What they're extruding over at fab@home

[fabathome.org]

The toy truck tire is pretty impressive. The food stuff is just silly and their attempt at doing a mould for a model airplane propeller that they cast out of epoxy, while not exactly a success, gives you an idea of what could be done.

They also give an idea of what the build time of their machine is, viz, five hours for the propeller mould. Before you get too excited about how slow their machine is you need to note that...

"We manually filled the mold with epoxy while it was being fabbed so that overhanging parts of the mold would not cave in."

Which says that they hadn't, at that time, tackled the problem of extruding support structures. I suspect that most of the five hours was spent stopping the build and carefully injecting tiny amounts of epoxy to top up the build level.

So why not build a base and extrude it in an upright position instead of on its side? Finishing is a process in and of itself anyway. Even molded plastic parts arent perfect until they are machined.

A pet project of mine is to construct an engine using plastic parts fabbed from a reprap and cast with the lost wax process. One of the more complex parts is the cranshaft, which cannot be constructed unless it has some support.

However I can construct it on its end so that I can fab almost all of it with one CAD model. The angular offest of the beam from the main to crank journal would ordinarilly be +/- 30*, obvously needing support if fabbed on its side. Fabbing it on its end solves that problem nicely because the +/- 30* is on the vertical plane, which never exceeds the 45* overhang problem.

For complex parts like this, what about modifying the program that interprets the CAD model such that a 3d space around the model is drawn, and everything in that space that is not part of the model gets assigned to filler material?