CNC Design of Parts: Easy Guide

4 weeks by cncdivi

The initial stage in Creating a CNC Part involves Designing the Perfect Part.

Design the part in the CAD software based on sketches, photos, specifications, and any other ideas we have for the part. The part is “Idealized” because we haven’t done any serious homework yet to evaluate how easy it will be to manufacture the part. Experienced Designers will have avoided many manufacturing problems at this stage while Beginners will discover they need to change quite a bit to make the part easy to manufacture.


– Beginners Guide to Choosing CAD and CAM Software: Don’t have CAD yet? This article will help you choose a package.

CNC Chair Plans:  Another way to think about idealized parts is from an ergonomic standpoint.  Check out this article to see what the ideal ergonomic dimensions for a chair are and how they were used to help design a CNC Chair.


Collect Up Design Resources

Deliverable: Gather and Organize Design Resources for Easy Reference During Design

One of my inspirations was the beautiful brass steam engine model by the “Engineman”

Collect up any sketches, photos, specifications, or other ideas for the part. Flip through them, organize them, and make them available for easy reference while you create your CAD Model.

One of my inspirations for the steam engine design was the beautiful brass steam engine model by the “Engineman”. In addition, my article on “Bobs No 1” describes some of the other resources I gathered on things like how the linkages were supposed to meter steam to the cylinder.

Another approach to consider is tracing or vectorizing bitmaps. If you have photos of a part you’d like to make, many CAD programs will let you load the photos, scale them, and either trace or automatically vectorize features you see on the photos so that they come out accurately modeled by the CAD software.  For more on converting bitmaps or images to CAD files and g-code, check our article.

In some cases, accuracy isn’t even all that important. For example, when you’re modelling a decorative detail in a carving application.



Deliverable: Locate the Origin on the CAD Model

Decide where to locate part zero (the 0, 0, 0 origin in the CAD Model’s geometric space) and situate the part in the CAD model so the origin is at part zero. Decide how we will locate part zero on the machine and notate the solution in the setup sheet.

You may hear the term “datum” used by CNC’ers. Datum is simply the name for some geometric reference that will be used to facilitate some aspect of CNC. You hear about Datums with respect to Dimensioning and Tolerancing and Part Zero is also a Datum.

For a big project, like the steam engine, you may want to break apart the assembly so each part can have its own Part Zero that will be convenient for the machining of that part.

To make your part, you will eventually have to tell your CNC Machine where Part Zero is using a procedure similar to this:

Touching off with a Haimer 3D Taster…

There are many ways to choose your Part Zero, and you’ll want to select one that’s convenient both for machine setup and when designing your part.



Create the CAD Model

Deliverable: Idealized CAD Model

In this step, we create the initial CAD Model of the part.



Deliverable: Idealized CAD Model with Dimensions, Tolerances, and Notes Called Out

Simple drawing with GD&T dimensions and tolerances called out…

The goal is to add all the Dimensions, Tolerances, and Notes needed to convey the additional information that will be important to making sure the manufactured part matches the Designer’s goals and specifications for the part.

Modern CNC Designers use Geometric Dimensioning and Tolerancing (GD&T) as their standard for how to add Dimensions and Tolerances to a design.

Be careful to keep tolerances as loose as possible.  It is tempting to specify very tight tolerances, but tolerances can add tremendously to the cost of making the part without adding any real value.  See our article on the High Cost of Tight Tolerances for more.



Photo Realistic Rendering

Deliverable: High Quality Photo Realistic Rendering

Modern CAD software has the ability to create high quality photo realistic renderings of CAD models using technologies such as Ray Tracing. These renderings make it possible to visualize what the finished part will look like.

A fishing reel rendered in Fusion 360…

High quality renderings can make it easier for Designers to communicate with the ultimate part consumer to determine if they’re on the right track.

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