B. Olbert, J. R. P. Angel, J. M. Hill and S. F. Hinman
Steward Observatory Mirror Lab, University of Arizona
Steward Observatory, Tucson, AZ 85721
email: bolbert, rangel, jhill, shinman@as.arizona.edu
Proceedings of SPIE conference on Advanced Technology Optical Telescopes V, 2199, p. 144 (1994)
Abstract
1. Introduction
2. Mirror Blank Handling and Cleanout
3. Magellan Mold Construction
4. Conclusions and Future Plans
5. Acknowledgements
6. References
| Figure 1 | Figure 2 | Figure 3 |
| Figure 4 | Figure 5 | Figure 6 |
Abstract
We report on the casting of two 6.5 meter diameter borosilicate
honeycomb mirrors at the Steward Observatory Mirror Laboratory. These
F/1.25 primary mirror blanks are for the MMT Conversion Project in
Arizona and the Magellan Project in Chile. The first 6.5 meter mirror blank
was cast in April 1992. After a three month annealing cycle it was
removed from the furnace using a fixture glued to the upper surface of
the blank. It has since been stripped of its mold material in
preparation for polishing. The mold material is broken up with a high
pressure water spray. The second identical 6.5 meter was cast in
February 1994. Each honeycomb 6.5 meter is cast from over 10
tons of E6 borosilicate glass manufactured by Ohara. This glass is
melted into a mold constructed of aluminosilicate fiber to produce a
honeycomb structure with roughly 20% of solid density. The hexagonal
voids in the honeycomb are produced by ceramic fiber boxes bolted to
the bottom of the mold with SiC bolts. The furnace rotates at 7.4
rpm during the casting process to produce a F/1.25 parabola on the
front surface. This rough parabola minimizes the amount of glass
which must be removed during the grinding process. The front
faceplate of the mirror will be 28 mm thick after generating. After
the second 6.5 meter blank has been cast, minor modifications will be
made to the furnace to allow casting of the first 8.4 meter mirror for
the Large Binocular Telescope.
1. Introduction
In the early 1980's Angel and Hill 1 began developing the technology
required to produce honeycomb mirror blanks up to 8 meters in
diameter. The honeycomb structure provides a high stiffness to weight
ratio while allowing ventilation to achieve a short thermal time
constant. After producing a number of small blanks including three
1.8 m and three 3.5 m mirrors, the Steward Observatory Mirror Lab cast
the first 6.5 m diameter borosilicate honeycomb mirror blank in April
1992. The progress of that blank through the opening of the oven
after annealing has been described by Hill and Angel 2 . A
description of the process of lifting the mirror and removing the mold
material is given below in Section 2. This first 6.5 m mirror blank
has now moved on to the polishing laboratory for finishing 3 . The
first 6.5 m F/1.25 parabola is destined for the MMT Conversion on Mt.
Hopkins 4 . In February 1994, a second 6.5 m mirror blank was cast.
That mirror is currently annealing in the furnace. A view of the
second 6.5 m mirror in the mold at 1180 C ° is shown in Figure 1.
Four views of the chunks of glass flowing into the mold are shown in
Figure 2. This second 6.5 m mirror is destined for the Magellan
telescope in Chile 5 . Section 3 describes in more detail the
process of casting these 6.5 m honeycomb mirror blanks. We also
describe below some of the technical details which we have learned
about the casting process.
2. Mirror Blank Handling and Cleanout
The MMT casting cycle was completed on June 29, 1992. After the
25-ton upper furnace enclosure was removed and the inner
and outer walls and tension bands were disassembled, the mirror blank
was thoroughly inspected. The mirror remained on the surface of the
oven from July 20 until October 15 awaiting tests and modifications to
the 6.5 meter lifting/turning fixture.
Inspection of the MMT casting showed that small gaps opened between the post collars on the cores and floorboard in many locations. Glass melt slowly flowed through the gaps and onto the bolt, nut, tile and furnace hearth, foaming whenever it contacted SiC. Only about 50 kg of glass leaked out so the effect on the blank was negligible except that the glass coating bound the SiC parts together, making part removal and cleanup difficult and time consuming. Approximately 50% of the bolts were broken during disassembly because threads along bolt/nut contact region were fouled by glass.
