revented.
It is not always feasible to use a chuck jaw as a driver and then a
special driver having the form of a small angle-plate or block is
sometimes bolted directly to the table. Another method of driving is to
set a brace between a spoke or projection on the work and a chuck jaw or
strip attached to the table. Drivers are not only used when turning
flywheels, but in connection with any large casting, especially when
heavy cuts have to be taken. Of course, some castings are so shaped that
drivers cannot be employed.

=Turning in a Boring Mill.=--The vertical type of boring mill is used
more for turning cylindrical surfaces than for actual boring, although a
large part of the work requires both turning and boring. We shall first
consider, in a general way, how surfaces are turned and then refer to
some boring operations. The diagram _A_, Fig. 4, illustrates how a
horizontal surface would be turned. The tool _t_ is clamped in
tool-block _t_{1}_, in a vertical position, and it is fed horizontally
as the table and work rotate. The tool is first adjusted by hand for the
proper depth of cut and the automatic horizontal feed is then engaged.
When a cylindrical surface is to be turned, the tool (provided a
straight tool is used) is clamped in a horizontal position and is fed
downward as indicated at _B_. The amount that the tool should feed per
revolution of the work, depends upon the kind of material being turned,
the diameter of the turned part and the depth of the cut.

[Illustration: Fig. 4. (A) Turning a Flat Surface. (B) Turning a
Cylindrical Surface]

Most of the parts machined in a vertical boring mill are made of cast
iron and, ordinarily, at least one roughing and one finishing cut is
taken. The number of roughing cuts required in any case depends, of
course, upon the amount of metal to be removed. An ordinary roughing cut
in soft cast iron might vary in depth from 1/8 or 3/16 inch to 3/8 or
1/2 inch and the tool would probably have a feed per revolution of from
1/16 to 1/8 inch, although deeper cuts and coarser feeds are sometimes
taken. These figures are merely given to show, in a general way, what
cuts and feeds are practicable. The tool used for roughing usually has a
rounded end which leaves a ridged or rough surface. To obtain a smooth
finish, broad flat tools are used. The flat cutting edge is set parallel
to the tool's travel and a coarse feed is used in order to reduce the
time required for taking the