When Should You CNC Machine Your 3D Printed Parts?

cnc machine cutting 3d part

In the beginning, there was CNC. Shortly afterward, 3D printing came along and machining hasn’t been the same since.

That might be a slight over-simplification, but it isn’t too far off. Both CNC machining and 3D printing are mechanical processes to machine a workpiece into a desired shape or part, but they both work in very different ways. CNC machining is a subtractive process; it removes material bit by bit, cutting or carving a workpiece into shape. 

3D printing, on the other hand, is an additive manufacturing process. 3D printers build a workpiece slowly, layer by layer, from the ground up. Using both processes on the same workpiece would appear to be working against yourself. You built a part up, only to machine it down? Why – and when – would you do that?

It turns out that while CNC machining and 3D printing would appear to be opposites, there are some times when using the two methods together actually does make sense. We’ve outlined a few of the circumstances when you should use the two together.

When to use CNC and 3D printing

Use a CNC machine on your 3D parts to:

1.) Save time

One of the traditional methods for manufacturing is to machine molds for injection molding, but this is a time-consuming process. While 3D printing isn’t exactly quick, it is faster than the alternative, so it can make sense, when time is pressing, to 3D print a part and then finish it on a CNC machine.

2.) Ensure precision

The technology behind 3D printing has advanced by leaps and bounds over the past decade, but it still isn’t capable of achieving low tolerances simply through the printing process. For high levels of precision, 3D printed parts need to be machined. CNC machine tools are capable of those low tolerances. If 3D printing is the most efficient way to produce a given part – say a prototype – but CNC machine tools provide more accurate results, then printing rough dimensions with a 3D printer and machining precisely with a CNC machine can make perfect sense.

3.) Finish well

As mentioned, CNC machine tools are capable of highly precise, smooth finishes in a way that 3D printers currently are not. Finishing 3D printed parts with a CNC machine makes sense when high tolerances or fine finishes are needed, as lathes and mills can take multiple passes and use multiple tools to achieve a smooth finish.

It also makes sense when machining certain materials. Cutting an entire part out of a particularly hard material might cause extra wear on a CNC machine, but 3D printing that same part and then simply finishing with CNC results in a better finish and less wear-and-tear on both machines.

How to use 3D printing and CNC machinery

If you think that your situation merits using both techniques on the same part, here are some considerations to keep in mind.

1.) Add to your dimensions

CNC machines remove material, so don’t 3D print your parts too close to the desired dimensions. Otherwise, you’ll remove too much material with your CNC machinery and the finished product will be too small. 

2.) Be careful with positioning

3D printers can produce part geometries that CNC machines can’t reach – at least, not without removing any material in their way and thus ruining the part. If you plan to use a CNC machine to finish the part, be sure to use a geometry that your CNC machine can reach.

3.) Remove any 3D supports

3D printers build in layers. When applicable, to prevent those layers from collapsing on each other, your 3D print design should build in structural supports. Those supports will need to be removed after the 3D print stage, but don’t remove them until you are sure that you won’t require that support through the CNC process also.

4.) Beware of fixturing

Curved, flowing parts are ideal for 3D printing. However, they can be extremely difficult to machine on CNC tools. Build-in extra material or even extra architecture to make fixturing the parts in the CNC mill or lathe much easier.

New potential for combined machining

There are many times when traditional methods are best and the most efficient method is to use 3D printing and CNC machining separately, not together. However, when flexibility and creativity are called for and the circumstances are right, skilled machinists can use the two methods in combination. Both methods are surprisingly flexible. This makes the combination of CNC and 3D printing ideal for one-off scenarios also, like prototyping. 

3D printing and CNC machining aren’t polar opposites; they aren’t even two sides of the same coin. Instead, the two methods form the yin and yang of modern manufacturing. Using both machining methods in combination opens up a world of new possibilities for machinists. As 3D printers get larger, more accurate, and faster, more and more manufacturers are combining both additive and subtractive machining. Increased flexibility, shorter production times, and more precise parts are just some of the results.

READ MORE:  Explaining CNC Milling
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