Monday, November 18, 2013

Waterjetting 15b - Making the Millennium Arch

At the time that we carved the Missouri Stonehenge, our abilities seemed limited to cutting only linear passes through thick blocks of rock. However, while this has an application in the quarrying industry there are also many applications where contour cutting to depth would find a market. We were challenged to demonstrate this when it became time for the MS&T campus to find a project to mark the Millennium.

Figure 1. The MS&T Millennium Arch by Edwina Sandys The piece is in two parts, the Arch in the foreground and the extracted figures form a second grouping near the building entrance.

As a little bit of a back story, Winston Churchill had come to Missouri in 1946 where he gave his “Iron Curtain” speech at Westminster College in Fulton. This led, inter alia, to the National Churchill Museum. At that time Scott Porter, a Rolla minister’s son, had gone up to Fulton and took a color photograph of the visit. Skip forward past the fall of the Berlin Wall and Westminster College had commissioned Edwina Sandys, the internationally recognized artist and sculptor, to create an appropriate sculpture to mark that event. The sculpture, "Breakthrough” was formed from pieces of the Berlin Wall that had been cut by a hand-manipulated abrasive waterjet nozzle to silhouette a male and a female shape, and to quote the sculptor:
In Breakthrough, from the blank former–Communist side, you see light through the male and female shapes, and when you walk through to freedom, from dictatorship to democracy, it’s as if you were living in a black-and-white world, and now you’re in glorious Technicolor.
At the Dedication Scott presented a copy of the photograph to Edwina, and they got to talking – long story short – the campus asked her to create a sculpture for the campus, funded by Scott as a memorial to his parents, and I became her “hands” in helping to carve “The Millennium Arch.”

Figure 2. Breakthrough, a 32-ft high sculpture on the Westminster College Campus – carved with a hand-controlled abrasive waterjet (Edwina Sandys)

By the time that the sculpture was commissioned the granite quarries in Missouri had re-opened and we were able to acquire three blocks of granite from which to carve the five pieces of the sculpture. One of the advantages of waterjet cutting, as both Breakthrough and The Millennium Arch illustrate is that the “cut-outs” are removed entire, and can thus become figures in their own right. The figures from “Breakthrough” are at the Roosevelt Library in Hyde Park, NY, forming the “Break Free 1990-94” sculpture.

Once the blocks had been brought to Rolla we decided to move them indoors, something we had to do manually as we wanted to continue cutting through the winter, and because the computer driven table that John Tyler and our students designed and built would be better protected inside. (Though we had forgotten that debris from the cutting would have enough energy to reach the roof). The blocks weighed around 35-tons each, and the first task was to trim the edges of the two legs of the piece, to a rectangular shape. Stepping motors were used to move the cutting lance along the first four sides of each block, then the pieces were moved outside and gently allowed, with timber block support, to fall over onto their sides before being returned to trim the last two sides.

Figure 3. Cutting the edge of a Millennium Arch leg.

Figure 4. Before and after picture - the block on the right is about to enter the cutting table, while that on the left has been trimmed to size/

Figure 5. Block laid flat under the cutting table. to allow cutting of the male figure (in progress). Note the heavy plastic sheet strips used to protect the bay.

The heavy plastic strips shown in figure 5 are heavy enough to absorb the energy of the flying debris from the cut, while being flexible enough that they will deform swaying and deflecting, rather than being cut themselves.

After the design had been finalized (we had cut a 1/12th scale model followed by a half-scale version to check the dimensions as we re-learned how best to cut this granite) the blocks were cut using the same concept of twin, spinning jets issuing from a dual orifice nozzle, and rotating at 90 rpm. Because the Missouri granite is stronger than the Georgia granite, the cutting pressure had to be raised to 20,000 psi. The granite was not as consistent as that of the earlier Stonehenge, and harder inclusions inside the rock were more difficult to detect as they slowed the cutting rate, requiring a closer monitoring of the cut to ensure that the nozzle was not fed forward too far in successive passes.

