3D Printing vs. CNC Technologies

Updated: May 12, 2021

Comparing 3D printing and CNC machining, it is clear that these are two different technologies with different histories and they are moving in different directions. But they aren’t as different as you might think. Increasingly, engineers see these technologies as complementary. That’s what we see here at Diabase Engineering. We believe engineers of the future will have access to complex software and a range of fabrication hardware—at an affordable price—so they can focus on choosing the right technologies for the job. The highest impact engineers won’t be relying entirely on point-and-click, plug-and-play tools.

3D Printing Growth, CNC Decline

Less than two decades ago, 3D printing was the scary new technology that many engineers weren’t sure how to use. Now a generation of engineers have entered the profession with 3D printing having been the most accessible manufacturing process to them, often the only one used in their educational curriculum.

Meanwhile, a generation of traditional machinists have left the field, taking with them their set of skills and tools. This flux of capabilities has led to an inversion of the status of additive manufacturing versus machining in the typical technology company and the typical classroom. There are multiple problems with this situation, not the least of which is that 3D printing is a limited process.

A high degree of technical sophistication goes into the creation and tuning of additive manufacturing machines, but the depth of user understanding of the machine is typically very shallow. This is partly by necessity. Additive processes are often very sensitive to a large number of variables, so for the sake of reliability, suppliers of these machines have to lock out the end user from anything but the most basic level of control.

The Role of a Machinist

The term “machinist” may conjure up a stereotypical image of an older character in an oil-stained shop apron puttering away with manual tools. This is a flawed picture. The term is more akin to the word “technologist.” It should evoke an individual that has a fluency in the operating principles of machines and who can efficiently make use of them to produce a desired output. It is our view that every engineer should also be a machinist in this broader sense.

Technical and Vocational Training in Digital Fabrication

The point of interacting with a machine in the classroom is not to learn the right buttons to push on a particular brand of device. The real value of a technical education is a deeper understanding of how things function. What are the principles that limit the capabilities of a particular machine in terms of travel, stiffness, material properties, fixturing strategies, etc? How might you get from an idea in your head to a physical object in your hand using the set of tools you have available? In this way, plug-and-play 3D printing is far from ideal as an education device. One of our main goals for the H-Series machine (made by our team here at Diabase) is to ease the transition from the simple push-button world of 3D printers to the deeper and more versatile world of CNC machine tools.

One of the first challenges for a machinist making a part with a milling process is the idea of tool access and workpiece orientation. This is not a subject commonly encountered in 3D printing because parts are simply deposited in a layered fashion from one side to the other. In the milling world, it is very rare that all of the features of a part can be completed from one side. The two ways that the machinist deals with this problem are by utilizing additional axes and using secondary operations. These two fundamental concepts are core to the design of the H-Series machine.

In the example below, 2 modular rotary axis units are located in the dovetail fixture plate, enabling a simple A-axis setup with support on both ends of the workpiece. This example also highlights the benefit of having tool changing between additive and subtractive processes available on the same machine. Rather than clamping a large piece of stock in the rotary axis vises, we first fixture a temporary build surface in the vises and print our stock in a near-net form on that surface. We then machine the part from 3 sides using a flat end mill for roughing and a ball end mill for smoothing. Because this is a very thin-walled component, we work our way from the extremities of the part toward the center, roughing and finishing as we go.

The above example represents the obvious way of combining 3D printing with CNC milling - printed near-net stock. However, another major benefit of additive / subtractive tool changing is the ability to create printed fixtures. This is especially useful for secondary operations on organically shaped parts or for reworking parts that were made using other technologies. The example in the next section highlights this use case.

Design Thinking in Practice

Every product designer has run into this situation in their prototyping process: In the CAD software the design is perfect. All of the components are modeled and everything mates up beautifully... But when the parts arrive and you start to build the assembly, you realize that something doesn’t work.

Maybe a cable bundle is larger than expected or a screw head needs a little bit more clearance for the install tool. Or maybe by the time the parts have arrived, you’ve already decided to swap out one of the components for a slightly different version.

Whatever the cause, the design has changed and some of the parts need to be reworked. Sometimes a rotary tool or rat-tail file are all you need to get the prototype built, but often you actually need to put new engineered surfaces in the parts.

Here is an example of a part that we recently needed to rework in our shop. This is a molded cover plate that allows a vacuum hose to be connected to the chamber that houses our nozzle cleaning station. We changed the drive mechanism on the cleaning station from a solenoid to a linear motor, so we needed to notch out the underside of the cover plate to provide clearance for the motor body.

The part is organically shaped with no rectangular features. So it was all but impossible to clamp effectively in a vise, even with custom soft jaws. This is where having additive and subtractive tools available in the same machine can provide real value. You can print your workholding fixtures to match your model geometry and easily locate your part in the machine.

In our case, the fixture geometry is very quick to create by using the “extrude up to body” feature.

Then after importing both the original and reworked models, Fusion allows you to use the original model as the “stock.” Then using “rest machining” and “adaptive clearing” you can optimize the toolpaths to give very short cycle times.

In our example, we printed the fixture in XA98 TPU. This material is semi-flexible which enabled us to use a press fit. The slightly tacky surface of the material combined with the layer lines provided a good grip on the part, so no fasteners were required to hold the part during machining. This gave adequate tolerances for our needs on this component. In order to achieve more accurate workholding, the fixture can be first printed in a rigid material, then milled to tighter tolerances and fasteners can be used to hold the part more rigidly in place.

The Future of Digital Fabrication

These several examples are by no means a comprehensive summary of the capability of the H-Series machine. They are meant only to highlight the ways in which technologies that have traditionally been seen as occupying separate realms can be combined to yield useful processes. These processes are accessible to those who are willing to learn the fundamentals of machine tool operation.

We believe the future of digital fabrication is one where a wide range of digital tools (software and hardware) are available to empower engineers to solve problems. We make machines, create parts, and work with educators every day to help make this a reality. If you’re interested in connecting with our team to discuss what’s possible, please get in touch.