Freeform grinding and polishing with PROSurf

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1 Freeform grinding and polishing with PROSurf Franciscus Wolfs, Edward Fess, Scott DeFisher, Josh Torres, James Ross OptiPro Systems, 6368 Dean Parkway, Ontario, NY ABSTRACT 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 metrologybased 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. Keyword List: polishing, deterministic, correction, freeform, metrology, UltraForm, PROSurf, grinding, conformal 1.1 Freeform and Conformal Surfaces 1. INTRODUCTION 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 freeforms. Complete freeform shapes have no single definition, but are frequently defined by complex mathematical equations (i.e. zernikie polynomials or others), point clouds, splines, or computer aided design (CAD) files. Due to breadth of possibilities for defining these complex optical surfaces, 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 Challenges of Manufacturing Traditional computer aided manufacturing (CAM) packages are tailored for use in metalworking, not optics. This distinction means that these packages will have difficulties meeting the specifications required for manufacturing precision optics. They also do not accept metrology feedback to figure correct surfaces. Traditional CAM packages don t have support for custom tooling used in grinding and polishing optics. For example, to use OptiPro s toric UltraWheel for polishing, it needs to be approximated as a sphere. To overcome the limitations of traditional packages, OptiPro created its own CAM package, PROSurf.

2. PROSURF 2.1 PROSurf Overview PROSurf is a CAM package developed by OptiPro tailored specifically toward the manufacturing of freeform optical shapes.

2 2. PROSURF 2.1 PROSurf Overview PROSurf is a CAM package developed by OptiPro tailored specifically toward the manufacturing of freeform optical shapes. The package can generate tool paths for grinding, UltraForm Finishing (UFF), and UltraSmooth Finishing (USF) on OptiPro Machines. The software supports a range of inputs to define the surface and a number of different metrology formats to correct the surface. There are a number of point spacing models and motion options built into PROSurf that have been developed from methods used on actual surfaces. The software contains built-in models of several OptiPro machine platforms as well the tool models that can be used for 3D simulation of the generated tool path in PROSurf. An example on an UltraWheel is shown in Figure 1 and Figure 2 shows a tool path simulation. The tool path simulation is important to detect any potential collision issues for complex 5-axis tool paths before they are run on the machine. Figure 1: PROSurf model of the UFF UltraWheel.

Figure 2: Sample of tool path animation in PROSurf. 2.2 Shape Definition PROSurf supports a number of input types to define the surface being processed.

The second option available to the user is to define the surface mathematically. To input an equation, it must be able to be written as Z = f(x, Y), as shown in Figure 4.

3 Figure 2: Sample of tool path animation in PROSurf. 2.2 Shape Definition PROSurf supports a number of input types to define the surface being processed. The first option is for the user to input a solid model of the surface. From the model, the user selects the surface that is desired for processing, as shown in Figure 3. The second option available to the user is to define the surface mathematically. To input an equation, it must be able to be written as Z = f(x, Y), as shown in Figure 4. As certain shapes become more prevalent, they will be generalized into predefined shapes in PROSurf. Currently, aspheric cylinders are a built-in shape and can be defined using the form shown in Figure 5. The final option for surface definition in PROSurf is to import a point cloud, as shown in Figure 6. With the point cloud, the quality of the processed surface will be directly affected by the density of the point cloud brought into PROSurf. Figure 3: Example of selecting the working surface from a solid model. The surface highlighted in white is the surface that will be processed.

Figure 4: Example of inputting an equation into PROSurf.

3 Point Distribution Control Figure 6: Point cloud definition of a surface in PROSurf.

4 Figure 4: Example of inputting an equation into PROSurf. Figure 5: Form for defining an aspheric cylinder in PROSurf. 2.3 Point Distribution Control Figure 6: Point cloud definition of a surface in PROSurf. In a traditional CAM package, the point spacing is controlled by the U and V directions in the shape definition. The user does not have very much control over the way that the points are distributed. In some cases, signatures are left from this definition and are very difficult to remove without being able to redefine the distribution points that the tool path is following.

In PROSurf, the user has more control over the point distribution. For equation and point cloud based shapes, the user s default option is an evenly spaced x and y grid.

One advanced spacing option is to evenly space points along curves that lie in a given plane.

