We have a confession. There’s no 3D printer in our fabrication shop. If that comes as a shock we apologize, but there are good reasons for not getting in to this technology, yet. That might change one day though, because it’s developing quickly. Already some pundits see 3D printing replacing conventional machining processes, although others argue it’s a complimentary technology. Here’s our take.
3D Printing Primer
We can’t talk about 3D printing, (additive manufacturing if you want to be more correct,) without first explaining what we mean. 3D printing is actually a range of processes that make parts by building them up, (hence the term “additive”.) The alternative is to machine material away from a solid billet (which we now call subtractive.)
Early 3D printers worked with plastics. More recently, machines have emerged that print metal. Most use a laser to fuse particles of metal powder into the shape required, building it up layer by layer.
Pros and Cons of Metal Additive Manufacturing
The major benefits of additive are:
- Build shapes almost impossible to make any other way. Lattice structures seem to be emerging as the “killer” application. These are especially valued in aerospace and motor sports for their weight-saving potential.
- Combine components that would otherwise be separate pieces requiring assembly. GE is doing this successfully with a complex part for their LEAP jet engine.
- Speed. 3D printing avoids the time involved in making patterns or mold tools, so it’s a faster way of getting initial quantities.
However, there are also two big negatives:
- It’s slow. The latest machines are getting quicker, but build times still stretch into hours for sizeable metal parts.
- Parts need finishing. The layers are visible and need milling or grinding to create smooth surfaces. Then there’s the matter of support structures. 3D printers struggle with overhanging features and holes that parallel the layers, so they add supports. These need cutting away after printing.
The Strengths of CNC Machining
Multi-axis CNC machines can sometimes produce a finished part in one operation. Often though, it takes two or three steps to go from bar or billet to usable part because the part needs presenting in different orientations.
Also in CNC machining’s favor, it’s accurate. Modern CNC machines, laser cutters and turret punches included, are very precise and repeatable. Tolerances tighter than +/- 0.002” are easily maintained. (That is somewhat part design and material dependent of course.)
Cases Where Additive Can Ceplace CNC Machining
We see two situations. One is where the design calls for really complex shapes that cannot be made any other way, such as the lattice structures discussed earlier. Notice though that this isn’t really taking away CNC work because it’s allowing designers to create entirely new geometric forms.
The other is where the part design calls for so much material to be removed that 3D printing is actually faster. For example, imagine a hollow box shape that can either be machined from billet or printed. Depending on size, printing could be faster, and as machines grow quicker, expect that breakeven point to move gradually in favor of additive. (We’d argue that fabrication will be faster than either of them for a while yet.)
Are Hybrid processes the Future?
Several machine tool manufacturers have launched hybrid machines. These combine material deposition, either laser-fused powder or conventional welding, with multiple machining axes. The idea is to allow production of a finished part in one go.
Impacting Design, not Manufacturing
It doesn’t look to us as though CNC machining is under threat. Rather, what’s happening is that additive is letting designers break free from the constraints imposed by conventional subtractive processes. Some parts previously machined from solid will move to 3D, but they’ll still need machining.
So is fabrication threatened? Not yet, and probably not for a long time to come. It is something we’ll continue to watch though.