Moving between 3D printing and CNC machining for plastic parts involves understanding their respective strengths, limitations, and the design considerations that apply to each. Here’s a practical guide for making the transition or choosing between the two depending on your needs:
1. Understand the Key Differences
| Feature | 3D Printing (Additive) | CNC Machining (Subtractive) |
|---|---|---|
| Material | Thermoplastics, photopolymers | Engineering plastics (e.g. Delrin, Nylon) |
| Geometry | Complex internal structures, undercuts | Simpler geometries, limited undercuts |
| Setup Time | Low | Higher (fixtures, toolpaths needed) |
| Volume Suitability | Prototyping, small batches | Mid-high volume, tighter tolerances |
| Tolerances | ±0.1–0.3 mm | ±0.02–0.1 mm |
| Surface Finish | Layered finish (post-processing needed) | Smooth, machined finish |
| Speed | Fast for prototypes | Slower for complex parts |
| Cost | Low initial cost, higher per unit | Higher setup, lower per unit in bulk |
2. Design Considerations for Transitioning
From 3D Printing to CNC Machining:
- Wall Thickness: Thin walls in 3D printing can be difficult or impossible to mill. Ensure minimum wall thickness suitable for machining (typically >1.5 mm).
- Support Structures: Remove any internal geometries or channels that can’t be accessed by cutting tools.
- Tolerances: Tighten tolerances if switching to CNC for high-precision needs.
- Fillets and Radii: Add internal fillets to reduce tool stress; avoid sharp internal corners unless needed.
- Tool Access: Ensure all surfaces are accessible to milling tools.
From CNC Machining to 3D Printing:
- Undercuts and Hollow Structures: Take advantage of 3D printing’s ability to produce complex internal geometries.
- Material Availability: Use printable materials like PLA, ABS, PETG, or Nylon if the original CNC material isn’t printable.
- Supports and Orientation: Consider build orientation for strength and aesthetics. Add necessary supports in the slicing software.
- Weight Reduction: Use lattice or honeycomb infills instead of solid volumes.
3. Material Cross-Compatibility
| CNC Material | Closest Printable Equivalent |
|---|---|
| Delrin (POM) | Nylon |
| Polycarbonate | PC (3D printing version) |
| ABS | ABS (FDM or SLA compatible) |
| Acrylic (PMMA) | Tough Resin (for SLA), PETG (FDM) |
| Nylon (PA6/PA66) | Nylon (SLS or FDM, sometimes blended) |
4. Workflow Tips
- Start with 3D printing for prototyping, especially during design validation stages.
- Switch to CNC for functional, end-use parts requiring precision, durability, and better surface finish.
- Use hybrid workflows: 3D print the base part, then machine critical features (e.g., tight holes or threads).
- Consider DFM (Design for Manufacturability) tools or plugins in CAD software (like Fusion 360 or SolidWorks) to auto-check for process compatibility.
5. Common Use Cases
| Use Case | Recommended Process |
|---|---|
| Rapid Prototyping | 3D Printing |
| Precision Fixtures & Tooling | CNC Machining |
| Lightweight Aesthetic Models | 3D Printing |
| Mechanical Functional Parts | CNC or Reinforced 3D Prints |
| Low-volume Custom Production | 3D Printing (SLS/SLA) |
