Small-scale surface features, whether desired — such as laser marking and dot peening — or unin- tended — such as scratches, nicks, dents, pitting,
edge break and local curvature — can greatly affect the
value, performance and lifetime of critical components.
Portable roughness gages are able to quantify surface
roughness on the shop floor. Coordinate measurement
machines, stereoscopes and laser line scanners can examine overall shape. But to date, it has not been possible
to measure small features with high resolution on the
A new measurement technology enables direct, noncontact optical inspection of defects and fine features on
precision surfaces. Instruments employing this technology combine the functionality and durability of handheld
gages with the resolution of high-end optical inspection
systems. This combination gives inspectors in shop floor
environments a powerful new option for quantifying
components during production and during repair/refur-bishment processes.
Determining the height or depth of fine surface features is critical for a variety of reasons. Small-scale defects that are just a few microns deep may be sufficient to
cause fracturing, corrosion initiation, excessive wear or
other effects that may seriously compromise the lifetime
or performance of the component. As such, a company’s
inspectors, as well as customers, generally err on the side
Unless a pit, scratch, nick or dent can be quantified,
there is no way to control and improve the production or
repair process. There is also no way to effectively challenge the decision of an inspector.
Similarly, fine features such as laser marking, dot
peening, blend radii, chamfer angles and edge break all
must be measured to ensure proper part performance.
Part marking that is too deep can exceed pitting or
scratching tolerances and reduce lifetime and perfor-
mance in the same way as unwanted defects. Part mark-
ing that is too shallow can cause the marks to wear away
prematurely, losing part serialization and the ability to
track components throughout their lifetimes. In addition,
proper radius of curvature of small features and edges
must be tightly toleranced, since this reduces edge frac-
turing and stress on components and ensures the proper
flow of air or liquids.
Traditionally, inspectors have had limited options for
quantifying defects or fine features. The most common
inspection methods tend to be qualitative, such as visu-
ally comparing a defect to a sample of known quality or
using a scribe gage or even a “calibrated fingernail” to
determine a feature’s depth.
Qualitative inspections, however, lack precision and
repeatability and are highly subjective. They also are
limited to assessing areas with a clear line of sight and
within easy reach. And since passing a bad part could
have far more severe consequences than rejecting a good
part — especially in industries such as aerospace, medi-
cal and automotive — inspectors will typically reject
anything even close to having an excessive defect.
Another method of shop floor inspection typically in-
volves using a rubber-like material to create a negative
impression of a defect. This replicated defect can then be
cross-sectioned, and the result examined using an optical
comparator. This method is more quantitative than mere
visual or tactile inspection. But it is time-consuming, and
accuracy relies on the inspector having sliced perfectly
through the most severe part of the feature.
Highly quantitative results can be achieved from pre-
cision metrology systems such as optical profilers and
stylus profilometers. These systems are capable of both
fine feature and roughness analysis, but they are highly
susceptible to environmental noise and vibration. Be-
cause of this, they are typically sequestered in metrology
labs, and their complexity is such that only highly trained
operators can use them.
Profiling systems are also typically designed in
microscope-stand configurations, which do not allow
measurement of large components, further restricting
their utility. Measurements are slow because of the instruments themselves and the need to handle parts multiple times to measure them in a centralized metrology
lab. These shortcomings, combined with the high cost of
the instruments, make these types of systems effectively
inaccessible to a shop floor inspector.
Measuring Surface Features With
High Resolution in Factory Environments
BY ERIK NOVAK AND MIKE ZECCHINO
4D TECHNOLOGY CORP.
➤ A measuring instrument used for surface
profiling and quantifying the roughness of a
material. The stylus is placed on the surface
of the material at a given contact force, then
is moved laterally around the material and
records vertical displacement as a function of
position. Since this requires contact between
the instrument and the material, the process
takes longer than noncontact techniques.
However, the process is not changed according to a material’s reflective properties, unlike
the noncontact variations.
See EDU.Photonics.com for this and other
definitions in the Photonics Dictionary and more
information in the Photonics Handbook.
Because of the lack of rapid,
accurate, shop floor measurement,
inspectors often lack confidence
in their assessments. The overly
cautious estimations that result
mean that rejection rates are typically
much higher than necessary.
Figure 1. A polarized structured light (PSL)-based gage measures scratches and nicks on a large shaft and voids
on the edge of a composite nose cone. Handheld operation and portability makes it possible to measure these
components without replication, right on the factory floor.