the drill
just digs in wherever it happens to hit, with no self-centering,
Hasn't anyone heard of,let alone used, a center drill? And/or spot face
with an end mill tool?
Apparently not. You wanna-be machinist need to take a Machinist 101 (no
pun) in your local community college.
I learned these fundamental techniques in high school, 1&2nd years.
K
On Fri, Aug 14, 2020 at 8:21 AM Norman Yarvin <yarvin@xxxxxxxxxxxx> wrote:
On Fri, Aug 14, 2020 at 12:56:40AM +0100, Peter Fairbrother wrote:
[1] we had an argument about that here several years ago, which I losteven
though I was right - since then I have found out how it works.
After we had that argument I figured out how to actually make the
self-centering effect work for me when at the controls of a lathe, and
realized I owed you a bit of a retraction. The trick is not to do
what I'd done before, which was to start drilling at normal speed; if
you do that you get what I'd complained of, which is that the drill
just digs in wherever it happens to hit, with no self-centering, and
with the tip of the drill wobbling around as the workpiece rotates.
Instead, bring the drill up to the surface slowly and let it scratch
around while trying to enter. It'll try to migrate to the point where
the scratching takes the lowest energy, which is on the axis of
rotation. After it's stopped wobbling, you start feeding at normal
speed. The way you put it is another way to think of it:
It might seem that everything is symmetrical in the rotational frame,but if
the work is rotating and the drill is off-axis the drill cuts more on onecause
side than on the other. If you get the angles and forces right this can
the drill to wander in order to cut evenly on both sides, ie it followsthe
axis of rotation of the workpiece.
Still, neither your version nor mine explains what breaks the
symmetry. One thing that might break it is inertial effects, if
anything were rotating fast enough for centrifugal force to matter.
But I don't think that's the case.
Indeed, though I haven't experimented with it, I think you could see
the same thing in a drill press: the tip of the drill seeking the axis
of rotation after scratching around on the surface. But it's a lot
easier to get the effect in a lathe since before drilling you commonly
face off the surface to get it flat and level; in a drill press you'd
need the surface to be flat and level to begin with. Also, in a drill
press the feed is less controlled, so it'd be harder to keep the drill
tip scratching around.
The difference I see that breaks the symmetry is a matter of
practicality: it's easier to align the axis of rotation with the axis
of the material when the material is what's being rotated. With a
rotating rifle barrel, for instance, you can use a dial indicator on
the outside of the barrel to get it running true before drilling,
doing so at both ends of the barrel. Achieving the same level of
alignment between a non-rotating barrel and a rotating spindle at the
other end of the lathe wouldn't be nearly as easy.
There aren't any easy reference surfaces like that, though, with the
task that prompted this discussion: drilling cooling holes in a
hyperboloidal nozzle+throat+chamber. That'd be a complicated setup
either way.
