A very useful piece of tooling for checking various machine problems is a
drill or reamer blank, as suggested by DoN.

The blanks I have are Cleveland brand, and are fairly precisely round,
straight and have a conical point on one end. Several sizes of blanks will
facilitate checking many different conditions of chucks, and different sizes
of collets.

Clamping a blank in a chuck or collet can reveal several aspects of
center-of-axis and runout, and likely to be very helpful in indicating a
condition of direction of travel of the inaccurately placed drill in this
situation, shown clearly by using a square or angle plate as recommended.

Some folks may recommend a rod from a printer, which may be ground fairly
accurately, but may also have worn spots (probably near the center of the
length) from many cycles of the printer's head assembly.

The chattering can be expected for the stated drilling situation, as one
cutting edge of the drill is very likely catching metal before the other
cuting edge, beginning the rapid rotational skipping/flexing of the drill.
After the centering alignment issue is corrected, chatter may still occur
since the 60 degree sides of the center drill's hole don't match the angle
of the drill's point.
One method that eliminates chatter is lightly contacting the drill point
with the workpiece before the power is turned on. When the power switch is
flipped on, adding moderate down-feed pressure will generally cause the
drill to cut without chatter.

I prefer to use a straight-drilled pilot hole, that is approximately the
same size of the web section of the larger drill. This eliminates the need
for the chisel point of the web in the large drill to displace the metal
directly under the web.
Split-point drills generally don't need a smaller pilot hole (with or
without a center punch or prick punch mark), since they begin to cut as soon
as the points contact the workpiece, and continue to cut without having to
force the web of a conventionally ground twist drill into the workpiece.

--
WB
..........


"Michael Koblic" wrote in message
...
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