Well you cant walk trough it at all. Why dont you create same sized part, make it red and set transparency to match the negated part? Then you could walk trough it whn you set CanCollied off! 
You need to connect the two parts that you want to be connected then press union and it should make the red block disappear.
The problem is that you haven’t unioned it yet.
Select both the Union and Negate parts and click the Union button on the “MODEL” tab, then the Negate part should disappear and create a hole in your Union where the Negate part was.
You can change the CanCollide property of a Negative Part in the Properties window.
Why you’d actually want this behaviour is another question entirely, when what Anix_Y suggested above works much better.
If you mean after Unioning the NegativePart to another Part, and you’re still having collision issues, you shouldn’t be making doorways with Unions anyways since that’s a bad practice. Instead you should just make doorways (or anything the player really needs to collide with accurately) with Parts.
Yes, Negating small areas out of large Unions doesn’t work well since the collision area of the Union is not what appears on screen.
There’s a setting that allows you to see the actual collision box of the Unions you create, but I’ve found it to be tedious to change the setting each time you want to check this property out.
I’ve found a Plugin that allows you to switch back and forth more easily and helps me out a lot while working with Unions/Meshes:
its actually a really large cube with hallways created by negated parts, on an other build of mine i can walk through the air i negated and here i cannot…
its to show where the part is, even when unioned i cant walk through
it worked on another build of mine though…
here are some videos to see whats wrong and what i want to achieve.
https://gyazo.com/70bb4565f7e0a93585768efda110dbc3
https://gyazo.com/342df9987dc5d5dc8e22ec2de106e441
more info, unioned without the colisions showing and the second picture is a colissions plugin. i have no idea why these colisions are like this? (camera is inside part)
First off, you should reply to multiple people in a single post. Making five posts in succession is just spamming. There’s almost no good reason to double post on the dev forums.
Second,
This is happening because, as I said in my first post, and Scottifly said following, Unions aren’t intended for this as Union collisions are terrible. Don’t use Unions to make tunnels through Parts, there are way better methods that are actually effective.
Can you give me some easy methods? Also I hope you don’t mean just make the cube hollow and add these hallways part per part, are there any plugins that can help me do this on other ways?
Considering collisions are your main requirement, Smooth Terrain comes to mind if you’re looking for a quick way to get caves and tunnels while still maintaining accurate collisions.
Meshparts and Unions just don’t have great collisions - nor should they in most situations when small and complex geometry is being used. If you still want the tunnel/cave to visually look like that Union, but still have collisions, consider making a collision map out of transparent Parts.
What were the other ways though? Aren’t there any plugins to change collisions and stuff, I can’t change it to terrain nor add transparent parts everywhere because that will make the player be able to see through walls.
Hi Kiri,
I’m going to assume you are just trying to make tunnels in studio. So if that’s what you’re trying to do I suggest you just build the tunnels with parts.
Also I do not see any problem with using terrain as players can easily clip their cameras through anything in Roblox using shift-lock. So even if you used union, part or terrain caves players can look behind the walls of the cave.
TLDR; Use Parts or Terrain for tunneling caves. Don’t use unions.
Aren’t there any plugins to change collisions or behaviour of unions?
I doubt there are, but feel free to look it up yourself. But may I point you in a different direction and suggest you mess around with these plugins:
Like @Aotrou said, make the Union the shape that you want and make it CanCollide off, then make a bunch of Transparent walls that are CanCollide on to block the players from going through the Union. You can also cheat a bit using this method to use one Part just inside the surface of a bunch of angles on the Union so you don’t have to match it exactly.
Their camera will still be contained by the Union, but their character won’t be able to move through them.
