I’ve been through this, and you can usually get FreeCAD to work with your design, but it takes some work:

Completely remove any use of minkowski, hull, and offset. Despite what the docs might say, these don’t work, and will just break the importer.
Try to build as much in 2d as possible and then extrude, rather than 3d-first
Make your design modular so you can test each part in the importer first
Be patient. I’ve had the importer complete successfully after 30 minutes of running.

Alternatively, it is possible to generate a BREP (IGS and STEP are BREP formats) model from a mesh, but it’s not great. There are some decent tutorial on the net about this. Here is a thread about using STL in Fusion’s CAM, which first requires BREPifying it: http://forums.autodesk.com/t5/hsm-support-forum/fusion360-cam-problems-with-stl-and-3dm-files/td-p/6075605 http://forums.autodesk.com/t5/hsm-support-forum/fusion360-cam-problems-with-stl-and-3dm-files/td-p/6075605

Best of luck!

Yona:

Thanks!  Removing hull() is not an option.  Nor is starting the entire
design process from scratch.  We have too much invested in the current
model.  I will take a look at the AutoDesk link.  I can't be the only
person for whom this is an issue...

On 1/11/2017 3:20 PM, Yona Appletree wrote:

You’re not, and it’s one of the reasons some of us would love to see a BREP based OpenSCAD-like modeler… but it’s not going to happen any time soon I suspect.

For this model, you may well be best off creating a BREP from the mesh, either automatically or using Fusion 360 to do it by hand (it has some helpful tools).

In the future, I highly recommend not using the 'forbidden functions' if you can avoid it, if there’s any chance you’ll be exporting to other CAD.

It’s worth noting that the reason why minkowski and hull don’t work isn’t a limitation of FreeCAD or OpenSCAD, but rather that those operations, to the best of my knowledge, only really make sense with meshes and simply can’t be represented in BREP.

~ Yona

2017-01-11 18:38 GMT-02:00 Yona Appletree hypher@gmail.com:

What is the distinction you make between brep and meshes?

This MAY have worked.  It appears to have worked.  Maybe someone more
experienced than I can confirm.  I was able to create an IGES file which
then could be loaded back into FreeCAD

https://stevegt.quora.com/Converting-STL-to-BREP-using-FreeCAD

Jon

On 1/11/2017 3:38 PM, Yona Appletree wrote:

On 11. jan. 2017 22:38, Ronaldo Persiano wrote:

If you excuse my intervention, they are very different things.

Brep models are much more complex and based on other principles than
mesh based libraries such as CGAL (or Carve for that matter).
OpenCASCADE and ACIS are examples of Brep representations. I used both
in the past (that is, way before OpenCASCADE, it was called CAS.CADE
back then and was very expensive).

Typically, in brep you have a large number of independent classes
describing pure geometry (planes & other surfaces, straight lines,
b-spline curves, etc) and other classes describing topology (in ACIS you
have VERTEX, EDGE, COEDGE, LOOP, FACE, LUMP, BODY, and more). If you
check the file formats of these systems, for example the ACIS .SAT
format you will find this refleced explicitly. You cannot expect to
create such files from a polyhedron mesh.

One could create software like OpenSCAD using a Brep-based library
instead of CGAL (OpenCASCADE would be a candidate), but it isn't a small
undertaking.

Once you have gone the mesh route, you cannot expect a meaningful brep
representation in the form of a STEP, SAT or IGES file. It will not happen.

To go from OpenSCAD to brep you will at least have to communicate the
primitives (not the resulting mesh), transformations and boolean
operations. This is in principle what the OpenSCAD CSG format contains.

In the receiving end,re-implement the primitives & boolean operations
using brep in the first place, I think that is what FreeCAD does using
OpenCASCADE.

Carsten Arnholm

"You’re not, and it’s one of the reasons some of us would love to see a
BREP based OpenSCAD-like modeler… but it’s not going to happen any time
soon I suspect."

I happen to be working on one right now (loosely based on OpenSCAD2), so it
may happen sooner than you expect. I might have something to demo in the
summer?

My system exports the CSG tree in JSON format. You can define your own
shape primitives (like defining OpenSCAD modules) and specify how they get
exported as JSON. To export to FreeCAD or IGS, you would figure out which
primitives you want to appear in the output, define any primitives you need
that aren't already in the standard library, then define your model using
those primitives. Then a separate program is needed to convert the JSON
into the format of your choice. The same trick will work in reverse for
importing FreeCAD, IGS and other high level geometry formats.

My system is not specifically a BREP system, but it's capable of
representing all of the BREP primitives and operations, and exporting them
in a CSG tree, so it still gets the job done. I suspect somebody could also
hack this capability into OpenSCAD.

On 11 January 2017 at 15:38, Yona Appletree hypher@gmail.com wrote: