The Math of Building: Adding or Subtracting?
If you want to create something from scratch, there are basically two ways to do it. You can take a chunk of raw material and chisel, mill, or saw away pieces of it until you're left with the object you intended to build, or take portions of raw material and put them together to create what you want.
These different types of creation are referred to as "subtractive" and "additive," respectively. Both have their inherent pros and cons, and one of the most exciting things about 3D Printing is that it introduces a new method of additive creation.
Manufacturing processes, particularly automated processes that involve the use of computer software to control machine tools, tend to be subtractive. While subtractive manufacturing allows for incredible precision (think diamonds and marble sculptures), subtractive processes have inherent limitations, according to Behrokh Khoshnevis, professor of Industrial & Systems Engineering and Civil & Environmental Engineering at the University of Southern California. Khoshnevis outlined four downsides to subtractive manufacturing:
- You need to create the machine tools, which is overly expensive and time-consuming, unless you plan to manufacture vast quantities;
- You cannot build very complex parts, because the manufacturing tool cannot reach inside to finish internal details;
- Items usually cannot be built in a totally unattended way. You need to mount your raw materials on a machine, take them off and change the orientation periodically, and so on;
- Subtractive manufacturing is wasteful, with unused material left over.
Additive manufacturing processes have historically had their own inherent limitations. Traditional additive manufacturing generally involves creating a mold, pouring fluid into the mold, and taking the mold apart after the fluid fuses. As with subtractive manufacturing, this severely limits the ability to create objects with intricate interiors. What's more, construction of the mold itself is both time-consuming and expensive.
3D Printing offers big improvements in both the cost of the process and the possible intricacy of the finished object. (Check out our slideshow for some examples of intricate 3D-printed objects.)
"There is no need for machine tools or a mold, and because the exterior of the object is not in your way when you're building the center, you can create intricate geometrical shapes inside," says Khoshnevis. "It's also much easier to build a series of 2D layers than to build a whole 3D object at once."
3D Printing has limitations of its own (your local car dealer isn't printing cars yet), but as with any new technology, the expectation is that the ease and capabilities of 3D Printing will increase over time.
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