Cura fractional line width and height

  • Does anyone have a reference source that explains how Cura deals with 'fractional' line widths and layer heights?
    To give an example: I have a wall that I am printing that is 1.6 mm wide. With the average 0.4 mm nozzle and 0.4 mm line width that would be 4 passes or four lines side by side. Suppose I now changed to a 0.6 mm nozzle with a 0.6 mm line width - do I get a 1.2 mm wall (2 side by side lines) or do I get a 1.8 mm wall (3 lines side by side) or do I get a 1.6 mm line (two full lines on the sides of the wall and a partial line down the middle? What do I get if I want a 0.9 mm wall and Cura can't do a partial middle fill layer ?
    With layer heights, the apparent action seems to be to never print partial layers but I am not sure. As an example, if I have a layer height of 0.2 mm and I want to print a height of 1.1 mm, it would appear that Cura will always stop at 1.0 mm and not attempt to print a layer that is only 0.1 mm tall to reach the total feature height of 1.1 mm.
    Are there any hard rules? Are there any settings in Cura to bend these rules ?


  • Moderator

    In short, it doesn't do very well. In Slic3r it will condense the inner walls to a thicker gap fill, but cura doesn't have any such logic. It's an even number of walls or nothing, or worse, it will try to stuff 4 walls into the space for 3. It's really frustrating. The cura solution is to adjust the line width for the entire print so that any thin wall areas are an even multiple. Or it will try and cram solid zig zag fill into the tiny gap and shake the hell out of your printer and probably jam the nozzle in the process. (But I'm not bitter.)

    The best improvements I've found are in the master branch, but it's still not great. At least it will print thin wall areas whereas the stock cura build will often just not print the offending section. I basically have it set to not fill any gaps because of the excessive amounts of over extrusion that will cause. The expectation is that you will use outer before inner walls and then the over extrusion will just get pushed to the inside...

    For layer heights I don't think any slicer will automatically print the small fraction left over from an uneven model height divided by the layer height. If you need a model to be exactly the right height you best choose a layer height that divides evenly.

  • Thanks for the bad news Phaedrux .... and I am glad you are not bitter 🙂
    I am running the Master branch so I am good for that but it still sounds like something just waiting to jump out and bite you in the rear.

    I have a print that is just over 31 hours with a 0.4 nozzle and I was thinking of trying a 0.8 nozzle but some of the dimensions are quite critical as it is a modular arrangement and modules need to interlock. I need a total of 12 of these prints and the waiting is getting a bit old.
    I did a tweak on speeds to bring it down to 22 hours ... will see how the test module fits with the older modules.

  • Moderator

    @jens55 Definitely check the gcode preview. Not that it will tell you everything you need to know in Cura like it might in prusa slicer but you might be able to see if it's overlapping walls in the actual layer Simulation.

  • Good idea .... didn't think that would help any but one never knows!

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