Why does my DIY printer start printing in air? I've built a DIY 3D printer similar to Prusa i3. It does homing and bed leveling very well with a PNP Lj12a3_4_Z_BY sensor using G28 and G29.



However, when I push the print button on Pronterface or Simplify3D, Z axis goes up about 20mm and starts printing in air. I can't figure out what is going wrong.
If the cad model was imported from Solidworks into Simplify3D. Push "Center and Arrange" button to place the model in the middle of the Heat Bed. There might be a gap between the object and bed level that is hard to see.