How to Design CNC Machined Parts

How to Design CNC Machined Parts

To take full advantage of the capabilities of CNC machining, designers must follow specific manufacturing rules to design. But this can be a challenge as no specific industry standard exists. In this article, we have compiled a comprehensive guide with best design practices for CNC machining.

We focus on describing the viability of modern CNC systems, ignoring the associated costs. See this article for guidance on designing cost-effective parts for CNC.

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CNC machining

CNC machining is a subtractive machining technique. In CNC, various high-speed rotating tools (thousands of RPM) are used to remove material from solid blocks to produce parts from CAD models. Both metal and plastic can be CNC machined.

CNC machined parts have high dimensional accuracy and tight tolerances. CNC is suitable for mass production and one-off jobs. In fact, CNC machining is currently the most cost-effective method of producing metal prototypes, even compared to 3D printing.

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Major CNC Design Limitations

CNC offers great design flexibility, but there are certain design limitations. These limitations are related to the fundamental mechanics of the cutting process, mainly related to tool geometry and tool access.

1. Tool geometry

The most common CNC tools (end mills and drills) are cylindrical with limited cutting lengths.

As material is removed from the workpiece, the tool geometry is transferred to the machined part. This means, for example, that the inside corners of a CNC part always have a radius, no matter how small a tool is used.

2. Tool Access

To remove material, the tool approaches the workpiece directly from above. Functions that cannot be accessed this way cannot be CNC machined.

There is one exception to this rule: undercuts. We'll see how to use undercuts in a design in a later section.

A good design practice is to align all features of the model (holes, cavities, vertical walls, etc.) to one of the 6 cardinal directions. Consider this rule a recommendation, not a limitation, as 5-axis CNC systems offer advanced workpiece retention capabilities.

Tool access is also an issue when machining features with large aspect ratios. For example, to reach the bottom of a deep cavity, a special tool with a long shaft is required. This reduces the stiffness of the end effector, increases vibration and reduces the achievable accuracy.

CNC experts recommend designing parts that can be machined with the largest possible diameter and shortest possible length of the tool.

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