How can i turn this object into a closed polysurface

gorna chast.3dm (6.0 MB)

Hi - The SelBadObjects command selects your object. You can use the ExtractBadSrf command to find the one surface that is bad. You will have to recreate this surface.

I’m not sure about the design intent here. This looks rather fragile:

Also, you have edges that are 0.02 mm and are working with a document tolerance of 0.01. It is recommended to have your tolerance to be one order of magnitude smaller than the smallest feature in your model. I would put it at 0.001 units. Changing the tolerance at this point in the model, though, will make it so that surfaces won’t join anymore and have to be retrimmed or rebuilt.