As freeform and conformal optics are becoming more broadly used in the optics industry, the manufacturing technology and processing conventions need to be developed to meet the requirements and challenges posed in the manufacturing of these parts. These optics reduce the number of total optics required in an assembly in addition to overcoming the limitations in rotationally symmetric optics. While these optics offer many benefits for system capabilities, these traits also make manufacturing and metrology more difficult. In order to manufacture these complex optics, OptiPro has been focusing on development of the software, hardware, and processing techniques required to meet the stringent part tolerances. For software, OptiPro has been working on the continual development of PROSurf, a CAM package geared specifically toward freeform optics, and the UltraSurf software, which controls OptiPro’s non-contact metrology system. For hardware, the main area of focus has been on fixture development: how should the part be held so that it can be transferred between machines repeatably while still providing adequate support during processing and access to the alignment features. One of the focus areas of processing improvement has been on developing the conventions for the surface datums used for locating the surface and referencing analyzed data against. This paper will present the challenges in freeform manufacturing along with the solutions that we have developed to meet these manufacturing needs.
OptiPro Systems, LLC has been recognized as a U.S. leader in computer-controlled equipment for manufacturing high precision optical components found in medical, military, aerospace, night vision and many other industrial and commercial applications. OptiPro’s success in winning several Small Business Innovative Research (SBIR) grants through the Navy, NASA and the Army have led to the commercialization of new machines, software and processes for grinding, polishing and measuring complex optics such as windows and domes. OptiSonic machines incorporate the latest in ultrasonic machining technology by adding 20-40 kHz of adaptive oscillation to the grinding tool, which reduces force on the tool and part during processing. OptiSonic machines have proven to increase processing speed and precision when grinding various hard optical materials that are ideal for hypersonic applications. UltraForm Finishing (UFF) machines are capable of deterministically finishing windows, domes, and other complex optical elements through a patented sub-aperture polishing technique. This method involves a moving belt of polishing material wrapped around a precision compressive wheel. UltraSurf machines deliver ultra-precision non-contact metrology though the utilization of high-quality machine components and the latest non-contact probe technologies. The measuring probe is scanned over the optical surface while maintaining perpendicularity and a constant focal offset. Finally, our PROSurf freeform computer-aided manufacturing (CAM) software is used to develop advanced toolpaths for each of these platforms. We will present an overview of each of these technologies and discuss how they are advancing the precision optics manufacturing world.
Recently, the desire to use freeform optics has been increasing. Freeform optics can be used to expand the capabilities of optical systems and reduce the number of optics needed in an assembly. The traits that increase optical performance also present challenges in manufacturing. As tolerances on freeform optics become more stringent, it is necessary to continue to improve methods for how the grinding and polishing processes interact with metrology.
To create these complex shapes, OptiPro has developed a computer aided manufacturing package called PROSurf. PROSurf generates tool paths required for grinding and polishing freeform optics with multiple axes of motion. It also uses metrology feedback for deterministic corrections. ProSurf handles 2 key aspects of the manufacturing process that most other CAM systems struggle with. The first is having the ability to support several input types (equations, CAD models, point clouds) and still be able to create a uniform high-density surface map useable for generating a smooth tool path. The second is to improve the accuracy of mapping a metrology file to the part surface. To perform this OptiPro is using 3D error maps instead of traditional 2D maps. The metrology error map drives the tool path adjustment applied during processing. For grinding, the error map adjusts the tool position to compensate for repeatable system error. For polishing, the error map drives the relative dwell times of the tool across the part surface. This paper will present the challenges associated with these issues and solutions that we have created.
Aspheric cylinders have the ability to improve optical performance over standard cylindrical surfaces. Over the last several years there has also been development into the application and functionality of utilizing freeform surfaces to improve optical performance. Freeforms have the ability to not only improve image quality over a greater field of view, but can open up the design space of an optical system making it more compact. Freeform geometries, much like aspheric cylinders, may not have an axis of rotation to spin the optic about during manufacturing. This leads to costly fabrication processes and custom metrology set ups, which may inhibit their use.
