I need to drill some holes in 6061 aluminum. No problem. I have drilled a lot of holes in 6061 aluminum.
The need to be deep holes. I've technically drilled deep holes in aluminum. Just maybe not this deep. Deep relative to drill diameter.
They need to be pretty straight.
This is for a 1/4" pivot/hinge pin. The hole can be large and sloppy (moderately). 5/16 might be to much, but an H or I drill might be tolerable. I might even be able to get away with a J drill. The goal is that there is enough room for the pin to be on center within a few thousandths somewhere in the open space of the hole.
On tiny (1/8 and under) deep holes (relative to the drill diameter) I've just bought carbide drills from Precise Bits. Stubby little carbide drills do drill pretty straight. This isn't like that.
I have to reach past almost two inches of obstruction on one side that prevents the chuck from being lowered beyond that point and then drill through 2 inches of metal for the pivot hole.
I did it with the prototype by spot drilling with an over extended spot drill, drilling a little over half way with a jobber drill, flipping the part (same issue on the other side), and repeating that process. The holes were pretty close, and I sort of wallowed out the little bit of ridge (not to bad) by forcing a drill through the hole off the mill by hand. (Cordless drill).
It was really sketchy, but it just barely fell within spec. Now I need to make 51 pieces like this. I can see it going badly after I get a little tired from making these parts for a few days. The drilling is done as a 3rd and 4th setup after machining on two other faces of the work piece. I have to do the drilling (I think) as two setups because the other side is two such pivot bosses that need to be drilled, and they are 2.005 inches apart. Drilling them from one side would be over4.25 inches of reach not counting break through. That's just to much for me to have any confidence in meeting my admittedly rather sloppy spec.
I have been looking at parabolic drills, but other than not needing to peck as often due to their ability to clear chips I don't see any benefit for drilling "pretty straight" holes. Only thing I can see that would help with staying "pretty straight" is spot drilling for a good start and a carbide drill to follow.
Of course mounting has its issues as well. The part has an "ear" that is 1.5 (apx) x 2 x .05 that can be clamped in the vise. It does have one saving grace. There is a square perpendicular boss on the back of the part than can register for height and squareness (perpendicularity) on the top of the vise jaw. Once I have a process that works "good enough" I should be able to repeat from part to part fairly quickly.