Exporting a STEP file with named surfaces from PTC Creo

When preparing geometry for numerical simulations, it is often required to mark individual surfaces in the model. These surfaces can then be used e.g. as an inlet or forces and pressures can be applied in a structural simulation.

The STEP format supports named entities. The question is: how to set the names in the CAD program? And how to achieve that they are actually stored in the STEP file? In the following, these questions are answered for the software PTC Creo.

Assign Names to Surfaces

Select “File” > “Prepare” and then “Open Model Properties”, then select “Names” in the model properties dialog:

You can then select faces by clicking on them and enter a name in the dialog box:

Exporting Names in STEP File

If export a STP file with the default settings, the names will not be stored in the file. You need to change the export setup for them to be kept.

Open the settings dialog through “File” > “Options”. Then go to “Configuration Editor”. Here, you need to add the option “intf_out_assign_names” and set it to “user_name”.

Accessing the Named Entity in ISCAD

It is now possible to access the faces through the assigned names, e.g. in ISCAD. Once the STEP file is imported, its sub-entities can be explored by typing Ctrl-I (see below). The named faces appear as “face_<NAME>” int the hierarchy:

Importing IGES File (*.IGS) from FreeShip to OpenCASCADE

FreeShip software is a convenient tool for designing hulls.
The capabilities of FreeShip are essentially limited to the design of the fuselage exterior. For everything else, a real CAD system is required. There is an IGES export function for the transfer.
There is also a successor: DelftShip. The IGES export function has been removed from the free version of DelftShip and is only available in the commercial version. This is not available to me, so I cannot test it.
I would like to use the geometry in our tool ISCAD. It is based on OpenCASCADE.
The hull geometry modeled in FreeShip 2.6 looks like this:
The export to an IGES file is unspectacular and 40 individual faces are created.
The next step is the import into OpenCASCADE. But there is a problem here. OpenCASCADE (v7.2.0) reports:

Report : 40 unknown entities.

Total number of loaded entities 41.

Nothing is displayed. Although it is reported that an import works with earlier versions (https://forum.freecadweb.org/viewtopic.php?t=1670), the import does not work with the current OCC version, nor with different older versions of Salome (and OpenCASCADE). On the other hand, the import works e.g. in the commercial CAD software Creo.

Finally, a study shows that the file exported by FreeShip contains only entities of type 128 (spline surface). In addition, there is a single color definition at the beginning. At the end of the parameter block of a 128 surface (see e.g. https://wiki.eclipse.org/IGES_file_Specification#Rational_B-Spline_Surface_.28Type_128.29) are the start and end parameters (min/max U and V values) of the surface. These entries are omitted by FreeShip and for OpenCASCADE this is an error.

One workaround is to patch the IGES import of OpenCASCADE. The corresponding code is located from line 188 in the file IGESGeom/IGESGeom_ToolBSplineSurface.cxx. I removed the error message and inserted default parameter limits:

  if (!PR.ReadReal(PR.Current(), aUmin) || !PR.ReadReal(PR.Current(), aVmin)){
    //Message_Msg Msg106("XSTEP_106");
    //PR.SendFail(Msg106);
    aUmin=0.0;
    aVmin=0.0;
  }

  if (!PR.ReadReal(PR.Current(), aUmax) || !PR.ReadReal(PR.Current(), aVmax)){
    //Message_Msg Msg107("XSTEP_107");
    //PR.SendFail(Msg107);
    aUmax=1.0;
    aVmax=1.0;
  }

With these modifications, the import works: