I dont want to do it "manually" witht the pen tool, i want perfect curves (semi circles).

If i try to do the curve by hand :
1: it takes too much time
2:it doesnt gets perfect
I know i could do this by creating a circle and then cutting it up to its half, but then the curve is no longer a curve , yes i can then weld them , but that is just a walkaound.
So how do the pro's do it ? whats the right way ?
thanx