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how to design parts for cnc machining

Design Guide: CNC Machining

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CNC machining is a subtractive manufacturing process suitable for a wide variety of industries, including medical, automotive, and aerospace. It is capable of producing functional parts and prototypes with a higher degree of accuracy and precision than other methods.

To take full advantage of this technology, it is important that your 3D model closely follows a set of geometric recommendations. This article offers an overview and considerations of the CNC machining process, basic & advanced design practices, and cost reduction tips.

 

how to design cnc machine parts both metals and plastics

 

CNC Machining Process

CNC machining is a subtractive manufacturing process that leverage a combination of computerized controls and a variety of cutting tools to rapidly remove layers from a solid block of material. At HLH Rapid, we can machine both metals and plastics.

 

CNC Considerations

  • Tool Geometry: As material is removed from the workpiece, the geometry of the tool is transferred to a machined part. This means, for example, that the internal corners of a CNC part always have a radius.
  • Specialised Tools: Such tools can help overcome certain design restrictions, but they come with trade-offs. For instance, a specialized tool with a longer shaft can help reach deeper parts of a cavity in the workpiece. However, it also increases vibrations which results in lower accuracy.

 

Characteristics of CNC Machining

Maximum Size 3000 x 1200 x 850 mm
Standard Tolerance ±0.05mm (metals), ±0.2mm (plastics)
Common Metal Materials Al6061, Al7075, SS316L, SS301 and more grades available
Common Plastic Materials ABS, PC, Acrylic, PEEK, HDPE
As Machined Surface Smooth to the touch (Ra3.2). Visible machine marks

 

 

CNC Design Guidelines

Certain features, such as minimum wall thickness and achievable tolerances, in CNC designs can be tricky and should be examined on a case-by-case basis, but staying within a set of geometric recommendations can help ensure better results and a successful machining process.

 

Tolerances

At HLH, standard tolerances are controlled to ISO 2768 Fine (F) for metals and Medium (M) for plastics. For features left unspecified, standard tolerances are used. Tighter tolerances can be achieved but is only recommended for critical features.

  • Metals: ±0.05mm standard
  • Plastics: ±0.2mm standard

 

Threads

HLH can accommodate and cut metric threads, imperial UNC and UNF, pipe threads among others. All threads should be clearly marked on your 2D drawings. The majority of the load applied to a thread is taken by the few first teeth (up to 1.5 times the nominal diameter); therefore, a thread length of 3x the hole diameter is recommended.

 

Designing threads for cnc machining (plastic and metals)

 

Wall Thickness

Designing wall thickness can be tricky and differ as decreasing the wall thickness reduces the stiffness of the material, which in turn increases vibrations during machining and lowers the achievable accuracy. Plastics are more prone to warping (due to residual stresses) and softening (due to temperature increase), so a larger minimum wall thickness is recommended.

  • Metals: 0.8mm at minimum
  • Plastics: 1.5mm at minimum

Designing wall thickness for cnc machining (plastics and metals)

 

Cavities & Pockets

End mill tools have a limited cutting length (typically 3–4 times their diameter). Tool deflection, chip evacuation and vibrations become more prominent when cavities have a smaller depth-to-width ratio. As such, we recommend limiting the depth to 4 times the length to ensure good results.

 

Designing cavities and pockets for cnc machining (plastic and metals)

 

Internal Edges

When designing internal edges for machining, experts recommend that the vertical corner radius be ⅓ or more of the cavity depth. This will enable the tool to cut along a circular path rather than a 90-degree angle. Increasing the corner radii slightly above the recommended value allows for higher quality surface finish.

 

Designing internal edges for cnc machining

 

Holes

Holes are machined using either a drill bit or an end mill tool. For high-accuracy holes with a diameter smaller than 20mm, use a standard diameter. Deeper holes (up to 10 times the diameter) can be manufactured, but we recommend limiting the depth to 4 times the diameter.

 

 

Text & Logos

For CNC machining, choose engraved over embossed text as it involves the removal of less material. To ensure engraved details are clear and visible, we recommend a minimum depth of 0.8mm and width of 0.5mm. For readability, ensure minimum font height is 5mm (font size 20 or larger) and suitable for every direction. Sans serif font is recommended not only for legibility but also to reduce details.

 

How to design engraved text and details

 

Summary of Best Design Practices

 

Thread Length 3x hole diameter
Minimum Wall Thickness 0.8mm (metals), 1.5mm (plastics)
Cavities Limit depth to 4x its length
Internal Edges ⅓ times cavity depth (or larger)
Holes Limit depth to 4x the diameter
Engraved Details Minimum depth of 0.8mm; minimum width of 0.5mm
Text Opt for larger fonts, font size 20 of larger

 

 

CNC Cost Reduction Tips

CNC machining is widely employed across various industries for its precision, speed, and efficiency. However, it can be more costly when compared to other manufacturing methods such as 3D printing. This section outlines several design tips that can help reduce the overall cost of your CNC parts.

 

  • Eliminate Deep Pockets: Deep pockets involve the removal of more material. Special tools may also be used to reach a specific depth. This translates to more machining time and higher costs. Therefore, limit the depth of pocket designs.
  • Limit Tight Tolerances: The tighter the tolerance, the higher the cost. Specify tolerances only when necessary (such as for critical features); leave the rest of the model within a standard tolerance range.
  • Choose Engraved Over Embossed: Engraved details often require less material removal than embossed and overall, less costs.

 

Discover more ways to further cut CNC costs in our How To Reduce CNC Machining Costs guide.

 

Getting Your Parts Machined

Have parts you need CNC machined? Use this CNC Machining Design Guidelines when designing your parts for machining. Once ready, upload your files here to get a quote. We’ll need:

  • a 3D CAD file in a .stp or .obj format
  • a 2D technical drawing if your design includes threads, tolerances, surface finish specifications or other notes

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