On 10/31/2010 04:58 PM, wrote:
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?

It's more that there's really no way to cut the center of the circle --
the best any drill bit or center cut mill can do is sort of mash the
metal out to where it can be cut, and to minimize the amount of material
that has to be so "mashed".

I'm no machinist, so take these with a grain of salt. But here are some
solutions that I can think of:

1: get a more rigid machine to work with. I can do what you're talking
about just fine in aluminum on a Smithy "3 in 1" (lathe, drill, lousy
excuse for mill), but I doubt it'd like me trying in steel.

2: Check your alloy -- are you using one of the absurdly easy-to-cut
alloys like 11L14? Can you?

3: Pre "drill" with a smaller mill, let it walk around, then finish with
a larger mill.

4: Put the part in a rotary table, and come down in the 'z' direction
while rotating the table -- this will keep your cut moving around, and
should make it easier on the machine.

5: I assume you'd be doing this already if you could, but consider
drilling a 1/16" pilot hole, and milling down over that. It'll leave a
hole or a dimple in the middle of your big hole, but even that small of
a pilot hole should span what your mill really can't easily get through.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html