Freeform polishing with UltraForm finishing

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Joel Mosley
5 years ago
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1 Freeform polishing with UltraForm finishing Franciscus Wolfs, Edward Fess, Scott DeFisher OptiPro Systems, 6368 Dean Parkway, Ontario, NY ABSTRACT 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. Keyword List: polishing, deterministic, correction, freeform, metrology, UltraForm

They can include a wide range of geometries, which may include off-axis sections of rotationally symmetric shapes, rotationally symmetric non-standard shapes, shapes that conform to the platform

Complete freeform shapes have no single definition, but are frequently defined by complex mathematical equations, point clouds, splines, or computer aided design (CAD) files.

2 1. INTRODUCTION 1.1 Freeform and Conformal Surfaces From a manufacturer s perspective, freeform optical surfaces are shapes that are not manufactured by standard spherical or aspheric manufacturing techniques. They can include a wide range of geometries, which may include off-axis sections of rotationally symmetric shapes, rotationally symmetric non-standard shapes, shapes that conform to the platform where they reside (i.e. conformal), and complete freeform. Complete freeform shapes have no single definition, but are frequently defined by complex mathematical equations, point clouds, splines, or computer aided design (CAD) files. Due to this, communication of the desired surface from the optical designer to the manufacturer can be challenging, but is extremely important. This definition may also present challenges to the manufacturer in how the surface is communicated to the CNC machine for processing. 1.2 UltraForm UltraForm Finishing (UFF) is a sub aperture polishing process that has been developed by OptiPro systems to polish spherical, aspheric, and free form surface geometries. It is a CNC deterministic process that uses surface metrology with a measured removal function (Figure 1b) to generate a tool path. This path will move the UltraForm polishing wheel (Figure 1a) along the optical surface in a controlled manner to reduce the form error. The wheel consists of a compliant material and can be manufactured to various diameters and hardnesses. Wrapped on the outside of the wheel is a belt of polishing material. The belt may be bound with abrasive materials such as cerium oxide and diamond, or made of a polyurethane material traditionally used in optical polishing. A combination of wheel diameter and hardness with the assortment of belts allows the operator to control the shape and depth of the removal function. (a) (b) Figure 1 (a) UltraFrom polisher machine, abrasive belt polishing an asphere, (b) UltraForm's removal function

3 To be able to keep a constant removal function during UFF processing, the UltraForm wheel will need to be kept tangent to the surface. For freeform surfaces, this will require 3-, 4-, or 5-axis of simultaneous motion depending on its geometry. OptiPro is developing an interface to integrate freeform geometry, surface error maps, and tool geometry to polish freeform shapes to optical tolerances. 1.3 Freeform Software Currently, there are no commercially available CAM packages designed for the deterministic optical manufacturing processes. OptiPro is working on the development of a CAM package that is tailored specifically to freeform optics. It is able to support a wide range of freeform shapes, one of which can be seen in Figure 2. Users have the options to define the shapes in a number of ways, including equation defined surfaces, importing point clouds, and importing CAD models. Once the surface is imported, the user has a variety of options for controlling the grid spacing. The benefits of which can be seen in Figure 3. With an even xy point grid, the surface of the arch had a ring-like signature in it. To get rid of the signature, the points were readjusted so that they created evenly spaced arcs along the normal direction of the arch. The tool paths based on this new point grid were able to eliminate a signature in the optics that was due to how the tool path was generated. There will be support for three, four, and five axes of simultaneous motion. Four axes of motion currently will support using the three positional axes as well as either the A or B rotary axis. Five axis motion supports the standard UFF configuration of the B and C rotary combination along with the new A and B rotary combination with the three positional axes. The software can perform metrology based correction for polishing. Figure 2: Freeform arch plotted in OptiPro's CAM software.

Figure 3: Surface finish of arch with even xy point grid (left). Even arc spacing along the wider dimension of the arch (right) 2.

3. Use even dwell times for polishing passes until the surface has cleared out and can be measured (uniform removal). 4. Measure the surface on a CMM, UltraSurf, or other metrology. 5.

Repeat steps 4 and 5 until form error has reached acceptable values. 3.1 Freeform Arch 3. PROCESSING OptiPro is working on polishing an arch shape to optical tolerances.

4 Figure 3: Surface finish of arch with even xy point grid (left). Even arc spacing along the wider dimension of the arch (right) 2.1 Basic Process Flow The process for polishing complex shapes is as follows; 2. METHODOLOGY 1. Input part definition into software using a CAD file, equation, or point cloud. 2. Generate a tool path for the specific surface. 3. Use even dwell times for polishing passes until the surface has cleared out and can be measured (uniform removal). 4. Measure the surface on a CMM, UltraSurf, or other metrology. 5. Generate a new tool path that will reduce the form error, adjusting the dwell time at each point on the tool path based on the relative magnitude of that point s error. 6. Repeat steps 4 and 5 until form error has reached acceptable values. 3.1 Freeform Arch 3. PROCESSING OptiPro is working on polishing an arch shape to optical tolerances. Since the shape varies along its two principal axes, the shape is not rotationally symmetric. This leads to the need for complex tool paths using up to five simultaneous axes of motion for polishing. The convex side of the arch required the part to be mounted on its side, as shown in Figure 4, since the only rotary combination available at the time of polishing was B and C. This orientation is used to reduce the occurrence of axis reversals. These reversals come near the tip of the arch when being rotated about its central optical axis, where the math breaks down in calculating the C angles.

5 Z Z Figure 4: Polishing the convex side of the arch on its side. The error maps of the convex surface, measured before the first deterministic polishing pass and after the final deterministic polishing pass, are shown in Figure 5a and Figure 5b, respectively. There are noticeable lines left from the tool path used in grinding the arch. The error maps of the surface are combined with removal parameters to determine where the wheel should dwell longer to remove more material and the areas to move more quickly to remove less material. The surface of the arch after the final correction pass is shown in Figure 5b. The final peak to value and the root mean square error improved more than a factor of five from the initial surface. Normal Error ( m) 1 Normal Error ( m) Y X Y X (a) (b) Figure 5: Convex arch before deterministic polishing, PV: 25.5 um, RMS: 3.97 um (left). Convex arch after deterministic polishing, PV: 3.94 um, RMS:.53 um (right). 4. CONCLUSION Freeform optics have the potential to revolutionize the precision optics industry. Advancements in manufacturing technology have allowed for creation of optical shapes previously thought to be impossible. Moving forward, collaboration between optical design and manufacturing will be required to successfully implement freeform optical systems. Advances in freeform metrology will greatly drive the manufacturing of freeform shapes and push them to tighter optical tolerances. OptiPro is continuing to explore new technologies and refining existing ones to further this development.

Freeform grinding and polishing with PROSurf

Freeform grinding and polishing with PROSurf Franciscus Wolfs, Edward Fess, Scott DeFisher, Josh Torres, James Ross OptiPro Systems, 6368 Dean Parkway, Ontario, NY 14519 ABSTRACT Recently, the desire to