US20130052919A1

US20130052919A1 - Graphite composite panel polishing fixture and assembly

Info

Publication number: US20130052919A1
Application number: US13/217,965
Authority: US
Inventors: John G. Hagopian; Jason G. Budinoff; Carl R. Strojny
Original assignee: National Aeronautics and Space Administration NASA; Space Administrationo
Current assignee: National Aeronautics and Space Administration NASA; Space Administrationo
Priority date: 2011-08-25
Filing date: 2011-08-25
Publication date: 2013-02-28

Abstract

A polishing fixture assembly for an optical element includes a first composite panel, a second composite panel, and a core member sandwiched between the first composite panel and the second composite panel and coupling the first composite panel to the second composite panel. At least one mirror mounting insert is coupled to each of the first and second composite panels.

Description

BACKGROUND

1. Field
The aspects of the present disclosure relate generally to the field of optical element formation, and in particular to a fixture for grinding and polishing meter class optical elements.
2. Description of Related Art
Meter-class optics require support during the grinding/polishing process with large tools. The use of aluminum as a polishing fixture is a standard configuration, with pitch providing a compliant layer to allow support without deformation. Unfortunately, with meter-scale optics, a meter-scale fixture weighs over 120 pounds (approximately 55 kilograms) and may distort the optics being fabricated by loading the mirror and/or tool used in fabrication.
Mounts that are typically used to support large mirrors during fabrication are especially heavy and difficult to handle. The mount must be especially stiff to avoid deformation during the optical fabrication process, where a very large and very heavy lap often can distort the mount and the optic being fabricated. If the optic is placed on top of the lapping tool, the weight of the optic and the fixture can distort the lap. Fixtures to support the mirror during fabrication are often configured from very large plates of aluminum, that are often two (2) inches or more in thickness (approximately 5 centimeters) and have a weight upwards of 150 pounds (approximately 68 kilograms). With the addition of a backing material, such as pitch, and the mirror itself, the panel polishing fixture can typically weigh over 250 pounds (approximately 113 kilograms) for a meter class optic.
For example, the 1-meter Spherical Primary Optical Telescope (“SPOT”) Primary Mirror Segment, developed by NASA, is somewhat light weighted with a scalloped back shape for radius of curvature control. The SPOT will utilize a single ring of point-to-point hexagonal mirror segments. There are three SPOT segments in total. Segment 1 is mounted on a traditional backing structure of two (2) inch thick aluminum plates that weigh close to 160 pounds. With the mirror configured in a “face-up” orientation, the polishing tool, also referred to herein as a “lap” tool, is positioned on top of the mirror. This configuration is difficult to achieve due to the weight of the polishing tool. With the mirror configured in a “face-down” orientation, the polishing tool is positioned underneath the mirror with the mirror resting on top of the polishing tool. Although it was thought that better results might be achieved with the mirror configured in the “face-down” orientation, the polishing tool in this configuration is loaded with the weight of the mirror and the aluminum panel polishing fixture. With the weight of the mirror and aluminum panel polishing fixture easily reaching over 250 pounds, the face-down configuration would likely result in a deformation of the polishing tool or lap and lead to high figure error.
Accordingly, it would be desirable to provide a method or device that addresses at least some of the problems identified above.

