Quality Test & Inspection: Not So Hard
May 15, 2007
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 and costly.
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.
Cartridge TestingBecause 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 process.
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 short travel.
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.
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.
The 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.
Image AnalysisEarlier 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 basis. Q
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.
- Automating 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 readings.