Make a heated bed with 9 calibration points.
Most likely you have learned that the new Prusa i3 from Prusa Research is including a nice self-calibration feature based on the special heated bed they use. That bed features a grid of nine disks that can be detected by an inductive sensor that moves with the hotend carriage. These spots double as bed leveling and XY geometry calibration. While the bed is flat an covered by a special (PEI) film to improve adhesion, the spots can be easily replicated if using an aluminium bed as my video below shows. Nine inserts of 8mm steel rod can be used by doing nine holes of the same diameter on the aluminium bed. The interesting feature is that steel is detected further away than the surrounding aluminium so the same algorithm should work if you build your bed like this. In fact you can just place the aluminium sheet on top of your existing heated bed (though doing so may increase bed weight needlessly). According to the calibration source code these are the locations of each poi