Hi Mark, if you re-load the file you just posted and try it again it seems to be working better now. There are a few cases where saving and reloading the file corrects some problem in the geometry.

But there's still a problem - you don't really have a clean intersection here, you can kind of see in this area here how the objects are just kind of barely skimming through each other:



You can kind of see there that the cylinder and the sweep have some parts where the cylinder is just barely sticking to the outside, and then other regions right nearby it where the sweep is actually the part that is sticking the most to the outside.

An inexact coincidence like that is very difficult for the booleans to resolve correctly, they may try to do something like make numerous intersection curves in that region and try to slice things up into really super thin slices with irregular boundaries, stuff like that.

You can have things that are coincident like that in booleans, but only when they are more exactly on top of each other instead of partially weaving in and out from one another like you've got here.

So to get a good result, you'll need to do something different to not get such a indeterminate and "paper thin" intersection zone like that, either have the cylinder recessed some more or punching all the way through some more.

- Michael