When metagrobologist David Singmaster gave his talk on Impossible Grilles at MathsJam 2015, it inspired Simon to print some of these “impossible objects” using the THREEDY 3D Printer he happened to have to hand. 
Whilst travelling around medieval cities, particularly during Italian holidays, David observed the heavy grills over windows of buildings. Many years ago, he noticed that the bars at the centre formed a pattern which seemed to be impossible to assemble. This “impossible” grill pattern is also depicted in M.C. Escher’s Belvedere print.
David later saw that this central area could be assembled by a simultaneous converging process but the pattern continues and this made his idea fail. David showed his idea to James Dalgety, who proceeded to make an example from heavy wire; David then saw how to assemble the whole pattern easily.
The grill rods are printed individually, then slotted together – simple as that!
A belated thank you to David Singmaster for his inspiring talk.