Mechanical Rigidity/Fisher
Fisher Delta
The Fisher is an experimental Delta printer that has a properly rigid frame.
Like the Ultimaker 2, this design uses panels to extremely good effect, providing complete rigidity of the frame. The use of plastic in the corners however means that it is not entirely a triangular frame, so the rigidity provided by the panels will be transferred via a "lever" effect into twisting of the plastic pieces. In reality however this is unlikely to be an issue.
Interestingly an experiment with straws, paper and tape quickly showed a potential design flaw in the Fisher: the upright struts could still potentially "bow", flexing the panels out of shape. This is easily fixed by adding a third (middle) set of horizontal (triangular) bracing (which may have to go on the outside in order not to interfere with the Z-axis mechanisms). The extra bracing ensures that the panels, across the top of the "open" part, do not bow / flex. https://youtu.be/6Fi1tcS3tn0
The Fisher is at heart a really good design.