How to rotate an object using mouse.KeyDown?

Im having trouble when rotating this part, it works fine for the first 3 rotates but when I try to rotate it again it just switches between two vector3 values.

So, the part starts off at (0,0,0) I press "r" once and it goes to (0,90,0) on the second click the rotation is: (-180,-0,-180) on the third click the rotation is: (-0,90,0) on the fourth click the rotation is (180,-0,-180)

and it just keeps doing (180,-0,-180) and (-0,90,0) after that..

Is there a better way to rotate a part using a script??

This is a part of my local script and its for a build preview

mouse.KeyDown:connect(function(key)
	
	for i,child in pairs(HQ:GetChildren()) do 
	
		if child:IsA "BasePart" then 

	key = key:lower()
		
if key == 'r' then
			
		child.Rotation = child.Rotation + Vector3.new(0,90,0)    --Rotate (Works ok for first couple times then theres a problem ....Eventually I need this to repeat so I can keep rotating it until I like where it is)
		
end
end
end
end)

Thanks for helping!

I'm new, but I'll take an educated guess. Sorry if I'm wrong. Maybe throw in a while?