When do you need it?
If the part is needed immediately, there are no other holdups to the operation, and you have a machine, operator, materials, and workholding available, you should CNC machine your part. If there are more urgent jobs, or the part can wait, 3D printing can usually get you a part in hand the next day, freeing up operator time for more critical tasks.
CNC machining usually is capable of removing material much faster than 3D printing can deposit it. Size usually does not play into the time constraint as much for machining; the amount of material removal required is much more critical. If the part volume/stock volume ratio is very low (material removal is high), 3D printing may be a good option.
With 3D printing, the part size does affect time; larger parts take longer to print. if a part fits in the palm of your hand, you can usually have it the same day. With smaller parts, it can often be faster to 3D print than to machine. With a CNC machine you have to spend time getting stock, writing G-code, figuring out work holding, setting up tools, and cleaning up after. Some parts can print in the time it takes just to get a CNC machine ready.
The process of outsourcing machined components to CNC machining services typically requires several days at minimum, between part review, creating drawings, sending out for quote, and reviewing with purchasing. This, plus shipping and lead time from the shop, can sometimes result in a very lengthy process. Printing, in this case, may be a suitable way to check fit and finish or have a part ready the next day while a permanent part is being cut.
How many do you need?
When you break down the cost of low-volume machined parts, the bulk of the cost comes from the time required for programming and setup; the actual time for cutting metal is generally fairly short. Scaling up production volume is generally achieved by making larger setups to cut more parts unattended. As part complexity and the number of features increases, programming time and the number of required setups may also increase. However, the cost of additional units drops off fairly quickly. Depending on geometry, CNC per unit cost scales well into orders of hundreds or low thousands of units per month since the programming and setup can be reused.
With 3D printing, the programming (slicing) happens in a matter of minutes, and complexity has little effect on the programming time. While the first unit cost and effort is low, the per unit cost is not affected much by volume. Scaling up production volume is generally achieved by bringing more machines online.
What is your budget for equipment and operators?
CNC machines can run unattended when properly set up, but typically trained full-time operators and programmers are critical to success. Machines are not always easy for owners to service, requiring expensive maintenance plans.
3D printers can easily run fully unattended, operators require minimal training, and programming is made easy by software. Machines are typically easy to service and have far less expensive maintenance costs.
CNC machines and 3D printers come in a range of prices based on features and build quality, but typically 3D printers can be owned for a significantly lower investment than a CNC machine.
The upfront cost and operating cost of additive manufacturing is low, making it ideal for low volume applications such as prototyping and tooling. However, when scaling to higher volumes, subtractive manufacturing and forming are more cost effective.