backlash was Re: Comments from expert PID programmers?

>
> Uhhh, No.
>
> There is no "pre-determined SAFE speed"
>
> You're forgetting that the cutter is ENGAGED with the material.

Very

> often the forces involved are enough to SLAM! the work from one

side of

> the "dead zone" to the other!
>
> Tools break, metal flies, folks get hurt.
>
> Part of the reason the FIRST things a manual machinist is taught

is NOT

> to climb cut on machines with ANY backlash...
>
> Ballendo
>

Never, Not, and No are so absolute. Does it really make sense to
have such a blanket rejection of climb milling if a machine has any
backlash? Must it always end in tragedy?

Seems to me, it requires careful thought and care, but it can be
done under the right circumstances. Isn't the worst case that your
chipload will suddenly bounce up to whatever it was plus the
backlash on your machine?

For example, if you are running a chipload of 0.001"/tooth for a
finish cut, and your machine has 0.003" backlash, suddenly the table
gets sucked in by the climb milling and you are running a chipload
of 0.004". That's worst case because the table has to move the
entire backlash distance before another tooth can engage and take
part of the load.

In this example, you're probably unhappy because your nice finish
cut is now taking the chipload of a roughing cut and you've got a
little divot you can see, but it does not seem like a scenario that
must involve breakage. Comparing the backlash to the chip load
seems like one approach to understand what the meaningful risk
scenario looks like.

Another thought is whether the cutting force for the particular
operation you're undertaking will be sufficient to drag the table
forward at all. For example, you've just lashed a trim router to
your mill head so you can run a 1/16" end mill at 20,000 rpm to cut
some aluminum. I've no doubt you'll break the end mill if your
table gets sucked in while climb milling, but how much force is the
little guy even generating to move that table? That's an extreme
example, but cutting force is going to vary with what you are
doing. In fact, climb milling will require less force to cut. If
you've tightened the gibs to increase the drag and chosen a small
cutter, you may never even trigger the dread trip through the dead
zone.

I mention all of this because I climb mill frequently with small
mills on finish cuts because it leaves a much nicer finish. I've
got about 0.003" of backlash in my IH Mill, it's got a big heavy
table, and the gibs are pretty tight even on a good day (hence
Aaron's recommendation to lap them). I wouldn't try climb milling a
big corncob aggressively into some tough material, but I don't know
if never climb milling is such a great plan either.

FWIW, here is one of many cutter force calculators on the net:

http://www.wenmag.com/workholding/cutcalc/

These guys make magnetic chucks. If you think the risk of climb
milling with backlash is bad, imagine applying more cutter force
than your mag chuck will hold!

In any event, a 1/4" endmill cutting steel with a chipload of
0.004"/tooth, 4 flute mill, 0.2" DOC, 3000 rpm spindle, 48.8 IPM
feed, is going to produce 117 lbs of cutter force according to that
calculator. The likelihood I am going to be running my mill on that
operation at 3000 rpm is just about nil (spindle won't run that
fast!), so the force will be less. If we look at a 1/8" endmill,
the same calculator now calls for half the force at twice the
spindle speed--50 odd lbs at 6,000 RPM (my spindle is SO not going
there either!). You see the trend.

One does read numerous accounts of climb milling gently with gibs
tightened and small cutters.

Not trying to start a fight, just seems like there are some ways to
analyze when and when not to climb mill.

Best,

BW