The problem with 3D printing / lathing / milling as a service is that particular parts can have orders of magnitude differences in machining costs, and typicall

The problem with 3D printing / lathing / milling as a service is that particular parts can have orders of magnitude differences in machining costs, and typically you need humans in the loop to figure out if you're asking for something ridiculous (or dangerous, or harmful to the machine...) These guys are close to that model: https://www.plethora.com/ https://www.plethora.com/ . Their supposed killer advantage is that they have spent a lot of time deriving a cost model for arbitrary designs which allows relatively unsupervised submissions, but because they're very dependent on that cost model, the number of operations and customizations they support is fairly limited.