3D printing and CNC (Computer Numerical Control) machining are vastly different processes. Both can be paths to the same end, whether that is prototypes or ready-to-use parts. However, there are quite a few differences that should be considered when selecting one to make your parts. These factors include the processes themselves, part geometry, cost, volume limitations, lead times, mechanical properties, and material selection. Below, each of these factors is discussed in detail.
Subtractive vs Additive
The primary difference between 3D printing and CNC machining is the process each technology uses to produce your part.
CNC machining is a subtractive manufacturing technology. This means that a block of material, or a blank, is carved away bit by bit until your part is created. Drills and rotary cutters are some of the main tools used to achieve this removal of material. This process is mostly automated and requires very little supervision once the machine has been calibrated and the workpiece has been mounted.
Due to the nature of cutting away material at high speed, most CNC machines produce a lot of noise, vibration, and mess. Waste comes off in tiny slivers that often cannot be recycled. However, this process produces a smooth surface finish for ready-to-use parts and has a significantly tighter tolerance than any 3D printing technology.
3D printing, on the other hand, is an additive manufacturing technology. Rather than removing material from a blank, 3D printing starts with an empty build chamber and adds material until your part is complete. 3D printed parts are created in cross-sectional layers, with each layer being printed directly on top of the last.
As a whole, this process typically produces much less waste than subtractive manufacturing since only the bare minimum of support material is used and discarded in post-processing. However, post-processing is much more involved. This post-processing can include solution baths, Waterjeting, gluing, sanding, and more.
Material Selection
Machining offers an incredible variety of both plastics and metals. While CNC machining is most commonly used to produce metal parts, machining plastic is not uncommon. Typically, machining plastics is reserved for parts that require tighter tolerances than can be achieved with injection molding. Additionally, there are some plastics that cannot be injection molded due to their extremely high heat resistance. These materials must be machined.
As for 3D printing, the material selection is much more limited. The majority of 3D printers run either thermoplastics, nylon, or photopolymer resins. While DMLS, or Direct Metal Laser Sintering, is available for 3D printing metals, the cost is extremely high. It may be to your benefit to CNC machine these parts unless the geometry can’t be reproduced by CNC machining. For this article we are focusing on comparing CNC vs 3D printing in the context of plastic parts.
Part Geometry
For CNC milling, part geometry is the driving factor for both cost and lead time. Parts with simple geometry require less sophisticated CNC machines and may be produced quickly and easily. However, parts with more complex geometry typically need to be cut with the more advanced CNC equipment, such as 5 or 6 axis mills. This drives up cost, especially if the workpiece needs repositioning and realigning, which is a manual task, to reach all surfaces that require milling.
Due to this, more complex geometry will take more time to mill and also produces more waste. Both time and waste drive up your total cost of production in CNC milling. Additionally, internal, or “hidden” geometries, cannot be achieved with CNC milling. Some parts may have geometry that requires custom jigs and fixtures to hold the part in a particular position during milling. If this is the case for your part, this will add to lead time and cost before production can even begin.
For 3D printing, geometry is practically irrelevant beyond the part’s overall bounding box. While CNC milling is capable of producing parts with more complex geometry than can be achieved with injection molding, 3D printing is still far superior. There are virtually no limitations on 3D printed geometry, outside of minimum design guidelines, such as wall thickness.
With 3D printing, lead times are far less dependent on geometry, particularly for technologies like PolyJet and MultiJet. 3D printers may be able to achieve some internal geometries with ease and also benefit from not requiring special tooling, fixtures, or jigs. They simply start printing and add support material as needed, allowing lead time and costs to be much lower.
Cost
As with any manufacturing processes, the biggest factor when deciding between CNC and 3D printing is cost. And in order to choose the most cost efficient process for you, both application and volume need to be considered.
If a part’s intended use is for prototyping or field testing, 3D printing provides the more cost effective option. This is because 3D printing requires no tooling or special setup beyond a design file. As such, you can easily make changes to your part’s design whenever you want without having to get new tools or jigs created each time as you might with CNC milling. While these setup costs are not as substantial as tooling costs for injection molding, they must be taken into consideration.
As for the volume, CNC milling will typically be more expensive than 3D printing for low volumes. This is due to the aforementioned setup costs. However, as you produce more parts with this method, CNC becomes much cheaper than 3D printing due to accounting for an economy of scale.
Volume Limitations
It is important to keep in mind that while CNC milling is not capable of producing the large quantities of parts that injection molding can, it is still better suited to high volume production than 3D printing. By nature of the subtractive process, material can be removed much quicker than it can be deposited by a 3D printer. This allows you to mill multiple parts in the time that a 3D printer may complete a single piece.
Lead Time
Injection molding lead times are typically several weeks because of the time it takes to cut your tooling. Neither 3D printing nor CNC milling are subject to this. However, depending on your part’s geometry, milling may require some custom jigs and fixtures to be made. These will typically not be as time consuming as an injection mold, but they do add to your lead times. If you need a fast turnaround, 3D printing can typically deliver a part in as little as 2-5 days. This is due to 3D printing’s lack of required tooling and positioning setup.
Mechanical Properties
In additional to the aforementioned factors, the mechanical properties of both 3D printing and CNC machining must be considered thoroughly. 3D printing creates parts in layers that may separate or break apart under extreme stress, depending on the part’s orientation and intended use. However, CNC milling makes a part much more durable under stress because the part is cut from one homogenous block of material. This makes it so that there are no layer lines along which it can easily break.
3D Printing at 3 Space
Here at 3 Space, we specialize in 3D printing and scanning. We use a variety of top of the line 3D printers to produce the best quality part for you. If you’re unsure whether 3D printing or CNC milling would be better for you, our technicians can take a look at your plans and provide some recommendations on what will be the best technology for you.