SUMMARY OF THE INVENTION

As described herein, the exemplary embodiments overcome one or more of the above or other disadvantages known in the art.
One aspect of the exemplary embodiments relates to a polishing fixture assembly for an optical element. In one embodiment, the polishing fixture assembly includes a first composite panel, a second composite panel, and a core member sandwiched between the first composite panel and the second composite panel and coupling the first composite panel to the second composite panel. At least one mounting fixture insert is coupled to each of the first and second composite panels.
Another aspect of the disclosed embodiments relates to a polishing assembly for an optical element. In one embodiment, the polishing assembly includes a polishing machine having a working surface and a grinding/polishing fixture. The optical element is positioned between the polishing machine and the grinding/polishing fixture. At least one pitch button is positioned on a surface of the grinding/polishing fixture between the grinding/polishing fixture and the optical element. A lift interface is coupled to a first side of the grinding/polishing fixture for coupling the grinding/polishing fixture to the polishing machine. A mounting fixture insert is coupled to a second side of the grinding/polishing fixture for coupling the optical element to the grinding/polishing fixture. The grinding/polishing fixture includes a first composite panel, a second composite panel
and a core member sandwiched between the first composite panel and the second composite panel and coupling the first composite panel to the second composite panel.
These and other aspects and advantages of the exemplary embodiments will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. Moreover, the drawings are not necessarily drawn to scale and unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein. In addition, any suitable size, shape or type of elements or materials could be used.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a perspective view of one embodiment of a panel polishing fixture assembly incorporating aspects of the present disclosure.

FIG. 2 illustrates pitch pucks disposed on a mirror-fixture interface of a panel polishing fixture assembly incorporating aspects of the present disclosure.

FIG. 3 illustrates a mirror disposed on a panel polishing fixture assembly incorporating aspects of the present disclosure.

FIG. 4 illustrates a mirror disposed on a panel polishing fixture assembly incorporating aspects of the present disclosure on a polishing machine.

FIG. 5 illustrates a bottom perspective view of the assembly shown in FIG. 1.

