There is honestly no debate at this point, 3D printing is here to stay. As far as manufacturing goes (specifically for prototypes) either manufacturers will get on board with 3D printing or they will eventually get left behind.
3D printing has already replaced CNC machining in applications such as prototyping and when producing only a single piece. CNC machining is still faster and better for most multi-piece production runs as well as parts that have tight tolerance requirements.
Obviously this is a hard and fast answer and there are loads of parts that will not be able to follow this logic. On the micro level of one off parts and prototypes 3D printing is the clear choice, on the macro level of high quantity and batch production CNC machining still holds the crown.
Is 3D Printing Cheaper Than CNC Machining?
For a single part 3D printing is generally cheaper than CNC Machining. This is due to the high programming, setup, and fixturing cost of CNC machining. In contrast 3D printing takes longer per part of machine time making it more expensive than CNC machining the more parts that you make.
In manufacturing time is money. If we had unlimited time to make parts then perhaps we would not make things as efficiently as possible but in the world of promise dates and deadlines faster is always better. There is also generally a cost per hour of on machine time designated for each type of machine on the manufacturing floor which is generally around $75-$150 per hour.
Is 3D Printing Faster Than Machining?
For a single part order 3D printing is generally faster than machining due to the amount of time that it takes to program, set up, and run a CNC Machine. Beyond the first piece CNC Machining will be faster than 3D printing because the initial time investment will already be met.
In CNC machining there is a large skill requirement needed throughout the entire process. From CAM programming, fixture design, machine setup, tool selection, tool spindle speed and tool feed rate, and troubleshooting issues there is a large skill requirement at every step of the way of CNC machining.
The average layman can not just step into a machine shop and expect to be productive in any way without extensive training. Many machinists train for years before they are proficient enough to set up a machine on their own and even more years before they can expect to move up to programming and designing fixturing for machining.
In contrast, 3D printing does not have a large skill requirement and can be taught to basically anyone. There are quirks with 3D printing just like there is with everything but honestly there is not a high skill level with 3D printing.
Let’s take 3D Benchy as an example. There are many features on 3D Benchy that are not even possible with standard CNC milling, specifically inside the cabin of the boat there are many features that you would need Wire EDM to achieve. If you were to simplify 3D Benchy and only take the external features, undoubtedly 3D printing will be faster for at least 5 pieces.
CNC milling will require custom fixturing as well as many hours of NC programming in the front end of the process but eventually when all the front loaded work is achieved you will be able to produce a single part faster than 3D printing.
Is 3D Printing A Machining Process?
3D printing is not a machining process. Generally a machining process is removing material to achieve your desired shape, 3D printing is additive manufacturing meaning material is added to the build plate to achieve the desired shape which can then be finished with machining if needed.
3D printing is not extremely popular in manufacturing yet but over time it will become more commonly used. 3D printing essentially creates a very accurate casting that will sometimes need to be finished with a traditional manufacturing process.
What Machining Processes Can A 3D Printer Not Do?
3D printing is a great method for producing prototypes and complex shapes that a traditional CNC machine is not capable of. Although 3D printing in some ways is more capable than a CNC machine there are some things that a 3D printer is not able to do and CNC machines are the clear winner.
3D printers can not print threads accurately. Perhaps the threads will work but if you have a 2B or 3D tolerance requirement on the tread it is very unlikely that the tread will be within tolerance. A better solution for 3D printing threads is just to print the hole at the minor diameter and then run a tap to cut the threads.
3D printers in general struggle with surface finish due to the natural finish caused by printed layers. There isn’t a great remedy for improving surface finish besides polishing your 3D print with sandpaper or scotch brite.
If you are needing holes with a tight tolerance such as an H7 press fit or a tighter slip fit your 3D printer will struggle with this. It is technically possible to print an engine block with a metal 3D printer but the piston bores will need to be precision machined to get correct pressure.
Manufacturing tolerances for aerospace and defense parts are very different from tolerances for fabricated parts. Generally you will need to achieve a cylindricity of less than .005 of an inch. 3D printing can simply not achieve such tight tolerances without CNC machining help.
3D printing in general cannot hold tight tolerances on parts. In the machine shop that I work in it is not uncommon to see a .010 profile or a .005 true position. A 3D printer can not hold tight tolerances on diameters, locations, and profiles.
Is 3D Printing Faster Than Injection Molding.
Similar to CNC machining, 3D printing is faster than injection molding when you are only producing one or very few parts. The time of producing a dye for an injection mold makes 3D printing faster for small batches and injection molding faster for large batches.