It doesn't matter whether you make your goods from gold, unobtainium, or rancid butter. Something will still have to convert the raw material into a desired form. You're proposing that machines will do that, unattended beyond roving repair crews. Statistically, that's simply not possible. It doesn't matter whether your robots are made from gold, unobtainium, or sculptured doggie doo—it becomes a simple issue of tolerance stacks, and there's simply no way out of that numerical reality. Human input will allow you to mitigate the damage a bit (but never eliminate it), but you've already stipulated no human input.