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104 Part II: Metrology
Part II.E.4
Optical Comparators. Many industrial products and component parts are so
small and of such complex configuration that they require magnification for accurate
discernment. For this purpose, a number of measuring and gauging instruments
using various optical systems (such as the toolmaker’s microscope, the
binocular microscope, and the optical projecting comparator) find wide application
for the inspection of small parts and tools.
The optical projecting comparator projects a magnified image of the object being
measured onto a screen. A workpiece is staged on a table to cast a shadow in a
beam of light in diascopic projection, as shown in Figure 11.4. The outline of the part
is magnified and displayed on a screen. In episcopic projection, the light rays are
directed against the side of the object and then reflected back through the projection
lens.
Optical projection provides a means to check complex parts quickly to small
tolerances. A translucent drawing is typically placed over the screen, with lines
drawn to scale for the contour of the part, the limits of the outline, or critical features
such as angles. For instance, the outline of a part can be compared with the
drawing on the screen and deviations in the whole contour can be quickly seen.
A fixture or stage may be supplied for a part to mount all pieces in the same way
in rapid succession. The table can be adjusted in coordinate directions by micrometer
screws or servomotors to 2 mm (.0001 in.). Table positions can be determined
from the micrometer readings or from digital readout devices. Thus, a part can be
displaced to measure precisely how far a line is from a specified position. In addition,
the screen can be rotated to a vernier scale to measure angular deviations in
minutes or small fractions of a degree. Magnifications for commercial comparators
range from 5× to as much as 500×. For example, at 250× magnification, 0.0020
mm (.0008 in.) on a part becomes 0.500 mm (.0197 in.) on the screen, which is readily
discernible. The horizontal optical comparator shown in Figure 11.5 is table
mounted and has a 356 mm (14 in.) diameter viewing screen. The designation
Chart
Projection
Screen lens
Condensing
lens
Workpiece
Mirror
Table
Lamphouse
Figure 11.4 Optical comparator system.
Reprinted with permission of the Society of Manufacturing Engineers, Manufacturing Processes and
Materials, 4th edition, copyright 2000.