I think that I recently ran a job out of 17-4, and I think that stuff just likes to sing when being drilled.
???
You are in a lathe?
A 5/8 shank on a .695 hole is pretty stiff, and 8" deep - the cutting end is going to be trying to relocate the shank end.
I am sure that you are mis-aligned more than the back-taper of that reamer. Alignments on a mill are pretty good (single holder (spindle bore) and a live tool) but a live part and turret/blocks/holders set-up just aint the same.
I'd say that you want to sweep the holder bore, AND THEN - after reaming, try jogging the reamer in and out of the hole. Can you see or feel it "bumping in" at all? Not only look from your current vantage point, but also from 90* around it too.
If you have to adjust the turret position AT ALL to git a good alignment, then your tool will be OK, but your shank will not be if you adjusted anywhere besides the sweeped location. And as you get deeper, your trailing edge flutes will be trying to move your shank to be in alignment, and you will be getting rub. This is likely where your lines come from.
???
I am always the champion of running floating reamer holders, but this is an extreem case for it's use.
It is even possible that the drill may benefit from one as well.
Is the gun drill superior to a tipped drill? The tipped drills of recent years have sure put the hurts to the gun drills.
Have you looked into an Allied Gen3 unit? IDK if they go that deep or not?
As above, a through coolant reamer would seem to be a requirement here. I've had those holes put in via a Hole Popper before*. Not sure if they can go that deep or not?
* Thank you to "you know who you are".
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