Hello Tom,,, I did get this to work by offsetting your Surface-Plane1 and using that to trim with. I found that if I offset anything less than .1mm the intersect would fail. Also clean up a few floating surface bodies with a body delete but that really doesn't seem to have any effect on the intersect working. If I suppress the body delete it still works. Seems to be in the Volute geometry just above the Surface Plane1. If you need the additional .1mm you might clean that up and and try an extend.??
Edit:.. Or boss extrude the .1mm back on??
That's perfect thanks a lot!!
I've tried to do the same with the other side of the volute, but I can't get that to work either, could you have a look at that please?