Materials comparison: In material selection (and successful delivery of material properties), CNC offers better options. Essentially the material choice is open to all engineering materials, up to and including spark-erosion CNC machining of pre-hardened tool steels. CNC-machined parts deliver the native properties of the billet material, largely undisturbed by processing characteristics.

While the processes differ in almost every respect, they are direct competitors for the provision of solid parts. Both differ in very fundamental regards as CNC machining is subtractive, while 3D printing is additive. This article will dive deeper into comparing 3D printing vs. CNC machining, their advantages, disadvantages, and alternatives.

Injection molding can be considered an alternative to 3D printing and CNC machining of parts, in that it is a single-operation process to produce a net shape part that is identical to the 3D file. In reality, the tool-making process rules out the use of injection molded parts as a substitute for most 3D print applications – unless the required production volume is sufficiently high.

The term 3D printing covers a range of methods and an even wider range of materials, accuracies, and costs. CNC (Computer Numerical Control) machining achieves similar results through subtractive manufacturing. It cuts the net shape of a part out of a block of material using rotating tools or rotating the workpiece or both. CNC machining has design limitations and virtually no materials limitations. If the material is rigid and can be cut, then any part can be made in any material. Some flexible materials, rubbers, for example, can even be processed while cryogenically stabilized. Geometric restrictions apply, and enclosed hollows may require the assembly of two or more parts.

The earliest commercially viable 3D printing systems began to come to market in the late 1980s. The technologies have broadened both in method and materials at an accelerating pace since then. Compared to CNC machining, 3D printing offers improved cosmetics and more effective use of materials. It also requires no direct handwork in executing complex parts. Figure 1 below is an example of an FDM 3D printer:

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Limited by undercut and internal access, tool path and tool type, axis-defined minimum radii, and the need for repositioning mid-task

CNC machining is a computerized manufacturing process that uses pre-programmed software and codes to control the movement of machine tools. It is a technology that uses a range of complex machineries such as lathes, mills, and grinders to accurately and precisely cut, create, and shape different parts. The first CNC machine was first developed in 1949 by James Parson. His research focused on producing helicopter blades and aircraft blades. In 1958, Richard Kegg together with MIT first created the CNC milling machine. Â

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3D printing is a family of processes that have a common thread of methodology. Virtual models of 3D parts are software-rendered as slices. The thickness is determined to suit a particular machine or setting, and each layer is printed sequentially. The stacking of these slices and their co-bonding, both internally and to the other slices allows the construction of a part from finite 2D steps. Some systems use a variety of materials including: extruded polymer filaments, light-sensitive resins, laser-melted powders, filament feedstock, waxes, and biological materials to print slices. To achieve this, several tools must be integrated, including: a CAD package to define the part design and save an STL file, a slicer software package to turn the 3D file into a sequence of 2D machine instructions, and a 3D printer setup.

Volume comparison: When the extensive setup effort can be shared over multiple parts, CNC machining can produce more cost-effective parts compared to 3D printing. 3D printing has fewer volume advantages, as each part takes the same material and machine costs, irrespective of volume.

Cost Comparison: CNC machining will carry all of the programming and setup costs and it will be expensive in comparison to 3D printing – often five to ten times the price. However, whenever a series of prototypes with small adjustments are required, the modified CNC-cut parts will carry a much lower setup cost, whereas a second 3D printed part will cost the same as the first.

3D printed parts are restricted to those supported by particular processes. The construction methods of the particular 3D print technology impose severe restrictions on the delivery of properties. Prints are often weakened by anisotropic “grain” in the construction method, porosity, poor layer bonding, and the substitution of printable but non-engineering materials.Â

Speed comparison: Preparation for the printing of a 3D component requires little time to set up before a print can commence. Although printing is slow by some measures, most prints will likely complete within a few hours and be ready to use. CNC machining, on the other hand, requires skilled preparation of programming for cutter selection and cutter path. It often requires custom or modified jigs to retain the part during processing (including possible repositioning of the part). This can consume considerable time before the first cut. Cutting, however, is generally fast, and complex parts can generally be completed in an hour or less of actual machining. Total time for preparation and machining can run into a day or more, depending on complexity.

CNC machining requires a complex setup and expensive outcomes. However, if the parts require long-term wear properties, strength, smooth surface, and high precision, it's preferable  compared to 3D printing. Figure 2 is an example of a CNC machine: