[SEL] sulfur and sulfurized lubricants for cutting metal

[Richard Allen]

The way sulfur works as a metal cutting lubricant is that it melts at a
fairly low temperature to make a thick oily liquid that is very
slippery.

Sulfur is peculiar because it melts twice. Just above the boiling point
of water at about 106° C (223° F) it melts, but when the temperature is
raised slightly it freezes again, then at about 115°C (239° F) it melts
again. Either molten phase works well as a cutting lubricant, but
because most machining operations create cutter point-of-contact
temperatures above the second melting point, it is in that form when
working.

The reason why sulfur is mixed with oil, grease, and lard is for
several reasons. At room temperature, sulfur is a solid a lot like hard
rosin (which is also a very good cutting lubricant but expensive), so
it has to be powdered and mixed with a liquid other than water to get
it into the cut. Sulfur by itself catches on fire at the temperatures
generated during cutting, so mixing it with oils and greases prevents
this. Sulfur is also very active chemically and it reacts directly with
the copper and zinc of brass, and when wet it rapidly attacks iron.
When heated in contact with air it forms sulfuric acid. All this is
very bad for the parts being made and the cutting machinery, so the
oils and greases are used to neutralize sulfur’s bad properties.
Because sulfur is very reactive chemically it forms several different
kinds of compounds with the oil and grease molecules, and those in turn
are also extremely good cutting lubricants. What actually goes on at
the point of contact is not fully understood, but apparently the
sulfur-oil compound molecules break apart at higher temperatures and
release the sulfur as minute or colloidal particles, which help push
the metal out of contact with the cutting tool. 

The 250-year old unbeatable standard cutting lubricant for drilling and
lathe work, as well as taps and dies, is sulfurized lard. I don’t know
how it is made, but because it was around 250 years ago it can’t be too
difficult. I have always used plain de-salted bacon fat or lard for
cutting steel because I can’t tell the difference between it and
sulfurized lard when using a torque wrench to determine how much force
is needed to turn a tap. Mice and rats eat lard but usually won’t touch
sulfurized lard, though, and more than a few times my dog got the can
of bacon fat off the lathe and ate it along with all the metal chips in
it, and the brush, too. Like I say, work and play safely, and that
includes your pets. 

Free machining stainless steels and steels have sulfur, selenium, or
lead introduced into the metal when being alloyed. When these alloys
are cut, the free sulfur, selenium (selenium is very much like sulfur
but EXTREMELY poisonous), or lead act as cutter lubricants, but the
down side is that free machining alloys are rather weak and have
limited usefulness, plus they don’t weld very well. Lead is also used
in some armor piercing bullets as a lubricant, and does an excellent
job judging from the holes I have seen all the way through 6-inches of
hot-rolled mild steel that were made using an ordinary hunting rifle.

Work and play safely,

Richard Allen