common mistake is to grind the lower or clearance side at too
great an angle. Fig. 34 shows the correct angle, and the dotted line A
illustrates the common tendency to grind the clearance.

THE PROPER ANGLE FOR LATHE TOOLS.--Now there is a reason why the angle
of from 10 to 15 should be maintained in the clearance. The point of the
tool must have suitable support for the work it is required to do, so it
will not chatter or yield in the slightest degree. A bit ground along
the dotted line has a cutting edge which will spring down, and
consequently break or produce a rough surface.

[Illustration: _Fig. 32. Fig. 33. Fig. 34. Proper Angles for Tools_]

Then, again, the angle of the clearance acts as a guide, or rather, a
guard, to prevent the tool from going in too far, as will now be
explained.

SETTING THE BIT.--In order to understand the correct setting, examine
the work A, in Fig. 35.

A is a cylinder being turned up in the lathe, and B the cutting tool,
which approaches it on a horizontal line, C, extending out from the
center of the cylinder A. This setting is theoretically correct, and in
practice has been found most advantageous.

In this case let us assume that the clearance angle D is 15 degrees, as
well as in the following figures.

[Illustration: _Fig. 35. Set of the Bit_]

Suppose we have a piece of tough steel, and the tool holder is raised so
that the point of the tool is at the 15 degree line E, as shown in Fig.
36, in which case the clearance line D is at right angles to the line E.
The line E is 15 degrees above the center line C.

THE SETTING ANGLE.--Now, it is obvious that if the tool should be raised
higher than the line E it would run out of work, because the clearance
surface of the tool would ride up over the surface cut by the edge of
the tool.

If, on the other hand, the tool should be placed lower, toward the line
C, the tendency would be to draw in the tool toward the center of the
work A.

In Fig. 37 the tool has its point elevated, in which case it must be
lowered so the point will touch the work nearer the center line C.

The foregoing arrangement of the tools will be found to be effective
where the material is soft and not too tough as with aluminum.

BAD PRACTICE.--Figs. 38 and 39 show illustrations of bad practice which
should never be resorted to. Fig. 38 shows the tool, held in a
horizontal position, but with its point below the center line C. With
any rough metal the too