CNC Machining Design Guidelines

By Luke Smoothy | 13/11/2020


Learn everything there is to know about CNC machining in our CNC Machining Guide.

Deciding the various manufacturing operations required with CNC machining will largely depend on the final design of the product; the design will determine the raw material size, weight, material, dimensions and complexity. 

These various factors, as well as the end-use of the product, will determine the all-important cost of the part and therefore, overall production cost. Often, parts that are CNC machined require the utmost precision and a high-quality finish. This is largely due to CNC machined parts being used in mission-critical situations. One of the main reasons why CNC Machining is so popular is due to the high-quality results, however, quality needn’t come at a high cost. Considerations need to be made early on, to ensure every possible element of the design and CNC process has considered not only longevity and quality, but cost efficiency, too. 

These CNC Machining design guidelines advise engineers and designers on how to design cost-efficient parts that can be successfully CNC machined with the least effort.

CNC design considerations

The design of a CNC part should consider various elements during the initial design phase, such as:

  • The type of material being machined
  • The type of CNC machine
  • The size and shape of the product 
  • How the final product will be used (e.g. strength requirements)
  • How many different processes or operations are needed
  • Common tool sizes, and whether bespoke tools are needed 

All of these elements are important, as they can help determine the final cost of a CNC machined part and therefore impact the final costs. One key rule designers and engineers can use to reduce costs is by reducing the required machining time. However, it’s also important to remember that optimising just one part of the process can only go so far in creating time and cost savings – all the elements have to be considered practically when looking to optimise the design.


Tolerances denote the dimensional accuracy of a part and refer to the amount of deviation in a specific dimension of a part compared with the nominal value.

Parts with tight tolerances are typically harder to machine and it may take more than one attempt to machine the part to the desired tolerance. Naturally, this can impact costs. Therefore when considering tolerances, try to make the tolerances as loose as possible to reduce costs.

Size recommendations 

CNC Milling and machining costs are impacted by the size of the part. Generally speaking, the larger the part, the larger both the material and machining costs. However, do also consider part complexity; the more complex the design, the more machining time will be required. The smaller and simpler the geometries are better. 

3 Axis Machining

Consider the type of machining process being used. Where possible, it is recommended to keep the design to 3 axis machining, as opposed to 4 or 5 axis machining. 3 axis machines are cheaper to run and are currently more prevalent in the CNC machining industry. Additional axes increase set-up time and require different machinist skills. Try to reduce the number of reorientations required to machine the part. Reorientations incur additional setup and programming costs.

Sometimes multiple faces will be needed to be cut. More complex designs can benefit from 5 axis machining. This will require additional set-up but can improve quality, occasionally 5 axis CNC machining will be the only suitable solution.

CNC Machining

Material selection 

Material selection can also significantly impact the cost and time of a CNC machined part. In general, plastic is favoured as a cheaper and time-saving solution as it is relatively easy to machine. There are many polymers with various strengths, uses and characteristics, however, in many cases, metal will be the right material. This is largely reliant on how the final part will be used. Aerospace and medical industries can often require demanding and expensive materials, however, for many parts, cheaper and general-purpose polymers can be utilized.

For more guidance on choosing the right material for your CNC machining needs, read Get it Made’s guide: Choosing the Right Metal for CNC Machining

Considerations that should be taken into account: 

  • Is a certain chemical resistance needed?
  • Will the product need to be thermally stable?
  • Does the product’s density need to be considered?
  • What are the aesthetic requirements?

The form, fit, and function are all important elements to think about.


Designers should be aware of the costly implications of undercut features. Undercuts are difficult to machine and may require costly custom tools to create the exact geometry – this will, of course, increase the final cost. If an undercut is required, designers should ensure the depth of the undercut is not too deep. The shallower, the better, as this creates undercuts the tool can reach. Often undercuts would need to be produced using Electro Discharge Machining (EDM), which is a process commonly used in the production of injection moulding tools. 

Wall thickness

The wall thickness is also an important consideration at the design stage. If wall thicknesses are too thin, this may require slower, more careful machining which increases costs. By having larger wall thicknesses there it can also mean less material to remove, further reducing machining costs. The wall thickness should be greater than 0.75mm; if walls are too thin this will of course impact the quality and durability of the final part.

CAD system

One final cost to consider is the time and cost spent designing the parts themselves. Using an efficient, easy to use CAD package could save many design headaches! We recommend the following CAD packages:

  • SolidWorks
  • Autodesk Inventor or Fusion 360
  • OnShape

The modern CAD systems can be particularly helpful with designers who need to design complex 3D surfaces and features and perform detailed analysis.

For more CNC Machining guidelines, check out our Injection Moulding Design guidelines here, or get in touch today for a 24 hour CNC Machining quote.

Low Volume 5 Axis CNC Machining

Luke Smoothy

Director and Founder of Get It Made Ltd.

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