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.
This is the first part I've made for my keyway cutter. Ten left. This will take a while.
Actually a dozen more. I forgot two spacers in my count. ☺
13 total parts to machine. I guess I'm lucky!
I think I finally understand why it wandered. Read an eight-year-old thread on hobby-machinist.
https://www.hobby-machinist.com/threads/carriage-drilling-adapter.21869/post-948875
@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?
@smellsofbikes Hmm, then it must exist somewhere, one would think... I don't see anything like it at Aircraft Spruce & Specialty. McMaster https://www.mcmaster.com/2986A59/ similarly looks like jobber-length flutes on a long stem, not mechanic-length flutes on a long stem. I could imagine starting with something like https://www.mcmaster.com/3171A44/ and then following with a long bit.
I guess I just need to get a diamond grinder and shorten the flutes on some of those aircraft drills. Because I need more tools!
@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.
Michael K Johnson
smells of bikes