FIGS. 6A-6E illustrate one embodiment of a mounting fixture insert incorporating aspects of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the aspects of the disclosed embodiments are directed to a panel polishing fixture assembly for meter style optics that is comprised of a lightweight graphite panel with an aluminum honeycomb core. The fixture assembly 100 is generally configured to support an optical element, such as a mirror, during the grinding and polishing process. As shown in FIG. 1, the fixture assembly 100 generally comprises a graphite composite panel 102, mounting or fixture inserts 104 and lift interface assemblies 106. The use of composite structures that are lightweight yet stiff provides for a decrease in fixture weight by almost 70 percent. The composite panel 102 includes a first side 108, a second side 110 and a core member 112 sandwiched between the first side 108 and the second side 110. In one embodiment, the composite panel 102 is a lightweight graphite panel that is approximately 1/16 inch thick. The core member 112 is an aluminum honeycomb member that is approximately 1 inch thick. The first side 108 of the panel 102 generally interfaces with a lift assembly (not shown). The second side 110 of the panel 102 interfaces with the optical element 120, which for purposes of the discussion herein will be referred to as a mirror. In one embodiment, the fixture 100 includes spacers 122. The spacers 122 generally comprise glass spacers that make up the gap 126 between the mirror 120 and the composite panel 102 of the fixture 100. The spacers 122 are generally shorter than a size of the gap 126. In one embodiment, pitch elements 116, shown in FIG. 2, are used to fill the remaining space, which is typically less than approximately 0.05 inches. Depending on the mirror 120, additional support may be required to keep the mirror 120 from flexing.
The mounting or fixture inserts 104 are generally configured to couple the mirror to the fixture assembly 100. For example, the SPOT mirror has invar inserts bonded into the glass with low coefficient of thermal expansion epoxy that matches the glass. In one embodiment, such as for the SPOT mirrors, threaded rod 132 is fed through the polishing fixture inserts 104 and engaged in the threaded inserts bonded in the mirror 120. A washer (not shown) and nut 136 is engaged on the other end of the threaded rod 132 and loaded against the polishing fixture inserts 104. In one embodiment, referring to FIGS. 1 and 5, there are six (6) threaded mirror inserts 138 on a central flat flange 140 that are used for mounting. Bolts 142 are passed through the central polishing fixture flange 140 shown in FIG. 1 and into the six (6) mirror inserts 138 and loosely tightened. The description above related to the SPOT mirror demonstrates one method of mounting. However, in alternate embodiments, other types of mirrors can be accommodated similarly. In the example shown in FIG. 1, three fixture inserts 104 are illustrated. In alternate embodiments, any suitable number of inserts can be used. In one embodiment, the fixture inserts 104 generally comprises one or two machined aluminum pieces that are inserted and typically bonded into holes in the composite panel 102. The inserts 104 provide a load spreading capability so that bolts and other fasteners can be tightened without damaging the faceplate of the composite panel 102, which is susceptible to high localized loads. The inserts 104 can be custom made to the mirror/fixture configuration to allow mounting to the mirror 120 and fixture 100.
The lift interface assemblies 106 are generally configured to provide an interface to a lifting apparatus (not shown) for lifting operations to remove the mirror from the polishing machine for metrology. In one embodiment, the lift interface assemblies 106 include a turnover bracket 114 that can be used as a connection point by the lifting machine to lift the fixture 100. In alternate embodiments, the interface assemblies 106 can include any suitable components that allow the fixture 100 and mirror to be removed from the polishing machine. The lift interface assemblies 106 generally comprise inserts flanges that are inserted from the opposite side of the panel 102 to allow the lift load to be spread out over a larger area (see FIG. 6, 6 Detail C). The SPOT fixture utilizes a 2-part insert 144 (see FIG. 6, 6 Detail D) that is threaded to allow a large lifting turnbuckle 166, shown in FIG. 1, to be installed to aid with lifting.
Referring to FIG. 2, in one embodiment, the mirror 120 is interfaced to the second side 110 of the panel 102 using one or more pitch pucks 116. The pitch pucks 116, also referred to as “pitch buttons”, support the optical element or mirror 120 under fabrication. The pitch pucks 116 are fabricated to fill the gap 126 between the mirror 120 and the polishing fixture 100. Pitch is melted and poured into molds that are shaped to provide a variety of sizes and thicknesses to accommodate the variety of gaps 126 between the mirror 120 and fixture 100.
FIG. 3 illustrates one embodiment of a mirror 120 interfaced with the polishing fixture 100. The pitch pucks 116 provide support and compliance during grinding and polishing. Pitch, as the term is used herein, generally refers to a Newtonian Fluid that allows the polishing forces to be equally distributed on the supporting surface 128 of the mirror 120. The polishing fixture 100 is configured so as not to deform to the extent that the pitch pucks 116 break and loses contact on the mirror-fixture interface 118.
FIG. 4 illustrates one example of a mirror 120 mounted in a face-down orientation on a polishing machine 130. In operation, the polishing fixture 100 acts as a reaction structure to the polishing machine 130. The polishing fixture 100 of the disclosed embodiments is sufficiently stiff to avoid imparting a distorted shape to the mirror 120 under fabrication and light enough to avoid self-deflection. The polishing fixture 100 is also configured to withstand the significant tangential loads from the polishing machine 130 during operations. The normal and tangential loads the minor 120 and polishing fixture 100 experience are quite high. The loads, which are generally dependent upon the surface area and fit of the tools, will vary during the grinding and polishing process. The polishing fixture 100 of the disclosed embodiments is configured to withstand these forces.
In one embodiment, the mirror 120 is supported on the polishing fixture 100 with the optical surface 124 facing downwards, in direction A towards the polishing machine 130 as shown in FIG. 4. In alternate embodiments, the geometry can be reversed, with the mirror 120 and optical surface 124 underneath the polishing machine 130. In either embodiment, the optical element 112 is on a polishing machine 110 with a lap 162. The minor 120 or lap 162 rotates on a spindle 134, while the arm 166 sweeps across in a repetitive motion. The polishing fixture 100 is configured to accommodate loads higher than the normal operating loads for transient operations, where the stiction of the parts increases exponentially as the parts fit in close communication with each other.
Referring now to FIGS. 6A-6E details of the mirror mounting inserts 104, 158 and lifting inserts 144 can be seen. In the exemplary embodiment shown in FIG. 6E, the lifting insert 144 includes two pieces; a threaded wide flange insert 157 configured to receive the lifting turnbuckle 166, and a washer 159. The threaded wide flange insert 157 is inserted through a hole in the composite panel 102 and is bonded to side 108. The washer 159 is bonded to the threaded wide flange insert 157 and to side 110 of the composite panel 102. The mirror mounting inserts 104 and 158 are of similar construction and both include a wide flange insert 151 and a washer 152 as can be seen in FIG. 6C and FIG. 6D. The wide flange insert 151 creates a through hole 153 in composite panel 102 to allow the threaded rod 132 and bolts 142 to pass through the composite panel 102. A washer 152 is bonded to the wide flange insert 151 and to side 110 of the composite panel 102. The inserts 104, 158 and 144, are bonded to the composite panel 102 in a fashion that can withstand the forces of lifting polishing and grinding.
In one embodiment, referring to FIGS. 1 and 6A-6E, the inserts 104 are bonded to the composite panel 102. Holes are cut in each side panel 108, 110, which can comprise carbon face sheets, and the inner core 112, or aluminum honeycomb. The inserts 104 are inserted and bonded in from one of the sides 108, 110 using a standard, low coefficient of thermal expansion epoxy.
The aspects of the disclosed embodiments provide a composite polishing fixture for meter-class optics. In particular, the panel is a lightweight graphite panel with an aluminum honeycomb core. Two composite panels are bonded together with inserts for mirror mounting, assembly handling and interfacing to the polishing machine. The mirror is interfaced to the polishing fixture using pitch buttons to provide a strong, but compliant interface. The inserts are bonded to the composite panels and are capable of withstanding the lifting and polishing loads. The use of composite structures that are lightweight but stiff enough to allow standard polishing and grinding techniques to be used, while providing for a decrease in fixture weight by almost 70 percent.
Thus, while there have been shown, described and pointed out, fundamental novel features of the invention as applied to the exemplary embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. Moreover, it is expressly intended that all combinations of those elements and/or method steps, which perform substantially the same function in substantially the same way to achieve the same results, are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.

