Fractions of a complete worm-shaft turn are obtained with a micrometer dial or with an index plate on
the worm shaft. An index plate is a disk and has in its surface several holes, arranged in circles. The
holes within each circle are equally spaced with accuracy. The circles of holes are concentric with the
worm shaft. A spring-loaded pin in the crank handle is adjustable to fit into holes of a particular circle to
be used.
The index plate with the crank and spring loaded pin provide an accurate means for obtaining desired
fractions of a complete worm-shaft turn. One space between two adjacent holes in a particular circle
may represent several degrees or minutes of angular movement of the rotary table. A vertical milling
machine provided with a rotary table may machine a complete or partial cylindrical surface having any
desired radius. Internal cylindrical surfaces with radius that is too small for available end mills may not
be cut. Partial cylindrical surfaces may be joined to flat surfaces by using rotary-table feeds with the
regular milling machine feeds.
UNIVERSAL DIVIDING HEAD
Like a rotary table, a universal dividing head may be used for rotating a work piece through precise
angles. A dividing head has a spindle that supports and rotates the work piece.
A center may be mounted in the tapered hole at the front end of the dividing-head spindle for holding a
work piece between centers.
Other work piece holding devices, such as a chuck, also may be mounted on the front end of the
dividing-head spindle. The work piece is mounted so that it will rotate accurately concentric with the axis
of rotation of the dividing head spindle. The work piece is rotated by turning the index crank. Since the
gear ratio is 40 to I, it takes, 40 turns of the crank to rotate the spindle and work piece through one
complete revolution. In comparison with a rotary table, that rotates only about a vertical axis. A dividing
head may rotate a work piece about an axis at any angle, and its indexing ability is much greater. The
dividing head spindle axis can be positioned at various angles from below the horizontal to slightly
beyond the vertical position.