This was partly due to the unit tolerance of your file being changed. You had it set at .1 mm but I think an object of this size with junctures like this will work better at the defaults of .001. The first thing I noticed was the bad edges in the screenshot below. This likely resulted from the unit tolerance change too but I’m not sure when it happened. I exploded that polysrf and used RebuildEdges then I joined again after setting the document to .001.
That won’t be enough to get a good intersection curve here though. Coincident surfaces are known for this type of headache and the best you can do is decrease the area in which they overlap perfectly and also use surfaces with enough control point info to get an answer. Here I split out a section of the straight surface shrinking it as I did so, then I rebuilt that new trimmed surface to have more control points and a degree of 3 in the long direction. This allowed me to get an intersection curve that I could use for the trim. There were still a few pesky naked edges in other areas present after the rebuildedges and join at a higher tolerance. I just deleted these and did a couple Sweep2 surfaces…
I hope this helps.
test_bjames.3dm (2.0 MB)
