"Paul Hovnanian P.E." wrote in message
...
Ignoramus10071 wrote:
A metalworking fluid perform several functions.
1. It is a great conversation starter
2. It cools the cutter
3. It lubricates the contact area where the tip of the cutter removes
material, and therefore less heat is generated. (not the same as 2.)
4. It prevents microscopic welding of material to cutter, thus
improving surface finish and effective sharpness of the tip.
5. For high speed machining, high pressure stream of coolant removes
chips. (not something we encounter with manual machines)
I would also like to learn when straight oil is more appropriate than
soluble oil.,
I suppose its a tradeoff. A lubricant provides coolant and (as in your #3)
a
coolant lubricates. The more heat you generate, the better the fluild's
cooling properties should be.
I'm also thinking about the use of a lubricant to reduce the load on the
workpiece/tool and resulting distortion. Cutting a thread by hand with
oil,
for example, seems to produce a better result than without, as the tap (or
workpiece) doesn't twist as much. Its not a heat factor, as I can just go
slow enough to allow conduction to dissipate that. The ultimate in
distortion when not using oil is busting a tap off. But short of that,
anything that reduces the forces will result in a more accurate cut.
--
Paul Hovnanian
----------------------------------------------------------------------
Have gnu, will travel.
Maybe it helps to look at where these coolants come from. Cutting oils,
animal, vegetable, or mineral, are so-so lubricants with very high film
strength but poor film-puncture strength; they can take a lot of pressure
but they collapse entirely when you exceed their pressure threshold. Thus,
they lubricate moderately well except right at the cutting edge. With
moderate power (old machines; flexible machines), they reduce cutting forces
and non-cutting friction but the oil is a poor conductor of heat.
Lubrication is more important than cooling at low speeds and low power.
Miscible-oil ("soluble" oil, which isn't really soluble) coolants came along
to answer several issues in production manufacturing. One was cost; they're
much cheaper, since they're mostly water. Another was tool materials that
allowed higher speeds and the generation of more heat, along with higher
horsepower and much higher spindle speeds. Cooling passed lubrication as the
generally dominant need. Miscible oils cool well because of the water. They
lubricate well enough, although not nearly as well as straight oils. They
were cheap enough that you could drain them into central sumps and keep
large reservoirs of them without breaking the bank. In the old days, you
could dump them in the nearest stream when you were done with them and they
wouldn't leave much of a slick. g
Where cutting forces or non-cutting friction is high, you can add sulfur to
oil and you get more film strength, with little influence on lubrication,
without increasing the puncture threshold. Tough cutting conditions, such as
tapping, benefit from the sulfur. You can also add chlorine or a variety of
other chemicals to get a somewhat mysterious reduction in shear strength at
the cutting edge. Somehow they get right into the shear area and reduce
cutting forces. This was the subject of a lot of research during the '50s.
Maybe they have it figured out now. They didn't when I was writing about it,
in the '70s and early '80s. The extreme example of this was carbon
tetrachloride, which produced a very large reduction in shear strength right
at the cutting edge. Don't use it.
And don't confuse water-miscible coolants with the newer synthetics. They
work by magic, or something like it. Or it seems that way to us
non-chemists. Somehow they produce combinations of reduced shear strength,
decent lubrication, and good cooling. They are not miscible oils, synthetic
or otherwise. They are magical chemicals.
Water-miscible oils are the common denominator today NOT because they
lubricate well, but because they became the industrial standard for many
types of machining by the '60s and they are readily available and still
relatively cheap. If you have an industrial machine and cutting tools from
that era or later, they're great. If you have older machines and if you use
a lot of HSS for turning and milling, particularly if your machine has low
spindle speeds, you're probably better off with straight oil -- sulfurized
or not, as you prefer. And, according to Doby Dave on AMC, they leave your
skin nice and smooth, and running sulfurized oil helps him keep his youthful
looks. g
--
Ed Huntress