I imported this into Rhino from an iges file. I removed the flange and would like to extrude/join in the center of the gap but following the arc. My efforts to far have failed. However, I’m certain Rhino has a tool for this but no sure which is best.
Well, if the outer rim is joined to the spokes - that is to say that there are holes cut into the tube, then extending the edge via subobject selection of the open edge with revolve about the center to meet the other will probably fail with a bizarrre result.
The cleanest solution might be to remake the outer tune rim (torus) as a new clean object and rejoin it with the rest. Otherwise, a revolve of one of the edges to meet the other as a separate piece, then join might work. Hard to say for sure without the file.
Try Sweep1 with the arc curve as the rail (selected in the gap) and the two surface ends as the cross section curves. Let us know if it works. If it does not work I have an idea of what may be needed to make it work.
Experiment!
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Untrim, perhaps.
-Pascal
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I know remaking the outer rim would be easiest. I was hoping to learn another method. Nothing I tried worked, and couldn’t spend more time on it. Remade the outer tube with sweep1.
If the surface was a single ring, then try what Pascal Golay suggested above - “Untrim”.
You can also fill the gap with the following tools in Rhino:
Blend surface, Sweep 1 rail (suggested by David Cockey), Sweep 2 rails, Loft…
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You all forgot to mention the best tool: _PostGeometryToForum
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Yep, I did mention the other one in my first post - _HardToSayForSureWithoutTheFile
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if the radius of the pipe was circular i would extract the isocurve then use arcblend then sweep.
Whoohoo, BlendSrf is what I was hoping would work. I’m unsure if the blend continues along the curve of the tube surface or if it is straight? (The ring – which I did not create – actually looks faceted.) The option box opened with “curvature” selected. Am I correct in assuming Rhino is recognizing the surface as curved?
Ring2.3dm (531.1 KB)
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Good!
Gracias. This thread has introduced me to several tools of which I was unaware.
De nada
That’s the fun of using Rhino .
the real fun starts when you stop using rhino and start riding it 
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I suggest to extract one of the main surfaces, untrim its end, then rotate the surface along its center (with the Center snap on) and make the untrimmed end parallel to the X or Y axis. Next, use “Revolve”, pick the center of the surface to set it as the start of the revolve axis, then press Enter to set the 2nd revolve axis to be vertical, then rotate with the mouse to a full circle around the shape to make it 360 degrees. That will create an exact copy of your ring from a single closed surface.
An alternative way to build that surface is to examine the two diameters of the ring (1466,850 mm 21,750 mm, respectively) and use the “Torus” command to build it really fast. Its radius must be 749,3 mm.
Once you get the basic surface using one of the methods above, you can extract the cutting shapes for the holes from the original model with the “DupBorder” command. Then group all the curves except the ones that were located around the blend surface that you created. You can delete these two. Then use split to cut the holes in the new ring. That’s it. Sounds long to describe, but it actually takes less than a minute to execute. 
Cheers,
Bobi
Ring2 Revolve.3dm (1010.7 KB)