Splitting 2 intersecting surfaces; Only the one I split first works?

I have a lofted surface and an extruded surface that intersect, and I am trying split both where they intersect.

I can split either of these surfaces using the other surface. But once one of them is split, attempting to split the second surface always fails. When I am selecting “cutting surfaces”, I have tried selecting with a window to make sure I don’t miss anything. But typically I will just select one or two of the halves of the previously split object. That has always worked perfectly until now.

Any ideas for how to troubleshoot? What could go wrong with a surface as it is getting split? I’m really not sure what to look for. Nothing looks wrong.

I found I can work around using the boolean union command, which split and joins both. It’ll do for this instance, but “split” is one of my go-to commands…so it’s a bit frustrating when it just doesn’t work!

(And BTW, I have recently switched to using a windows machine. I doubt that matters, but just in case it does?)

If surface A is larger than surface B , you will be able to split B using A , but not vice-versa (check their boundary/naked edges)
Anyway… can you attach a .3dm with the 2 surfaces so others can try to replicate and see the problem?

I don’t want to post the file. It isn’t a hugely impressive drawing, but it is the result of a few years of trial and error to get to the current design. Throwing it on the internet for anyone to see doesn’t seem like a good idea.

I did check for naked edges, but that didn’t show anything. There must have been something, but I couldn’t find anything.

But I did find a solution. I extracted several surfaces in the area I seemed to be having the problem, and then I used extendsrf to extend their edges past their intersections. And then split and re-joined, which effectively got me back to where I started. (I didn’t expect extendsrf to work on these surfaces.)

After doing this I was able to split both surfaces normally.

I feel like I got really lucky. I still wish there were suggestions about how to locate the cause of a problem like this. My only clue came from experimenting with boolean union, where it sometimes created a group of exclamation marks…and the group properties said the edges end at a naked edge. But still, I looked for those naked edges but couldn’t find them!

How large are the objects?
How far from the world origin? (Not Cplane origin unless the Cplane origin is at the world origin.)
What is the absolute tolerance in Units? Has the absolute tolerance been changed since the project began?

The lofted surface is about 12"x5"x4". It is maybe 20" from absolute zero? I have gotten in the habit of drawing a couple steps, and then making a copy next to the original to preserve the work up to that point. So I have a bunch of different steps and different components on various layers, and they’re spread around the origin.

My absolute tolerance is set to .00001". I did change it from .001".

The only things that remain that were drawn with the .001" tolerance are the curves I used to create the main lofted surface. I might have re-built them at the finer tolerance, but I’m not 100% sure.

I re-lofted the main surface after the tolerance was changed, so all the surfaces in question were drawn at the finer tolerance.

I changed the tolerance because 1) RhinoCam simulations were not turning out as accurate as I wanted and 2) when I started the project I didn’t know Rhino had a user adjustable absolute tolerance.