Chris Borges has constructed a milling machine using a 3D printed outer shell filled with concrete, within which he has placed a steel skeleton, creating a rigid structure to attach the various components to.
He has done an excellent job documenting the process, and the resulting machine looks pretty good and has a solid feel to it.
However, some commenters have argued that the structure is mechanically inefficient, with the steel bolts attaching the components to the plastic interior simply bending the plastic, which then bends the concrete, rather than the concrete and steel working in unison to provide a rigid structure.
It is also noted that a $800 vibrator would have been a better use of funds than the 3d printer, although having the vibrator would not have helped with the mechanical design.
Others have defended the design, suggesting that for a DIY build it is actually not that bad, and that most of the flex comes from bending forces rather than tension or compression.
The designs and documents are all available online for anyone who would like to trymaking a similar machine.
