Ignoramus13320 fired this volley in
:

Second question, I will need to tap #4-40 holes in same aluminum. How
risky is this as far as tap is concerned and should I use lubrication?

In revers order, Iggy: Yes, you should lubricate the tap, and in a hole
that deep (which is deep for 4-40) you should not try to power through in
one pass, but back it out and clear chips several times.

On the first part: it's no risk whatsoever to the tap, so long as you
don't torque it to the breaking point G.

Whatever you do, don't try to freehand tap those. Even if you don't use
power, use the mill with a center chucked in it to keep that tap straight
over the hole the entire time (if you're using a tap wrench), or just
chuck up the tap and use your hand to turn the spindle. Off-line tapping
breaks more taps than the character of the metal being tapped.

LLoyd