M

f I

272

FLUIDITY AND PLASTICITY

that the operation was not as satisfactory as at the beginning.
The disk shown in the left of Fig. 86 was perfectly smooth, and
there was little evidence of heating and on inspection the tool
was found to be not even slightly dulled. The chips shown below
the disk were only slightly serrated.

On substituting the mineral oil heating began at once, the
surface of the disk shown at the right of the figure was very-
rough, the chips were deeply serrated, and the tool so dulled that
it failed completely on cutting the fourth disk. On examining
the cut in the "bar at the time of failure, shown in the middle of the

FIG. 87.—Illustration of the forming of a chip in the cutting of metals and of
the function of the lubricant.
figure, one can plainly see two beads of metal flowing ahead of
the tool and gouging into the bottom of the cut. A burr is
being thrown up at the left.
The operation of a tool in cutting is illustrated diagrammati-
cally in Fig. 87. The metal 6 is being cut away by the tool c, a
chip / being formed which bears down heavily upon the tool at
a point d some distance back from the point. That this is the
actual case is proved by many facts. Per example, a tool in use
is often, gouged out by the shaving at some distance back from
the point, and there is sometimes found a "bead" of metal