Claims

1. A polishing fixture assembly for an optical element comprising:

a first composite panel;

a second composite panel;

a core member sandwiched between the first composite panel and the second composite panel, and coupling the first composite panel to the second composite panel; and

at least one mirror mounting insert coupled to each of the first and second composite panels.

2. The assembly of claim 1, wherein each of the first and second composite panels comprises a composite graphite material.

3. The assembly of claim 1, wherein the core member comprises aluminum honeycomb core structure.

4. The assembly of claim 1, wherein the inserts are bonded to each of the first and second composite panels.

5. The assembly of claim 4, wherein the insert includes an internal thread.

6. The assembly of claim 1, further comprising one or more pitch buttons coupled to one of the first or second composite panels and configured to support the optical element under fabrication.

7. The assembly of claim 1, wherein the optical element comprises a mirror.

8. The assembly of claim 7, wherein the mirror is a three-meter mirror.

9. The assembly of claim 1, wherein a diameter of each of the first panel and the second panel is approximately 900 millimeters.

9. The assembly of claim 1, wherein a thickness of the panel is approximately 1/16 inches.

11. A polishing assembly for an optical element, comprising:

a polishing machine having a working surface; and

a grinding/polishing fixture, wherein the optical element is positioned between the polishing machine and the grinding/polishing fixture;

at least one pitch button positioned on a surface of the grinding/polishing fixture between the grinding/polishing fixture and the optical element;

a lift interface coupled to a first side of the grinding/polishing fixture for coupling the grinding/polishing fixture to the polishing machine;

a mirror mount insert coupled to a second side of the grinding/polishing fixture for coupling the optical element to the grinding/polishing fixture; and

wherein the grinding/polishing fixture comprises:

a first composite panel;

a second composite panel; and

a core member sandwiched between the first composite panel and the second composite panel, and coupling the first composite panel to the second composite panel.

12. The assembly of claim 11, wherein the optical element is configured in a face-down orientation relative to the working surface of the polishing machine.

13. The assembly of claim 11, wherein the optical element is configured in a face-up orientation relative to the working surface of the polishing machine.