Hi, I’m tryin to model a tunnel portal. I want to crop out the part of the portal which is inside the sphere. Or can i extrude my portal section to the surface of the sphere?
Thanks.
Hi Bunver,
Group the two items separately
Treble click on the portal to enter the edit mode into the portal
Treble click once more to select it all. You should only have the totality of the portal selected now - if you don’t, there are other items you might want to group together so they are out the way
Now right click on the selection and choose intersect>with model.
Click outside the selection box, or hit escape a few time to come out the selection.
Hide the sphere (edit>hide or right click and choose the hide option from there)
You should now see a line where the sphere has intersected the portal. You can now delete the bits you don’t want.
That should work 
Thank you very much IanR, this was very helpful This is the result
Can I ask how to close that empty face in the drawing?
To a certain extent, this depends on what you want to end up with and how it will be used. If you want the remaining geometry to be a single, integral object without trapped internal faces (a solid suitable for 3D printing, let’s say), you shouldn’t have the geometry grouped when you intersect it. In fact, the only reason you might have for separately grouping these objects is to avoid getting them prematurely stuck together while moving them into position, but that actually suggests that more practice is needed with the Move tool. Grouping is just a hedge.
So @bunver, may I ask, do you want the part of the tunnel inside the sphere removed or the part of the sphere inside the tunnel? Or both? Is the tunnel supposed to open into the sphere? If you just want the part of the tunnel inside the sphere removed, as appears to be the case from your description, who will see that anyway?
-Gully
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Unless we want to keep both objects, move the sphere away and use it for another purpose later? But in this context Gully_Foyle, since it’s a tunnel entrance, I expect you’re right. 
There’s a plugin made by Fredo called Curviloft, which would do it…
One way to think about what Gully said is that if you leave the sphere and the tunnel geometry ungrouped, when you delete the unwanted geometry, you’ll be leaving a bit of the sphere behind as the surface that’s missing in your screen shot.
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I have for a long time been trying to understand how solid objects work. Is the sphere in this context a solid and the portal object not? There have been so many occasions where I’ve had problems with sticking geometry that I have always group things. It’s a revelation for me. It’s only now that I’ve seen what I suspected it literally meant- a solid object really is solid. But I still don’t know how to make an object solid. I’ve tried and tried… I’ve much to learn. Thanks Gully and Dave.
In simple terms a SketchUp solid is a group or component which contains only edges and faces and in which every edge is shared by exactly two faces. No more and no less. In order to use the Solid Tools, both the group/component get modified and the one doing the modifying must be solid.
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@bunver, (Edit: initially, I mistakenly addressed this to @IanR. Sorry, Ian)
A solid object is simply a volume that is completely enclosed by faces, with no stray bits trapped inside and no holes or gaps. To be recognized as a solid by SU, this closed, watertight object must be either a group or a component, containing only raw geometry–only edges and faces–and only enough geometry to enclose the volume. Entity Info identifies a solid as such as gives its volume as an attribute.
Intersect Faces has several options, as you might have noticed, relating to “context.” The geometry making up a group is in the group’s context. To edit the geometry in a group, you must enter the group’s context. The geometry in Group A does not stick to the geometry in Group B because they are in different contexts. Therefore, in order to use the Intersect Faces command to cut two objects along their intersection so that they fit together to form a single object, the two objects must be in the same context.
What happens when you use Intersect to cut two objects to fit each other is that the original two objects are essentially consumed in the process, giving birth to a new object that has bits of geometry from both parents. So position the two objects appropriately, making sure they end up in the same context. If they are in separate groups, explode the groups. In this situation, you’ll generally want to select both objects, then right-click > Intersect Faces with Selection. Once the two objects have mutually cut each other along their intersection, all that remains is to remove everything that is not the thing you want to end up with, which is sometimes as awkward to do as it is to say, using Erase and Delete (and occasionally Undo, which is Ctrl-z).
-Gully
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The simplest way i guess.
Thanks a lot…
I couldn’t download the plugin.
I always get this error
Group the arch-form and the sphere separately.
Place them as desired.
Edit the arch-form, set the View > Component Edit toggle to Hide Rest of Model - so you can better see what’s happening.
Select all and use the context-menu to intersect with model.
The outline of the sphere is now ‘imprinted’ on the arch.
Don’t delete anything just yet…
Now exit the edit and edit the sphere.
Using the context-menu intersect the sphere’s surface with the model.
You should now have a ‘imprint’ of the arch on the sphere.
If edges are not showing toggle Hidden Geometry On in View menu, over-drawn any smoothed edges that coincide with the sphere’s segmentation.
Delete the unwanted parts of the sphere’s surface to leave the part that will infill the arch’s cut.
Select the surface of the sphere and reverse it using the context-menu - this is needed later when it becomes the merged ‘cut face’.
Edit the arch and delete the unwanted parts.
Exit the edit and the arch should now appear to have its cut ends faced - although currently the geometry is separated.
Select the two groups and make one new group of them.
Edit that group and select the two sub-groups and explode them.
Now the geometry will merge,
No need for any Plugins to do this…
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And as one more way of the many ways to do this.
Edit>Cut the ungrouped sphere.
Open the tunnel group for editing.
Edit>Paste in place.
Show hidden geometry.
Select faces involved in intersection.
Intersect them.
Hide hidden geometry
Delete the waste.
Keyboard shortcuts are a huge time saver.
If the sphere is wanted, there’s still a copy of it on the clipboard. Or you could have made the sphere a group or component, copied the surfaces and left the sphere intact.
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