machines, such as are used for turning very
large flywheels, sheaves, etc., have a swing of 16 or 20 feet, and
larger sizes are used in some shops. The size of a vertical mill, like
any other machine tool, should be somewhat in proportion to the size of
the work for which it is intended, as a very large machine is unwieldy,
and, therefore, inefficient for machining comparatively small parts.

=Holding and Setting Work on Boring Mill Table.=--There are three
general methods of holding work to the table of a boring mill; namely,
by the use of chucks, by ordinary bolts and clamps, or in special
fixtures. Chucks which are built into the table (as illustrated in Fig.
2) and have both universal and independent adjustments for the jaws can
be used to advantage for holding castings that are either round or
irregular in shape. The universal adjustment is used for cylindrical
parts, such as disks, flywheels, gear blanks, etc., and the independent
adjustment, for castings of irregular shape. Chucks which have either an
independent or universal movement for the jaws are known as a
"combination" type and usually have three jaws. There is also a four-jaw
type which has the independent adjustment only. This style is preferable
for work that is not cylindrical and which must be held very securely.
Chuck jaws that do not form a part of the machine table, but are bolted
to it in the required position, are also employed extensively,
especially on comparatively large machines.

Most of the work done in a vertical mill is held in a chuck.
Occasionally, however, it is preferable to clamp a part directly to the
table. This may be desirable because of the shape and size of the work,
or because it is necessary to hold a previously machined surface
directly against the table in order to secure greater accuracy.
Sometimes a casting is held in the chuck for turning one side, and then
the finished side is clamped against the table for turning the opposite
side. Parts which are to be machined in large quantities are often held
in special fixtures. This method is employed when it enables the work to
be set up more quickly than would be possible if regular clamps or chuck
jaws were used.

Work that is to be turned or bored should first be set so that the part
to be machined is about central with the table. For example, the rim of
a flywheel should be set to run true so that it can be finished by
removing about the same amount of metal around the en