Step calibration



  • I have the theorical number of steps/mm in the firmware, with the pulley teeth count and belt pitch and motor step.
    In practice, the calibration cube is not 20.0x20.0x20.0mm, but 19.96x19.91x19.98 for example.
    X and Y are drive by pulley, and Z by lead screw T8 with backslash.

    The tension belt have an effect of dimensions ? they are streched, but I don't know if it's more or less than it could be.



  • If the belts are not parallel to the guide rails in all three planes it will lead to dimensional errors that will vary depending on where the print is located on the bed. This is a common error (the top sketch):

    alt text

    If the attachment of the belt to the bed (assuming a bed flinger) looks like that, the physical steps per mm will vary with position of the Y axis, as will the tension of the belt.

    Print one cube near the center and one near the edge of the bed and compare measurements. If the belts are parallel to the guide rails, the dimensions will match.



  • @pipersw said in Step calibration:

    In practice, the calibration cube is not 20.0x20.0x20.0mm, but 19.96x19.91x19.98

    That doesn't sound that bad tolerance. You may want to watch this https://www.youtube.com/watch?v=kh7y_CARmmY

    Before changing step constants, I would first establish that this is a scale error and not an offset error. E.g. what happens if you print something twice as long, does the error doubles?



  • @pipersw said in Step calibration:

    the calibration cube is not 20.0x20.0x20.0mm, but 19.96x19.91x19.98 for example.

    That could very well be down to extrusion factor. Have you calibrated the e steps yet, or the slicer extrusion factor?



  • Yes the extruder is calibrated. I have slightly change the estep and it's ok now



  • @phaedrux After check the distance on guide rails with the caliper, I return to firmware value for steps for X and Y axes.
    The step of extruder have been calibrated, but not the slicer extrusion factor.
    I have difficulties to have the same lenght move each print, so I think the belt length is sligtly change (overtighten ?), or the acceleration are too high.

    I have take the default value from the RRF configurator for Anet A8.



  • @pipersw said in Step calibration:

    I have difficulties to have the same lenght move each print...

    How much differences you see between measurements and for what expected length?



  • 0.2mm for 20mm cube on x and y



  • @phaedrux said in Step calibration:

    @pipersw said in Step calibration:

    the calibration cube is not 20.0x20.0x20.0mm, but 19.96x19.91x19.98 for example.

    That could very well be down to extrusion factor. Have you calibrated the e steps yet, or the slicer extrusion factor?

    hat is a very good tolerance i would say...

    Regular filament can vary 0.1mm in diameter in total(plus minus 0.05mm) so that itself can easily add a few percent in difference...

    Personally i aim for plus minus 0.02mm on printed parts that has to be good... that ofc makes it easy to design stuff since i dont have to add or substract 0.2mm from a model af many ppl need to...

    Pulleys can easily make it go bad... ive measured my 20 tooth pulleys in diameter.. some are as low as 11.85mm while other are 11.97mm... thats quite a lot of difference for to identical pulleys...

    Also i have found them to be very oval shaped many of them, so that could be quite wobbly.. but if that will affect distance, i dont know..



  • Bit late to the party here but here are a couple other factors that can affect the printed part sizing.

    1. Backlash in axis. Backlash due to loose belts, excessively compliant mechanics, etc can present as oval holes if one axis suffers from significant backlash and the other does not. This is pretty easy to check. If you have a dial indicator you can use that to take two measurements at the same axis position, approaching the measurement point from opposite directions for each reading. If you don't have a dial you can print a large and small part to see if the error is an absolute value or scales with part size. Absolute error would indicate something mechanical like backlash rather than a material setting / steps/mm issue.

    Another easy backlash check is to look at the first layer of the part. IF the diagonal fill lines appear to be in sets of two (two lines close together, a small gap, then another two lines close together) this indicates excessive backlash in an axis.

    1. Material shrink. Plastics will always shrink a little bit while cooling. For a PLA I print with a lot I see shrink rates around 0.5%, around 0.1mm over 20mm (for my printer and material). Extrusion factor can't correct for this error and it's also not really a positioning error as my axes move very accurately. It's possible to scale x/y steps but I prefer to keep the positioning steps correct and scale up my part in the slicer in XY by my shrink rate. Z tends to be pretty stable and not need scaling for shrink in my experience.

 

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