Over the last several years, OptiPro Systems has developed and optimized our eSX grinding, UFF and USF polishing, UltraSurf metrology, and ProSurf software programming technologies to make the processing of these complex geometries much easier and deterministic. In this paper we will discuss the challenges associated with manufacturing complex shapes like aspheric cylinders as well as freeform geometries, and how several technologies working together can overcome them. The technologies focus on metrology feedback to a grinding and polishing machine that is controlled through an iterative computer aided manufacturing software system. We will also present examples of these hard to manufacture shapes with results.
As optical geometries become more precise and complex and a wider range of materials are used, the processes used for manufacturing become more critical. As the preparatory stage for polishing, this is especially true for grinding. Slow processing speeds, accelerated tool wear, and poor surface quality are often detriments in manufacturing glass and hard ceramics. The quality of the ground surface greatly influences the polishing process and the resulting finished product.
Through extensive research and development, OptiPro Systems has introduced an ultrasonic assisted grinding technology, OptiSonic, which has numerous advantages over traditional grinding processes. OptiSonic utilizes a custom tool holder designed to produce oscillations in line with the rotating spindle. A newly developed software package called IntelliSonic is integral to this platform. IntelliSonic automatically characterizes the tool and continuously optimizes the output frequency for optimal cutting while in contact with the part. This helps maintain a highly consistent process under changing load conditions for a more accurate surface. Utilizing a wide variety of instruments, test have proven to show a reduction in tool wear and increase in surface quality while allowing processing speeds to be increased.
OptiSonic has proven to be an enabling technology to overcome the difficulties seen in grinding of glass and hard optical ceramics. OptiSonic has demonstrated numerous advantages over the standard CNC grinding process. Advantages are evident in reduced tool wear, better surface quality, and reduced cycle times due to increased feed rates. These benefits can be seen over numerous applications within the precision optics industry.
Recently, the desire to use freeform optics has been increasing, including shapes such as torics and anamorphic aspheres. Freeform optics can be used to expand capabilities of optical systems. They can compensate for limitations in rotationally symmetric optics. These same traits that give freeform optics the ability to improve optical systems also makes them more challenging to manufacture. This holds true for grinding, polishing, and metrology. As freeform optics become more prevalent in the industry, tolerances will become more stringent, requiring deterministic manufacturing processes.
To generate freeforms, it is crucial to have control over all aspects of the process. Controlling the surface definition is important for achieving a better surface finish during processing. Metrology will be required to adjust tool paths at various stages in manufacturing. During grinding, metrology will be used to adjust tool positions relative to the nominal tool path to compensate for repeatable machine and tooling error. For polishing, metrology will be used to deterministically adjust dwell relative to the amount of the error in different surface locations, allowing for convergence towards the desired surface at a uniform rate.
OptiPro has developed PROSurf, a CAM software package for creating freeform tool paths and applying metrology-based corrections. The software can be used for both grinding and polishing freeform optics. The software has flexibility to allow for different methods of modelling the surface: mathematical equations, solid models, and point clouds. The software is designed to make it easier to manufacture and polish complex freeform optics.
Hard ceramic optical materials such as sapphire, ALON, Spinel, or PCA can present a significant challenge in manufacturing precision optical components due to their tough mechanical properties. These are also the same mechanical properties that make them desirable materials when used in harsh environments. Premature tool wear or tool loading during the grinding process is a common result of these tough mechanical properties. Another challenge is the requirement to create geometries that conform to the platforms they reside in, but still achieve optical window tolerances for wavefront. These shapes can be complex and require new technologies to control sub aperture finishing techniques in a deterministic fashion. In this paper we will present three technologies developed at OptiPro Systems to address the challenges associated with these materials and complex geometries. The technologies presented will show how Ultrasonic grinding can reduce grinding load by up to 50%, UltraForm Finishing (UFF) and UltraSmooth Finishing (USF) technologies can accurately figure and finish these shapes, and how all of them can be controlled deterministically, with utilizing metrology feedback, by a new Computer Aided Manufacturing (CAM) software package developed by OptiPro called ProSurf.
