I will try this and get back, thanks in advance.
No problem, hope it helps and fixes your problem
Tried it and doesn’t work. It was already set to default.
I have read that it is still an approximation, not the mesh collision for obvious reasons.
I will try to look into other solution maybe like you said, I could use two meshes where one is invisible with collision activated, but it is still twice the work.
I will keep you all posted if I find anything.
Try using Blender maybe? I am guessing your program does things different.
I believe you misread my suggestion.
I suggest you add two parts as collision walls and make the mesh collisions off.
Oh, I am sorry. I will look into it too.
thx
It was made with Openscad. I just used Mesh lab to check if there was flaws in the design. Sometime I use thinkercad for fast and easy design.
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Try Blender and see if it works, also how are you exporting the mesh (fbx, obj, etc)?
I will try it, thank you. I am exporting the mesh as .obj.
Yeah, try using blender and also FBX too
Software and file format don’t affect hitboxes unfortunately.
What is your solution then?
I suppose your solution is great 
Although I don’t appreciate the attitude, here is my solution.
I believe its better to turn off collisions on the mesh, and instead create a invisible part behind inside the mesh with the shape of the part being close to the arch, and have that be the collision box.
This is a hit-box issue.
To summarise:
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This is caused by Roblox making a less detailed version of the mesh. This can be visualised using this plugin.
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There is no way to fix this without using different software.
Recommended Solution:
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Make the problematic mesh CanCollide False
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Place down multiple parts and resize them to match the intended collision walls
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Set the part ‘Transparency’ to 1
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I recommend you group them all together for easier editing in future
Memory Consuming Solution:
- Set the part to PreciseConvexDecomposition. This is a relatively new tool which is very memory consuming so should be used in moderation. Read the DevForum post here.
Yep!
That’s what I said! 
I understand, but you recommended unions which are as you said memory heavy, and its better to use parts in my opinion, even if it doesn’t match the shape exactly, its better to have no unions at all.
I’m pretty sure I said “Non-Unioned”
Oh my bad, I misread your post, you are correct then.
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roblox has alot problems with hitboxe’s i think. i had alot problems with the same thing.