Hi Khris, thanks for posting your files. I took a look at "Pb boolean-union (top of cylinder disappears) (nose v2).3dm" and it looks like the problem there s the circular parts of the 2 solids are not quite precisely aligned, they are pretty close like around 0.002 units apart but that adds a lot of complexity to the boolean process when 2 objects have overlapping surface area.

Part of the boolean process involves intersecting surfaces of each solid with surfaces of the other solid, and when surfaces have overlapping area it adds more stages to the boolean process and it can be difficult for the intersector to decide if the surfaces are exactly overlapping or if they need some kind of intersection generated between them. So it can be good to avoid having overlapping surfaces or when you do it's good to have them more exactly aligned.

I also took a look at your base shapes file, and the issue is that these profiles come close to being aligned but are just off by enough to cause all kinds of intersection difficulties:






So that's likely to be the root of a lot of your problems in this case, to get a good boolean on those pieces you'd probably need to have those base shapes on the left side be exactly aligned with each other instead of what you've currently got. Either they need to be more closely aligned or less closely aligned, it's when they're about as close as you have them that will tend to get things tripped up because it's close to the intersection calculation tolerance.

- Michael