tomcas writes:

Like Dan G said- use your drill as though it were a reamer. Just
predrill first with a drill that's .005 to .010 undersize and then
follow up with your final drill size.


Hmm, you tried that? Or is this just what you imagine? In my
experience, drills cannot ream. You can't use such a small step up in
drill size, it won't cut, it just rubs and smears the metal. That's
why there has to be reamers for reaming.


I hope I'm not imagining it, I've done this a couple of hundred times
in the past 26 years. Depending on the hardness of the metal and
sharpness of the drill any material removal over approximately .002
cuts and does not abrade. Sure reamers do a better job and when the
desired size reamer is available I naturally go that route but in
pinch I've found this method works fine.

I suspect there are three regions, (1) too small an increment to possibly
work, (2) small increments that may or may not work depending on a lot of
factors like drill sharpening, speed/feed, lubricant, etc., and (3) big
enough increments that almost have to work. What the incremental
dimensions between these regions are is the real question.

All I know is trying to go up one letter size (e.g., D=0.246 to E=0.250 to
get a nice 0.250 hole) is too small an increment, at least in mild steel
with the import TiN drills I have. Maybe two steps would work. But then
you're missing the purpose of roundness, accuracy, and finish.