The introduction of laser-assisted technology by the OPTIMUS platform has enabled the efficient ultra-precision diamond turning of tungsten carbide, wherein a laser beam is precisely delivered through a diamond cutting tool to augment the diamond cutting edge with additional photon energy. This method effectively reduces the reliance on mechanical energy by a diamond cutting tool alone. Building on our prior research that explored the impact of tool rake angle on cutting depth under constant load, this study explores the combination of the optimal tool rake angle with relatively high laser powers useful for high Material Removal Rate (MRR). Initial results show the possibility to complete a pre-polish ready optic of nearly 5 mm diameter aperture within 20 mins versus an estimated 2 – 3 hours when compared to a similar process using traditional ultra-precision grinding, while preserving form irregularity and surface finish. With the MRR nearly 10 times higher than conventionally using ultra-precision grinding, laser assisted diamond turning of tungsten carbide is a revolutionary technology that can be considered for high-volume manufacturing of tungsten carbide molds used in glass optics. Furthermore, this study includes a comparison of advantages and limitations of each technology.
Tungsten carbide (WC) offers high-strength, high-melting points, and exceptional toughness, with critical applications in industries such as optical molding. Precision machining of WC typically uses grinding operations where tool wear is a significant issue, especially for small geometries required for the consumer electronics industry. Single-point diamond turning (SPDT) is another option for precision machining small features but excessive tool wear prevents this from being a viable option. Innovative approaches, such as in-situ laser-assisted diamond turning, have demonstrated remarkable potential in alleviating tool wear issues and generating optical-quality surface finishes. Laser-assisted techniques, leveraging laser energy for ductile mode machining, mitigate material cracks or fractures. This study delves into the intricate relationship between diamond tool geometry, particularly the rake angle, and ductile regime machining dynamics. Precise selection of diamond tool geometry and rake angle is crucial for desired surface quality. The experimental setup involves specialized equipment like a UMT Bruker tribometer with a modified OPTIMUS system to investigate the impact of tool geometry, specifically the rake angle, in micro laser-assisted material removal on tungsten carbide. The goal is precise and controlled material micro laser-assisted ductile mode removal while minimizing damage or subsurface defects. Results highlight that the rake angle significantly influences the critical depth of cut, with a -25° rake angle proving advantageous, especially when combined with higher laser power. Laser power and tool geometry are pivotal parameters for optimizing hard and brittle material machining, offering valuable insights for precision engineering applications.
Laser-assisted diamond turning has been shown to reduce tool wear, improve productivity, and achieve better surface specifications (including roughness and form) for traditionally diamond turnable materials for infrared optics. Amorphous glass being typically harder than IR materials, thus, diamond turning is less effective compared to traditional grinding and polishing methods. However, traditional grinding and polishing come with drawbacks, such as introducing significant subsurface damage ranging from 20-60 μm, necessitating removal during the polishing process, known as grey out. During grey out polishing, the optical axis can wander, leading to errors between the mechanical axis and optical axis when polishing aspheres. Moreover, sub-aperture polishing steps add mid-spatial frequency errors with each subsequent iteration before form convergence to a low irregularity. Laser-assisted diamond turning for amorphous glass shows promise as a method for rapidly producing near-net optics with minimal sub-surface damage.. This enables two critical gains for optics manufacturing: 1) glass optics can be polished to finished specifications much more quickly than with traditional grinding and polishing; and 2) mechanical tolerances such as wedge and sag can be maintained with precision, reducing manufacturing errors in aspheric optics. In this work, we present data showing that subsurface damage can be reduced to <3 μm for glass optics. Additionally, we demonstrate that form accuracy remains better than 500 nm for even after 10 or more diamond turning passes, indicating extended tool life and high level of conformity to near-net shape.
Tungsten carbide is a material of interest to the optical molding industry because of its suitable thermal properties in molding at higher temperatures. Tungsten carbide is typically ground and polished as tool wear from conventional machining is too high to be feasible. Laser assisted machining developed through Micro-LAM has allowed direct machinability of this material. A machinability study was performed on five grades of tungsten carbide that have been specially developed for glass lens molds. The primary difference between the grades studied is the grain size. With advances in material technology, there is an ability to provide finer grain structures in binderless alloys of tungsten carbide. Standardized trials were then performed across the different grades to evaluate machinability and surface roughness using Laser Assisted Machining (LAM) on a Single Point Diamond Turning (SPDT) platform. The trials proved that there is a strong dependence and correlation of grain size versus final achievable surface finish after LAM turning. Larger grain materials have larger voids and gaps which may cause larger pull outs. These voids then must be polished in post-processing to get beneath the sub-surface damage which is a function of the void depth. Laser assisted machining of fine grain tungsten carbide can achieve a mirror-like surface finish suitable for optical molding applications with minimal post-polishing. Using this technology allows for producing tungsten carbide molds through a more deterministic process. Also, given the range of diamond tool sizes, this method is suitable for complex geometries such as those used in the molding of collimation optics for 5G applications or biomedical applications such as endoscopes.
A wide range of materials including metals and alloys, ceramics, glasses, semiconductors and composites are manufactured to meet service requirements to a given geometry, accuracy, finish and surface integrity. Metals and alloys in general are easier to machine because of their high fracture toughness, low hardness, non-directional bonding, low porosity, large strain to fracture and high impact energy. Non-metals, on the other hand, such as ceramics, semiconductors and optical crystals are characterized by covalent or ionic bonding, limited slip systems for plastic deformation, high hardness and low fracture toughness. It is due to these major differences that ceramics, semiconductors and optical crystals are considered more challenging to machine.
Conference Committee Involvement (1)
Optical Manufacturing and Testing 2024
20 August 2024 | San Diego, California, United States
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