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I am looking for a method to import my 3D models made in AutoCAD to Blender.
I have tried .stl and .fbx import but both of these triangulate the mesh and also I have to do a lot of clean up work to get rid of doubled geometry.
Is there a way to import those models in quad dominant form (the way blender handles the meshes)?
Below is the example when I import something in .FBX format
There is no quick one-button way to do that.
CAD models generally have either native NURBs or ACIS Solid based geometry, which are mathematical models describing surfaces, rather than polygon based meshes common in subdivision based modelling. Blender can neither read nor import any of those, and only has very basic support for builtin NURBS geometry.
File formats Blender can read are always mesh centric, like FBX, OBJ, STL or 3Ds, and most don't even support holding NURBS or solid data, like say IGES or STEP.
At export those models are always converted into meshes by the very CAD application they were imported from, before they even reach Blender. There is no way to guarantee arbitrary CAD shapes can conform to a good quad based topology, let alone convert them cleanly. Even when there are, CAD applications meshing algorithms are often concerned with fidelity and performance rather than what we perceive as "good topology".
MoI is an example of a CAD application with a relatively decent meshing algorithm, that outputs good quad dominant meshes, but even it fails to produce proper quad exclusive geometry on every occasion.
Good quad based topologies are purposefully planed from scratch along with your shape, and your model has to be built around them from the ground up to guarantee good results. Your mesh is your topology, not an afterthought or something you tack at the end.
There is no one-button-press solution to turn a triangulated topology into an elegant quad dominant edge-flow, good topology is an art, and there is no easy way to fix your models. It often required so much time and effort that modeling from scratch is often quicker than trying to salvage anything.
You can try Retopology which is largely a manual process.
You can look into the Remesh Modifier which is semi automatic method, but has limitations and drawbacks. It can speed things up at the expense of quality or manual control, no automated solution will beat well thought out manual topology in terms of quality.
If quad based topologies are not required and you just want simpler geometry you can also try to simplify with a Decimate modifier, using the Planar option.
My experience is that dealing with model cleanups always ends up being more work than doing it cleanly from scratch.
You end up spending a lot of time doing menial tasks, micromanaging, dealing with corner cases and unforeseen consequences, for average end results at best. The same time would probably be best spent, learning good modeling techniques using proper topology and actually refining your work, rather that fixing issues after the fact.
