Michael K JohnsonMay 15, 2022

I've never before bored a hole this long with such a high aspect ratio through steel; ⅝" almost 5" long.

First try, I broke about an inch off the end of my pilot drill right in the middle and after a lot of work failed to rescue.

Second try, I used a larger 3/16" 6" aircraft drill as a pilot, but it wandered. I ended up using a 2-flute ⅜" end mill in both ends for alignment and drilling through, which got me close enough for final drill and ream.

#machining

Show threadsmells of bikesMay 15, 2022
@mcdanlj wow that looks like a kinda awful project.
I've had fairly good luck with pulley drills that only have maybe 4cm of flute on the end. I've had terrible luck with full flute length drills in anything over about 10cm deep. Too flexy and they appear to expand to larger diameter as they cut, and bind.
Show threadMichael K JohnsonMay 15, 2022

@smellsofbikes I'm still learning what's a reasonable pilot drill size in steel. Most of my work has been aluminum. Anything with a split point?

Maybe I should have started with a stubby as deep as I could drill, then switched to the 6" aircraft drill?

I don't know "pulley drills" — can you enlighten me?

Show threadsmells of bikesMay 15, 2022
@mcdanlj the one I have I got from boeing surplus. It's 1/4inch probably 15 inches long with just a short twist fluted section at the very end. It's been really good for high aspect ratio. When it dies I don't know where I'll find another.
Show threadMichael K JohnsonMay 16, 2022

@smellsofbikes Down the rabbit hole I've gone, learning about gun drills. I have not run coolant in my lathe, only kool-mist or oil, and a gun drill requires relatively high-pressure coolant through the bit for chip evacuation, which I don't think the existing pump in the G0709 would supply.

So now I know why it could be hard to find this particular kind of twist drill. I'll bet that in practice most shops are using something else entirely for this kind of work.

Show threadMichael K Johnson
@smellsofbikes I suppose the next step into this rabbit hole is cutting my own drills, and regretting that I got a BS-1 semi-universal instead of a BS-2 universal dividing head.
May 16, 2022 at 11:57amWeb