Printable Modular Machine Components

This is an experimental set of printable machine parts, the idea being that you can configure these into whatever 3-or-fewer-axis machine you want. The basic parts are:

1) Linear rail and ball carriage.
2) Rack and pinion
3) Structural blocks with 2 (angle), 3, 4, and 5 mating faces.

Hole patterns for all parts are NEMA 17 - 31mm square, and holes are sized for M4 fasteners.

With the right tool holders it should be possible to make a small milling machine, drilling machine, maybe a pick-and-place machine, who knows? I invite people to please post suggestions or modifications.

Notes:

1) I haven't printed any of these yet, but I have skeinforged them, so I think they'll print ok.
2) I haven't arranged multiple parts into one file so they can be printed together, but it would be smart to do this. I'll post assembled files when I get a chance.
3) I don't know how accurate the resulting machine will be, so please keep in mind that this is still something of a work in progress. I hope to add and anti-backlash split gear as a start, but I'd love to hear other suggestions.