Optical Dividing Head (Metrology)

8.4.
Optical dividing head consists of a circular scale, suitably calibrated in degrees and
fractions and fitted concentrically to the axis of the machine spindle. The scale is viewed
through a microscope. It can measure directly upto 12″ of arc and readings can be estimated
upto 3″ of arc. Fig. 8.9 shows the diagrammatic arrangement of the optical system employed
in optical dividing head. It would be noted that the light passes through a system of lenses and
illuminates the divided scale (mounted on the main spindle) thus projecting the readings on a
ground glass screen. This arrangement thus dispenses with the focusing eyepiece and errors
due to parallex are eliminated. The spindle on which the glass divided circle is mounted, runs
in hardened and lapped plain bearings. The worm which is used only for rotation purposes is
mounted in bearings independent of those for the main spindle. The worm wheel is driven by
the worm, which is connected to a hand-wheel, and a lever is provided for engaging or
disengaging the drive. If handle is turned against the locked spindle, it would simply override.
In order to make a rough check on the required angle, an indexing ring graduated in degrees
is provided at the end of the spindle.