Four Ways CNC Machining Kicks 3D Printing’s Butt

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 that CNC Machining still kicks 3D Printing's butt.

Better at Tight Tolerance Parts

cnc machining, cnc millingCNC 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

3Diligent Industrial Machining, 3Diligent Machining.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

3Diligent Water JettingA fourth way in which CNC Machining still kicks 3D Printing's butt 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.

Powder Bed Fusion vs. Binder Jetting

 

Powder Bed Fusion and Binder Jetting are two of the most common classes of metal 3D Printing technology. Each one provides unique advantages and considerations as it relates to meeting the needs of different applications. Powder Bed Fusion has grabbed headlines in the industry for the longest, but binder jetting's emergence has grabbed its fair share of headlines as well. Could either process be the solution for one of your applications? Here we provide a quick tale of the tape, comparing these two popular metal 3D Printing process families.

Process Overview

Powder Bed Fusion

 

Powder Bed Fusion involves the use of a focused energy source - commonly an infrared laser or electron beam - to selectively melt layers of metal powder.  This process results in highly dense parts that provide strengths typically surpassing cast parts (and occasionally forged parts).

binder jetting metal 3d printing process

Binder Jetting

 

As its name would suggest, Binder Jetting involves the targeted jetting of a binding agent to hold particles of powdered material together. This process takes place layer-by-layer to produces a "green part," basically a fragile matrix of metal held together with adhesive. This part can then be used for non-stress applications or more commonly undergoes post-processing steps, most notably sintering.

Accuracy

How accurate are these processes? How reliably do they deliver that accuracy? Or said in the most straightforward engineering terms, which tolerances can they hold?

As Low As .001"

Powder Bed Fusion technologies are very accurate, with some technologies capable of achieving tolerances as tight at .001" (25 microns). It should be noted that metal Powder Bed Fusion is generally expected to meet tolerances of .005" +/- .002" per inch, and certain powder bed technologies like Electron Beam Melting have global tolerances that are looser than that. Generally speaking, Powder Bed Fusion is more accurate than Full Sinter Binder Jetting.

+/- 3%

Binder Jetting is a relatively accurate 3D Printing technology, but much of its accuracy depends on what level of post-processing you have in mind for your print. Since we are specifically looking at metal 3D Printing, then for the purposes of this discussion we are weighing Full Sinter Binder Jetting and Infiltrated Binder Jetting.  Full sinter involves the creation of a green part, but then sintering that part in an oven once the shape is set — which results in roughly 20% shrinkage and creates a challenge in delivering tight tolerances. This is not as noticeable with certain types of geometries and smaller parts. Additionally, this can be improved through repeated manufacturing. Nonetheless, its default tolerances are higher. Infiltrated Binder Jetting doesn't experience the same degree of shrinkage because the metal matrix is filled with another lower-melting-temperature metal instead of allowing the matrix to sinter down on itself.  Still, the nod generally goes to powder bed when we talk about accuracy.

Cost

How expensive are these processes? Both relative to each other and to more traditional metal manufacturing processes like CNC, Casting, or Metal Injection Molding?

Cost Effective for Intricate Designs

Powder Bed Fusion prints are generally pretty expensive. The underlying energy source - an expensive laser or electron beam - is quite expensive. The powder commonly required for these machines is also more refined than that required for binder jetting, also driving up cost. Generally speaking, Powder Bed Fusion becomes the most cost effective solution when a part has been designed with the process in mind. This means the designer has eliminated unnecessary mass and structured the part to be self-supporting throughout the build.

Lower Cost Inputs = Lower Cost Outputs

By jetting binding agent rather than accurately tracing a path, Binder Jetting can effectively arrive at a geometry much faster than Powder Bed Fusion. While this analogy oversimplifies things (especially for Electron Beam Melting), Binder Jetting is a paint brush to Powder Bed Fusion's pen or pencil. This approach gives Binder Jetting particular usefulness for chunkier parts that still have a good bit of complexity. Augmenting this reduction in machine time, Binder Jetting leans on lower cost solutions to create geometries. Glue and an oven are decidedly cheaper than a fiber laser or electron beam. Additionally, Binder Jetting doesn't require the same fine granulated powders that powder bed systems do — this means cost savings as well.

