10 CNC Mistakes to avoid to Improve Your CNC Machining Design

10 CNC Mistakes to Avoid to Improve Your CNC Machining Design

Avoid these common mistakes when designing CNC parts to ensure precision. Learn best practices to improve your CNC design and optimise your manufacturing process

10 Common CNC Mistakes to Avoid to Improve Your CNC Machining Design

Design is a crucial aspect of the CNC manufacturing process. In addition to providing machining instructions, it directly impacts machining time, cost, and effort. This article explores 10 ways to improve your CNC machining design to increase machining efficiency and save cost.

1. Avoid Non-machinable Features

While CNC machining can produce highly complex geometries, certain features simply cannot be machined using conventional CNC machining. These features include:

  • Undercuts: Undercuts are sections that require material removal from underneath overhanging structures. Some undercuts can be machined using complex, specialised tools and multi-axis machining.
  • Complex internal channels: Machining tools are typically cylindrical and follow straight paths. Some internal channels, such as curved holes and helical channels, do not present compatible access paths for the tool. As a result, the CNC milling process cannot create such channels. Advanced techniques such as EDM (Electrical Discharge Machining) are more appropriate for these features.
  • Complex internal cavities: Like internal channels, machining complex internal cavities geometries, like hollow spheres or intricate internal lattices, is impossible with conventional CNC, as the tools cannot navigate within a fully enclosed internal space.

cnc design mistakes: curved hole

CNC design mistakes: Complex internal channels

See our CNC machine design guide to learn more about designing for CNC machining.

2. Prioritise Functionality and Simplicity

While it should not come at the expense of functionality, simplicity reduces the time and cost of manufacturing. Designing unnecessary complex features is a common CNC design mistake. When designing a feature in a part or an assembly, consider the processes and tooling it would take to achieve that part. The design should satisfy all functional and structural requirements, with the aim of featuring features that do not require advanced 5-axis machining or Electrical discharge machining (EDM), as these cost more.

3. Limit the Use of Tight Tolerances

CNC machining is highly accurate and can achieve tight tolerances. However, in many instances, applying tolerance to all the dimensions in your design is unnecessary. Improve CNC design by only specifying tolerance when it is crucial for the part's functionality, such as mating or moving parts. CNC machining operations, such as CNC milling and CNC turning, typically apply a default tolerance of ± 0.13 mm, which is quite accurate. Tighter tolerances are achievable but require more time and effort. See our guide on CNC machining tolerances for more information on tolerancing.

cnc design mistake, limit tolerance

CNC design mistakes: Over tolerancing

4. Minimise Aesthetic Features

Improve your design by minimising aesthetic features. Considering the capabilities of CNC machining services, it may be tempting to get carried away with decorative patterns, embossments, engravings, lettering, and other aesthetic features. However, features that have no functionality and only serve to improve aesthetics unnecessarily increase machining time and effort, and including them is a major CNC design mistake. On the other hand, if aesthetics is a major consideration for your part, then feel free to include such features.

5. Design sufficiently thick walls

During machining, the workpiece is subject to continuous vibration on contact with the cutting tool. Similarly, the tool or workpiece may bend or deflect slightly. Thinner walls are less stiff and more susceptible to bending, breaking, and warping due to vibrations and deflections. Their susceptibility to vibrations also lowers achievable accuracies. Design sufficiently thick walls with enough stiffness to withstand vibration or tool deflection. We recommend a minimum wall thickness of 0.8 mm for metals and 0.15 mm for plastics.

Maintaining a good wall width-to-height ratio is also important, as taller walls are also more susceptible to damage and warping during machining. We recommend a width-to-height ratio of 3:1 for non-supported, free-standing walls to ensure stability.

Improve cnc design: Wall thickness

Improve CNC design: Sufficient wall thickness

6. Assign Radii To Internal Edges

The cylindrical geometry of CNC milling cutting tools makes them unable to machine sharp internal edges. These tools produce a radial internal edge that is a minimum of the tools’ radii. Improve your CNC design by adding radii to internal edges. Another common CNC design mistake is adding an internal edge radius smaller than the tool’s radius. We recommend adding an internal radius 30% bigger than your cutting tool’s radius to mitigate tool wear and tear. For example, if your cutting tool is 10 mm, design internal edges with a 13 mm radius. This allowance reduces tool stress and increases cutting speed.

Improve CNC design: Internal radius

Improve CNC design: Assign Radii To Internal Edges

7. Use Standard Hole Sizes

Standard hole sizes can be efficiently and accurately drilled with readily available standard drill bits. Non-standard holes, on the other hand, require end mill tools to machine out the dimension progressively. This increases machining time and effort. Furthermore, for threaded holes, standard hole sizes have corresponding thread sizes programmed in CNC machines, making it more efficient to create threaded holes.

CNC design mistake: Non-standard hole size

CNC design mistakes: Non-standard hole sizes

8. Limit Thread Depths

The strength of thread connections usually resides in the first few threads. Improve your CNC design by limiting the depths of your threads to a maximum of three times the hole diameter. For through holes, you can design threads at the top and bottom. For blind holes, we recommend leaving an unthreaded length of half the hole’s diameter at the bottom.

Improve cnc design: thread depth

Improve CNC design: Limith thread depth

9. Limit Cavity Depth

CNC cutting tools have a limited depth, typically 3 to 4 times their diameter, beyond which they are highly susceptible to deflection and fracture. Design cavities with a suitable depth-to-width ratio to prevent tool hanging and deflection and to facilitate chip evacuation. Milling tools mill cavities three times their diameter in depth most efficiently. Cavities deeper than six times the tool diameter are considered deep. Such cavities should have a maximum depth of four times their width to allow for sufficient machining space.

Improve cnc design: cavity depth

Improve CNC design: Limit cavity depth

10. Specify Standard Surface Roughness

CNC machines typically produce a default surface roughness of 3.2 µm Ra and can produce surfaces as smooth as 0.06 µm Ra. However, machining time and cost increase exponentially with specified surface roughness. We recommend specifying the default surface roughness of 3.2 µm Ra when surface roughness is not critical. Smooth surface roughness is crucial for functionality in certain applications, such as load-bearing and mating, moving parts. For example, Medical CNC machining applications, such as joint replacement parts where loading and movement are continual, require very low surface roughness. See our CNC machining surface roughness article to learn more about specifying the right surface roughness for your part.

Conclusion

By avoiding these CNC design mistakes, you can significantly improve your design and optimise the CNC manufacturing process. At Geomiq, our instant quoting platform analyses your design in seconds, providing valuable DFM feedback. Upload your CNC design to get started and receive your part in as little as three days.

About the author

Sam Portrait

Sam Al-Mukhtar

Mechanical Engineer, Founder and CEO of Geomiq

Mechanical Engineer, Founder and CEO of Geomiq, an online manufacturing platform for CNC Machining, 3D Printing, Injection Moulding and Sheet Metal fabrication. Our mission is to automate custom manufacturing, to deliver industry-leading service levels that enable engineers to innovate faster.

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