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Various methods to scan the top surface exist to improve the bed adhesion to prevent prints to come loose from the bed during printing: e.g. BLTouch and clones or inductive and capacitive sensors/probes.

In order, for the print head, to use this so-called auto-bed leveling (ABL) you need to add the G29 command in your start G-code of your slicer.

Is it necessary to call G29 before every print?

The rationale behind this is that scanning the surface takes up some time, certainly on very short prints, it would be great if the surface geometry could be saved.

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No, it is not necessary to call G29 before every print to "auto level the bed" 1) provided that:

  • the bed surface has not changed (e.g. large load or force has been exerted on the build platform, leveling screws are accidentally adjusted, a substantial different bed temperature is used causing different thermal stresses, etc.),
  • the carriage of the hotend is stable (some printers, e.g. the cantilever type, or single side Z lead screw driven printers are more prone to an unstable or level axis), and
  • the scanned surface geometry is saved in the controller board memory.

There are several solutions to solve this. You could manually run the G29 command once in a while storing the scanned surface with an M500 command to save the mesh to the EEPROM (memory) of the controller board (this can be done from the printer controller display for Marlin operated printers, an interface like a terminal or a print server application, or from pre-stored .g/G-code files on an SD card). If you use the SD-card, note that it is possible to auto-launch G-code files from the root of the SD-card upon inserting.

Do note to remove the G29 command in the start code of the slicer. The G29 command needs to be replaced with M420 S1 for Marlin firmware operated printers. This command will load the saved mesh at the start of the print from memory. This is especially useful when using a large amount of probing points (e.g. a large bed mesh using a 10 x 10 mesh of 100 probing points, to ensure the mesh is up-to-date, once in a while initiate the scanning sequence to store an updated mesh).

1) Please note that auto-bed leveling might be confusingly indicating that some magic leveling of the build platform/surface itself is taking place (this is also possible in Marlin when there are multiple Z steppers and lead screws used), but, that is not actually what is meant with this phrasing. The process of the auto-bed leveling actually scans the surface of the build surface and compensates the height of the print head/nozzle during a predefined printing height (usually 10 mm, set in the firmware or through G-code: M420 Z10 ; Gradually reduce compensation until Z=10), during this printing process the nozzle gradually be less and less compensated until there is no compensation and the print nozzle will print parallel to the guide axis (e.g. the X-axis in i3 style printers and X-Y axes in CoreXY kinematics printers.

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