Some parts are reproducible via 3d printing but I don't brake drums are one of them. Brake drums are critical components. 3D printing might work. In fact it probably will work (depending on the process) and eventually it will be made to. But the problem with critical parts is that they MUST work. Because of that you need a proven track record. For example, cutting a replacement for my hub from a chunk of steel WILL work. That is one way such parts are made. One might THINK that cutting a replacement brake drum from a giant chunk of steel on a lathe would be expensive but would work. But it would not because the particular metal and even the manufacturing process a brake drum is made from is critical. To my (admittedly very limited) knowledge no one is my making brake drums with 3D printing yet so there will be a steep learning curve. Such things would be very researchy.
Another example of a critical components might be a tie rod end or a brake linkage. You stick with the tried and true manufacturing process because you don't just think it will work, you know it will. Eventually 3D printing will be used for those purposes, but someone had to do the R&D first to get it there. For example, 3D printing is being used to make jet engine parts. But the aerospace companies did the testing and R&D to make sure it works right. (It should be noted, while the quality of jet engine components must exceed that of a brake drum or wheel hub or tie rod end, the specific details of the process like the materials and quality control proceedures are very different)
Of course in some cases there might be overlap. For example, if you found a ball joint end was being made by 3D printing to operate a critical component like a wing flap or the landing gear on a passenger jet, it might be more reasonable to assume that same process could be used for automotive tie rod ends. But you also might not find it as cost effective as you'd hoped if part of the way they made it work was to look for cracks and voids in the printed part with neutron diffraction.
I have no doubt we'll get there though. It will start on the high end where people are along to pay 1000s of dollars for custom parts with a fast turn around time. Then it will trickle down. Once Airbus does the leg work to figure out how to make tie rod ends for some application for example, and the vendor that's doing it buys a one those little dental CT machines to check the parts, and the process is validated for use on actual aircraft, that cost is paid for by the initial application of the tech,and then the company starts asking "who else can we sell to to pad our bottom line".
Of course non critical components like an alternator bracket, a headlight knob or a window crank would work just fine. If they break you don't lose braking or steering.
