Export of Onshape assembly not using .step (but with the features thereof)

erik_knudsen

Hi all,

I'm having a bit of trouble with the .step-export feature. The generated files sometimes contain extraneous features that cause problem downstream in our toolchain.

Examples:

- a sphere being exported as a (degenerate) toroid.

- a spline cutting a conical object leaving the entire cone in the generated file.

- a sphere that cuts part of a complicated object leaves the sphere in.

As of yet I have not been able to pinpoint a smoking gun which triggers this.

What I really need for my use-case, is a way to extract a boundary representation of an onshape assembly where I can keep the part names intact. In theory this ought to be solved by the step-exporter but in practice for my purposes this feature is not reliable enough.

Any ideas anyone? I've been through most  (if not all) file export formats provided by onshape, but none seem to provide the same functionality.

cheers
Erik

NeilCooke

Hi Erik - what are you importing the STEP file into? What does it look like when imported back into Onshape?

S1mon

You say you have been through most of the export formats. Have you tried Parasolid? That’s the native kernel of Onshape so it’s generally the most robust and accurate.

erik_knudsen

Hi Neil,

I've imported our files into several different pieces of software - among them gmsh (via its OCCT-layer), cadquery (through a different import layer also based on OpenCASCADE), CUBIT, and FreeCAD. To a varying degree the features show up. The first example, which was a very simple model case - showed the oddity in all of them.

That simple model case was simple enough that I could interpret the step-file manually, and it is simply not the correct geometry that's being exported.

To answer your second question, I can't see (with the naked eye) anything amiss when re-importing into OnShape, which makes it even more strange IMHO.

I have generated a support ticket on the issue so I assume that someone is working on fixing the step-exporter. The reason for this discussion was I hoped to find some kind of work-around in the meantime.

erik_knudsen

Hi S1mon,

I have looked at parasolid, but not pursued the route further due to it being a proprietary format. This unfortunately means

means none of the other tools downstream (which are all open source efforts) can handle it. It could work as a stepping stone though. I should probably take another look at it.

NeilCooke

Hi Erik, the acid-test of export quality is how it looks when importing back in. This means that your other tools are not able to interpret the STEP format properly and therefore there is nothing Onshape support can do to help you. Have you tried STEP AP203? Have you tried IGES? These are much older formats which might work with your other tools. 

erik_knudsen

Hi Neil,

Thanks for the suggestions - I have so far tried all of the available versions of STEP with the same results. IGES worked geometry-wise but has the unfortunate side-effect that the naming scheme gets lost along the way.

I do beg to differ wrt. to acid test. Among the reasons for thinking so is: as I mentioned, in the simple case I can read the created .step-file manually and it simply is not the correct geometry.

To elaborate a bit more on this: Here are some screenshots from step imports into gmsh (image2.png) and FreeCAD (image5.png) as well as a screenshot from a text editor (image6.png).
The geometry here is a sphere (radius 5cm ) with a spherical void (radius=2.5 cm) inside.

1) For gmsh we can see that the so imported geometry somehow produces an extra curve with a slight offset.

2) for FreeCAD we find the same thing as indicated by the yellow curves in the screenshot.

3) Looking directly into the step file: I've highlighted the problematic entity (#13) that seems to cause the effect in this case.  As you can see, item #23 is in fact a sphere, whereas the inner void gets transformed into a degenerate toroid.

The upshot of these things is that the inner void only cuts out a ~.7mm thick shell of the sphere - not it's entire volume.

This is however not the discussion I was looking for - this is already handled (and acknowledged) by the support team.
What I had hoped for was suggestions for working around the issue through some other means than direct .step export. Maybe I was not quite clear about that in the original post.