Trying to better understand mesh compensation



  • I'm trying to wrap my head around what, exactly, mesh bed compensation does in regards to actual prints. I realize that the functionality builds a grid-based "mesh" mapping of the build plate, but I'm not sure how the "compensation" is handled in regards to the actual printing.

    For example, assume I have a very wavy build plate. If I now print a perfect 100mm cube with mesh compensation enabled, will the resulting print:

    1. have a wavy bottom and top, but has a perfect 100mm height at any given point...

    or

    1. have a top that's perfectly flat, but a bottom that's wavy (to match the build plate) and the part has varying heights.

    or

    1. Something else?

    Thanks
    Gary



  • By default the compensation tapers off over 10mm of Z height. At least I think that's the default.

    This is user configurable. So if you wanted it could behave like #1, or on prints taller than your taper height it could behave like #2.

    https://duet3d.dozuki.com/Wiki/GCode#Section_M376_Set_bed_compensation_taper

    M376: Set bed compensation taper ¶
    Parameters

    Hnnn Height (mm) over which to taper off the bed compensation
    Example

    M376 H10
    This command specifies that bed compensation should be tapered linearly over the specified height, so that full bed compensation is applied at Z=0 and no bed compensation is applied when Z is at or above that height. If H is zero or negative then no tapering is applied, so compensation is performed throughout the entire print.

    RepRapFirmware does not adjust the extrusion factor to account for the layer height varying when tapered bed compensation is used. Therefore it is recommended that the taper height be set to at least 20x the maximum error in the height map, so that the maximum amount of the resulting over- or under- extrusion is limited to 5%.



  • ah! Thank you. So, given the example of a 100mm cube, if I set M376 Hn where n is <100, then I'll get a perfectly flat top, and if n >100 then I'll get a wavy top.

    I wonder, then, how the tapering works. If I have M376 H90, and y "units" of compensation are needed, doesn't it apply 1/90th unit of compensation per layer for 90 layers? I guess it would have to....

    Add in all the other magic that the duet performs (and the complications of a delta printer) and it really makes for an impressive package!


 

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