Digital Manufacturing Essentials: 7 Tips to Help You Design for CNC Machining

Digital Manufacturing Basics: How to Design for CNC Machining

CNC Machining: Key Tips and Checklist for Quality Design ​​

CNC machining is the manufacturing method of choice when you need simple parts with tight tolerances, excellent mechanical properties, and scalable low volume production. To achieve the highest impact from each of these strengths, there are several key factors that you should consider during the design process.

Your decisions will vary depending on the technology used, i.e. 3-axis, 4-axis or 5-axis milling, and mill orientation, i.e. horizontal or vertical. Every variant, however, will circle back to four key questions:

  1. What size and shape drill bit can I use?
  2. Can this tool access the feature that I want to mill?
  3. How much vibration will this make (at the cost of precision)?
  4. How will the heat of the bit affect the material I am working with?

With these four questions in mind, here are seven basic rules to follow for effective CNC machined part design. Tip: A chart of standard bit sizes and diameters for reference throughout this article can be found here.

Rule 1: All roads lead to radii

As the majority of drill bits are cylindrical by design, this means any internal cuts you make will also create a curved corner/edge, also known as a fillet. When designing a part containing internal fillets, “the bigger the better” is a good rule to follow. The resulting corner will be half the diameter of the tool used.

Use a non-standard radius, e.g. 1.25 mm rather than 1 mm, to give a tool clearance to cut the corner. Where possible, design using a different wall and floor radii too so the same tool can be used throughout.

The exact measurement for internal corners will be relative to the depth of the cavity being machined. When inserting internal corners and edges, account for a radius more than one third the depth of the cavity.

CNC radii

Rule 2: Undercut for right angles

To create right angles in a CNC machined part, it is better to add undercuts to the design rather than attempting to reduce the radius of your corners for a similar effect. To avoid the added cost of custom tooling, design an undercut with a standard dimension, i.e. 3 mm to 40 mm wide in whole mm increments. Due to the shape of tools used, keep undercuts shallow where possible. The maximum achievable depth of undercutting tools will be double the width of the head.

CNC Plunge
Source: Sub-Category-2017-CNC-Plunge.jpg

Rule 3: Fillets give you cavities

Cavity/pocket depth is typically relative to the diameter of the tool used to make internal fillets. As a guide, pocket depth should be up to 3 - 4 times the tool diameter. Any deeper than 6 times the tool diameter will require a larger tool. This will result in sacrifices to the radius of your corners.

Cavity width should also be considered when machining a pocket. Keeping depth to a maximum of 4 times the width is a good guide measurement.

Rule 4: Tall features, bad vibrations

As with the depth of cavities and pockets, the maximum height for tall features is up to 4 times the feature’s width. The taller a feature, the more prone to vibration it is, reducing the machined precision of your part.

Rule 5: Avoid thin walls

Generally speaking, it is better to have thicker walls in the design of your part. As with tall walls, vibrations increase when producing thin features. When machining plastics, heat also has to be taken into consideration. Thinner walls will be more susceptible to softening and warping due to the friction of the toolhead.

As a guide, between 1.0 and 1.5 mm is an appropriate minimum thickness for plastic walls. Minimum walls within the range of 0.5 mm and 0.8 mm are possible in metal parts. Walls should be thicker if they are supporting or taller to avoid vibration and chatter.

Rule 6: Stick to the standards when making holes

There are two types of holes to choose from in CNC milling: blind holes and through holes. No matter which of these types is chosen, the recommended depth and diameter are the same. Hole diameter should correlate to standard drill bit sizes from 25.5 mm (over 1 mm diameter) and above. The maximum hole depth relies on the nominal diameter of a hole. It is common to create a hole depth equal to 10 times the nominal diameter of a hole.

Example of through and blind holes.

Rule 7: Stick to the standards for threads

Sticking to standard sizes is also important when creating threads. The larger the thread, the easier it is to machine. Length should be kept to a maximum of 3 times the nominal diameter of a hole. Avoid extra costs by sticking to off-the-shelf thread sizes in your parts. Tip: A list of standard thread sizes supported by Xometry is available here.

Design for CNC machining checklist

To keep track of the recommended rules for CNC machining, here’s a quick reference checklist to get you started. Of course, there are always exceptions to the rules as they’re listed below. For specialist projects and designs, contact our applications engineering team to answer specific questions and guide you through the process.



Internal corners/edges

Radius ≥ one third cavity depth


Design using standard dimensions: 3 mm to 40 mm


Guide maximum depth is 4 x the width

Tall features

Guide maximum height is 4 x the width


Minimum wall width for plastic: 1.0 - 1.5 mm, Metal: 0.5 mm - 0.8 mm


Depth should be kept within 10 x the nominal diameter of the hole


Length should be kept within 3 x the nominal diameter of the hole

For more tips on cutting the cost of CNC machining through design, check out the video below.


About Xometry

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