ient form for lathe roughing tools showed that the nearer the lip
angle approached sixty-one degrees, the higher the cutting speed. This,
however, does not apply to tools for turning cast iron, as the latter
will work more efficiently with a lip angle of about sixty-eight
degrees. This is doubtless because the chip pressure, when turning cast
iron, comes closer to the cutting edge which should, therefore, be more
blunt to withstand the abrasive action and heat. Of course, the
foregoing remarks concerning lip angles apply more particularly to tools
used for roughing.

[Illustration: Fig. 14. Grinding the Top and Flank of a Turning Tool]

=Grinding a Lathe Tool.=--The way a turning tool is held while the top
surface is being ground is shown to the left in Fig. 14. By inclining
the tool with the wheel face, it will be seen that both the back and
side slopes may be ground at the same time. When grinding the flank of
the tool it should be held on the tool-rest of the emery wheel or
grindstone, as shown by the view to the right. In order to form a curved
cutting edge, the tool is turned about the face of the stone while it is
being ground. This rotary movement can be effected by supporting the
inner end of the tool with one hand while the shank is moved to and fro
with the other.

Often a tool which has been ground properly in the first place is
greatly misshapen after it has been sharpened a few times. This is
usually the result of attempts on the part of the workman to re-sharpen
it hurriedly; for example, it is easier to secure a sharp edge on the
turning tool shown to the left in Fig. 12, by grinding the flank as
indicated by the dotted line, than by grinding the entire flank. The
clearance is, however, reduced and the lip angle changed.

There is great danger when grinding a tool of burning it or drawing the
temper from the fine cutting edge, and, aside from the actual shape of
the cutting end, this is the most important point in connection with
tool grinding. If a tool is pressed hard against an emery or other
abrasive wheel, even though the latter has a copious supply of water,
the temper will sometimes be drawn. When grinding a flat surface, to
avoid burning, the tool should frequently be withdrawn from the stone so
that the cooling water (a copious supply of which should be provided)
can reach the surface being ground. A moderate pressure should also be
applied, as it is better to spend an extra minute or two in g