- July 4, 2025
Deep cavity parts, especially those with narrow internal geometries, are among the most challenging tasks in CNC milling. Common issues include excessive tool overhang, poor chip evacuation, and inadequate cooling. These problems can reduce tool life, degrade surface quality, and lower production efficiency.
Key Considerations for CNC Machining of Deep and Narrow Cavities
When machining deep and narrow cavities, the following factors require special attention:
- Tool Deformation: Longer tools are more prone to bending, which affects dimensional accuracy and surface finish.
- Chip Evacuation: Chips tend to accumulate at the bottom of deep cavities, increasing the risk of tool breakage or wall damage.
- Vibration: Excessive tool overhang and lack of support can lead to vibration, which reduces accuracy and shortens tool life.
- Depth-to-Width Ratio: Keeping the cavity depth-to-width ratio between 3:1 and 4:1 helps reduce machining instability.
- Depth-to-Fillet Radius Ratio: When the ratio of cavity depth to the corner fillet radius reaches 10:1, the structure can be considered a deep cavity. The larger the ratio, the longer the required cutting tool becomes. So this is a primary indicator of machining difficulty.
Aluminum Deep Cavity Machining
This article will provide a detailed analysis of practical solutions to deep cavity parts. It is based on a real aluminum cavity project featuring an ultra-deep and narrow cavity structure with a depth of 113 mm, a minimum width of 14.5 mm, and an internal fillet radius of 6 mm at the corners.
Part Overview
- Material: AL7075-T6
- Dimensions: 175.2 × 103 × 122.65 mm
- Features: Deep internal cavity with maximum dimensions of 146.2 × 83 mm. The narrowest section measures 14.5 × 14 mm. All internal corner radii are 6mm, with a depth of 113 mm. The resulting depth-to-radius ratio of 19:1 classifies this as an ultra-deep cavity.
Key Challenges
- A ∅12 mm tool requires over 115 mm of overhang, which leads to insufficient rigidity (tool depth exceeds 5 times the diameter).
- Aluminum chips accumulate faster than they can be removed, wrapping around the tool and increasing the risk of failure.
- The internal walls must meet a strict surface roughness requirement of Ra ≤ 0.8 µm.
- The perpendicularity of the internal cavity wall is highly demanding (0.1 mm perpendicularity required).
How to Optimize Process Strategies?
The following strategies were used to improve tool stability, chip evacuation, and overall roughing efficiency.
1. Optimize Tool Entry Strategy
Before roughing, pre-drill pilot holes to reduce cutting load during tool entry and to aid chip evacuation.
In this case, two ∅22 mm through-holes were drilled at the bottom of the cavity. These holes provided entry points for roughing tools and channels for chip removal. The roughing tool entered vertically along the Z-axis through the holes, then performed XY-plane milling.
This approach avoided the heavy “impact force” typically encountered when the tool plunges directly into stock material along the Z-axis. It is an issue especially problematic in cavity slot roughing.
2. Stage-Based Rough Machining
A three-stage roughing strategy was used:
Stage 1: High-Efficiency Dynamic Roughing
An ∅18 mm solid carbide three-flute wavy end mill (total length 100 mm, projection 70 mm, depth 0–65 mm) was used. Adaptive dynamic roughing was applied (S4000/F1800, depth 25 mm, width 1.8 mm) to maximize roughing efficiency.
Stage 2: Stable Deep Roughing with Insert Cutter
Anti-vibration extended ∅20 mm insert cutter (overall length 200 mm, overhang length 130 mm, machining depth 65–113 mm) used for stepwise roughing (S2800/F2000, depth of cut 0.5 mm, width of cut 14 mm), aiming for stable and safe roughing down to the bottom of the cavity.
Stage 3: Corner Refinement for Uniform Finishing Allowance
Secondary roughing using an extended ∅12 mm solid tungsten carbide end mill (overall length: 200 mm; overhang: 125 mm; machining depth: 0–113 mm) at S3000/F1500 with a depth of cut of 0.35 mm. The purpose is to remove the large corner radius left by the previous large-diameter roughing tool, so that all internal cavity wall surfaces have a uniform finishing allowance of 0.2 mm.
3. Select a Suitable Tool Material and Geometry
Tool selection and roughing strategy are crucial for stable deep cavity machining. In this case, YW-type carbide inserts outperformed YG and YT-type inserts in heat dissipation and anti-adhesion performance.
Optimizing Finishing Tool Paths
The table below shows two types of finishing tool paths:
Left: Layer-by-Layer Finishing
On the left is the layer-by-layer finishing method, where after completing each layer, the tool moves to the next level via auxiliary entry and exit paths. The advantage of this method is its “high efficiency,” but the downside is the visible entry and exit marks on the workpiece.
Due to the large tool overhang, the deflection at the tool’s tip and root is inconsistent, resulting in a conical shape after rotation. This leads to noticeable layer marks on the inner wall after finishing, as well as a taper that fails to meet the 0.1 perpendicularity requirement.
Right: Optimized Tool Path (One-Pass Spiral Machining)
On the right, the optimized tool path employs the one-pass continuous cutting technique (single entry and exit throughout the process). The tool path spirals downward from start to finish. While the tool deflection issue remains, the spiral one-pass technique ensures that the tool’s tip maintains a consistent, low-load cutting condition with uniform speed.
As a result, the impact of tool deflection does not vary with machining depth. This allows the inner wall of the workpiece to achieve a uniform surface finish from top to bottom, while also meeting the drawing’s perpendicularity requirements.
Dual-Channel High-Pressure Coolant System
Even with pre-drilled chip evacuation holes, aluminum chips are generated rapidly during roughing. Continuous coolant is essential. Not only for cooling the tool but also for flushing away chips in real time.
A dual-channel high-pressure coolant system, with both vertical and side outlets, was used to ensure reliable chip removal.
(Note: In the picture, the high-pressure coolant from the vertical outlet was not activated.)
Final Results and Summary
Through the use of standard high-performance equipment and process optimization, we achieved:
- 42% reduction in rated cycle time per workpiece
- 125% increase in tool life
- Consistent surface roughness (Ra ≤ 0.8 µm)
- Verticality (≤ 0.1 mm)
Key Takeaways
- Tool path strategy is as important as tool selection.
- Segmented roughing reduces vibration for long tool overhangs.
- Spiral one-pass finishing avoids the instability caused by excessive tool overhang.
This project explained that deep cavity machining does not require special tools or machines. With careful planning, proper sequencing, and tight process control, high-quality results are achievable using standard setups.
Need help optimizing your next deep cavity machining project? Contact WayKen for expert support.
