The purpose of center drilling is to start the hole exactly where intended
without the drill point wandering all over the place, yes? This is then
normally followed by a twist drill of the desired size etc. From this
concept I would assume that the axes of the drills are concentric, or in
other words the hole drilled by the twist drill is exactly concentric with
the hole started by the center drill.

This does not seem happening in my case and I am wondering why.

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.

I tried the same experiment with a No.2 and No.3 - same result.

I thought I'd better find out which is the true center: The "center drill"
or the "twist drill" one. This was even more complicated than I expected. I
used two centere finders on small punch marks. They both showed center
differently! The centre found by the barrel-type coincided with the center
drill point, the wiggler type was quite significantly off (I use 10x
magnifying glass to get the best accuracy with both).

So the questions at this stage we
1) Is this a normal behavior? I thought unlikely...
2) Is this because of cheap Chinese center drills?
3) Is this a function of the mill chuck?
4) Is there some other reason?

I was wondering about the way the drills are clamped in the chuck and I
tried different degrees of tightening. The last effort involving only light
tightening of the chuck both for the centre drill and the twist drill I
managed to hit the centre-found punch mark with both the centre and twist
drill.

Is it possible that over-tightening the chuck throws things out of kilter? I
hope to repeat this with the bigger center drills tomorrow but I would
appreciate any insight.


--
Michael Koblic,
Campbell River, BC