NEW VERSION 2!!! The original spirals had problem with some slicer, so I redone them with more accurate geometry. Spirals useful for leveling and calibrate delta beds (or other) from 40mm to 400mm. The spirals are actually a small square of 0.25mm side that spins in circles.Set the layer height in your slicer from 0.2mm to 0.5mm for best results. Lower height than 0.2mm will result in 2 layers or more, greater than 0.5mm will result in no layers detected. You can combine the spirals on your bed if you need more time (or fun). The spirals will match together in 20mm diameter increment. You can also combine the 40mm with a bigger one, validating the flatness of your plate.