I think David was originally expecting/hoping that a metal 3D printed part wouldn't need machining. From previous comments and the photos I've seen that's very unlikely to be the case.
It's easy as a complete novice in 3D tech to think it's just a case of drawing the part and getting a functioning part made. I think those of us who have actual experience of doing this will know that there's a huge lot more to the process and there are many pitfalls on the way to a basic level of competence.
The two other factors that have barely been mentioned are;
Metal printing to a high standard is hugely expensive.
Designing working live steam models needs a lot of experience in adjusting scale sized parts to a real size that will actually work and perform as you'd expect.
