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Dimesions problem


haldemir55
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Posted · Dimesions problem

Hi every one

I made a simple panel design with openscad. and I got stl file. When printing finish I saw the outer dimensions of the rectangular panel exactly true but cannot be said same for the shapes inside. All inside dimesions wrong. Wrong means smaller then the real value. Is there any problem abouth slicer software, cura or hardware?

 
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    Posted · Dimesions problem

    Please show a photo of what you mean.

     

    If you are talking about vertical holes (not horizontal holes) through your part then yes, it's normal and common for vertical holes to be about 0.5mm too small.  This has to do with the fact that the plastic, while liquid, shrinks in the first few milliseconds and acts like snot or mucus and is pulled inwards while printing inside corners and holes.

     

    I add around 0.4 or 0.5mm to most vertical holes (regardless if it's a 2mm hole or a 30mm hole).  I do this in CAD.  Sometimes I need very tight tolerances.  For those cases I add 0.3mm and then drill the whole out with a drill later (even if 20 holes it goes quick if they are all the same diameter).  For holes that can be a little loose I usually add 0.5mm.  For the rest I usually add 0.4mm to the diameter.

     

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    Posted · Dimesions problem

    I have had fairly good success by tweaking the "hole expansion" setting in Cura. I do use this in combination with the "horizontal expansion" settings as well as the outside of my parts were coming out oversized slightly. If redesigning the part isn't an option on something, this is a good place to start.

     

    I work in a design engineering office where the other designers are used to dimensioning for machining tolerances, and sometimes I do have to print parts that will later be machined (out of metals), so we cannot specifically dimension the models for the effects of 3D printing. This is where the settings mentioned earlier come in. I still have to drill out holes for press fit items, but they are very close to start, and often can be drilled with a pin vise instead of a full powered drill.

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    Posted · Dimesions problem

    I did a test on vertical holes from 3mm to 30mm diameter.  I found that below 10mm diameter that a Hole Horizontal Expansion of .2 worked well and then from 10 to 30mm diameter .1 worked well.  Above 30mm it wasn't required.

     

    Hole Horizontal Expansion has drawbacks - "If it's vertical then it's a hole and I don't care what shape it is" and "If there is a cross hole then the vertical feature isn't a hole anymore".

     

    Here is a piece of tubing with a chamfered end and a cross hole and HHE set to 2mm for demonstration.

    In the yellow area you can see that HHE doesn't work in the area of the cross hole as the vertical hole isn't a hole in that area.  HHE also doesn't work in the area of the chamfer for the same reason - the hole ends and so there is a ridge there.  If an outer wall is a loop (like around a U section), it doesn't describe a hole.

     

    Untitled.thumb.png.6c25fefedc2073f52cf53a7917b1645b.png

     

     

     

     

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