Computer numerical control, commonly referred to as CNC, is a term used to refer to the system in which a toolhead moves along predetermined paths to accomplish a variety of tasks, such as drilling, cutting, printing, or milling.
In maker communities, the term “CNC” is often used to refer to CNC milling, a process involving a spindle as a toolhead. However, it is worth noting that CNC can also be used for much more than just milling.
As far as this article is concerned, CNC milling is a process in which a cutting tool systematically removes material to produce some final geometry. In this subtractive process, a computer handles the conversion of a CAD design into coordinates, which are turned into instructions used by the cutting tool to remove material and reveal the final object.
CNC milling machines with five axes are simply advanced variants of regular milling machines, where the cutting tool moves along five different axes simultaneously. It’s only with this freedom of motion that some complex geometries can be produced. Regular 3-axis CNC machines, popular among enthusiasts, simply cannot move the cutting spindle in any way that will produce partial cavities and overhangs, for example, without manual adjustment of the material midway through the process.
Because of its ability to manufacture complex parts from strong, durable materials, 5-axis CNC is a common manufacturing and prototyping solution in the aerospace industry, an industry which requires complex parts with outstanding strength.
Mathematically-inclined readers in the audience might be wondering where exactly the two extra axes come from. After all, the Cartesian coordinate system only has 3 axes: X, Y, and Z. But it’s really quite simple: 5-axis CNC adds one additional “axis” of rotation around each of the X and Y axes.
The above diagram shows six axes. The rotation around the vertical Z-axis does exist in some machines, but in reality, 6-axis CNC milling machines are a rare breed, since the sixth axis adds few benefits.
Swivel head machines, as the name implies, can maneuver the toolhead around the block of material to get into tight spaces from different angles. One benefit of this method is a larger, heavier object can be machined, since the block of material remains stationary throughout the process.
On the other hand, CNC milling machines that move the object on the table, also referred to as trunnions, achieve their additional two axes of freedom by rotating the table on which the material is placed. The benefit of this approach is speed and stability, although objects that are too large or heavy can’t be rotated in this fashion.
But 5-axis CNC milling has some complications, particularly when we’re milling things. For instance, there’s a clear distinction between the two approaches used to achieve 5-axis cutting abilities: the continuous method and the 3+2 axis method. The names give away the definitions, sort of…
Continuous 5-axis CNC involves continuous adjustment of the cutting tool on all 5 axes to keep the tip optimally perpendicular to the cutting surface. In contrast, the 3+2 axis method locks the part at a certain angle, determined by the rotational axes around X and Y, while the toolhead moves in 3 axes to cut the part.
Maybe you’re wondering, what’s the main benefit of continuous CNC? Speed. Without the need to stop cutting to reorient the part multiple times during the process, continuous 5-axis CNC is faster. It’s worth noting, however, that a continuously-moving toolhead necessitates more moving parts (more wear and tear) and advanced crash detection. Thus, it’s more complicated mechanically and programmatically to execute successfully.
That brings up an important consideration in the 5-axis world: collisions. With heavy cutting tools and bits of material flying all over the place at high speeds, it’s quite important to ensure that things don’t bump into each other. But teaching a machine to keep track of where everything is in 3D space turns out to be a big programmatic challenge.
Speaking of programmatic challenges, the conversion of a 3D design from CAD software file to a physical tool path is quite a feat of engineering as well. Only some of the most advanced software tools are capable of producing truly smooth real-world results.
It’s not difficult to spot the similarities between 5-axis CNC milling and 3D printing, so you might be wondering how these two processes compare. The short answer is that 3D printing and CNC are complements, not substitutes, where CNC is used for the strongest and toughest of parts.
An obvious difference is 5-axis CNC is mainly present in industrial applications. 3D printing has a presence in that world, too, in the form of SLS machines that use a laser to additively produce an object from a large basin of powder. Although both are capable of working with professional-grade materials, such as aluminum, one problem with SLS 3D printing is efficiency.
In the SLS 3D printing process, the entire volume of the print chamber must be filled with powder to produce a single part, no matter its size. Not all of the unused powder can be reused due to the fact that the entire chamber containing the powder must be heated during printing. As a result, print jobs must be queued and arranged systematically to maximize the volume of powder used in each run and reduce unused space, because some of that unused powder will simply be wasted.
CNC milling in general faces no such limitation: as long as a block of material roughly the size of the finished model is supplied, the excess will be chipped away to produce the final geometry, no matter how large or small the model is. Not as much material will be wasted provided the original block of material is similar in size to the final part. Besides, in the case of some CNC materials, like aluminum, the material that is cut away can even be collected and recycled.
Though not as commonly used as SLS, electron beam melting (EBM), another industrial 3D printing method, uses less material. That’s because it doesn’t require a particular volume to be filled, essentially “shooting” the build material at the part as it forms.
The future of 5-axis CNC is one where a raw block of material is placed into the machine and a finished, production-ready part is removed hours later. Currently, machined parts still require processing after the cutting is over, and this manual labor can prolong the delivery time significantly. Therefore, the possibility of more automation is an exciting one in the world of 5-axis CNC.
Nevertheless, this technological wonder of a process can already do some impressive things. Naturally, we were obliged to curate some particularly impressive videos that will make you wish you had a few thousand dollars to spend on one of these machines yourself.
If you’re an average maker, you may be lamenting your lack of a CNC machine. However, there’s no need to be sad. You see, 5-axis CNC lives primarily in the industrial world for a reason: it’s only needed for parts that need to be both complex and very, very strong.
We’re going to assume that most of our readers aren’t designing parts for a commercial jet engine on weekends in their garage. And if that’s the case, there are already heaps of affordable consumer machines capable of producing some intricate geometries, and they’re practically a click away from arriving at your doorstep.
Whether you want some tiny details from an SLA machine like the Peopoly Moai or some detailed nylon sculptures from an SLS machine via Shapeways, you’re pretty much covered in terms of options.
So, don’t fret. Leave 5-axis CNC milling to the rocket scientists and automotive engineers. Most of us home makers will do just fine with our Ultimakers.
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If you now suffer from an insatiable thirst to learn more about the captivating world of 5-axis CNC, here are some resources you can explore:
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