Fusion
While it does not apply to this particular situation, a low res tri mesh can be directly converted into a BRep.
Problem with the two proposes software solutions is that they are only available in form of subscriptions at rather rediculous prices.
As such I’d experiment with Instant Meshes, Meshmixer to repair meshes and Meshlab. Those are all available for free.
Today, there is not a single, great approach to the workflow you are trying to achieve. If the area of the skull mesh you are trying to match is localized, or you are very patient, you can use "Object Snap" in Sculpt mode to create a Sculpt body that snaps to the mesh, and that can be converted into a BRep body, which STEP supports. Here is a short sample screencast:
A bit more speculatively, we are working on some technology for converting triangle meshes to quad meshes, which can then be converted to Sculpt (TSpline) bodies. If you want me to try this technology on an example mesh, I'd be happy to try it out, and send you back a BRep body. That's not a general solution, I realize, but if this particular case is urgent, I'd be happy to help you out. We'd just have to figure out a way for you to share the STL file with me (confidentially, of course).
Jeff
Jeff Strater
Engineering Director
Hi @bapca,
I'm very interested in the problem you have and would like to get a bit more insight into your workflow. You mentioned that you are working with someone who requires "solid .STP files for machining another part of the device". I'd be curious to understand the specific workflow here - do they need to machine the actual skull geometry? Does this device need to use surfaces from the skull geometry as part of the device geometry? How much of the skull geometry do they need to use, is it the whole thing or perhaps just a small region of it?
Thanks
Sugi
Autodesk
Fusion 360 Development Team
@bapca do you understand the essential difference between .stl and .stp files and the data contained in them and why the machining engineer needs a .stp file ?
OK, I did not think so. while the resources for that are easy to find online, they are not necessarily presented in a cohesive fashion.
A mesh, no matter whether it is comprised of triangles, quads, or N-gons has a finite resolution. The specialty with Quads is that they can be "cleanly" subdivided into ever smaller quads, but even then a mesh has a finite resolution.
.stl files are one the earliest form of least common denominator 3D data and were primarily developed as a means of one-way data exchange between a computer and a 3D printer. A high resolution mesh consisting purely of triangles also does not have any topology (in the mathematical sense) , which makes it very difficult to recreate the original surface topology from it.
The .stp or STEP format, which can also contain meshes, more commonly contains BRep information, so solid geometry and NURBS surfaces. These are mathematically precise surface definitions that are free of any resolution, down to the precision of the computer they are created on.
This brings us to the purpose of CAD software, which is for the most part precision, accuracy. Geometry in a CAD software is created by the geometric modeling kernel. That is also the same part of the software that will create the CAM tool path based on that mathematically precise, resolution free surface representation.
That's the reason your CAD/CAM engineer wants a .stp file.
Most CAM software packages can also machine .stl files but it's usually not used for applications requiring precision and it is also not simple creating a smooth tool path from a segmented surface.
Then there are quad meshes as used in many Sub-D modeling packages. Sub-D means subdivision surface.
The subdivision control cage is is (or at least should be) a quad mesh that is used to control the shape a subdivided surface. The vertices of that mesh are basically control points.
In concept that is very similar to how NURBS surfaces are created. The shape of a NURBS surface are created by using a quad mesh of control points. Quad meshes as well as NURBS describe a topology!
Initially computer graphics and animation software such as Maya actually used NURBS surfaces to create models. However NURBS surfaces are relatively "stiff" and were difficult to deform for the purpose of animation and also computatiopally heavy , so Sub-D meshes were developed to deal with these problems.
This similarity brings us to T-Splines, PolyNurbs and other technologies that allow us to convert a quad mesh into a NURBS surface. Thus with high resolution .stl files the recommendation is often to re-mesh or re-topologize them with other software into a quad mesh , which in most cases will allow output as a .obj file and that qualmish can then be converted by any of these technologies.
But there is no direct conversion form a .stl into usable .stp for your purpose.
Is there any way that you can share some sample mesh files with @furutas and I? We would, of course, keep them under strict control, and only use them for internal use. We could demonstrate the mesh-to-brep conversion I mentioned above, and I can also provide a better explanation of the "Object Snap" workflow. We can take this communication out of the public forum if that works better for any privacy intellectual property concerns you might have.
Thanks,
Jeff
Jeff Strater
Engineering Director
Really ? You have a quad mesh and then convert that to a BRep ?
Did you read any of what I wrote above ?
If you truly have a quad mesh then you can convert it first into a T-Spline.
Then you can convert it into a BRep. You'l immediately see the difference as the BRep surface created from the T-SpLine is smooth as opposed to the faceted surface created by converting a mesh directly into a BRep.
