Just look, a simple model that I created quickly (easily) and a .dwg file that was exported. Why is Out so different from the original? 
Original.3dm (106.7 KB)
Out.dwg (47.6 KB)
Out.dwg (47.6 KB)
I think this may be both a tolerance thing - your object is about 0.6 meters long and the file tolerance is 0.01 - as well as the fact that your original surfaces are trimmed out of some much longer pieces and not shrunk after.
In the file below, I did the following:
- shrunk the polysurface
- exploded it and untrimmed all the surfaces at the joint
- raised the file tolerance from .01 to .001
- re-split all the parts where needed and joined back into a polysurface (no naked edges at the joint)
- exported .dwg 2007 Solids
- re-imported the .dwg.
The re-import appears to be fine and the edges within tolerance.
Original-rebuilt.3dm (146.9 KB)
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Okay, but that’s not normal … Why is this so?
And if some big project … Exported, everything seems to be fine, and then suddenly we find out that everything is terrible there … To redo?
I think the main issue is to make sure your tolerances are set correctly before you start modeling.
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Excellent! And how to set the correct tolerance?
Here’s an excellent FAQ to teach you the details of modeling tolerances in Rhino:
https://wiki.mcneel.com/rhino/faqtolerances
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DocumentProperties > Units > Units and tolerances
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The video in this link has good information about defaults and templates. http://tips.rhino3d.com/2014/02/templates-defaults-in-rhino.html
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To be honest, I already saw all this and I know. I don’t understand how to calculate the correct tolerance so that everything will be fine later. I often doubt my choice 
The general rule of thumb is your absolute modeling tolerance should be 1 order of magnitude tighter than the smallest modeled edge or curve segment.
For most things modeled in Rhino, this means 0.001" for table sized objects.
Your model should also not be too far from the 0,0 coordinate origin.
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Well, but there are still units of measure that are also very influential. 0.001 meters or 0.001 centimeters, do you feel the difference?
Another rule of thumb is the absolute tolerance in Rhino should be one order of magnitude tighter than the accuracy needed. So for an object where the accuracy needs to be 0.01 use an absolute tolerance of 0.001.
Starting with a tighter than needed tolerance is much better than starting with a looser than needed tolerance.
If the absolute tolerance is initially tighter than needed then some operations such as intersect will create heavier geometry than needed. It is always possible to reset the absolute tolerance looser and proceed.
If the absolute tolerance is initially set too loose then numerous problems can occur. It may not be possible to fix the problems by resetting the absolute tolerance to a tighter value without also remodeling some of the geometry.
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Well @Modeler3D does have a point which has not been addressed here though. The original parts are modeled ‘correctly’ for the original file tolerance in that if you explode the original polysurface, rebuild the edges and re-join the parts, the thing does join without any naked edges. The edge tolerance report is something like .003 (within the 0.01 file tolerance).
It’s only when the thing has been roundtripped through .dwg that the problem shows up - so what is it in the export/import of the .dwg is problematic here?
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Why is DWG so sensitive? It is he who has problems.
DWG is hot garbage.
Fine
In general, I have the feeling that DWG has a different tolerance from the Rhino project
And why do I always appear this layer “0”? This is when I open .dwg in Rhino
Layer 0 is default layer in AutoCAD and you can not delete it, every .dwg file must contain a layer 0.
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Just to mention: no .dwg file created by any CAD program other than Autodesk is a true .dwg file. Autodesk never released the code for .dwg and has tried to reclaim it over the years. The .dwg code used by other CAD programs is a close approximation (I think overseen by a consortium of some kind). Though my knowledge about all the ins and outs of this is no more than this.
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