"Ignoramus13011" wrote in message
...
On 2009-01-31, Ed Huntress wrote:
That's what I'm trying to address. I think Iggy understands the issues,
but
an implication was creeping in there that coolant is just better overall.
Actually, water-miscible coolant is a good solution for balancing several
competing demands, but the big ones just don't apply to hobby machining.

Are you comparing coolant with no coolant, or coolant with oil or
other lubricants?

Coolant (water-miscible oil, often called water-soluble oil, but it doesn't
actually dissolve) with straight oil.



The others that earn their living making chips deserve to do whatever
they
think is best for them.

Sure. And the cost equations that weigh all of these factors are the
bread
and butter of industrial engineers, as well as some manufacturing
engineers.
Commercial shop owners know about them but apply them somewhat unevenly.

In high-volume production today, in the car industry supply chain and in
the
making of many consumer products, the hot topics are dry- and near-dry
machining, where tools cutting at 4,000 sfm in hardened steel are
throwing
red-hot chips, making noise like a machine gun, and the workpieces are
coming out cool.

So, Ed, what I do not understand is how does the cutting tool;, under
these conditions, managed to stay cool enough to retain cutting
qualities.

There are three things involved. First, many of these tools are not steel or
tungsten carbide. They're cubic boron nitride (CBN) or any of various
ceramics.

The ones that do have a carbide substrate and are capable of high-speed dry
machining are multi-coated. Some of the layers are for edge-wear resistance,
some for crater resistance, some for insulation, and sometimes a top coat of
moly disulphide or a low-friction ceramic is laid on to ease the break-in
for the layers below, which tend to be rough and to drag a lot until they're
broken in.

A thick aluminum oxide later provides some bulk insulation but its primary
insulating is done through sublimation, which requires a *lot* of heat, and
the vapor thus produced forms an insulating gas layer on top of the tool.

Third, at higher speeds the heat tends to concentrate in the chip, rather
than in the tool or the workpiece. When you have one of these processes
well-tuned, the chips are red-hot but the workpiece remains cool enough to
touch. The cutting tool is hotter, but much cooler than you would expect.
This is the realm that is almost exclusively about dry machining.

This is all highly engineered stuff that requires high spindle speeds, lots
of horsepower, and machines that are as rigid as a headstone, and is of no
use to hobby machining.


Different things happen in different metalcutting speed/power realms. And
the hobby-shop realm has little to gain by applying techniques from the
higher-speed realms. Our relative cutting conditions dictate a whole
different set of solutions.

Some day we can talk about the realm above 10,000 sfm. That's where it
*really* gets interesting.

Let's talk about it and have some fun.

It's confined mostly to aerospace. The initial experiments, at Lockheed and
at Carnegie Melon Univ., used .30 caliber bullets shot across the edge of a
cutting tool, and spark-gap high speed photography to help figure out what
is happening.

At around 10,000 sfm (3,000 m/minute -- in the neighborhood of 100 mph) the
chips thin out and extrude ahead of the cutting edge. The shear area
narrows, which changes the heat distribution, until almost all of the heat
goes off with the chip.

Most interesting is that the energy required to remove a volume of metal
falls off sharply somewhere between 5,000 sfm and 10,000 sfm (I forget the
details) and horsepower requirements actually start dropping.

There are 50 hp, 100,000 rpm and 100 hp, 50,000 rpm spindles made for custom
aerospace applications. They're something to see.

--
Ed Huntress