Designing for CNC Milling: A Practical Guide for CAD Designers

When it comes to CNC milling, great part design is just as important as machining skill. Thoughtful design can significantly reduce manufacturing costs, shorten lead times, and improve the quality of the final part. This guide from XRC Engineering provides essential tips for CAD designers to ensure their parts are efficient, machinable, and accurate — especially for 3- and 5-axis milling processes.

Whether you’re working on prototypes or production components, these best practices will help you design smarter.

1. Understand Tool Geometry and Limitations

📏 Avoid Deep, Narrow Cavities

• Why it matters: End mills have limited reach. Long, narrow pockets can introduce chatter, deflection, and poor surface finish.

• Best practice: Keep cavity depth ≤ 3x tool diameter when possible. If deeper pockets are necessary, use stepped or open designs.

  
  

🌐 Avoid Internal Sharp Corners

• Why it matters: Milling tools are round — internal corners will always have a radius.

• Best practice: Add a minimum 1.5x tool radius fillet to inside corners. Match internal radii across features for consistency and easier toolpathing.

  
  

2. Optimize Wall and Feature Thickness

🧱 Avoid Thin Walls

• Why it matters: Thin features tend to flex or chatter during machining.

• Best practice: Maintain a minimum wall thickness of 1.5 mm (metal) or 2 mm (plastic) unless necessary for function. Thicker is better.

  
  

⚖️ Balance Material Removal

• Why it matters: Parts with drastically uneven mass can warp, especially in long runs.

• Best practice: Distribute wall thicknesses and avoid isolated material “islands” when possible.

  
  

3. Consider Workholding and Fixturing

🧲 Add Flat Surfaces for Clamping

• Why it matters: Secure fixturing is key to part accuracy. Parts with irregular or rounded bases are hard to clamp.

  
  

• Best practice: Include fixturing flats or tabs in non-critical areas. For 5-axis parts, leave space for trunnion clearance or support.

  
  

🧮 Minimize Part Reorientation

• Why it matters: Every new setup increases cost and tolerance stack-up.

• Best practice: Design parts that can be milled in a single orientation when possible. If multi-face access is required, consider 5-axis-friendly geometries.

  
  

4. Standardize Hole Sizes and Features

🕳️ Use Standard Hole Sizes

• Why it matters: Drilled holes are quicker and more accurate than milled ones.

• Best practice: Stick to standard drill sizes (e.g., M3, M6, 6.5 mm, etc.). Avoid odd diameters unless necessary.

  
  

🔩 Account for Threading

• Why it matters: Internal threads require specialized tools or inserts.

• Best practice: Include clear thread callouts (e.g., M8 × 1.25 – 6H). Ensure there’s enough wall around tapped holes to avoid cracking.

  
  

5. Simplify Where Possible

⛓️ Avoid Excessive Small Details

• Why it matters: Tiny fillets, logos, and decorative cuts can add hours to a job.

• Best practice: Minimize cosmetic features unless they serve a functional or branding purpose.

  
  

📦 Use Design for Manufacturability (DFM) Principles

• Avoid non-essential undercuts unless machining on a 5-axis

• Round over external edges when possible to improve toolpathing and reduce deburring

• If mating parts are being designed, build in clearance (~0.1–0.3 mm for sliding fits)

  
  

6. Include Clear and Useful Drawings

📐 Make it Easy for the Machinist

• Include 2D technical drawings with key dimensions, tolerances, material spec, and finishes

• Call out critical features and note any areas that are purely cosmetic

• Use GD&T (Geometric Dimensioning and Tolerancing) when applicable, but don’t overuse it

  
  

7. Know When 5-Axis Adds Value

If your part has:

• Undercuts or compound angles

• Features on multiple faces

• Complex contours or freeform surfaces

Then it may be ideal for 5-axis milling, which allows full access to nearly any geometry in a single setup. Design with smooth transitions, generous radii, and clearance for rotary axes.

At XRC Engineering, our new 2025 Haas UMC-400 gives us full simultaneous 5-axis capability, so we can help you take advantage of these design freedoms without compromise.

Bonus: CAD Design Checklist for CNC Milling

✅ Avoid internal sharp corners

✅ Maintain consistent wall thickness

✅ Use standard hole sizes and thread callouts

✅ Minimize deep cavities and thin features

✅ Include generous radii and chamfers

✅ Design for fixturing and tool access

✅ Provide clear drawings with tolerances

✅ Keep features accessible for milling tools

✅ Know when 5-axis access is required

Final Thoughts

Designing with manufacturing in mind doesn’t mean limiting creativity — it means enabling your parts to be made more efficiently, with better accuracy, lower cost, and fewer delays.

Great CAD design = fewer headaches at the machine and better results in your hands.

At XRC Engineering, we love working with engineers and designers to turn great ideas into perfect parts. If you’re unsure about a feature or want feedback before release, our team is happy to provide DFM reviews as part of our quoting process.

Have a complex part you’re designing?

Send us your STEP file for a quick manufacturability review — we’ll help you get it right the first time.