Lower cost additive manufacturing (AM) machines which have emerged in recent years are capable of producing tools, jigs, and fixtures that are useful in optical fabrication. In particular, AM tooling has been shown to be useful in lapping glass workpieces. Various AM machines are distinguished by the processes, materials, build times, and build resolution they provide. This research investigates the impact of varied build resolution (specifically layer resolution) on the lapping performance of tools built using the stereolithographic assembly (SLA) process in 50 μm and 100 μm layer thicknesses with a methacrylate photopolymer resin on a high resolution desktop printer. As with previous work, the lapping tools were shown to remove workpiece material during the lapping process, but the tools themselves also experienced significant wear on the order of 2-3 times the mass loss of the glass workpieces. The tool wear rates for the 100 μm and 50 μm layer tools were comparable, but the 50 μm layer tool was 74% more effective at removing material from the glass workpiece, which is attributed to some abrasive particles being trapped in the coarser surface of the 100 um layer tooling and not being available to interact with the glass workpiece. Considering the tool wear, these additively manufactured tools are most appropriate for prototype tooling where the low cost (<$45) and quick turnaround make them attractive when compared to a machined tool.
Sensors capable of measuring the quasi-electrostatic field of traveling projectiles have been developed to detect the
passage of a bullet in flight. These sensors provide an alternative to existing optical chronograph technologies, which are
sensitive to variations in environmental lighting, and magnetic chronographs, which require close proximity to the
bullet’s path. In contrast, electric field sensors are insensitive to lighting changes and prior testing has demonstrated the
ability to reliably detect bullets at distances of at least three meters. A linear array of these sensors has been used to
measure the time of flight between the sensors, which with the known distance between the sensors can be used to
calculate the projectile’s velocity. These velocity measurements are compared to established chronograph technology as
a measurement validation. By extending this array of sensors along the projected path of the projectile, a profile of the
projectile’s position and velocity through flight can be calculated. This expected utility of this data is in refining the
calculations that are performed to determine a ballistic solution, particularly in long range engagements, where there has
been limited availability of accurate projectile velocity measurements. This robust sensor array that can easily be
deployed represents an inexpensive way to experimentally investigate numerous phenomena related to ballistics
Additive manufacturing technologies have the ability to directly produce parts with complex geometries without the need for secondary processes, tooling or fixtures. This ability was used to produce concave lapping tools with a VFlash 3D printer from 3D Systems. The lapping tools were first designed in Creo Parametric with a defined constant radius and radial groove pattern. The models were converted to stereolithography files which the VFlash used in building the parts, layer by layer, from a UV curable resin. The tools were rotated at 60 rpm and used with 120 grit and 220 grit silicon carbide lapping paste to lap 0.750” diameter fused silica workpieces. The samples developed a matte appearance on the lapped surface that started as a ring at the edge of the workpiece and expanded to the center. This indicated that as material was removed, the workpiece radius was beginning to match the tool radius. The workpieces were then cleaned and lapped on a second tool (with equivalent geometry) using a 3000 grit corundum aluminum oxide lapping paste, until a near specular surface was achieved. By using lapping tools that have been additively manufactured, fused silica workpieces can be lapped to approach a specified convex geometry. This approach may enable more rapid lapping of near net shape workpieces that minimize the material removal required by subsequent polishing. This research may also enable development of new lapping tool geometry and groove patterns for improved loose abrasive finishing.
Mating parts often experience repetitive relative motion termed fretting which results in friction, wear, as well as
acoustic emission signals. Acoustic emission signals have the potential for monitoring the condition of the surfaces
participating in the frictional process. In structural health monitoring studies, where the focus is on quantifying crack
growth related acoustic emission signals, the signals generated by other mechanisms give rise to undesirable false
positives. A major source of such false positives are fretting related signals. The present paper describes an experimental
approach for characterizing the friction related acoustic emission signals. A test fixture is developed to obtain fretting
related signals under controlled conditions. The waveforms are analyzed to extract features common to these signals. A
comparison of acoustic emission signals related to fretting and crack growth is provided.
In this research, the acoustic emissions from simulated crack growth and incremental crack growth in a cyclically loaded
aluminum panel were detected by acoustic emission sensors. One of these sensors was comprised of an array of thin
strips of piezoelectric material bonded to the specimen and electrically connected. The geometry of these sensor strip
arrays and their orientation to the fracture site enabled the sensors to capture the shear component of the acoustic
emission waveform. Cyclical loading was used to grow the crack, allowing sensor performance to be assessed in
comparison to bonded and resonant sensors. The detection of the shear wave is of particular interest as the shear
component of fretting events is often small, providing a possible means of discriminating between critical events (crack
propagation) and sources of minimal concern (fretting). Shear modes were detected in the acoustic emissions from both
the simulated crack growth and the crack growth due to cyclical loading.