A finished metal part's blueprint specifies its shape, dimensions, and tolerances — but it doesn't specify how the part actually has to be made, and that's deliberate. The exact same final geometry can often be produced by casting, by machining, or by additive manufacturing, three fundamentally different processes that shape material in opposite ways. Choosing between them is a manufacturing engineering decision made on cost, production volume, and material behaviour, not something dictated by the part's shape alone.
Three different relationships between material and shape
Casting starts with molten metal poured into a mould shaped like the desired part, letting the material solidify into that shape directly — efficient for producing large numbers of identical parts once the mould exists, since the mould, not repeated machine work, does most of the shaping. Machining starts with a solid block of material larger than the finished part and removes material, typically by cutting, until only the desired shape remains — a subtractive process capable of very high precision and excellent surface finish, but one that wastes material and takes machine time proportional to how much needs removing. Additive manufacturing, commonly known as 3D printing when applied to metal, builds the part up layer by layer from digital data, adding only the material the final part actually needs, at the cost of generally slower production and, for many metal processes, different material properties than casting or machining produce.
The decision is about everything the blueprint leaves out
Which process actually gets chosen for a given part depends on considerations the finished blueprint alone can't answer: how many units need producing, since casting's up-front mould cost only pays off at volume while machining and additive methods scale down to single prototype units more easily; how the part will be loaded in use, since casting can introduce internal defects that matter for high-stress applications where a machined part's more predictable internal grain structure is preferred; and how geometrically complex the part is, since additive manufacturing can produce internal structures and shapes that would be difficult or impossible to cast or machine at all. A manufacturing engineer choosing between the three isn't just picking the cheapest option — they're matching a specific process's real strengths and weaknesses to a specific part's actual requirements, decisions that leave no visible trace on the finished geometry itself.
What we're still unsure about
The basic tradeoffs between casting, machining, and additive manufacturing — in cost structure, achievable precision, and material properties — are well established manufacturing engineering knowledge. What continues to shift, and isn't settled the way the basic tradeoffs are, is exactly where the economic and technical crossover points between the processes fall, since additive manufacturing technology in particular has been improving in speed, cost, and achievable material properties, gradually moving the volume and complexity thresholds at which it becomes competitive with casting or machining for a given part — a moving target manufacturing engineers have to keep reassessing rather than a fixed rule.
This sits inside Manufacturing Processes, one of eight topics in Mechanical Engineering, one of four domains in Engineering, one of seventeen subjects the app can quiz you on.