270 FLUIDITY AND PLASTICITY
shops all over this country, with concurring evidence from Great
Britain, establishes the fact that fixed oils, preferably lard oil,
are superior to all others. This is particularly true in operations
such as •'parting off" soft steel, in threading wrought iron or
steel, in drilling deep holes in steel as in the manufacture of gun
barrels. The tool keeps its edge longer, the machine runs more
smoothly, there is less heating, a much greater speed may be
attained, the chip is less serrated and therefore longer, the cut
surface is smoother and much closer dimensions may be obtained,
when using lard oil or its equivalent.
On the other hand, there are certain operations such as planing
and reaming where a lubricant is not required. In others such
as sawing metals a liquid may be used merely to cool the work.
No lubricant is ordinarily used in cutting cast iron, brass or
aluminum. Wrought iron and "draggy" metals require a
lubricant.
Between the two extremes of those operations and materials
which absolutely require a fixed oil and those which require no
liquid at all, there are a great number of classes of work in which
mineral oils are satisfactory but where aqueous soap solutions
or oil-emulsions are widely used and found to be highly satisfac-
tory. In these cases the oil or water serves to reduce the heating
of the work and the tool, and the soap or soda prevents the rust-
ing of the machine. Fixed oils are often a needless extravagance
or positively disadvantageous.
Where lard oil is required it is not primarily to conduct away
the heat, for the operation may be a light surfacing operation
where the heat developed is slight as in the cutting of fine micro-
meter screws. Its superiority does not depend on its peculiar
viscosity because a mineral oil possessing the same viscosity in
no way shares its superiority.
It is true that mineral oils increase in fluidity, when heated,
more rapidly than fatty oils, but castor oil is exceptional in this
respect resembling the mineral oils and yet it appears to be a very"
useful cutting oil and lubricant.
It has also been suggested that pressure might decrease the
fluidity of the mineral oils less rapidly than that of the fixed oils,
but this explanation appears to be not even qualitatively correct
(cf. page 89, Report of the Lubricants and Lubrication Inquiry