I’m trying to take this
to enclose the hole, but this is what I get
what am I missing? thanks!
Can you post the .3dm file. You can use the vertical arrow icon in the toolbar above where you type a post.
What are you using for the rail? How many sections?
Use revolve for something like that.
If he uses either revolve or sweep he may be next wondering why his boolean or filleting fails.
If he wants clean trouble free geometry he will delete
all the relevant surfaces and make all the geometry anew by extruding clean curves.
There’s nothing to be gained by pursuing this method of cobbling together geometry. It doesn’t save time and it may well sabotage future geometry operations.
The main thing you are missing is that Rhino is not capable of understanding what you are trying to do.
Rhino doesn’t understand that a swept arc should be the same thing as and extruded arc.and Rhino doesn’t understand that a swept planar surface should be equivalent to a standard planar surface. As a result what Rhino will build is a booby trap that is likely to blow up on you if you want to use the geometry for additional modeling.
The best approach to keep Rhino from building a land mine that you may later step on is to just extract and delete all the surfaces that touch the hole and build new surfaces by extruding clean curves and then join and cap.
What in the illustrations indicates bad geometry?
Its not a question of bad geometry. The issue is do you want to build geometry that is robust and will work well for subsequent operations.
Besides being more robust and trouble free, its also easier and faster and more accurate to build the hole as a complete cylinder rather than cobbling it together as a joined partial cylinder and freeform surface.
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In cases like this where the main object is just an extrusion of equal thickness everywhere, i.e as if it was cut from a flat sheet, I usually duplicate the edges of the top or bottom surface with DupFaceBorder, delete the object, change the remaining 2D curves as required and extrude the lot again to the original thickness. In David’s case that would mean redrawing the circle that describes the hole, offset the circle to the width of the additional part-ring, trim the unwanted bits from the curves, select all curves and ExtrudeCrv.
I tend to do this multiple times during the development process, it takes some time (not a lot) but I feel it is the best way, and it maintains the integrity of the object.
And to answer the original question: your cross section is at an angle to the sweep rail, and it just maintains that angle along the rail.
Max.
here’s a youtube.
(sometimes a lot faster to just screen record than to type out the thoughts 
)
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I found the way to put the rectangle at the start and end. I went back and followed your suggestion and made the arc and extruded the solid. That worked really well. I would like to join the two solids, but that fails unless I pull back the adjacent surfaces back to the intersecting edge. Does that make sense?
P.S. thanks again!
An easy way is to extract and delete all the surfaces that touch the hole make new extruded surfaces for the hole and the outer surfaces then join and cap then cut the hole. .
Another easy way that someone else suggested is if this entire part is a flat extrusion just delete the whole thing and edit the original curves it came from and then make the whole part again in one simple operation.
That is probably how I would do it, if is flat. Just extract the bottom surface then duplicate border of the top polysurface. Draw my new curves, split and / or trim as need then rejoin and extrude solid.
Thanks everyone!