metal part

Design for Machining Toolkit

Get in-depth design advice to optimise your plastic parts for CNC machining. Designing with machining in mind can accelerate production time and reduce production costs

Optimising Part Design for Machining

There are many design elements involved when creating plastic and metal parts for CNC machining — design for cost, design for quality, design for assembly, design for manufacturability. And navigating that landscape can be challenging at times. At Protolabs, we provide automated design analysis on CAD models that highlights features in your part design that can be adjusted for manufacturability. It’s a great design resource to have at your fingertips. To keep those manufacturability advisories at a minimum and optimise your part design, we created this helpful kit of different CNC machining resources.

Designing for Machined Parts

Deep Features
Threads and Inserts

Navigating Critical Machining Advisories

Material Left Behind
Thin Walls
Holes That Can Be Threaded
Part Too Large

cnc stainless steel part

Designing for Machinability

CNC machining has been around for decades, and for good reason. It’s one the fastest manufacturing technologies for prototypes and end-use parts. Want to learn how you can optimise your product development cycles with CNC machining? This white paper shares how to design for the subtractive manufacturing process, select the best material for your application, and streamline new product development.


Design software

Optimising your part for CNC Machining

Manufacturing prototypes and production parts fast and cost-efficiently is often a balancing act of on-demand CNC machining capabilities and an optimised part designed for those capabilities. As such, there are a handful of important considerations when designing parts for Protolabs’ milling and turning processes that can accelerate production time while reducing costs.
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Designing for Machined Parts


cnc part tolerances

Tolerances represent dimensional accuracy of a part. It is the amount of acceptable variance in the dimension of a part. Typically, Protolabs will maintain a general machining tolerance of ±0.1 mm or better, however, our network partner can attain down to ±0.010 mm. The best thing about a standardised general tolerance of ±0.1 mm.. you don’t need to produce a 2D drawing, we can start manufacturing as soon as you have the 3D model.

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CNC radial holes

In general, at Protolabs, most holes are interpolated with an end mill rather than drilled. Apart from allowing for greater flexibility in hole sizes but also offers better surface finishes than drilled holes. The same tool is also used for slots and pockets, which means a reduction in cycle time and part cost. The principle trade-off is that holes more than 6 diameters deep become challenging as they normally need specific drills, which may mean the part needs to be machined from both sides.


cnc threading

We take a more modern approach to thread making, rather than the traditional "taps" used to manually create a thread. We use a single point thread mill to helically cut the thread profile, this CNC process produces an accurate thread and one tool can be used to cut multiple threads that share the same pitch (number of threads per inch). UNC and UNF threads from #2 up to 1/2 inch M2 to M12 (metric) are possible within a single tool set. The full list is here.

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cnc text

Protolabs is capable of machining text, as long as space between characters and the stroke used to write is at least 0.5 mm
It is worth noting text should be:

  • recessed not raised, 0.2 to 0.5 mm deep is normally sufficient, because it is quicker to remove material.
  • fonts with radii, this includes arial, verdana, sans-serif or similar, because this suits circular tools.
  • 20 point or larger, to ensure the text widths are greater than 0.5 mm.

Walls and Features

cnc part walls

Protolabs tool sets contain materials such as tungsten carbide, a super rigid material that offers maximum tool life and productivity with minimal deflection. But even the strongest tools deflect, as do metals and plastics that are machined.
Minimum feature thickness in the Protolabs factory is 0.5 to 1 mm and maximum single cut depth is 50 mm high and 1 mm wide, has a slenderness ratio of 50:1 - this can work in some materials, but 20:1 is a good general rule.*


cnc radii

As a part of the CNC machining process, sharp corners on a part will be radiused (rounded). This will be identified before the part is milled. In softer materials, tools are flat bottomed so only the tool diameter needs to be considered. In Harder metals it is typical for tools to have a tip radius too – this will leave a small radius in the bottom of pockets. **


* Try adding steps to buttresses to stiffen your design.
Watch out for deep, narrow pockets, or part features situated alongside tall walls. Cutter or workpiece vibration could cause deflection, and a loss of accuracy or surface finish.

** Generally, radius is a good thing – fillets spread loads well, sharp corners can act as stress raisers and can initiate fatigue cracks.
We deburr every part, but if you are considering adding external radii to improve cosmetic and handling, for CNC a 45° chamfer is quicker to cut and so much more cost effective.
Remember for CNC:

  • Internal corners – Fillet/Radii
  • External corners – Chamfer

Any part that requires square corners will cost you heavily, as the only way to produce them is to burn them out with Electro-discharge machining (EDM) or cut slowly with extremely small tools.

Navigating Critical Machining Advisories

Material Left Behind

cnc material left behind

We advise what percentage will be milled and what will remain un-milled, deviating from your original design. Dark blue areas are those where material will be left behind or indicates where features will not be formed. You can decide whether to modify your design or leave with the changes as they are.

Reasons for un-accessible material come in two categories:

  • General design for CNC for example deep holes, galleries or internal features, deep threads, undercuts that can’t be reached, or tool access. These may be pushing the limits of any CNC process and might be candidates for 3D printing.
  • Trade offs to manufacture quickly; holes deeper than x6 diameter, threads deeper than x3 diameter, undercuts that need special tools (t-cutters, woodruff cutters or Lollipop Cutters), large 5-axis parts or restricted tool access. This is because we stock a standardised toolset. We are always happy to offer advice, and have a huge range of CNC capabilities available via our digital network.

Holes That Can Be Threaded

cnc threaded holes

As part of your design analysis, you can easily highlight and assign holes which need threads, by simply clicking on your 3D model. 
We offer a selection of metric and inch (UNF, UNC) threads along with coil and key inserts, however, we do not install or supply inserts.
To make a hole eligible for threading it must be modelled at the pilot drill diameter. The software will detect eligible threads, enabling you to pick one-by-one or all of them in seconds.


Too Thin

cnc thin walls

Standard part tolerance at Protolabs is +/- 0.1 mm (or ISO-2768 medium). For features that are less than 1 mm, will be automatically highlighted as a thin wall geometry by the quoting system. You should keep in mind that we'll still allow it to be machined, so the machined part may differ slightly from your original design. Any thin walls below 0.5 mm risk breakage during machining, and flex or warp afterwards. Wherever possible, thickening them up is advised.

CNC Materials

Part Too Large

Maximum extents refers to the raw material and machining envelope. Depending on part geometry and fixturing/clamping requirements the maximum part size may be slightly smaller. For every upload we will analyse and provide feedback.

Find out the max extents for each, individual, material we stock at Protolabs.

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CNC silver parts

Enhanced Machining Capabilities with Our Digital Network 

Get anodising, tighter tolerances, and volume pricing options through our network of manufacturing partners powered. You'll find plating (black oxide, nickel), anodising (Type II, Type III), and chromate coating at scale; tolerances down to ±0.020mm; and cost-efficient machined parts at higher volume.  

Try Digital Network