Rotating a part on its edge?

I have no idea where to start for this except for a basic rotation script, but I don't know how to make it rotate on an edge so it does not rotate from center. (i.e. swinging open doors.)

This is all I have:

local b = script.Parent
local rot = 0
local db = true

b.Touched:connect(function(hit)
    if hit.Parent:FindFirstChild("Humanoid") and db then
        db = false
        for i = 0, 45 do
            wait()
            rot = rot + .04
            b.CFrame = CFrame.new(b.CFrame.X, b.CFrame.Y, b.CFrame.Z) * CFrame.Angles(0, rot, 0)
        end
        wait(2)
        db = true
    end
end)