We had thought about the possibility of making the cuts through the granite using an abrasive waterjet cut, however single pass cutting would have been incredibly slow, and a test using a multiple pass system with a single non-rotating nozzle showed that after a couple of inches of cutting that the edge quality was beginning to deteriorate. With the prospect of this getting worse in depth, and the need to separate the two part of the sculpture after the cut this led to the decision to use the plain waterjet system that we were familiar with. It did mean that the figures ended up two-inches shorter than the holes they came out of.

As with the earlier Stonehenge the slots were cut in the block by traversing around the path one time, and then lowering the nozzle a third-of-an-inch and then repeating the pass. The slot was roughly an inch wide, to allow for variations in the crystal sizes on the edges of the cut, and to allow the nozzle to move around all the contours of the geometry.

Once the internal figure had been released from the surrounding leg we had to separate the two pieces. It was easier to slowly jack the outer leg up, first one end then the other. the narrow gap between the pieces restricted the tilt we could make on any one lift, but it took less than an hour to get the leg high enough that we could slide the figure out.

Figure 6. Raising the leg so that the cut figure of the female can be slid out from underneath, after which the leg is lowered back onto rollers to remove it from the frame.

The surface quality of the three Arch and two figure pieces had then to be adjusted. The surface was hand-polished for the inner surface of the legs and the figure surfaces. The rough cut crystal-outlined surface was first ground flat using special graded grinding disks, and then the final polish achieved with the industrial polishing disks. While it took 22-hours to cut out a single figure from a leg, it took us two months to grind the surfaces, which could have been done faster had we mechanized the process. Hand polishing was a poorer selection that I made at the time.

There were two additional problems with surface texture – the sides of the legs appeared too “regular” after cutting and still showed the striations from the individual passes down the walls. To overcome this problem we used a hand-held lance, at 20,000 psi, to retexture the surface, and this turned out to give a relatively natural –looking surface.

The capstone gave a different problem, since the intent was to make it appear as a natural shape, and yet it was, as delivered, very clearly shaped by the splitting wedges that had separated it from the massif. Again a hand-held waterjet was used to smooth some of the sharp corners and provide a rough contour for the piece, while the secretarial and other staff in the RMERC (Vicki Snelson, Diane Henke et al) helped mix up our own brew of glue and granite chips to fill some of the splitter holes left in the block.

Figure 7. Dr. Galecki re-contouring the capstone for the Arch.

The two vertical legs were erected, and then a template taken identifying their exact position. This was then set against the underside of the capstone, and two pockets were milled out two-inches larger on each side than the leg sizes. When the capstone was then lowered onto the legs these pockets allowed the legs to penetrate six inches into the capstone, and provide some additional stability to the stones.

Before the capstones were set, however, it was noticed that the waterjet finishing of the surface of the legs had shown where there were two weakness planes within the legs that might, in the millennia that follow, fail. To prevent this from causing the Arch to collapse three holes were drilled down through the legs, and carbon rods anchored and post-tensioned through the possible weakness planes. Carbon rods were used to prevent the problems that corrosion might otherwise, in later years, cause to the sculpture as metal might expand and fracture the rock more that support it.

(Those of us there then autographed to tops of the legs, before we set the capstone on them.)

After the capstone was in place the gap between it and the legs was filled with the same glue:rock chip mix that had been used elsewhere to fill undesired holes.

The two figures were installed on a separate plinth, some 50-ft from the Arch itself, and these two were anchored in place with carbon fiber rods that extended up from anchor points in the plinth through the lower legs of the figures.

The Arch was dedicated in the Fall of 2000, in the presence of both Scott Porter and Edwina Sandys.

Figure 8. Arch Dedication ceremony, Scott Porter is escorting Edwina Sandys, while I follow with Chancellor John Park.

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