Recently, the desire to use freeform optics has been increasing. Freeform optics can be used to expand the capabilities of optical systems. These same traits that give freeform optics the ability to improve optical systems, also makes them more challenging to manufacture. This holds true for grinding, polishing, and metrology, and, as freeform optics become more prevalent in the industry, tolerances will become more stringent. OptiPro Systems has developed a method of deterministic freeform polishing to be used with its UltraForm Finishing (UFF) process. This method uses the error map of the surface to determine the appropriate feed rates for removing a portion of the error from the surface of the optic. The material removed varies across the surface of the optic to allow for the error to decrease across the surface at a uniform rate. The flexibility of this method allows for the deterministic polishing of surfaces that can be mathematically modeled. In addition to deterministic polishing, OptiPro is also developing a software package for generating freeform tool paths. This software can be used for both grinding and polishing freeform optics. It has the ability to generate the freeform tool paths for deterministic polishing. This software will make is easier to manufacture and polish complex freeform surfaces.
Optical systems that utilize complex optical geometries such as aspheres and freeform optics require precise control through the manufacturing process. As the preparatory stage for polishing, this is especially true for grinding. The quality of the grinding process can greatly influence the polishing process and the resultant finished product. OptiPro has performed extensive development work in evaluating components of a precision grinding machine to determine how they influence the overall manufacturing process. For example, spindle technology has a strong effect on how a grinding machine will perform. Through metrology techniques that measure the vibration characteristics of a machine and measurements of grinding forces with a dynamometer, OptiPro has also developed a detailed knowledge of how the machine can influence the grinding process. One of the outcomes of this work has led OptiPro to develop an ultrasonic head for their grinding platform to aid in reducing grinding forces. Initial results show a reduction in force by ~50%.
Hard ceramic optical materials such as sapphire, ALON, Spinel, or PCA can present a significant challenge in manufacturing precision optical components due to their tough mechanical properties. These are also the same mechanical properties that make them desirable materials when used in harsh environments. Tool wear and tool loading conditions during the grinding process for these materials can be especially problematic. Because of this, frequent dressing and reshaping of grinding wheels is often required. OptiPro systems is developing an ultrasonic grinding process called OptiSonic to minimize the forces during grinding and make the grinding process more efficient. The ultrasonic vibration of the grinding wheel allows for a grinding process that has the capacity for longer tool life and reduced tool wear for a more deterministic process. This presentation will discuss the OptiSonic process and present current results.
Future optical systems are moving away from traditional spherical optics. The anticipated benefits are numerous for freeform optics as they provide better aerodynamic characteristics for aircraft, lighter weight for space missions, and smaller size for medical procedures.
Currently the design and utilization of conformal and freeform shapes are costly due to the difficulties introduced with fabrication and metrology of these parts. Techniques for creating these complex optical surfaces are still in development for traditional optical materials. OptiPro has a unique opportunity create manufacturing solutions through computer controlled multi-axis optical generating, polishing, and metrology machines. OptiPro Systems is continuing to develop advanced optical manufacturing technologies. OptiPro has made toric and freeform arch shapes. OptiPro’s existing manufacturing platforms include its eSX grinding, UltraForm Finishing, and UltraSurf non-contact surface scanning system, which will be used for grinding, polishing, and measuring conformal and freeform shapes.
Freeform surfaces are initially generated using deterministic micro-grinding with diamond bonded tools. Tool paths with up to five axes of simultaneous motion are required to generate and polish the optical figure of conformal surfaces. Sub-aperture corrective polishing will need to vary the amount of time the tool contacts at each location in order to remove the proper amount of material. These locations and dwell times are derived from a surface figure error map provided by OptiPro’s UltraSurf. Research and development of the freeform manufacturing process will be presented.