Post by Bruce Klawiter
I do a ton of work with small cutters and in aluminum, I use a coolant
mister with alcohol, I use as much air as possible and just enough
alcohol to keep the part wet. The parts feel like they were in the
freezer when I am done. As Jon said keep the work cold.
Guy's, maybe I don't understand cutting alu as well as I thought.
All along, I have believed that it was more important to keep the oxygen in
the air away from the cutting surface in order to slow the formation of alu
oxide on the surface, which in normal air, not blown, can get a good start
in 0.001 seconds or less behind the cutting tools edge as alu is a VERY
active metal, oxidizing (rusting of ferrous material is exactly the same
reaction at a rate millions of times slower than the alu rate) very
rapidly, and its this thin film of oxide that is its own protective
barrier, putting out the fire so to speak. This oxide is also the 2nd
hardest substance known to man and can take the edge off a carbide tool
that has to cut thru it in seconds under the right set of wrong cutting
params, which my slow feed made worse. Dig cutting being worse in this
Sealing the cut surface against the air and its oxygen, blown or otherwise,
that causes this instant alu oxide film with its subsequent wear on the
cutting tool has always been the reason for my use of a cutting oil, deep
enough to flood and seal the surface, or misted, particularly when I don't
have the spindle rpms to throw it away from the cut. Misting works better
because it blows the cut chips away, preventing recut damages on the
surface. But my air compressor is outside and I didn't want to throw off
the tarp and open the shop door so I could plug it in.
Consider also that the higher rpm spindles put the cutting edges past the
cut surface so fast that the only alu oxide they see is on the original
surface before the cut and doesn't have much of a chance to reform before
the cutting tool has moved on.
The majority of the heat you are referring to is not the heat of the tools
cutting action, but is the result of the chemical reaction that forms this
alu oxide film so rapidly. So my theory has always been to seal the oxygen
away from the cut surface as well as you can with whatever you can that is
not oxygen bearing. Some oils, including the particular cutting oil I
used, can have quite a bit of available oxygen & therefore will not be as
effective as one would think at slowing this 'rusting' reaction.
Based on that same theory, it would be my contention that if you were to
hook a bottle of dry nitrogen up to that mister, and arrange a cover over
the workpiece so as to contain it, totally excluding the air, (this isn't
at all healthy for the machine operator for obvious reasons) then it should
be possible to machine alu with excellent tool life even running dry. Any
noble gas would do, but dry nitrogen is generally an almost let it vent
gas, saving only what they can sell, in the grand scheme of trying to make
a profit from an air reduction facility, so nitrogen is, compared to the
other gasses, dirt cheap.
However, unless you have your own air reduction facility, the cost of the
dry nitrogen would exceed the cost of the tooling saved, which puts this
theory into the real world as practical only for the most critical work.
Worth it only to the extent this cost can be passed on to the customer.
Now, I an not enough of a chemist to know how well an alcohol mist keeping
it wet would function, but if it was truly being kept wet and sealed so
there was little or no chance of the oxygen in the air supply actually
getting to it until the machining is completed, its possible that this
could be close to the ultimate, effective AND cheap method.
FWIW, this rapid oxidation is one of the reasons powdered alu is used in
some explosives, fireworks being one that comes to mind.
Its entirely possible that my choice of cutting oils (comes in a quart
plastic container labeled 'Cutting Oil' from ACE Hdwe in this case) was
actually not a very good product for this usage as it had lots of free
oxygen. Obtaining this sort of info off the label of a plastic container
of a product normally sold as a pipe thread cutter lubricant simply isn't
going to be done as that isn't deemed important enough to list amongst the
other carnival barker text on the back panel.
Its also possible that because there is available oxygen in this particular
oil, and that combined with the slowness of the cut, that my tool was
actually swimming in what gradually turned into a mud that was not alu, but
nearly pure oxide, wearing the tool just as if I'd spun it by hand on a
green wheel. Finding the broken off piece and giving it a good look under
a microscope would tell that tale.
So we each are left to be doomed to finding a method that seems to work,
and often without a real, controlled condition set of test results to guide
us. Obviously I found one that didn't work. :( I was using the slow feed
rate to reduce the deflection of a 1/2" long 1/16", 4 flute, upcut spiral
TiCN coated tool, it was the only one I had on the shelf at the time. And
a .005" deep cut for the same reason, figuring on multiple passes to do the
whole thing. With a 1/8" long bit I would have pushed it 6 or 8x faster.
But that's me, making excuses to explain my failure too, so everybody knows
that I know most of the "why".
Is there some other, even more important condition we need to maintain at
the cutters edge other than an oxygen free environment?
This to me, seems like the single, perhaps even 'only' in comparison with
other effects, important item when machining alu. Or any other highly
reactive material, but alu is about 10,000x worse than all the others
Has anyone with a machining center with relatively tightly closing doors
ever checked this out by closing the doors and bleeding only nitrogen thru
the misters instead of using compressed shop air with a cutting fluid?
Its my belief you would be surprised at the tool life obtained, and with
the quality of the finish too. You should, as the doors open, see the
dulling effect on the finish, the result of the air finally getting to it.
That is my take on it. How right or wrong am I?
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