In article , Dave Baker says...

The thinner the drill the more it flexes and wanders around. Once you have a
hole that's on the **** the larger drill tends to follow it and the job is
****ed. If you seriously think you can drill an accurate 2 inch deep hole with
a 1/16" drill then I have to suggest you've never tried it.

It can be done. Key is to use a high quality drill. I wound up drilling
an 0.043 hole in a two inch long piece of permaloy. But then I cheated
and drilled from both ends.

This was done with a Gurhing parabolic flute high-cobalt drill, with
the chuck mounted on the lathe compound. Infeed was done with the
compound aligned with the bed, and chip clearing was done with
the carriage traverse, running up to a stop on the bed.

There is one good reason for double drilling and that's to get good tolerance
on hole size.

Here I have to disagree with you Dave, even though you're clearly
well-skilled in general. Larger drills don't cut on their dead center
and the forces that occur at this point do tend to induce wander.
I prefer to drill a straight hole with the small drill first, as
you say the larger one will follow that, spot on.

I would say it's easier to drill a straight hole with a drill that
has the smallest dead center - sure you can get the 135 degree
split point drills, but my tiny drill was ground that way from
the manufacturer too.

Key in any event is to use the highest quality cutting tool you can
get. I was trying to do the permaloy tube thing using cheapistan
HSS drills and was getting about a quarter of an inch of action out
of them before they dulled and had to be swapped.

I drilled three two inch permaloy tubes with the *same* guhring
drill.

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