On Nov 9, 9:27*pm, "Michael Koblic" wrote:
...
Example: Using my mini-mill, I start the hole with a No.1 center drill and
then change to a twist drill (say 7/64"). X and Y are locked. As I bring the
drill down it is clearly off centre - today I measured it and it is quite
consistent: The drill point moves 0.010" "east" and 0.005" "south" to enter
the starter hole. If the full hole is then drilled it is slanted ever so
slightly - perhaps 0.001" over 0.25" length. This happened with two
different 7/64" twist drills.

I tried a different No. 1, I tried both ends, same result. Looking at the
slowly rotating point with a magnifying glass it describes a small circle
which is not obvious when I bring it down on the metal. However, there is
perceptible vibration of the mill which is absent if I drill with the twist
drill. I interpret this that the mill head is doing the circles while the
point is embedded. If I had a more rigid set-up the circle would perhaps be
apparent....

Michael Koblic,
Campbell River, BC

I've seen those problems and think I've traced them to wear and play
in the quill and dirt or an accumulation of tolerances in the chuck
tapers and jaws. My cheap drill press shifts visibly when I pull on
the handle enough to make a drill bite into steel, and the loading on
the quill rack changes from supporting it to forcing it down.

I've see a center drill make the mill head vibrate, don't remember
which chuck but the fix was clamping the drill in a collet. Possibly
there was a chip in the taper?

You can check runout and shift under load by indicating a chucked rod,
and quill perpendicularity by chucking the indicator and running it
down an angle plate. This isn't the same as tramming it since the
spindle axis may not be parallel to the quill.

jsw