Bed Leveling

The AON-M2 uses active bed leveling while printing to compensate for any irregularities that may occur in the bed surface. This is especially critical when repeatedly cycling the plates from room temperature up to 200ºC and back again which can cause slight warping and deformation of the plate surface. The bed-probing process should be run every time your printer may undergo changes in dimensionality. This includes but isn’t limited to:

  • print-plate changes
  • nozzle changes
  • significant temperature changes (> 10ºC)
  • machine relocation

AON3D recommends probing the bed before every print, however if you are running multiple prints consecutively without significant changes to the bed or tools, you may find it unnecessary to probe again.

If using both toolheads, you will need to probe the bed using both. This prevents any error that may exist on one tool from influencing the other, as well as ensuring that if one tool is out of service, the other can function normally.

The AON-M2 compensates for bed irregularities by using a multi-point topographic mesh and tool probe offsets to compute a value that is then applied to the machine’s Z position ensuring that the tool is always the correct distance away from the bed surface. The process is as follows.

Probing a Mesh

Each tool has its own mesh and probe offset and should be treated as an independent calibration. By default, meshes are set to 20mm at all points, and the probe offset is 3mm. This results in the bed having a zero point 23mm below the Z end stops.

These default values are only present until overwritten! Once you have probed the bed, the mesh will be saved to the last recorded values.

Preheat Time

Preheat your bed and chamber to the desired printing temperatures. Use the Standard Preheat Times to wait for the machine to reach a steady state. Note that it is necessary to wait for the full recommended preheat time, rather than waiting only until the bed or chamber has reached its target temperature on-screen. This will allow for the plate to fully expand and reach thermal equilibrium before probing a mesh.

Filament Retraction

Before probing, make sure the filament has been removed from the meltzone of the hot end. The meltzone refers to the area between the nozzle and heat break of the heater block assembly where the filament is actually melted. The filament can be removed from the meltzone by heating to the material’s operating temperature, extruding until material begins to come out of the nozzle, then retracting by 20 mm. Failure to retract the filament will make it much more difficult for the hot end to trigger the bed probe mechanism. This will introduce errors into the calibration results.

Nozzle Temperature

Probing requires lowering hot end temperature so as not to damage the plate. Lower the hot end temperature to 200ºC. To use a higher probing temperature, check the G29 parameters.


Home the machine by tapping the HOME XYZ button in the control tab of the touchscreen interface.

System UI Home Procedure

Enter the command G29 in the terminal tab with the desired tool active. This will initiate the probing sequence. See G29 G-code command parameters to use different grid dimensions, location, and/or number of grid points. To see what a portion of the typical probing sequence looks like please refer to the short video below.

Probing Sequence GIF

Rough Offset Calibration

Once probing is completed, the tool will move to the front centre of the bed. You will see that the nozzle will move very close or possibly touch the bed. To correct this and “touch off”, use the command M290 Znnn to modify the probe offset so that the nozzle is just barely touching the bed.

For example, the command M290 Z0.1 will move the bed 0.1mm away from the nozzle, while M290 Z-0.2 will move the bed 0.2mm closer to the nozzle.

If using a computer, in the terminal page of the user interface, you can use the up and down arrows on your keyboard to quickly cycle through past commands.

Once the initial calibration is complete, you may switch tools, and repeat the process for the opposite tool if required. It is important to note that this is only a rough calibration, and small adjustments will likely be necessary when printing your first layer. It is for this reason that we recommend always printing parts with a brim, skirt, or raft that may be treated as sacrificial.

Once all probing has been completed, either run the command G28 X or use the HOME XY button in the system user interface to move both tools to their parked positions.

Raise the temperature back up to the extrusion temperature required for the material being printed.

Re-extrude the 20mm of filament you retracted earlier.

Fine-tune Offset Calibration

After you have probed the bed and done a rough calibration using the M290 command, the distance from the nozzle to the bed will be nearly calibrated, but not as precise as required for printing perfect first layers. This must be fine-tuned by printing a first layer and making further adjustments to get the initial spacing correct.

The offset value that is modified in this process is stored in the machine memory, so once the calibration has been completed, only small adjustments should have to be made for future prints.

We provide two calibration G-code files, as well as their corresponding factory files for you to use. Each file will simply print a single-layer rectangle shape, giving you time to make small adjustments using M290. You will need to run this process for both toolheads to ensure the offsets are correct on both tools.

File Description Usage
t0_z-calibration.gcode Left Tool Calibration Use with ABS for T0
t1_z-calibration.gcode Right Tool Calibration Use with ABS for T1
z-calibration.factory Base Factory File Modify to suit your needs and material choice

A typical process would be as follows:

  1. Load filament for the tools (ABS filament is required to use the G-code files provided).
  2. Clean and secure the print bed.
  3. Preheat both the chamber, and the bed to the desired temperature for printing your parts.
  4. Run the bed probing procedure once temperatures are stable.
  5. Coarsely adjust the probe offset.
  6. Start the t0_z-calibration.gcode print job.
  7. While the first layer is printing, fine-tune the offset using small increments such as M290 Z-0.05.
  8. Once you are satisfied with the print distance, you may stop the print and repeat the process for T1 using its respective calibration file.

If using a computer, in the terminal page of the user interface, you can use the up and down arrows on your keyboard to quickly cycle through past commands.

In order to get a feel for what would be considered a good offset for producing a good print, please refer to the following set of images, which show prints with offsets that are too small (bed too close), too big (bed too far), and just right.

  • A print with the probe offset being too small, ie. the bed is too close to the nozzle:

Print Outcome with Head too Close to Print Surface

  • A print with the probe offset being too big, ie. the bed is too far from the nozzle:

Print Outcome with Head too Far from Print Surface

  • A print with a perfectly adjusted probe offset:

Print Outcome with Head Properly Adjusted to Print Surface

Printing Your First Layer

While the initial calibration will bring the nozzle-bed distance close to its ideal, you will still need to manually fine tune the offsets on each print while extruding a sacrificial skirt or brim for the first layer. Once calibrated, this should only require very small changes (e.g. M290 Z0.05) to account for small changes in machine dimensions that accompany changes in machine temperature due to thermal expansion.

AON3D recommends probing the bed before every print, especially if you have changed the printing temperatures or made physical changes to the machine, including changing nozzles, repairing toolheads, or swapping build surfaces. You may have to fine-tune the offset again when printing at different temperatures and after making changes to the machine.

Last modified: September 13, 2019