Rib Cracks
Seventeen rib cracks were discovered in the MMT mirror during cleaning and inspection. Fourteen were single, slit-like cracks that started at or very near the cross pin hole and ran into the rib, the lengths ranging from ~1 cm to ~12 cm. The rest had more than one crack branch with the longest branch being ~12 cm. Two rib sections containing cracks were cut out for analysis. Fractography studies done on the two sections by Dr. Edwin Beauchamp of Sandia National Lab showed that at least one crack was associated with an impact as evidenced by a Hertzian-type cone fracture on the surface opposite the crack origin. It was then suspected that all cracks were due to an inadvertent, intense impact of the cleaning wand on the rib. Working under this assumption, the wand was encased in soft rubber and the jet pressure was reduced by 25%. With these changes, and the cleaning crew with heightened awareness of the problem, no more rib cracks were observed while cleaning the remaining half of the blank. The crack tips of all rib cracks were eliminated by stop-drilling, and the drill-hole boundary was etched with HF paste. Once treated to round the crack tip, the cracks do not threaten the integrity of the honeycomb. A detailed inspection of the blank and stop-drilling of the cracks was completed on May 30, 1993.
Geometry
The rib and core geometry of the MMT 6.5 meter blank turned out very well. All the ribs in the blank are within a few millimeters of their expected absolute positions. The faceplate thickness was measured to be 32.4 ±0.7 mm, somewhat thinner than the 42 mm expected, but still more than the 28 mm required. It appears that the cores compress slightly and creep under the hydrostatic pressure causing the ribs to be slightly thicker than the original mold geometry. This results in the finished blank being 2% heavier than nominal. Faceplate curvature indicates that the rotation speed of 7.40 rpm was held to within 0.5%.
Stress Spot measurements of residual stress in 47 ribs showed an average stress-induced retardation of 4.8 nm cm -1 (20 psi tension) with a standard deviation of 2.2 nm cm -1 . The maximum observed retardation was 7.2 nm (30 psi). This is consistent with the expected amount of residual annealing stress combined with some small amount of inhomogeneity of the expansion coefficient. The stress magnitude was calculated from the measured stress-optic coefficient of E6 = 4.22 psi nm -1 cm. Additional measurements will be made after both sides of the mirror have been polished.
Bubbles Approximately 150 bubbles of ~1 cm diameter were found in the upper 5 cm of the MMT blank. These bubbles are located mainly in the outer third of the faceplate. These seem to have been caused by not holding at maximum temperature long enough to allow all the trapped air to escape. An additional hour of cooking time would probably have cured the problem. The number of 1 mm size bubbles is noticeably lower than was seen in the 3.5 m mirrors. While many of the bubbles will be ground away during generating, about two dozen will intersect the final surface. These large bubbles will be plugged prior to polishing to avoid trapping polishing compound as has been done on occasional bubbles in previous mirrors.
3. Magellan Mold Construction
Once the oven was cleaned and inspected, floor tile layout began for
the Magellan 6.5 meter mold on February 8, 1993. Tub walls, inconel
tension bands, and pneumatic cylinders were installed and the oven was
fired to seat the inconel bands in March 1993. After the 2-week
firing, the tub was lined with refractory and machined to size. Core
machining and installation began on May 26, 1993 and was completed by
the end of July after which the mold was inspected and fired once
again to 1180 C °.
As with the MMT mold, the Magellan mold was built within the furnace area on the 10 m turntable. The base of the casting tub was constructed from a close-packed array of individual hexagonal-shaped floor tiles, all tiles installed with high-accuracy templates. The nuts used to engage the bolts were individual hex-shaped parts that were positioned with the tiles. To prevent rotation as the bolt was engaged, the nut meshed into the tile feet. All tiles were made undersized by ~0.8 mm to guarantee no overlap problems during layout. To insure stability, the tile corners were pinned together by pyramidal wedges cemented into the array's tri-corner junctions. The nearest neighbor tile separation distance was 192.2 ±0.13 mm. Over the mold radius of 3.25 m, the p-v spread in tile separation distance was 1.8 mm.
