Machining vs. 3D Printing for Functional Prototypes

In making a short-run functional prototype, when does CNC machining make sense and when is 3D printing the option to use instead?
#prototyping #machining


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Proto Labs, a company focused on fast short-run production, routinely employs both processes. The company posted an article on its website (protolabs.com) comparing the strengths of both. Here are some of the considerations:

1. Material selection. Advantage: machining. A wider range of materials can be machined than can be applied additively. This is true of both metals and plastics. (Though 3D printing certainly does offer a wide range of resilient materials.)

2. Geometry. Advantage: 3D printing. Sharp internal corners, undercuts and deep pockets are features that machining is challenged to produce. Curving internal cooling channels and other internal features are among those that are impossible for machining to produce. And certain overall forms are impossible for it as well. As Proto Labs points out, machining could never produce a wiffle ball, but 3D printing could grow one easily.

3. Tolerances and surface finish. Advantage: machining. Delivering tight dimensional accuracy and fine finishes is routine for machining. 3D printing can produce relatively smooth and accurate forms, but holding narrow tolerances is a challenge.

Then there is a consideration Proto Labs did not list, but I’d like to include:

4. Unattended operation. Advantage: 3D printing. Human attention is not a consideration if the job is to be sent away to an outside source, but it potentially is a consideration if the prototype is made in-house. Machining generally requires employee attention for setting up the machine and overseeing the job. By contrast, 3D printing is inherently a low-labor process. The printing cycle could simply be left to run through the night so that the part(s) are there in the morning. This might be attractive for the shop that has plenty of other work for its people and machine tools to do.

When prototypes are needed in a small batch instead of just as a one-off, Proto Labs says there is also a third option to consider: short-run injection molding. This option might be more attractive than either of the two options discussed here. 

(But then, should the injection mold itself be machined or 3D printed? Read about 3D printing of molds at short.additivemanufacturing.media/3dpmold.)