Is it possible to build Lego bricks with a makerbot replicator 3D print? Is it possible for plates, tiles or small pieces ? Is it possible to use MLCAD with a Makerbot replicator?
(fair warning: I work for a 3D printer company [Type A Machines])

I thought this cross-posting up on the RepRap forum would be useful here.

Summarizing that post:

It is possible to 3D print LEGO-compatible parts on desktop 3D printers, but with caveats:


The fit will likely never be as tight as real LEGOs. Those are tough tolerances to match. LEGO Corp are (I've heard) the masters of injection molded plastic.
The fit we achieved is not bad though, but to make it work you can't just print the holes at the exact diameter and just hope it works.
I did my models in CAD software and (CRUCIAL POINT) had to expand hole sizes by about 1/2mm to get the right fit.
I added 1mm chamfers to the holes as well.
I also widened the holes after printing with a 4.8mm drill bit. No power tools needed, just a few turns manually.


...but with those caveats, it's totally doable.  The holes I printed stuck quite nicely to the top of a standard LEGO 8x8 plate. Of course, it's crazy to try and print any standard LEGO/Technic brick. LEGO corp does that so much better than you ever will. The interesting thing is making custom pieces that you'll never see from LEGO. There are a lot of interesting 3D designs out there that - with the methods above - allow you to turn them into a LEGO component.