Hi Involute,

> Still, is there a test, other than zooming in, to tell whether a point is on a surface, instead of just really, really close?

Not really - well I guess if things seem to be behaving oddly that's a sign to do a closer inspection.



> If not, how do I know I'm zoomed in enough during inspection?

I go in something like 6 or 7 pushes of the mouse wheel. You don't want to go in too extreme because when things are only say 0.0000001 units apart that's totally ok. It's when things are more than 0.001 units apart that they're going to be considered separate pieces and won't intersect with each other.


> E.g., as far as I can tell, the end of the curve in this attachment is on the surface, but I still can't open the end of
> the tube on that surface (I've left it un-unioned to make it easier to inspect).

That's actually normal - union doesn't really open holes in things, it combines things together and only removes pieces that are contained inside both objects. So the part that you want to have as a hole isn't contained inside the hollow tube you have, it's only in the main body volume so that part will be kept.

For drilling a hole you want boolean difference for that instead of boolean union. If you want the inner tube to be totally open all the way through you want to have 2 separate tube objects an outer and an inner one - first do a boolean union with the outer tube and your main object, then follow that by a boolean difference using the inner tube as the cutting object.

Here's a simplified example - if you have this tube solid and this sphere solid:



When you union them, it gives a result like this:



They are combined into one object now but there's no hole through the sphere, basically when you union 2 solids together the result is going to look from the outside the same as before, the only stuff that's been removed is all the extra inner structures.

Hope that makes sense! I can understand how it seems like the hole in a hollow tube is part of the object and so you're expecting it to persist during a union, but the way it works is when you have a hollow tube, it's only the thick wall that is really part of the solid, the hole in the middle is empty space same as empty space all on the outside of everything...

- Michael