John Emmons wrote:

Why do holes made by normal twist bits almost always end up being out of
round? Even with my drill press.

A reamer cleans them up but what causes the problem in the first place? And
is there anything I can do to minimise it?


The drill bit has two "points", the outer edge of the cutting flutes,
that try to
rattle around in the bore that it is starting. Very frequently, you
will get 3-lobed
or 5-lobed shapes in thin sheets. As the drill progresses into thicker
material,
the twisting of the flutes gives it more secure support around the
circumference,
and the chattering of the point is reduced. But, in a thin sheet, the
drill has
gone all the way through before this can happen. The point angle and rake
to the cutting flutes have some bearing on this, and there are a bunch
of drill
point designs that reduce the problem (split-point, brad point and many
others).
These can all be optimized for specific workpiece materials.

A very stiff mounting of the workpiece and a short drill bit, such as a
lathe
center drill will greatly reduce this problem. The Jobber's length
drill bits
are quite flexible, as you can see when you start these bits, the point will
start to bounce around.

The reamer follows the centroid of the hole, which has no guarantee of being
anywhere near where you wanted the hole. So, better drilling techniques
will also give you a better final hole size. For almost any job where I
want
a hole to line up with something, I use a milling machine, clamp the work in
a vise or to the table, and start the hole with a center drill first.
Once the
center drill has made a pretty good starting hole, the twist drill will
follow
with a lot less of this shuddering, and the final hole will be both more
round
and closer to where I started it. I usually get hole locations within a few
thousandth of an inch of intended position this way. Using a center punch
and then drilling directly with a jobber's length drill, the hole
position can
drift 25 thousandths of an inch, or even more.

Jon