CNC Machining Design Guide
Design parts that are faster, cheaper, and easier to machine
Smart design choices can cut CNC machining costs by 30–50% without sacrificing part quality. This guide covers the most impactful DFM rules — wall thickness, internal radii, hole depth ratios, and tolerance specifications — based on real-world factory feedback from our network of 179+ audited manufacturers.
Core Design Principles
Minimize Setups
Design parts that can be machined in as few setups as possible. Each flip adds time, cost, and tolerance stack-up.
Use Standard Tools
Design features that can be cut with standard end mills and drills. Custom tooling adds lead time and cost.
Specify Only Needed Tolerances
Tight tolerances on every dimension dramatically increases cost. Apply tight specs only where function demands it.
Balance Rigidity
Thin walls and tall features vibrate during machining. Ensure adequate support for cutting forces.
DFM Rules & Guidelines
Wall Thickness
Thin walls vibrate and deflect during machining, causing chatter marks and potential breakage.
Minimum 0.8 mm for metals, 1.5 mm for plastics. Ideal: 1.5 mm+ for aluminum, 2.0 mm+ for steel.
Walls thinner than 0.5 mm on metal parts. Tall thin walls (height-to-thickness ratio > 4:1).
Internal Corner Radii
End mills are round — they cannot create perfectly sharp internal corners. Forcing small radii requires smaller tools, slower feeds, and multiple passes.
Internal radius ≥ 1/3 of cavity depth. Minimum R0.5 mm. Use R1.0 mm or larger where possible.
Sharp 90° internal corners. Radius smaller than the smallest standard end mill available.
Hole Depth
Deep holes require special tooling (gun drills) and are prone to wandering. Standard drill bits have practical depth limits.
Depth ≤ 4× diameter for standard drills. Up to 10× with peck drilling. Through-holes are cheaper than blind holes.
Holes deeper than 10× diameter. Small-diameter deep holes (e.g., Ø2 mm × 40 mm deep).
Tolerances
Each tolerance level requires slower machining, more passes, and often secondary operations like grinding.
±0.1 mm for general features (achievable on standard CNC). ±0.025 mm for critical fits. ±0.005 mm only where absolutely necessary.
Blanket tight tolerances across all dimensions. Geometric tolerances on non-functional surfaces.
Undercuts & Internal Features
Standard 3-axis CNC can only reach features accessible from above. Undercuts need special tooling or 5-axis machines.
Design features accessible from one direction. If undercuts are needed, use standard T-slot cutter dimensions.
Complex internal cavities that require EDM. Multiple undercuts on different faces of the same part.
Thread Design
Internal threads deeper than 3× diameter add risk of tap breakage. Thread milling is available but slower.
Thread depth 1.5–3× nominal diameter. Use standard thread sizes (M3, M4, M5, M6, M8, M10). Specify thread class only where needed.
Threads in hardened materials without specifying thread milling. Non-standard thread pitches.
⚠️ Common Design Mistakes
- ⚠ Specifying ±0.01 mm on every dimension — this can 10× your machining cost compared to standard tolerances.
- ⚠ Designing pockets with sharp internal corners — the machine literally cannot make them. Always add a fillet radius.
- ⚠ Creating parts with features on all 6 faces — requires multiple setups and fixturing, dramatically increasing cost.
- ⚠ Forgetting to add draft or clearance for assembly — parts that are perfect on paper may not assemble due to tolerance stack-up.
- ⚠ Calling out surface finish (Ra) tighter than needed — Ra 0.8 µm requires grinding; Ra 3.2 µm is standard for CNC.
💡 Pro Tips
- ▸ Group critical tolerances on features machined in the same setup to minimize tolerance stack-up.
- ▸ Add chamfers instead of fillets on external edges — they are cheaper and faster to machine.
- ▸ Design symmetrical parts when possible — eliminates orientation errors during setup.
- ▸ If you need a sharp internal corner for mating, use a relief groove (dogbone) instead of forcing a tiny radius.
- ▸ Provide 2D drawings with GD&T only for critical features. Over-dimensioned drawings slow down quoting.
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