension generated during rough machining and to decrease the part’s hardness.

 

(6) Semi-Finishing
On a five-axis CNC machine, a ball end mill is used 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) Finish Machining
The inner circle is precisely turned to its finished size using a CNC lathe. Threaded and pin holes are created on a CNC milling machine. M6 threaded holes are milled to ensure they are perpendicular and meet the requirements of go/no-go gauges, while pin holes are bored. The cavity blades are machined to a fine finish on a five-axis CNC machine tool, with a machining allowance of 0.08 mm on each side. A small allowance is also left for manual polishing, with the tool mark depth typically measuring 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.

 

PART.05 Product Processing Solutions

During the roughing process, the bladed disk maintains good overall rigidity, and with a 1mm allowance on each side, roughing is relatively straightforward. However, during the semi-finishing and finishing stages, the blades become exposed, which leads to reduced blade rigidity. Additionally, because the blades are positioned deep, the tool clamping length must be sufficiently long to ensure that the blade root is reached, resulting in a significant loss of tool rigidity.

To address these issues, the following considerations are proposed: 1) Utilize an extended thin tapered HSK tool holder (as shown in Figure 3) to minimize the tool extension length and enhance tool rigidity.

 

2) When finishing the blades, use modeling clay (see Figure 4). The modeling clay absorbs vibrations, which helps mitigate the vibrations produced by the ball end mill during the cutting process.

 

3) Ball end mill selection. A 10mm four-flute ball end mill with a chisel edge was used for the operation. The material of the tool was carbide, and it had a cutting edge length of 15mm, with an approximate rake angle of 16 degrees. The tool was relatively sharp, which resulted in low cutting resistance and minimal vibration during use.

 

PART.06 Conclusion

To tackle the machining challenges associated with deep-cavity, thin-walled parts, we optimized both the toolpath and cutting method. 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 decrease blade chatter. This approach successfully addressed 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
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