When exporting a solid cylinder from Rhino to DWG (Solid 2007) it arrives there as two circular caps and TWO HALF-CYLINDERS. The same occurs when exporting solids with circular holes.
Why is that? It makes it utterly difficult to export fabrication models and have a CAM-software identify drilling features. (Not that feature recognition would be a good way of creating cam-data, but it’s the state of the art, unfortunately…)
In the image below,
the red cylinder was created as an extrusion in Rhino6
the green cylinder was created as a brep in Rhino6
the black cylinder was created as cylinder in ACAD2020
Both Rhino-cylinders were exported to DWG Solid 2007. The black cylinder was added in ACAD2020 (where the two others where already solids with two half-surfaces…). Then the DWG was re-imported into Rhino 6.
Believe it or not, this is a “feature”… back in the day, there were MCAD programs that did not like closed surfaces with seams, and preferred the two half cylinder approach. I say “back in the day” because my knowledge on that is perhaps 15 years old, MCAD programs probably have evolved significantly since then.
There are options in both STEP and IGES exports to not split cylinders and the like:
However this option is not exposed in DWG export options. @lowell may know if it’s possible.
(DWG V 2007 is also 13 years old)