You're missing the part where you set the C0 (and optionally C1) of the weld.
To explain how this works, I will quote the wiki:
Sometimes, you want to just weld together two parts in their current positions, so that they remain in the same relative positions. This takes just a basic understanding of CFrame math and welds. As previously mentioned,
1 | Part1.CFrame * C1 == Part0.CFrame * C0 |
And understanding that
1 | CFrameA:inverse() * CFrameA |
cancels out, we can apply some basic algebra
1 | Part0.CFrame * C0 == Part1.CFrame * C1 |
2 | Part0.CFrame:inverse() * Part0.CFrame * C0 == Part0.CFrame:inverse() * Part1.CFrame * C1 |
3 | C0 == Part0.CFrame:inverse() * Part1.CFrame * C1 |
(http://wiki.roblox.com/index.php?title=Weld)
So you need to set the weld's C0 to:
1 | weld.C0 = PartsToBeWelded[i].CFrame:inverse() * WeldTo.CFrame |
I've run into other problems with welds involving trying to weld Anchored parts, which can usually be solved by setting the weld's parent after you set its other properties, i.e.
01 | WeldTo = script.Parent:WaitForChild("Base") |
02 | PartsToBeWelded = script.Parent:GetChildren() |
03 | for i = 1, #PartsToBeWelded do |
04 | if PartsToBeWelded[i] == "Part" then |
05 | local weld = Instance.new("Weld") |
06 | weld.C0 = PartsToBeWelded[i].CFrame:inverse() * WeldTo.CFrame |
07 | weld.Part0 = PartsToBeWelded[i] |
09 | weld.Parent = PartsToBeWelded[i] |
