At Autodesk University 2021, Greg Paulsen of Xometry outlined how to decide if a part should be machined or printed.

Manufacturing can be expensive and time-consuming, especially if you don’t choose the optimal process for your part. For modern manufacturing needs, it usually comes down to a choice between additive and subtractive manufacturing. So, how can you decide if a part should be machined or printed? At Autodesk University 2021, Greg Paulsen of Xometry led an industry talk to answer that exact question. Over the course of an hour, Paulsen provided an overview of the two main types of manufacturing and explained how to choose the right process when making a new part.

Overview of additive versus subtractive manufacturing processes. (Image courtesy of Advanced Additive Manufacturing.)

The subtractive manufacturing process begins with a large starting material that is reduced through milling, cutting, drilling, and so on. This includes CNC machining, sheet metal fabrication, injection molding, stamping and more. When CNC machining is used to manufacture a part, it is cut with a high-velocity tool using a CAD model to create the final product. CNC machining is a highly precise process, but it can be expensive depending on the setup requirements and operation costs.

As machining is the traditional form of manufacturing, Paulsen spent most of the session discussing the different types of 3D printing and explaining how to choose between additive and subtractive manufacturing for any given part.

CNC machining. (Image courtesy of Xometry.)

In contrast to subtractive manufacturing, 3D printing involves making a part layer by layer. The broad umbrella of additive manufacturing includes stereolithography (SLA), selective laser sintering (SLS), fused deposition modeling (FDM), HP Multi Jet Fusion (HP MJF), direct laser sintering (DLS), direct metal laser sintering (DMLS) and more. Each method uses a unique process and can utilize materials as diverse as plastic, rubber and metal. Unlike machining, 3D-printed parts usually have a rougher finish and will vary based on the type of printer used. For example, resin-based 3D printing is generally smoother, whereas powder-based printing can be grainy.

With so many types of additive manufacturing, it can be challenging to know exactly which process is right for your purpose. Paulsen provided an overview of a decision tree that Xometry uses to help guide its customers through selecting a 3D printing process. For example, if a part needs to be made of metal, you are much more limited than if a part can be made of plastic or rubber. Overall, Paulsen summarized that material, function, finish and size will influence the type of 3D printing you ultimately choose.

When comparing the final products of different 3D printing processes, Paulsen provided an excellent analogy of different writing materials. He compared powder-bed processes like SLS and DMLS to a fine point Sharpie: good detail with some bleeding. Powder-bed processes can significantly impact your tolerances, especially if you are looking for precision. Liquid resin processes like SLA have finer precision and are closer to a Sharpie pen: fine details with clean edges. With reduced bleed, tolerances are tighter, and liquid resin offers some of the highest detail resolution of any 3D printing process. Filament processes like FDM are usually the go-to for larger objects as they are similar to a drafting pencil, creating broad and deliberate lines.

Industrial 3D printing. (Image courtesy of Xometry.)

Although 3D printing is an exciting new technology, there are many reasons to stick with traditional machining. One of the main reasons is that machining inherently includes precision tolerances and a smooth finish. As the traditional manufacturing choice, machining materials are well-characterized with predictable outcomes. If you need a material to always behave a certain way, machining is usually your best option. Turnaround time for the manufacturing process can be as little as two to three days, but this will vary depending on the required setup and operations.

During the session, Paulsen described an example from Xometry of Phase Four, an aerospace company that was looking to scale an electric propulsion system. The company’s goal was to reduce the manufacturing cost of its satellite system without sacrificing performance to facilitate mass production. In terms of the actual parts, Phase Four needed an exact product with well-characterized components to ensure that it would function in space. Using Xometry’s CNC services cut the cost of producing the satellite by 80 percent without affecting its function.

If you are looking for tight tolerances and a precise final product, 3D printing is likely not the best option for your team. Printers ultimately choose the tolerance of your part, and nearly all 3D-printed parts will be less precise than a machined version of the same product. 3D printers are considered to make “near net” shapes, and the final product will vary based on the printer, the material used, and the overall design and orientation.

The main benefit of 3D printing is its incredible flexibility. Compared to traditional CNC machining, increased part complexity does not usually increase production costs with 3D printing. This makes 3D printing a great choice for complex, organic shapes like lattices, corals, or other objects that require fine detail. The flexibility of a 3D printer also allows you to use sacrificial support structures throughout the printing process, which can be removed from the final product.

3D printing is the leading manufacturing process for prototyping and low volume production. With no requirement for a machining setup, manufacturers need only a CAD file to get started with production of a new part. Then, in as little as one to three days, companies can print and test a new part, making additive manufacturing integral to engineering research and design.

However, the overall volume of material required will affect the cost, and any necessary secondary finishes can impact expenses. Industrial 3D printers also tend to have higher overhead rates than machining tools.

Paulsen shared the example of Dixon Valve, a company that required fixtures of a very specific geometry to be added to its device. The company needed a metal component that was ultimately 3D printed using DMLS in aluminum with the help of Xometry.

For the final segment of his session, Paulsen walked through a series of manufacturing examples that could impact a company’s choice in selecting machining or 3D printing for a part. When it comes to large, bulky geometries, Paulsen said this usually drives costs down in machining if you already have a setup—but can increase costs with 3D printing as you will lose precision and require additional materials. However, the reverse is true for organic features and designs, which are usually easier with 3D printing. Machining is usually more expensive than 3D printing for other complex shapes and designs like coring, lattices, deep holes and channels. Finally, if a part requires tight tolerances or a smooth surface finish, machining is usually the way to go to limit any manual modification of parts following 3D printing.

When it comes to selecting a manufacturing process for a part, Paulsen encouraged consideration of the part’s function, design and purpose. For example, if you need a precise part with a smooth finish for mass production, you are likely looking at a product that will need to be machined. However, for prototypes or complex, organic geometries, 3D printing is likely the way to go.

In his recap of the session, Paulsen summarized the three main types of manufacturing to be considered. For metal 3D printing, you are usually looking at a purpose-built part, a part being used for a low volume prototype, or one with complex pieces in the final assembly. With polymer 3D printing, your part is likely a one-off, research prototype or a small piece that can be scaled for production. Finally, for CNC machining, your part is likely being mass produced and requires tight tolerances.

In concluding the session, Paulsen highlighted Xometry’s services for custom machining or additive manufacturing on demand. The company offers free CAD add-ins for instant pricing in Fusion 360, Inventor and SOLIDWORKS, and it provides free design guides, videos and webinars.