“- + + = >” is a formula that describes a useful way to think about the relationship between subtractive and additive manufacturing processes. Subtractive processes include those that take away material to create a workpiece’s shape. The traditional metal removal processes such as milling, turning, grinding, electrical discharge machining (EDM) and so on, fall into this category. Additive processes include those that add material, usually layer by layer, to build up a new desired shape. Direct metal laser sintering, stereolithography and 3D printing are examples.
The point of the formula above is that our focus should be on the complementary nature of these seemingly opposite approaches to forming a workpiece. Another way to think about this formula is that subtractive and additive processes can be combined to develop innovative manufacturing methods that are superior to conventional methods. Using additive and subtractive processes together helps us make things that could not be made as well—or at all—by other means. One-piece objects with complex, enclosed interior spaces and detailed, external features are examples. This might be a mold in which conformal cooling lines were produced with direct metal laser sintering, but the cavity was finished with EDM and/or high speed milling. Another example could be using an additive process to build up a turbine blade while creating curved air cooling channels within, followed by creep-feed grinding to finish contours on an outer surface.
It is true that additive processes, which have come on the scene only in the last few decades, are likely to replace subtractive processes in some applications, at least in part. In that sense, the processes will compete with each other. Nevertheless, the focus should stay on the creative freedom enabled by their combination. This creative freedom is both an appeal and a challenge to the imagination. Sometimes the design team will take the lead in exploiting this freedom. Other times the manufacturing team will have to show the way by suggesting new possibilities to bring inventive part shapes into reality. Dynamic tension between these teams is a normal sign of genuine innovation.
Successfully combining these processes means that “additive thinking” and “subtractive thinking” will have to blend. These modes of thought are different. With an additive process, the blank workpiece is empty space. This is an advantage when components are characterized by internal voids or enclosed cavities—the absence of material. The inherent efficiency lies in the fact that unneeded material is left out from the start. Yet, many parts still will be primarily chunks of matter. Starting with a solid mass will be the advantage, because material that will yield the finished shape is already present in a known condition.
Of course, adding additive processes as an option is an opportunity for jobber machine shops and prototypers. Most of them have been moving toward greater diversity in manufacturing capability for a long time now anyway. The key is offering a sum of specialties that add up to greater value for the customer and more profit for the shop.