Validating Using Laser Scan Micrometry to Measure Surface Changes on Non-Concave Surfaces, 08-R8562
Sean C. Mitchem
Kerry J. McCubbin
Inclusive Dates: 06/22/15 – 11/04/15
Background — Contact stylus profilometry is a staple instrument in the field of surface metrology due to its high sensitivity to detect sub-micron level surface deviations in the vertical plane. To evaluate changes in surface topography caused by wear phenomena, contact stylus profilometry requires accurate overlays of the worn surface profile relative to that of the original unworn surface profile using common unworn surface features that are present in both worn and unworn surface profiles. This limits the articles that can be measured and is not applicable to newer engine technology, where previous parts designs only had partial contact between two surfaces but now involve contact that causes wear across the entire surface and eliminates the unworn reference edges that were previously available. SwRI developed a method documented in U.S. Patent Application 14-620,020 utilizing laser scan micrometry (LSM) to measure wear on cylindrical objects that have no unworn features on the surface. The focus of this research involved proving equivalency of the SwRI-developed technique LSM method with the industry-accepted technique of contact stylus profilometry for measuring wear on camshafts that have undergone lubricant testing. For clients to feel comfortable with our new measurement technique, it was important to produce data and demonstrable testing results that showed a level of equivalency in wear measurement results.
Approach — Our approach involved measuring wear on cam lobes with different wear severities using both methods and then comparing the wear measurement results to determine equivalency (see Figure 1). To validate the accuracy of our method, we needed to improve the angular positioning accuracy over our existing proof-of-concept system (see Figure 2). This was accomplished by integrating an absolute encoder on the rotational axis and developing motor drive algorithms that use encoder feedback to provide precise and repeatable angular positioning. By being able to validate the accuracy of our angular positioning at each measurement point, we were able to compare the measurement data with data collected from measuring the same part on the contact stylus profilometer.
Accomplishments — Our results in achieving precise angular positioning by using the encoder feedback were 100 percent. We were also able to improve the motor controls to eliminate any potential issues with backlash or partial steps. Measurement results showed that we were able to achieve a measurement equivalency of 81.4 percent on a 10µm-deep wear scar, and 76.5 percent on a 20µm-deep wear scar, well within what would be acceptable to our client base (see Figure 3). Additionally, we were able to validate that we can achieve a 93.4 percent repeatability of ±2µm between two separate measurements of the same test sample, a major goal of the research. With the additional capability to measure surfaces that have worn reference surfaces, which contact stylus profilometry cannot measure, we feel confident that we now have a measurement technique that our clients will seek to use in their testing needs.