automatic Vickers system for testing cartridges is shown. Source: Wilson Instruments/Instron
Performing accurate micro/macroindentation hardness-Vickers
and Knoop-tests can be a difficult process, even under the best conditions.
Preparing the surface properly to allow the optical measurement, and the manual
indent measurement process, using a high-powered microscope, is time consuming
Some applications for micro/macrohardness testing are particularly difficult to
perform, such as testing bullet cartridges. Manufacturers of these cartridges
have to test a sample of their products by performing a series of Vickers
hardness tests down the length of the cartridge. The shear volume of cartridges
generates the need for a large number of Vickers hardness tests. The fact that
the cartridges have a poor surface, are tapered and have a necked-down area
adds to the difficulty.
Fortunately, current state-of-the-art, micro/macroindentation hardness testing
automation systems can handle these difficult challenges with relative ease.
fixture is used for testing five cartridges. Source: Wilson Instruments/Instron
of the numerous cartridges being made and the significant magnitude of testing
required, cartridge testing is an excellent candidate for automation. There are,
however, several problems to overcome.
Outside diameters of the cartridges are not straight; they have one taper at
the large end and another at the small end. It is desirable to test both ends
of the part in the same fixture; therefore, a compromise must be made in the
design of the anvil, which results in both surfaces being slightly
non-perpendicular to the indenter. This causes a constantly changing focal
point for the optics, which has to be accounted for when automating the
The next problem involves the need to test on the necked-down area of the
cartridge, which is significantly smaller in diameter than the body.
The testing instrument must be able to accommodate the vertical (Z axis)
travel, which is necessary to focus and test on all diameters. The relatively
large Z axis movement does not present difficulties when performing manual
tests. However, automating the Z axis movement is a problem for most
micro/macroindentation testers because, typically, they only use the Z axis for
performing the auto focusing step of the automation, which requires a very
The surface finish of the cartridges presents another problem. The normal
surface is far from smooth when viewed under a high-powered microscope. In
order to make the best use of their time and keep costs low, manufacturers need
to conduct a large number of tests in a short period of time. It is best to
test the cartridges as they come off the last production operation.
is a typical 2.5-kg Vickers indent on the unfinished surface of a cartridge.
Source: Wilson Instruments/Instron
Fortunately, the industry has settled on a 2.5-kg Vickers
test, which produces a relatively large indent. However, it is not unusual for
the indent to be unsymmetrical. The uneven surface around the indent is typical
for an unfinished cartridge. These features present a difficult task for the
image analysis software of an automatic system.
An automatic system can overcome all of these challenges and successfully
support the production line requirements for testing cartridges. To address the
need for large volumes of testing, a modular fixture was designed to allow
testing of up to five cartridges at one time. The fixtures are designed with
the right taper and support areas to hold the samples properly during the
testing process. The fixture holds the cartridge on the anvils and provides an
easy method of loading and unloading the samples to be tested.
To use multiple fixtures, a large stage is necessary. The normal XY motorized
stages used for automatic systems have 2 by 2-inch of travel. This range is
okay for routine applications, such as case depth studies and small samples,
but it cannot accept large area samples or multiple samples. Single cartridges
can be tested with a small stage, but the most effective use of the software is
to mount multiple samples on the same stage.
The software has enough capability to handle several samples; today, new
oversized stages can handle multiple samples to take full advantage of that
feature. This capability allows the cartridge manufacturer to load up to five
parts at one time, then walk away while the testing is completed and results
generated. In addition, new oversized stages have a higher load capacity, so
other applications, such as Rockwell scale testing, can be performed on them.
need to have significant Z axis movement dictates the capability of the
instrument. All automatic testers have motorized XY motion stages, which are
necessary for the movements associated with the testing of flat samples. They
typically add motors and belts to provide some vertical movement for the
automatic focusing process. This works for the small movements performed when
auto focusing the optics, but the testers are slow and do not work well when
trying to accommodate large changes of elevation in the test surface inherent
in the cartridges. By using a tester designed with a standard motorized
vertical movement, the problem is easier to solve.
Vickers indents across a weld joint are shown. Source: Wilson
versions of image analysis software had difficulty with uneven surfaces and
changing test conditions. Measuring the indents on test blocks is easy for all
the systems, but the earlier programs had problems with anything that deviated
from the ideal. The cartridge indent and the weld transition area indents are
typical images that earlier image analysis systems had difficulty measuring
consistently without manual input. Current versions of image analysis software
have made great strides to overcome these problems. The software can now
automatically compensate for lighting and reflectivity variations that
previously caused significant errors in the readings.
Another problem for image analysis systems is roughness around the indents.
Today’s software is better able to ignore the surroundings and focus only on
the indent. It can even change the measuring technique when it is necessary to
suit the shape and lack of uniformity of the indent. Another capability of
newer systems is their ability to auto focus over a longer distance. The extra
range is critical when testing the cartridges because of their taper and the
resulting, ever-changing focal/test points.
Another problem the cartridge creates for testing instruments is that spec
writers for the cartridges have standardized on a Vickers test with a test
force of 2.5 kg. Most likely, there is a good reason for this load, but most
dead weight testers do not have a 2.5-kg test force as standard. Therefore, a
special weight must be obtained to do the testing and the electronics have to
be modified to allow its use. Modern closed-loop Vickers testers solve this
problem because they can apply almost any force within their load range.
Therefore, odd forces such as 2.5 kg are no longer a problem.
Hardness automation technology has finally caught up to applications, such as
bullet cartridges, which have been difficult to do in the past. Better image
analysis software, closed-loop testers and larger stages are the pieces that
enable historically difficult applications to be easily performed on a routine
Ed Tobolski is hardness product manager for Wilson Instruments/Instron
(Norwood, MA). For more information, e-mail firstname.lastname@example.org, visit
www.wilsoninstruments.com or call (781) 575-5840.
the Z axis movement is a problem for most micro/macroindentation testers.
- Earlier versions of image analysis software had difficulty with
uneven surfaces and changing test conditions.
- Modern closed-loop Vickers testers can apply almost any force within
their load range.
- New software can automatically compensate for lighting and
reflectivity variations that previously caused significant errors in