"anorton" wrote in message
news snip-
The tendancy to go around in circles hints that there might be something
slightly asymmetric in the mill. In my minimill, the collets are well
centered on the rotation, but the rotation axis angle is slightly off from
the z-axis, but not enough to cause problems with the coarse stuff I have
done so far. The perpendicularity of the Z-axis to the stage was way off,
but at least that was adjustable.

The harsh reality is that end mills do not make good drills.

A study of the geometry of an end mill quickly discloses the fact that there
is little chip relief between the flute and the centerline of the cutter.
All it takes is a slight chip load, one side greater than the other, for the
end mill to begin oscillating as has been described. The problem can be
addressed by relieving the end mill towards center, creating greater chip
space, but that comes at the cost of losing considerable strength in the end
mill, possibly resulting in a broken lip.

Do note that an end mill does not normally create a flat bottom, but leaves
a high spot in the center.. They are ground with a slight taper towards
center to insure that the periphery is the lowest point in contact, creating
a flat cut when the cutter is advanced on material.

Part of the problem with using an end mill for drilling is the fact that the
periphery is relieved with a primary and secondary land. They are intended
to cut on the periphery. Drills, by sharp contrast, are circular ground, so
they pilot. The circular portion is generally relieved to reduce the
contact area in an effort to lower friction.

If one desires to create spot faces or shallow counterbores, the option to
use a counterbore with a pilot is always open. That yields not only a flat
cut, but prevents the oscillation that is so troublesome. Negative
aspect is that it's easy to break the pilots.

Harold