This is a part I printed yesterday for my mini Sherline lathe. I've been using the lathe to machine parts for my Tri hotend. Then I put the Tri hotend on my Rostock, printed out a custom steady rest/center for the lathe, and now I can use it to machine more parts.
It seems like most of the 3d prints people are doing now are artistic, aesthetic, or non functional, but this is a good example of a perfectly functional machine tool part that would normally cost a ton of money. Actually, something like this isn't even for sale as far as I know! Around a dollar in plastic to print.
You can see some half finished Tri hotends on the side. It's true reprap stuff here; the lathe makes the Tri hotend parts, the Tri hotend makes the lathe parts (and everything else). Once we get more rigid plastics for printing, it is definitely possible to make a small lathe or mill. Some parts will still have to be steel, like chucks, collets, cutting tools, and ball bearings. But it is definitely possible to make functional parts like this.
I'm going to try to make an entire indexing tailstock soon, which would help a lot with tool change times. That will be another thread.
I'm not making all of the Tri hotends on this, just the early beta units.