as micrometers, are also very useful for
measuring critical dimensions, such as
the diameter of a shaft.
Important characteristics of
measurement instruments include
their accuracy, how quickly they can
obtain individual coordinates and their
ability to measure small features in
con�� ned spaces. ‘Accuracy’ is still widely
used as a descriptive term, but strictly
speaking should not be used to quantify
performance since it does not have an
agreed de�� nition. Instead, ‘uncertainty’
should be used to measure the accuracy
of an instrument, since uncertainty can
be fully quanti�� ed using internationallyagreed
procedures. However, such
methods are demanding and most
scanning measurements are not able to
meet them. This means that scanning
measurements should generally not be
used for critical dimensions.
INSTRUMENTS
Laser scanners are able to rapidly
measure millions of coordinates on 3D
objects using the principle of triangulation
(see box, p50). Laser scanners have two
main parts – a laser source and a camera.
The simplest type of laser scanner is a
laser point scanner in which the laser
source projects a laser dot on to the part.
The laser scanner is calibrated so that the
direction of the laser is accurately known
in terms of an azimuth (what direction to
face) and elevation angle. The distance
between the laser and the camera, and
the orientation of the camera, are also
accurately calibrated. The position of
the red dot on the camera’s image plane
can be converted into an azimuth and
elevation angle of the dot relative to the
camera. The scanner therefore knows the
direction of the point on the component
from two di�� erent locations and the
coordinates can, therefore, be calculated.
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OBSOLESCENCE – REVERSE ENGINEERING
inspection sequences. Programming a
CMM for a reverse engineering job can,
however, be time-consuming.
SOFTWARE
When reverse-engineering a mechanical
product, special software is normally
required to convert the measurement
data into a useable CAD �� le. Scans
produce huge point-cloud data-sets,
containing millions of individual points
measured on the surfaces of the part.
The �� rst job of the software is to
align multiple point-clouds obtained
from di�� erent measurement datums
(information) into a single coordinate
system. Often optical issues such as
re�� ections will mean there are holes or
spikes in the combined point cloud data.
The software is used to clean the pointcloud
data, �� lling holes and removing any
spikes that don’t relate to real geometry.
These smoothing and repair operations
result in a ‘watertight’ polygon mesh.
Feature recognition may then be carried
out to identify geometrical elements
such as �� at surfaces and cylindrical holes.
Reverse engineering has a wide
range of uses. Many of these are entirely
legitimate, such as documenting
legacy parts or checking for patent
infringement. Non-contact
scanning can rapidly provide
3D models of complex
parts while CMMs
provide highlyaccurate
measurements.
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As the laser
point is moved
over the object,
di�� erent coordinates on the surface
are measured. A laser point scanner
gives four direction angles for each
coordinate – two from the laser and two
from the camera. This is more than is
required, since only three dimensions
are required to �� nd a point in space. The
laser can be passed through lenses so
that a continuous line is projected on
to the part. Now, for a single position of
the laser, each coordinate along the line
can be identi�� ed as an individual pixel
on the camera’s image plane. Laser line
scanners can, therefore, scan objects
much more quickly.
Structured light scanners are similar
to laser scanners, but rather than project
a line of laser light, they project a pattern
of white or blue light. The pattern may
appear as gradually shaded bands or
a grid. Structured light scanners can
provide improved accuracy compared
to laser scanners, especially when
scanning objects with challenging optical
properties, such as polished metal.
CMMs are machines that physically
move a probe around a part. They have
encoders within their joints so that the
position of the probe is always accurately
known. CMMs have some advantages
over optical instruments. They allow
automated measurement of points on
parts from multiple directions and at
relatively low uncertainties. This makes
them very useful for carrying out de�� ned
Autumn 2020 www.operationsengineer.org.uk 51
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