LUBRICATION 273
welded to the tool at this point. The tool therefore pries the
chip away rather than cuts it, and the point of the tool merely
clears up the surface, so long as the tool is well lubricated.
The surface of the chip is serrated and of about twice the
thickness of the cut. We have here evidently a case of plastic
flow. The explanation of the serrations and the thickening
is probably as follows:—As the tool moves into, the metal, the
strain gradually increases and a certain accommodation takes place
due to the elasticity of the metal and the machine. When the
shearing stress reaches the yield point, the metal flows, and the
more rapidly as the temperature rises rapidly in the region of
flow. In this process the pressure on the tool is relieved, the
stress falls again below the yield point, and the process is repeated.
If the machine is very sturdy with very little play, the cutting
will be steadier, but here comes the advantage in the use of a
good lubricant, that it is drawn into the space TO, contaminates
the under side of the freshly formed surface of the chip and there-
fore substitutes viscous flow for the energy-consuming plastic
flow to a greater or less degree depending upon the efficiency of
the lubricant (cf. Taylor, "The Art of Cutting Metals")-
If the lubrication is not effective, the pressure on the tool must
be relieved to a greater extent by means of plastic flow of the
material. The result is greater fluctuations in pressure, the metal
flowing outward during the period of flow, producing serrations
of increased height, and possibly flowing downward into the
space m. It is this metal, flowing inward toward the work and
the point of the tool which creates the most serious condition,
for it tends to break off the edge of the tool and to gouge into the
face of the work.
'With brittle substances such as cast iron, it is readily per-
ceived why a lubricant is not necessary. The chip breaks
as it is pried off and there is comparatively little if any plastic
flow. In cutting very hard and brittle materials such as glass
and some varieties of steel, a lubricant as such is not needed, but
something which perhaps has just the opposite property of
causing the tool to adhere to the material, i.e., will cause the
tool to "take hold" or "bite." Turpentine is used for this
purpose on steel and turpentine with or without camphor is
used on glass. It is difficult to see how these substances act
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