you don't say what depth or what type of aluminum your cutting so it's impossible for you to get the right answer to your question , if your tapping very deep then you may need to peck tap it , also if your dealing with soft gummy aluminum then once again you may need to peck ,
you could check the drawing to see what class of thread you need to cut then drill at the top of the tolerance ,
what brand of taps are you using
A poet knows no boundary yet he is bound to the boundaries of ones own mind !! ........