Inner and outer ``tub wall'' sections were assembled around the tile base in the manner of barrel staves. Inner walls were locked into position by keystone pins. Pre-rolled inconel 601 bands were wrapped around the outer wall and the band ends hooked to air cylinders shackled to steel posts located outside the furnace shell. The posts were effectively tied to the the main I-beam spokes of the turntable. Band tension was applied by feeding a well-regulated supply of 2.4 MPa nitrogen to the cylinders. Tension of 500 pounds on each of 80 bands serves to constrain the tub against the hydrostatic pressure of liquid glass. As usual, the casting tub was prefired to 1180 C ° to seat the bands and fix the tub geometry.
The tub was lined with soft refractory fiberboard, and the floor machined flat. The tub sidewall liner was machined to the skirt contour of the blank. The soft refractory hex boxes, cross pins, post collars and box caps needed to make the honeycomb structure were cut and machined from various preforms. All regular hex core shapes were NC machined from one hex preform shape. The irregular partial cores along the inner hole boundary and outer boundary were made by cementing pieces of flat board to various hex box sections.
Each machined box was guided to its particular hex tile with a special box placement tool. The tool was located by locking it into the center holes of two nearest neighbor tiles. In this way, the core positions were referenced directly from the tile positions. The box was pushed snugly into a V-shaped guide plate on the tool, and slid down until the post collar made contact with the floorboard. The post collar separates the core from the floorboard to form the perforated backplate of the mirror. The box sidewall was clamped to the tool, the bolt and washer inserted into the box cavity, and the bolt tightened to a fixed torque. Ceramic fiber cross pins are used to stabilized the cores laterally. The cross pin holes were reamed by hand to fit, and the tapered fiber pin cemented in place. The cores were capped after all cross pin connections were made. The assembly process is shown on Figure 3 of Hill and Angel2.
Since both molds were geometrically identical, all fit SiC parts recovered from the MMT mold were used in the Magellan mold.
The most significant departure from the MMT involved use of soft refractory material prefired to 1180 C ° by the manufacturer and use of a new, specially post-treated hex box material. The MMT parts were prefired in the 10 m furnace. The new box material gave cores with much higher compressive and flexural (tensile) strength and reduced core-to-core strength variability. A strength improvement was needed for Magellan because the compressive washer/core contact (bearing) stress in some MMT partial cores exceeded the design limit stress by ~40 %. A selected property data summary for MMT and Magellan hex preforms used to make cores is shown in Table 1.
| Preform | Avg. Compressive Strength at Damage Limit (MPa) |
Avg. Flexural Strength (3 point bend) (MPa) | Avg. Axial Burst Force (corresponding to buoyancy force at failure) (N) |
| MMT | 0.101 | 0.384 | 975 |
| Magellan | 0.384 | 0.794 | 1,827 |
Table 1: Selected Material Properties of 6.5 m Hex Box Preforms
After the 2-week prefire of the assembled mold, an inspection revealed delaminated partial cores, which necessitated a 4-month mold repair cycle, much of which was failure analysis and testing of repair processes. This was completed on January 11, 1994. There was no observable delamination of the one-piece hexagonal cores.
Approximately two thirds of the Magellan partial cores showed a delamination-type crack in the board along the board-hex box or board-board glue joints. The delamination occurred sometime during or after the mold prefire step. A few delaminations were found in the core caps along the cap-core glue joint.
Crack growth was empirically proven time-dependent. Forty-five cracks were found in partial cores during inspection completed a few days after the prefire. Thirty-seven previously undelaminated partials showed cracks along the glue joint after a second inspection was completed several weeks later. Because of time-dependent crack growth, every partial core and every core cap were considered potentially defective at some level.