5 In PROSurf, the user has more control over the point distribution. For equation and point cloud based shapes, the user s default option is an evenly spaced x and y grid. For a solid model, the default option is to use the built in U and V directions to follow the surface. For most surface types, there are additional options for spacing distributions. One advanced spacing option is to evenly space points along curves that lie in a given plane. For example, even spacing along XZ plane curves will change the distribution so that the linear distance between consecutive points in a given XZ plane curve is constant instead of the x-axis spacing being even. This method was developed to remove the patterns that was common on cylindrical and near-cylindrical shapes. The difference in surface finish from these two spacing distributions is shown in Figure 7. Figure 7: Surface ground with traditional CAM package (left) and with PROSurf (right). A second advanced spacing method that can be used is even arc spacing. This method defines arcs that are evenly spaced along one of the two centerlines of the surface, with the requirement that the surface is symmetric across each of the centerlines. The effect of this distribution is most noticeable on steep geometries, as shown in Figure 8. Initially, this part was ground and polished using an evenly spaced XY point distribution, shown on the right in Figure 8. This distribution left a ring-like signature on the surface, which resembled the layout of the points in the tool path. Since the signature matched the layout of the points, a different point distribution was needed to remove it. To do this, the even arc spacing method was created. A sample of the point cloud for this method is shown on the right in Figure 9. Using this point distribution to polish the surface, the signature disappeared from the surface of the part, as shown on the left in Figure 9.

4 Motion The basic motion in PROSurf to generate passes across the surface uses raster motion staying in either normal or tangent contact

6 Figure 8: Signature left in the surface (left) using an evenly spacing XY point grid (right). Figure 9: Surface finish (left) using the even arc spacing point distribution (right). 2.4 Motion The basic motion in PROSurf to generate passes across the surface uses raster motion staying in either normal or tangent contact with the surface. In order to improve the efficiency of the process time and finishes, more complex motion tool paths are also defined in PROSurf. These include rapid material removal, compensated head angle motion, and radial-in feed.

In addition to requiring the machine operator to restart the tool path each time it finished, running the entire tool path repeatedly involved a substantial amount of time wasted cutting air in the

7 In order to efficiently go from a blank to the rough shape of the surface, PROSurf has a motion called rapid material removal. Before implementing this motion, roughing generating a surface with PROSurf required looping the same program over and over until the surface was fully ground. In addition to requiring the machine operator to restart the tool path each time it finished, running the entire tool path repeatedly involved a substantial amount of time wasted cutting air in the initial passes. Rapid material removal solves both of these issues. The tool path that is generated will start with a flat top of the blank and rough the entire surface out in a single program. This method also detects whether there will be any cutting between consecutive step over points. If the tool will be just cutting air between the step over points, the lines will be skipped over to reduce the amount of wasted time. In the left diagram of Figure 10, the arrows show that the path being followed by the tool will continue along the curve of the shape. As can be seen, there is a significant amount of distance to the path. The right diagram of Figure 10 shows the more efficient tool path created using rapid material remove, which picks the tool over and moves across to the next point of cutting. Figure 10: The left diagram shows the tool path profile without rapid material removal. The right diagram shows the profile using rapid material removal. In both diagrams, the arrows represent the path that the tool will follow across the area with no material. Certain part geometries and/or machine travels may make it impossible to always stay in normal contact with the surface of the part. For polishing, it is important to stay in normal contact to keep a more constant removal rate. In situations where staying normal to the part in polishing is not possible, staying close to normal is the next best option. PROSurf gives the user the ability to input a range of normal motion that the B-axis can safely carry out. When the B-angle requirements fall outside of that range, the tool will rotate to the B-angle limit allowed and then contact the surface tangentially from there. As Figure 11 shows, it is still possible to figure correct a surface using a compensated B-angle tool path.

8 Figure 11: Before (lighter) and after (darker) figure correction on a cylinder using compensated 4-axis motion. Regions where non-normal motion is used are indicated on the diagram. The head angle is capped at +/-20. Ideally, the head would go between +/-44.5 to stay normal across the surface. Currently, a motion called radial-in feed is being developed. This motion starts at the point furthest from the center of the surface. The tool path spirals in towards the center, continually rotating the C axis in the same direction, as shown by the point distribution in Figure 12. Some of the anticipated benefits of this motion include improved surface quality and reduced cycle times Y (mm) X (mm) Figure 12: Example of radial-in feed point distribution. The line connecting the points is the order that the tool path will go to the points, starting on the edge. 2.5 Figure Correction To be able to meet the tolerances on freeform surfaces, it is crucial to have the ability to figure correct the surface based on metrology taken on it. PROSurf has the ability to perform metrology-driven figure correction for both grinding and polishing processes. During grind correction, the most recently run tool path can be compensated based on the quality of the surface that tool path created. PROSurf adjusts the z-coordinates based on the vertical error left in the part on the previous run; the tool is raised in areas where too much material was removed and lowered in areas where not enough was removed. The grind correction allows for a better surface to be presented to the polishing machine, leading to reduced cycle

times in polishing. If a single pass does not sufficiently correct the surface, the user can feed in additional maps to perform cumulative corrections.

