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The Open Air design starts with the principal of making a reprap that can build parts significantly bigger than the machine itself. To do this, I am designing a machine that will allow extending an existing piece by building more onto it. Adding new layers to an existing, already finished piece has many implications. The exiting piece must be supported during the add on construction, but it must be mostly outside the main machine to leave the most room possible for the extension. To do this, arms with adjustable clamps will be added that swing horizontally from the sides and extend out to grip securely the existing part. It also means that the bed must be stationary, otherwise the old part extending out would be flopping around with every X and Y movement. It also means that at least one side must be open to allow parts to stick out. I have chosen to have two sides of a cube, plus the corner or edge between them removed, thus the team name "OpenAir".
To be able to extend parts outside the framework, the bed must be stationary. This implies that all three axes have to be built into the extruder. The stationary bed will also help modifying the bed to be heated. With the bed not moving, there will be less induced airflow over the exposed heated parts, thus draining less electrical power to keep them heated. It also means that delicate objects being made are not shaken back and forth during layering, so that thinner sections and more unsupported overhangs can be used. Another benefit is that if welding, casting, or other hot metal work requires a controlled atmosphere, since the bed does not slide out it is easier to enclose the whole structure to fill with CO2, Nitrogen, or Argon.
Gantry Style Extruder Movement
To use the maximum possible working area inside framework, the Open Air will use a gantry style movement system. With the extruder head moving in both X and Y it will be able to reach every corner of the stationary bed. This allows for making the biggest possible within an operating volume. Any system the moves the bed instead of the extruder in X and Y has to have a large footprint on the desktop than the size of the bed. With a gantry style X, Y, and Z axes motion system, the biggest pieces can be only the thickness of frame and gantry less than the size of the total machine footprint. To accommodate the open sides and missing corner, I will use a full, square platform for the Z axis and run it up and down with three threaded rods linked together, possibly with a chain drive. X will then move a beam from one side to the other inside the platform, and Y will move the extruder across the beam.
To make the 3D motion platform strong enough to handle light milling even with two sides and one corner pillar missing, I plan to use a truss-box beam square. This should make the corner floating in air very strong and stable, and allow milling to take place