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Problem with correct size.


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Posted · Problem with correct size.

Hi eweryone.

I have prusai3 printer and i use cura to slice and preper gcode. i have problem with correct calculation of size. it seems that slicer calculate sizes acording to center of extruder nozle. and since i have 0,4mm diameter nozle all of my prints are +0,2 mm wider form all sides. The worse thing is that hols that are printed are 0,4 mm smaller. how can i set cura to eliminate this?


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    Posted · Problem with correct size.

    I know you want to keep your location secret but often it's helpful simply to know what country you live in. I can't tell you to get "nappy's at the telecom store" if you live in USA. In this case though I don't care.

    Cura does *not* treat the center of the nozzle as the path but instead uses another number. That number is very simple as long as shell width is a multiple of your nozzle width (e.g. .4, .8, 1.2 etc). If you keep that correct it will draw the circle .2mm larger than the STL file shows.

    I know this is hard to believe.

    The problem with vertical holes being .4mm too small is very common! It isn't really Cura's fault. It is caused by 3 factors but the main contributor is that PLA acts like snot in the first few milliseconds as it is cooling - it is a sticky liquid. Kind of like a liquid rubber band. As it is laid down in a circle with air in the middle it is pulled inward because it shrinks so fast (cools so fast) yet is still a liquid.

    This problem also happens on exteriors with a small radius of curvature (like a zero radius of curvature for corners) although it is automatically coincidentally compensated by minor over extrusion when it slows down for corners.

    The best solution by far is to increase all vertical holes (not horizontal though!) by about .4mm.


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    Posted · Problem with correct size.

    To complement George's comments. In my experience printing what I will call "rings", i.e. an outer circle and an inner circle, it is easy with the outer circle to get a diameter that is at least accurate to 100 microns; indeed I would be disappointed if it were not better than 50 microns. But the inner circle is different! Often the error will be close to 400 microns but can be 500 even 600 microns. I have not done any specific testing on different PLA filaments (manufacturer/colour) but no doubt these come into play too.

    The only answer I think is trial and error. With a specific filament you will soon get clarity on the size of the error and you can then make an adjustment in your design dimension to achieve the dimension you require. On the current "ring" I am printing I am using 19.7mm to achieve a diameter on the inner circle of 19.2 and with different outer diameters I have achieved 19.15, 19.2 and 19.25, bearing in mind that measuring the inner diameter accurately is a lot harder than measuring the outer diameter. Certainly an "error", if indeed it is, of 0.05mm is good enough for my particular application.


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    • 3 years later...
    Posted · Problem with correct size.


    This postings are 3 yrs old now, but the problem seems to persist - I've got the same problem, and I found notifications about it in other forums.


    The workaround "tweak values in the STL" is not very charming when you use 3D prints for mould injection prototyping - at least for me it's hardly avoidable to make nasty mistakes with that.



    Is it really impossible to correct the behaviour of the filament within the slicer? - I think, it already does that in many places. And it should be possible to identify vertical "rings" and apply some correction. Maybe adjustable (I love CURA for its abundant controls!!), as filament qualities may vary a lot ?




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