The algorithm adjusts the dwell times at each point based on the relative magnitude of the error at the point.

9 times in polishing. If a single pass does not sufficiently correct the surface, the user can feed in additional maps to perform cumulative corrections. During polishing, the surface can be corrected using a dwell based correction algorithm in PROSurf. The algorithm adjusts the dwell times at each point based on the relative magnitude of the error at the point. The tool path will spend more time in areas with higher amounts of error and less time in areas with smaller amounts of error, as shown in Figure 13. The error converges towards the ideal surface at an even rate across the surface. Figure 13: Error mapped to surface (left). Calculated dwell times to correct 50% of the error in a single run (right). The software supports multiple different inputs for error maps: dat, xyz, mod, csv, and opm. The opm extension is a custom, simple-to-use OptiPro Metrology format that allows the user to have the option of inputting error map types that are not currently available from other software. This also allows for additional offline manipulation of the data knowing that it can be brought into PROSurf if it is formatted properly. 3.1 Basic Process Flow The process for processing complex shapes is as follows; 3. METHODOLOGY 1. Input part definition into software using a CAD file, equation, or point cloud. 2. Generate a tool path for the specific surface. a. If the tool path is grinding, generate a tool path for the nominal surface with no corrections. b. If the tool path is polishing, generate tool path using even dwell times (uniform removal) and loop until the surface has cleared out. 3. Measure the surface on an interferometer, CMM, UltraSurf, profilometer, or other metrology. 4. Analyze the metrology and determine if the surface has met specifications set. 5. If the surface has not met the specifications, a corrective tool path needs to be generated and run. a. If the process is grinding, generate a new tool path that will reduce the form error based on the error map input and regrind the surface. b. If the process is polishing, ProSurf will generate a new tool path to reduce the form error by adjusting the dwell time at each point on the tool path based on the relative error at the point. 6. Repeat steps 3 through 5 until form error has reached acceptable values. 4.1 Spinel Toric 4. PROCESSING OptiPro worked on figure correcting a 150 mm by 150 mm spinel toric. The surface was defined using a CAD model imported into PROSurf. Initially, the surface started with a peak-to-valley (PV) of µm with a

root-mean-squared (RMS) of 9.313 µm, as shown in Figure 14. After figure correction, the PV of the surface was 9.926 µm with an RMS of 1.540 µm.

313 µm RMS; after figure correction with UFF (right): 9.926 µm PV, 1.540 µm RMS. 4.

10 root-mean-squared (RMS) of µm, as shown in Figure 14. After figure correction, the PV of the surface was µm with an RMS of µm. The surface was then transferred to a USF machine for smoothing and additional processing. Figure 14: Initial surface of spinel toric (left): µm PV, µm RMS; after figure correction with UFF (right): µm PV, µm RMS. 4.2 Radial-In Feed OptiPro is working on the figure correction of an anamorphic asphere, shown in Figure 15, using grind corrections. The surface started with a PV of 139 µm, with an RMS of 45 µm, as shown in the left diagram of Figure 16. After figure correction, the PV improved to a fourteenth of its original value to about 11 µm, as shown in the right diagram of Figure 16. The RMS improved by a factor of 15 to about 3 µm. Figure 15: In-process anamorphic asphere using radial-in feed grinding.

Advancements in manufacturing technology have allowed for creation of optical shapes previously thought to be impossible.

11 Figure 16: Initial surface after nominal grind using Radial-In Feed (left) and surface after a number of figure correction passes (right). 5. 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 has developed PROSurf as a solution for manufacturing freeform optical surfaces. The software will continue to adapt to meet the changing needs of customers.

Freeform polishing with UltraForm finishing

Freeform polishing with UltraForm finishing Franciscus Wolfs, Edward Fess, Scott DeFisher OptiPro Systems, 6368 Dean Parkway, Ontario, NY 14519 ABSTRACT Recently, the desire to use freeform optics has