"Anthony" wrote in message
...
snip----
IMHO and experience, you only want the undersized hole to be about 1/2 or
less of the dia of the finish drill. What is important for the hole size
is that the pilot hole is just slightly larger than the web wedge on the
following drill.

That isn't very effective when you're trying to achieve proper hole size,
but it's a good way to reduce drilling pressure.

The reason for leaving a minimum of material in the hole for the second
drill is to avoid the possibility of the cutting edges winging. It's
virtually impossible for that to happen when there's only a few thou in the
hole, even if the drill is sharpened improperly.

A drill sharpened with one edge longer than the other will compromise
between running on the cutting edge and pressing against the opposite
margin, trying to center the drill. This happens because the drill is
flexible enough for the pressure of the cut to force the drill off center,
balancing the forces. The result is a hole that is bell mouthed. The
hole gradually forces the drill to center, but the result can be a scrapped
part, depending on the nature of the work at hand. Tapped holes, for
instance, won't tolerate that condition.

Harold