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Chapter 11: E. Specialized Inspection Equipment 101
data recorded on a polar chart. Precision spindle instruments are those in which
the spindle supports and rotates the part with the gage tip remaining stationary,
and those in which the spindle rotates the gage tip about the part, which remains
stationary (Drews 1978).
4. OPTICAL EQUIPMENT
Describe and apply optical comparators,
optical flats, microscopes, reference
specimens, etc. (Application)
Body of Knowledge II.E.4
Part II.E.4
Optical inspection instruments fall into four basic categories: optical comparators,
video systems, microscopes, and laser systems (Klepp 1998).
Optical comparators use traditional optics to magnify and project an object’s
image onto a glass screen. This type of optical inspection instrument is by far the
most widely used and is also the least expensive method of optical noncontact
inspection. It also offers the greatest versatility because lightweight parts as well
as ones weighing hundreds of pounds can be inspected on many available instruments.
The optical edge finder, developed more recently, eliminates the operator’s
subjectivity from the measurement and allows the system to be fully automated.
Optical comparators are available in numerous styles and configurations from
domestic as well as international manufacturers. Typical options include projection
screen sizes from 10" to 80", horizontal or vertical light path configurations,
profile and surface illumination systems, various stage travel options, magnifications
from 5× to 200× and digital readout options such as a basic two-axis display
and automatic computer numeric-controlled systems. Accuracy and repeatability
of instruments currently available can vary depending on the feature being
inspected but usually fall within ±0.00010" under certain conditions.
Video systems using a magnification lens and camera are relatively new to
the mainstream inspection instrument market. Although these systems typically
cost more than optical comparators, they offer less versatility with regard to the
size and weight of parts that can be inspected. However, these shortfalls often are
more than offset by the advantages offered. Vision systems, generally small and
compact, work best with relatively lightweight and/or flat parts. Camera and computer
advancements offer magnifications in the 1× to 1300× range; some even offer
full three-axis measurements under CNC control.
Video edge-detection also is available, which eliminates operator subjectivity
from the measurement process. The image digitization process used by video
systems also allows for computer manipulation and storage of the displayed
image. When connected to a printer and driver, a video inspection system can
print out a pictorial view of the inspected part for documentation purposes. Due