strange problem with export

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  • Hi, netgen experts,

    I am running into a strange problem during export from netgen.  Say, I have a cube created in CAD program.  The dimensions are 0.005mX0.005mX0.005m.  I exported the file to step format.  I imported it into netgen, meshed it, then, export it into stl format.  In the stl file, I got 5 m X 5m X 5m.  But, if I exported the part in CAD program to stl format, imported the stl into netgen, meshed it, then exported it to stl format.  I will get the new stl file correct, ie, 0.005m X 0.005m X 0.005m.

    Have anyone encountered this?  Or can anyone confirm if you can also reproduce this?  Maybe I am not doing this correctly?!



  • Hello again Pei,

    This seems to be a problem with the way the Units are set up in your CAD STEP Export module. Netgen is units-agnostic, and does no kind of manipulation to the imported data.

    The STEP import function in Netgen also does not use the Units normally present within a STEP file.

    However, it looks like the CAD system you use converts the model from "m" to "mm" during the export. Hence, what was "0.005 m" before the export, would become "5 mm" after the export into STEP.

    When Netgen imports the file, it sees only "5", which is exported as it is when you write the STL file from Netgen.

    There must be an option in your CAD package to control this feature.

    Have a nice day!


  • Hi, Philippose,

    Thanks for the quick reply!

    I suspect that the problem is in OpenCascade.  I did the following tests:

    1. Created the part in Solidworks.  Exported it to step format.
    2. Import the step format part into SpaceClaim (another CAD software).
    3. The dimension is correct.
    4. Imported the step format part into gmsh (also uses OpenCascade I believe), mesh it - dimension was 1000 times higher, just like in netgen.

    I created a new part in SolidWorks using meters, ie, 1m X 2m X 3 m.
    When imported into netgen, I got 1000 X 2000 X 3000.

    I am planning to head over to OpenCascade forum to see if anyone experienced this.


  • Hello again Pei,

    Good Evening to you!

    Well, the problem is technically speaking, not a problem with OpenCascade.

    If I remember right, STEP files have standard "raw data" units which I think is fixed to be "mm"…. and then, in the STEP file, there is an additional entry which specifies what the original units of the geometry was. So, during the export operation, the STEP export module of the CAD software automatically converts from whatever units you used to create the geometry, to "mm", and then also exports the name of the original units.

    When you import this STEP file into Netgen using the OpenCascade module, the OpenCascade mSTEP module reads in the raw data, but does no automatic conversion of the data back to the original units.

    I remember having looked around in the OCC STEP documentation to see if I can somehow get it to either automatically convert the data or to let me know what the original units were, but I was not able to find anything conclusive, and hence I left the matter.

    Maybe I should look into this again sometime :-)!

    Have a nice day!


  • Hi, Philippose,


    for the time being, I wrote a quick perl script to scale points by 1000.  I think that when I tried gmsh, I encountered the same issue.



  • Anonymous

    Helllo i want to know that how can i import the solid work model in NETGEN.

    which format is supported .please help me

  • You should start a new thread otherwise people won't see your message.

    But use STEP. There's also IGES but it's an older standard and has some problems. In both cases you need Netgen with OCC (Open Cascade).

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