Electron Beam Powder Bed Fusion

Materials

Which materials can each technology process? Is there a broader range of materials available to one process or the other? How do material options compare to traditional manufacturing technologies?

Selective Due to Economics

Nearly every metal can be used in Powder Bed Fusion systems. Certain powder bed systems are better at processing high temperature materials whereas others are more conducive to lower-melting-temperature materials. Generally driving this is the extent to which the powder bed chamber is heated during manufacturing. Lower temperature chambers can be more prone to cracking and internal stress when processing high temperature materials. More than capability is actually economics when it comes to commonly available materials for Powder Bed Fusion. Materials commonly associated with less critical applications (e.g., iron) are not as prevalent as higher value metals like Titanium, Aluminum, Stainless Steel, Copper, and Nickel Inconel superalloys (e.g., 625, 718).

Limited Due To Nascency

Because the first step of Binder Jetting's process simply involves binding particles of metal together with glue, it offers tremendous material flexibility, in theory.1 Materials such as sand, gypsum, metal, and plastic can be bound with the Binder Jetting process. With regards to the next step — sintering — some limitations emerge. The extreme temperatures required for melting titanium, for instance, are challenging within a sinter furnace. Stainless Steel is the most common material used in the Full Sinter Binder Jetting process, in part because its behavior with regards to shrinkage in the sintering step is perhaps best understood.

Sand Print to Cast

Density

Will the process deliver me the density I need for repeated cycles and fatigue?

99.5% and Up

Powder Bed Fusion delivers dense parts.  Straight off the machine, density is typically north of 99.5%.  This is significantly above the density of cast parts, which typically run around 98% dense. This is one key consideration in how PBF parts can deliver better-than-cast material properties. Additionally, parts can undergo a Hot Isostatic Press (HIP) post-processing step to bring density within a hair of 100%.

93%-98%

The Binder Jetting process generally cannot deliver the same density as Powder Bed Fusion parts. Quite simply, while the sintering process can create density typically on par with cast parts, it does not achieve complete density. This level of density is appropriate for many applications. However, for certain tasks with high cycle time fatigue concerns, this may be a limiting factor. Again, Hot Isostatic Press (HIP) as a post-process step can be used to improve overall density.

Design Constraints

Does one process provide unique advantages when it comes to available geometries? Do each operate with the same design constraints?

Master of Complex Lattices

When it comes to design, Powder Bed Fusion offers a great deal of design freedom. Relative to traditional manufacturing processes, powder bed is capable of processing extraordinarily complex designs just as fast - actually faster - than standard blocks of material. Especially with laser systems, very fine features can be achieved. We generally don't recommend features smaller than 1mm, although we commonly can resolve them. In general, this allows for extraordinarily complex lattice structures to eliminate weight while retaining strength. Notable to consider is that Powder Bed Fusion systems generally require the inclusion of supports, made from the same material as the part, to prevent the metal from warping under the rapid heating and cooling that occurs during the process. Certain powder bed platforms operate with a heated chamber which reduces the need for such supports, but that can create other design considerations. Either way, you should try to design your part to be self-supporting. Just imagine if your part was a skyscraper — would any aspect of it need scaffolding to be built? This is critical, because parts designed without supports in mind can drive the price of a part up more than 50%.

Support Needs Reduced, But Sintering Must Be Considered

The Binder Jetting process does not have the same support restrictions as powder bed systems because there isn't a thermal component to creating the green part. With that being said, the post-processing step can be problematic for fine features, as the green part shrinks in size by 15-20%. This can also come with the threat of internal stresses. As a result, binder jetting parts are generally limited to sizes smaller than a fist for cost-effective printing and sintering.

The Bottom Line

It's really a tie.  Depending on the part you've designed, the quantities you seek, the material you desire, and the performance you require, either process might carry the day. The good news is that 3Diligent offers both of these 3D printing technologies (any many more!), and our experts can help you design to take advantage of either process.