Backlash is present in moving mechanical designs. Think of backlash as the delay between you saying “Move!” and when it actually starts moving. This can be caused by gears within a motor, play between leadscrew drive systems, the stretch of a belt, and even any binding in a specific direction.
The LulzBot Mini 2 uses the G29 auto bed compensation feature to measure the print surface plane. Auto Bed Level compensation will slightly move your tool head axis up and down across the calculated print surface plane help get the perfect first layer.
Here’s the new built-in Z-axis backlash compensation process:
- Establish an electrical connection between the washer and the hot end nozzle.
- Send the move Z-axis up command (this happens automatically after each probe.)
- Measure the steps needed for the Z-axis stepper motor movements to break the electrical connection.
- This machine-specific backlash amount is added to your Z-axis movements, 100% at first, and is faded out after the first few layers.
Cura LulzBot Edition version 3.2.23 and newer uses this backlash compensation to put in very small extra movements by the motor to compensate for this lash. As this backlash gets worked out after a few layers, we use less and less backlash compensation as the print progress. This allows for accurate prints while ensuring solid adhesion on the first layer. We have seen excellent results with the LulzBot Mini 2 and with our 0.25mm tool head (still in development) where we need to use a 0.2mm initial layer height.
This issue occurs on all moving machines (not just 3d printers) since you have to allow room for movement. We are exploring applying this backlash compensation to the X- and Y-axis as well, follow the progress here: https://github.com/MarlinFirmware/Marlin/pull/11061. So far, individual users across many different types of machinery are reporting benefits from this marlin inclusion.
There can be slight variances with probes. For example, if you have a slight amount of plastic on your nozzle, it will push through it before making contact with the washer. This will cause the printer to read that corner as lower than what it actually is, and affect your base layer across the build volume. A clean wiping pad will help mitigate this. Update your wiping and probing temperatures if you are using a filament brand that is not included within Cura LE. This will help ensure longer wiper pad life, cleaner nozzles, and better probes.
We tighten our Z belts to a range of 35N (Newtons) - 45N. Directions for this process can be found in step 5 in our Open Hardware Assembly Instructions (OHAI): https://ohai.lulzbot.com/project/final-mechanical-assembly-mini2/mini-2/. During the final calibration, we run a test print (attached) to measure the skirt height to determine flatness and Z offset. When measuring the height of the skirt we expect the X+, X-, Y+, and Y- measurements to be between 0.3mm and 0.5mm. If the skirt measures a thickness outside of this range, we will diagnose and re-work the printer to ensure it meets our quality checks and tolerances.
Would mind running the rocktopus test print and share your results?
octopus_rev05_callibration.gcode (1.61 MB)
We’d also like to check the dimensional accuracy from prints. Would you be willing to print 5 calibration cubes (one in each corner and one in the center) and compare their overall dimensional accuracy?