There has been a lot of hype in the last few years for 3D Printing. This is understandable, given its capabilities of delivering geometries that other technologies cannot, and to do so without external intervention — like fixturing. That being said, 3D Printing is not the be-all, end-all for Digital Manufacturing. In fact, CNC Machining, the longtime-standard for Digital Manufacturing, surpasses 3D Printing in a wide variety of tasks — sometimes even at quantities of one. In this article we will lay out four ways CNC still beats 3D Printing.
Better at Tight Tolerance Parts
CNC Machining is generally better than 3D Printing at creating tight tolerance parts — for a handful of reasons. First, and most notably, CNC Machining's been around longer. That means that precision control has been refined to a point where regularly delivering tolerances of .005" is commonplace; additionally — in the 3Diligent network — certain precision machinists can deliver .0005". In contrast, the 3D Printing processes are all relatively new and, as a result, have not been refined and optimized over as many decades to deliver super tight tolerances. It's worth noting that certain 3D Printers have been built to service micro-scale parts, and these can and do deliver tight tolerances, but that is limited to tiny parts, and more than just an exception for the technology than the norm.
Beyond sheer technological maturity, most in-market 3D Printing processes will struggle to ever achieve tolerances consistently on par with CNC machines because of the thermal nature of the printing process. Typically, 3D printers melt material from one form (e.g., filament, powder) and reconstitute it as another (the final shape). This means rapid heating and cooling, and therefore the possibility of warpage (among other potential microstructural impacts). In contrast, machining takes a hard part and simply chips away at it. All the while, the coordinates of the main work piece don't materially change — in shape or temperature.
Excels with Bulkier Shapes
The second area where CNC Machining outperforms 3D Printing is in bulkier designs. Quite simply, CNC machines are capable of processing a wide variety of stock materials that come in standard shapes (e.g., block, sheet, rod). CNC machines can simply chip away from these shapes and provide you a solid part in short order. 3D printers, by their very nature, additively construct parts. At their very fastest, you're laying down one thin layer of material on top of another. At their slowest, the 3D Printer is basically etching the part geometry one voxel at a time.
To use an analogy, imagine you're tasked with creating a black circle for your niece's grade school art project. You're given some white paper, some black paper, a black pen, and some scissors. What approach would you take? Sure, you could grab the pen, draw a circle, and start filling in the blank. But you could alternatively just grab those scissors and cut your circle out. You'd finish faster. And to the earlier comment, you'd be assured a degree of consistency that might not come if there were slight variances in the way you precisely filled in the circle shape.
In brief — big, blocky shapes do not print fast, but they tend to cut quickly. Additive would better shine on those designs where you'd pick the pen instead of the scissors.
Gives Consistent Material Properties
The next area where CNC outperforms 3D printers is in delivering reliably consistent material properties. To extend our grade school crafts analogy a step further, you may end up with a perfectly black circle if you precisely filled it in with your pen. However, there's also the possibility that the ink didn't flow quite right at a given moment, or you missed a spot ever so slightly because you had a hiccup. In contrast, if you cut the corners off of a piece of paper that you already can see is black, there's not much guess work.
With 3D Printers, you are oftentimes melting material on the fly. Sometimes you're setting a shape with a binding agent and curing that part's material properties in a secondary step — there are a few other ways as well. However, the main takeaway is that additive manufacturing typically establishes the material characteristics of the part on the fly as you reconstitute the matter in the build chamber. In contrast, with machining, you are simply chipping away from a forged piece of material billet. The material properties of that raw stock are already checked and confirmed.
All of this is not to suggest that 3D printers cannot deliver production parts that can meet and exceed the needs of many real-world applications. In fact, 3D printed parts consistently deliver material properties on par with or superior to cast parts, and you likely know that castings are literally EVERYWHERE. But it is to say that there are variables in play with 3D Printing (and casting) that don't exist to the same extent with CNC Machining. Microstructural features (like porosity and grain orientation) matter much more, especially as it relates to parts that experience high cycle fatigue. You may want to consider post-process solutions like Hot Isostatic Pressing.
Offers a Better Material Selection
A fourth way in which CNC Machining still beats 3D Printing is in material selection. It should be noted that 3D Printing is making tremendous strides in this area — and in fact certain materials that cannot be manufactured with any other process are now becoming available to 3D printers. Things like custom alloy powders are being developed just for the powder bed fusion 3D Printing process, for instance, that outperform conventional stock casting or CNC materials. With that being said, machining still currently offers a much wider variety of material options than 3D Printing. Materials like brass, for instance, are machinable and not generally available to the world of Additive Manufacturing. The same holds true in the polymer world. Whereas polyethylene, polypropylene, and acetal (Delrin) are viable options for a capable machinist, printing in those materials is still not heavily commercialized. To the extent that those materials are available in market, it is on a relatively niche basis for specific machines. The tradeoffs of different 3D Printing processes is a discussion for a different day though.
So Is CNC Machining Better Than 3D Printing?
In truth, anybody who tells you machining or 3D Printing is "better" is just offering their own opinion, based on their own applications. But suffice it to say, CNC Machining does currently outperform 3D Printing on a number of dimensions. From delivering tight tolerances right off of the machine, to delivering bulky shapes faster and cheaper, to providing more reliable material properties, to offering a broader range of materials: CNC Machining remains an incredibly useful technology that is driving Industry 4.0 forward.