Both computer numerical control (CNC) and 3D printing are technologies used for lower-volume production and fast prototyping. Yet the two technologies achieve this in two completely different ways to attain the necessary precision. CNC machining uses subtractive production techniques that involve removing material to form a finished workpiece. In contrast, 3D printing produces a finished workpiece by building up layers in an additive process.

Neither technology can be deemed a good fit for production runs of more than a couple hundred components, as the injection molding process offers a much more cost-effective means for mass producing products. However, for smaller production runs both precision CNC machining and 3D printing offer advantages for certain functions, while being disadvantageous in others. Understanding the situations in which to use these two technologies will help manufacturers make the best decision for their application.

Precision CNC Machining vs. 3D Printing

In many ways, both production methods are capable of fabricating components that require greater precision. CNC machining and 3D printing are at the leading edge of manufacturing, commonly used in the production of a wide array of parts for various industries.

The similarities between precision CNC machining and 3D printing include:

• Capable of using both OBJ and STL file types.
• Producing three dimensional products.
• Programming of instructions for production are done via computer.
• Their use in both prototyping and smaller production runs.
• Utilizing models made in three dimensions.

Beyond these similarities, each production method offers different benefits, with each having its unique complications as well.

With precision CNC machining, the main cost in both money and time comes from programming and setting up the machine. The amount of time it takes to cut a workpiece is minimal, while scaling up production will most likely involve automation. The more complex the part, the longer it takes to program and set up the machine. Depending on the necessary precision, CNC machining allows reuse of programming and setup instructions to comfortably scale manufacturing from hundreds to the low thousands of components produced on a monthly basis.

For precision 3D printing, programming is simple and can be done in just minutes. The complexity of the part won’t affect programming time or setup much at all. However, though production of a single unit isn’t very high, the costs don’t go down much when producing higher volumes, so it becomes more expensive than CNC machining when scaled up. Additionally, the only way to scale up production with 3D printing is for the manufacturer to use more 3D printers.

Precision CNC Machining and How It Works

The first CNC machine was developed by researchers at the Massachusetts Institute of Technology (MIT) in 1952, with the first commercial model patented in 1958. Using subtractive production methods, it involves carving blocks of material to fabricate components of sufficient precision. CNC machining works well for applications that require particularly robust parts that require tight tolerances, such as those needed in the aeronautic, automotive and defense industries. CNC machining is also commonly used for engraving and lettering, along with woodworking applications.

Engineers first make two-dimensional or three-dimensional models with computer-aided design (CAD) software. These CAD files then instruct computer-aided manufacturing (CAM) software that’s used to program the CNC machine with specific commands to fabricate the component. To achieve the desired precision, CNC machining utilizes an array of tools that rotate with sharpened blades to cut away material. While more basic machines work on three axes, precision CNC machining is done with more advanced equipment that works on four or five axes.

Common tooling for precision CNC machining includes:

• Drills: Carves out material from a stationary workpiece with a spinning drill.
• Grinders: Uses an abrasive wheel to remove material from a workpiece.
• Laser cutters: Cuts wood, metal or plastic workpieces with a powerful laser tool.
• Lathes: Rotates the workpiece as stationary tools cut it.
• Mills: Rotating cutting tools for sculpting a workpiece.
• Plasma cutters: Cuts sheet metal along two-dimensional profiles.
• Routers: For cutting larger workpieces of plastic, sheet metal or wood.

CNC machines can use multiple tools to make the necessary cuts to the material. Oftentimes these can simultaneously cut into a workpiece to boost efficiency.

Precision 3D Printing and How It Works

The father of 3D printing is William Masters, an American designer, engineer, entrepreneur and inventor. In 1984, he filed the first of three patents for 3D printing, all of which laid the foundation for today’s 3D printers. As a method for additive manufacturing, precision 3D printing involves building workpieces in layers, one layer at a time. Though 3D printing offers less precision than CNC machining, it can still build components with sufficiently tight tolerances for many applications, including for the automotive, biotech, dental, engineering, food, footwear, jewelry and medical device industries.

As with precision CNC machining, engineers can use CAD software to make three-dimensional models. 3D scanners can instead be used to create a digital model from an actual, physical three-dimensional object. Another means for creating a model involves photogrammetry software to help construct three-dimensional objects from photographs. Whichever means is used, this model is then analyzed to ensure there are no errors. Once the modeling is done, slicing software scans each layer to show the printer the areas that must be filled, after which it goes to be printed, one layer at a time.

Some common methods used for 3D printing of layers include:

• Fused deposition modeling: Utilizes plastic filament to produce workpiece.
• Selective laser melting: Fuses metallic powders together by completely melting the material.
• Selective laser sintering: Fuses metallic powders together through heat.
• Stereolithography: Uses light to create molecular chains that link together to form polymers.

The actual printing process can take from just a few hours to days, depending on the method used and complexity of the model. In some cases, there’s also a post-processing phase where washing, polishing and sealing are done.

There are a number of key differences between the two types of production processes.

The differences between precision CNC machining and 3D printing include:

• File types: Though precision CNC machining and 3D printing can both utilize OBJ and STL file formats, none of the other types of computer files are compatible between the two.
• Geometry: While 3D printing can achieve a certain level of accuracy, they have limitations in their level of precision; CNC machining can produce components with extremely thin walls, while there are limitations for such geometries for 3D printing.
• Materials: CNC machining allows for production with a wide array of materials that include alloys, acrylics, metals, modeling foam, thermoplastics and wood, while 3D printing works with more limited materials like metals, plastics and polymers.
• Noise: As 3D printers don’t vibrate during the manufacturing process, they’re less noisy than CNC machines.
• Precision: CNC machining offers better accuracy and tighter tolerances to within a micrometer, whereas 3D printing hasn’t yet achieved this level of precision.
• Quality: Along with tighter tolerances and greater precision, CNC machining produces more aesthetically pleasing workpieces that don’t deform during fabrication; bending and warpage are more common with 3D printing, while layers can often be seen, particularly at curved points in the workpiece.
• Raw materials: While a CNC machine can work with all manner of raw materials, a 3D printer can only use a specific material.
• Replication: Though 3D printing technology is markedly improving its precision, CNC machining reliably produces identical products each time.
• Size: A CNC machine can make products across a broad range of sizes, while a 3D printer can only fabricate products up to as large as their printing bed; though larger workpieces can be made with 3D printers, these need to be broken down into their component parts and printed separately, which adds considerable time to the process.
• Speed: Larger scale production is slower with a 3D printer, as it fabricates the whole product in one go, whereas a CNC machine is generally part of an assembly line with each machine set up to produce one type of component.
• Waste: This is one of the major differences between subtractive and additive manufacturing, with CNC machining producing far more waste as it removes material, much of it not recyclable; in contrast, 3D printing only uses precisely the amount of material needed to produce a component.

Precision CNC Machining by Spaulding Composites

Spaulding Composites Inc. provides expertise in every area of precision CNC machining, including design of highly complex geometries. Our capabilities allow us to achieve tolerances as close as +/-0.002 inch (50.8 micrometers), enabling us to bring your designs to life. To learn more about what we can do for you, contact us today to request a quote or to learn more about what we can do for you and your company.