The cracks were believed caused by a destructive superposition of cement shrinkage and material property differences between the mated box and board sections. Specifically, the expansion coefficient, high temperature shrinkage rate, stiffness and strength of board were different from that of the box at some variable level, with the board always weaker than the box. Although the property differences between box and board were small and some apparently changed in sign (e.g., expansion coefficient and shrinkage rate), an unfavorable superposition could open and drive a delamination-type crack. Time-dependent crack propagation is well known in this material. Comparison of core tops with failed and good glue joints showed that failure occurred most commonly with board lacking a strong ``rind'' which is naturally created by the manufacturer's post-treatment process.
Manufacture of all-new hex boxes, boards and glue to make new cores that wouldn't delaminate was considered to be a time-consuming and risky option because the manufacturer had no experience in solving this type of problem. Rebuild of delaminated cores with some ``all-new'' cores made from in-house stock was considered most attractive, and rebuild methods were developed at top priority. From this priority effort, new patching and part construction techniques were perfected and the finished parts were aggressively proof tested and qualified for use.
With exception of the triangle-shaped cores made from board stock only, all partial cores with joints that could potentially be breached and filled by glass melt were pulled from the mold. The residual joint strength was tested by inserting a flexible air bladder into the core interior and pressurizing it to induce an average tensile joint stress of 14 kPa. If the board split from the box section, the gluing surfaces on box and board were impregnated with rigidizer to give a strong, thick rind layer to resist delamination stresses. The board was then re-glued to the box section and the two edges pinned through and into the box with four sets of mullite ceramic pins (four pins along each edge for a total of eight pins). Boards that remained attached to the core after pressurization were pinned-only. The sidewalls of the triangular cores were pinned together at the corners while still in the mold because they were easily accessed and removal would likely damage them beyond repair. All new and patched cores were pre-fired to 1180 C ° and re-tested to a 14 kPa joint stress with the same pressurized bladder method used in the patching pre-test. Cores judged too damaged for patching or cores that failed the proof test were replaced by new cores. All caps were pinned through to the core with three mullite pins placed symmetrically around the edge at 120 degree intervals.
After all repairs were completed, the mold was inspected, cleaned and prepared to accept 10,233 kg of E6 borosilicate glass chunk. Once the glass was loaded on top of the mold, the furnace and lid and were lowered into position and the casting started. Glass loading is shown in Figure 6 and the casting and annealing cycle is shown in Figure 8 of Hill and Angel2. Based on real-time video images of the casting, the Magellan mold held up well with no evidence of significant glass leaking or of significant bubble content.
4. Conclusions and Future Plans
Two successful 6.5 meter mirror blanks have now been cast at the Steward
Observatory Mirror Laboratory. These castings have demonstrated that the
technology to cast honeycomb blanks of this size is now in hand.
Once the Magellan blank is moved into the washout fixture, work will
commence to upgrade the rotating furnace to 8.4 meter capability.
This effort should be complete by November 1994 at which point
construction of the 8.4 meter F/1.14 LBT primary mirror mold will
begin. Casting of this blank, the world's largest honeycomb monolith,
is expected to begin in late summer 1995. Current plans call for
casting a second 8.4 meter LBT blank following the first one, although
the possibility exists for casting one or more additional 6.5 meter
blanks, depending on funding at the Mirror Lab.
5. Acknowledgements
The production of the 6.5 meter mirrors is a team effort in which
dozens of people are involved. In addition to the authors, the
following Steward Observatory staff members were involved in the
cleanout of the MMT mirror and the casting of the Magellan mirror:
E. Anderson, J. Astier, D. Baxter, D. Campbell, M. Cline, J. Collins, R. Cordova, S. DeRigne, D. Dan, W. Davison, K. Duffek, P. Esterline, M. Hunten, K. Johnson, K. Kenagy, D. Ketelsen, A. Klocko, R. Kraff, R. Lutz, R. Meeks, P. Muir, B. Powell, S. Schaller, E. Smith, W. Stoss, P. Strittmatter, T. Trebisky, D. Watson and S. West.
Thanks to Lori Stiles for providing the photographs in this paper.