CNC Machining Design Guides for Cost Efficiency
Optimizing your CAD design for the CNC process can reduce part costs by 30% or more. These rules are based on tool geometry, machine capabilities, and material behavior.
Table of Contents
1. Internal Corner Radii
CNC milling tools are cylindrical (end mills). When cutting a pocket, the tool cannot leave a sharp 90° internal corner; it will always leave a radius equal to the radius of the tool.
- Rule: Specify a radius that is slightly larger than a standard tool size (e.g., use 3.2mm instead of 3mm). This allows the tool to move smoothly through the corner rather than stopping.
- Best Practice: Keep internal corner radii ≥ 1/3 of the pocket depth. Small, deep corners require long, thin tools which vibrate and break easily.
2. Blind Hole Depth
Deep holes are difficult to evacuate chips from and require specialized tooling.
- Rule: Keep hole depth ≤ 4x the hole diameter. For deeper holes (up to 10x), costs increase significantly due to slow feed rates and higher risk of tool breakage.
- Bottoms: Standard drills have a 118° or 135° point. Specify a conical bottom unless a flat bottom is strictly required for function.
3. Minimum Wall Thickness
Thin walls can warp due to machining stresses or vibrate (chatter) during the cut.
| Material Group | Recommended Min Wall | Absolute Min Wall |
|---|---|---|
| Metals (Aluminum, Steel) | 0.8 mm | 0.5 mm |
| Plastics (POM, ABS) | 1.5 mm | 1.0 mm |
4. Threading & Tapped Holes
Avoid specifying threads that are deeper than necessary. The strength of a thread is largely concentrated in the first 3-5 turns.
- Rule: Keep thread depth ≤ 3x the diameter. Standard taps are rarely longer than this.
- Standard Sizes: Stick to common metric (M3, M4, M5, M6, M8) or imperial (4-40, 6-32, 10-32, 1/4-20) sizes to avoid custom tooling costs.
Engineer's Tip: Use the largest possible internal radii and the shallowest possible holes. These two changes alone typically account for the majority of CNC cost savings.