Perimiter of intersecting srfs is not perfect and i do not know why

HI, I have given you a example dwg , you can see that at the edge of the trimmed and intersecting srfs that they are a mess. A simple dwg that started out with the closed crvs on the same plane, lofted and patched via the crvs and points. It took several attempts to trim them, often such efforts require extending an intersecting crv, creating a srf in both directions at 90 to the Cplane and trimming using that cut srf. I still get bad intersections. Offsetting these surfaces gives me even worse intersections at the perimeter, often srfs are in inexplicably overlapping. Then I have to fight with the dwg to get closed perimeter crv which of course I need to define the area to be machined. Rhino cam usually will machine these imperfect vessel shapes for me but Madcam etc often will not.
I usually draw with the mesh settings at smooth and slower on R5. Appreciate any help.

thanks rjk

vessel test.3dm(181.8 KB)

The problem is in using Patch. Patch does not guarantee that the trimmed surface edges lie on the trim curves exactly, this is an approximation. So the intersection between the two surfaces may not be complete and thus trim/split may fail.

In your example, I untrimmed the Patch surface and trimmed it off at the 0 plane, then the split/trim worked between the two. There is however a small deviation of the Patch surface from the input curve by around 0.05mm.