In addition to Tim's suggestions, use a quality cutting lubricant, a quality
endmill, and recheck tightness of head-to-column and column-to-base screws.

Forces like those you're encountering can cause the material around the tips
of setscrews to be displaced, effectively allowing them to walk.

If the drill point/web is fed to the desired final depth, the endmill will
then be cutting everything but the center of the hole, until it stops
without using more downward Z force.
Retract the endmill immediately to prevent it from just rubbing, which will
dull a good endmill fairly rapidly.

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


wrote in message
...
I am sorry to ask something very basic. My excuse is that I am testing
the Forte Agent :-)

I need to make round holes in steel, 1/8" deep, with flat bottoms. The
diameter is usually either 1/4" , 5/16", 3/8" or 1/2" and is not
critical.

I do these on my mini-mill. I pre-drill the hole to the required depth
undersize and then use a 4-flute mill to square up the profile.

All is well until I start getting near the bottom. The machine begins
to labour and there is a tendency for the table and/or the head to
want to move in a circular fashion, clamping x and y notwithstanding.
I assume that this is due to the mill beginning to remove more
material, particularly the center portion of the hole.

The end-mills are supposed to be centre-cutting. There are two flutes
that meet in the middle, the orthogonal two flutes do not. All the
pictures I could find suggest that these are indeed centre-cutting
end-mills.

So the question is: Is this a normal behaviour for end-mills in this
situation? If not, what is the likely cause? Is there a better
procedure to do this?

Michael Koblic,
Campbell River, BC