Deep M3 Counterbore Question

[miseree]

I have a project that is using a piece of 6061-T6 approximately 2" thick. One side has some pocketing and the opposite side is where the counterbore is (pilot is through the whole piece). I want to drill the 3.5mm pilot through (~2") and then rotate the work piece 180 degrees and chase the 3.5mm pilot hole with a M3 counterbore (6mm) approximately 1.8" (not extending to the other side obviously). The reason I don't just drill and counterbore from the same side is for the purposes of aligning with another piece that will be cut from the same stock (i.e. the pilot hole is shared between the two pieces and is ideally best drilled at the same time).

My question is can the 3.5mm pilot be trusted to be "straight enough" for the counter bore coming in from the opposite side? I plan to re-align the work piece in my mill to the best of my abilities. Are there any specific drilling procedures that may help minimize the wandering of the bit?

Thanks in advance for thoughts and any advice.

[RICHARD ZASTROW]

Try a step drill with a 6mm dia. c'bore and a 3.5mm pilot on the tip. The 3.5 pilot should follow the original pre-drilled 3.5mm dia. hole.

Dick Z

[juergenwt]

Richard - Step drill will work.
If you want to drill 3.5 from one side (from the side that needs to line up with the other part) peg drill it. Keep removing the chips and use coolant or cutting oil. C'Bore from the other side with a 6mm drill is OK. It will follow the 3.5mm hole. Once you are close to the depth you want to have - grind your 6mm drill flat and touch up the bottom.