In the world of rapid prototyping and small-scale manufacturing, there are two principal methods vying to be the standard. Additive manufacturing processes, such as Stereo lithography and Fused Deposition Modeling, create parts and prototypes by fusing layers of material together to build the part from nothing. Subtractive manufacturing, such as CNC milling and turning, uses a block of material and removes unnecessary excess until only the desired shape remains. But the question many customers ask is, which one is better for my needs?

The answer to that question depends on what exactly your needs are. Both additive manufacturing and subtractive manufacturing methods have their own advantages which makes them appropriate for different circumstances. By examining what your own goals are for your project, you should be able to choose the appropriate method for production.
Part Complexity
The biggest difference between additive manufacturing and subtractive manufacturing methods is the complexity of the parts they are able to create in a limited timeframe. For parts with complex geometries or intricate designs, additive manufacturing methods can produce far superior results than subtractive methods can.

The reason for this is simple. In additive manufacturing, the part is “grown” layer-by-layer, a few thousandths of an inch at a time. You can imagine this process as the assembly of thousands of thin cross sections of your prototype. Each cross section can be extremely detailed, and it is positioned precisely with all of the others. The interior of a hollow or open part is constructed at the same time as the exterior with no additional challenge. The result is a part with extremely high design tolerances both inside and out. And every part is assembled directly from the CAD files, with no human input required. The process is almost completely automated.

In contrast, subtractive methods must remove materials from a larger source. Because of the limits of cutting and routing technology they cannot create hollow parts in a single piece, and they are limited in the amount of detail they can create on a single machine. This is not to say that subtractive manufacturing isn’t precise – given enough time, a CNC router or a skilled machine operator can create extremely accurate and complex parts. However, the time and cost it takes to produce these parts, when compared to additive manufacturing, makes them far less efficient. They also take many more man-hours to produce, adding to manufacturing times and costs. Part Function Along with the complexity of the part you’re making, you’ll need to consider what the part’s eventual function will be. In general, parts that are used for fit checks, presentation models, and for short-use functional testing can be produced with additive manufacturing, while parts intended for long-term or high-stress use are best made with subtractive methods.

In many cases, however, additive methods can produce parts that are suitable for long-term functional use. One good example of this is medical technology, where pieces such as medical inserts made with Selective Laser Sintering are used in direct applications by hospitals and surgeons. Another case is for softer or more flexible parts like hoses or seals, which can be made with Polyjet printing more easily than with any subtractive method.
If you need to create a part or prototype out of a particular material, your choice of manufacturing method depends on what your needs are. Additive manufacturing methods are almost always used for manufacturing parts from plastics, though some new technologies can print parts in other materials such as metal. However, for most metal manufacturing, as well as wood, foam, or other materials, CNC routing and milling is likely the only option.
Part Quantity
For some customers, a single prototype is all they need. Others may want a short run of finished parts. In general, if you need fewer parts you’ll want to use additive manufacturing methods, whereas if you need more you’ll want to use other methods.

Additive manufacturing is particularly suited to making single prototypes or very small batches, as it allows for quick production without much lead time. There is no special setup required for a SLS or SLA machine to make a particular part beyond ensuring it is loaded with the correct base material. In contrast, subtractive manufacturing methods require a great deal of setup in order to create a single part. Programming a CNC router’s tooling path, for instance, takes a great deal of time and skill, especially compared to the nearly-automated process of printing a 3D piece.

However, for projects on a larger scale, subtractive manufacturing becomes more cost-effective. Creating an optimal tooling path for a single part is prohibitively expensive and time consuming, but doing the same for a thousand parts makes perfect sense, since each part should be exactly the same.

What if you’re going to be casting your parts? In this case, there are both additive and subtractive solutions. Additive manufacturing can be used to create a prototype part for a short-run casting mold, suitable for about 100 uses. This makes the additive process suitable for small runs of prototypes or limited-production products. However, for more durable molds suitable for mass production, it’s more effective to use a subtractive method.

Many times the ultimate decision for how to manufacture a prototype or part comes down to the budget. In these cases, all of the above considerations must play a part – lead times, materials, quantities and functions all carry a cost, either in money or time.

As we said above, additive manufacturing is best suited for single prototypes or very small batches, for parts requiring a high degree of precision, and for parts that have a specialized function. Subtractive manufacturing is more applicable for large runs of parts, for parts with a simpler design, and for parts that can be manufactured exactly the same way many times without changes.