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.

Waterjetting 2b - crack growth and granite sculpture

The last post in this series showed that the main way in which waterjets penetrate into materials is by growing cracks that already exist within the material, and I used glass as an example to show that this was true.

It is this process during which water penetrates into cracks, and then comes under pressure, either by the impact of more falling water (say under a waterfall in nature) or because the water freezes and then thaws, that causes the cracks in the rock to grow under natural attack, and the rock to slowly erode. As this happens the cracks slowly grow and extend to the point that they meet one another, separating small pieces of rock from the solid.

Within the body of a piece of rock the largest cracks that exist are normally at the boundaries of the grains of different minerals that make up the bulk of the rock. (Back in 1961 Bill Brace showed that the strength of a rock reduced as the square root of the increase in the grain size of that rock ).( Brace, W. F. (1961): Dependence of fracture strength of rocks on grain size. Bulletin of the Mineral Industries Experiment Station, Mining Engineering Series. Rock Mech. 76, 99± 103.) More recently, though still back in 1970, my second grad student, John Corwine, showed that it was possible to predict the strength of a block of granite, knowing the size of its crystals.

Which makes a good time to tell a little anecdote. Back when I was doing my own doctorate at the University of Leeds (UK) we were looking at how waterjets drilled through rock, and how that might be used to make a drill. We had already run some tests of different rocks that we placed under a nozzle, and gradually raised the pressure of the jet to see what pressure it took to make a hole in the rock. Tests on granite had shown that the jet (with a maximum pressure of just under 10,000 psi) would not drill a hole into those rock samples, and so the granite had been set aside. But, with the equipment just finished and yet having to go to lunch, I asked Dennis Flaxington, the lab technician helping me, to put a new sample into the rig so that we could run a test in the afternoon. When I came back I found that he had used a piece of granite. I made several disparaging remarks, at which point he noted that, having spent some significant time putting the rock in the apparatus, I should just go ahead and run the test (which normally took about 5 minutes) rather than being an unmentionable. And so we did, and as I posted earlier, this is the resulting hole in the rock, which we were now able to drill right through in a process that took about half-an-hour.

Figure 1. 9-inch thick block of granite drilled through by a 10,000 psi waterjet at Leeds University. It took over 30 minutes. (Summers, D.A., Disintegration of Rock by High Pressure Jets, Ph.D. Thesis, Mining Engineering, University of Leeds, U.K., 1968.)

How could this now work, when a single jet clearly did not penetrate into the granite in the earlier tests? The answer is that as we moved the rock under the nozzle (we were slowly spinning the rock under the nozzle, and then raising the rock, since at the time there were no high-pressure swivels available for us to use) the jet passed successively over the edges of the different crystals in the granite. As it entered and pressurized these small fractures, the pressure in the crack was enough to grow the crack and remove individual crystals along the jet path. By starting at the center, and taking successive passes around the axis a large depression was cut into the surface, and the rock could then be raised, and a second smaller layer removed. Repeating this slowly removed the rock in front of the nozzle, and at the end of the test we had drilled through 9 inches of granite.

From this experience, over time we went on to cut, for a University, a lot of granite. Obviously, to cut at a competitive rate we had to cut at a higher pressure that just 10,000 psi. But, after showing that we could cut Georgia granite at a competitive rate in tests run at 15,000 psi down in Elberton, Georgia, Dr. Marian Mazurkiewicz and I led a group of our students in cutting 53 blacks of that granite to form the MS&T Stonehenge that now sits on the University campus.

Figure 2. View of the Stonehenge at Missouri University of Science and Technology, the vertical blocks are some 11 ft tall. The entire sculpture was cut by high pressure water jets operating at between 12,500 and 15,000 psi. (MS&T RMERC ).

Cutting commercially is not quite as simple as it might appear, since larger blocks such as those shown in Figure 2 will contain rock that varies quite significantly in properties as the cuts progress. In the Stonehenge case the rock came from close to the top of the quarry, and the cracks in the rock were quite well defined. Some fifteen years later we were fortunate enough to be asked to cut a second sculpture, but this time working with the internationally acclaimed artist, Edwina Sandys. Edwina had designed a sculpture for the campus, the Millennium Arch, which required that we cut two figures from blocks of Missouri granite, and polish them to create one group, while using the original pieces as part of an Arch that would stand some 50 ft away.

Figure 3. The Millennium Arch at Missouri University of Science and Technology. (Each vertical leg of the Arch is some 15 ft long, and the figures removed and in the background, are 11 ft tall). Better images can be found here.

The vertical legs were first cut to shape, and then the figures cut out from them. In order to contain the crack growth to limit the amount of material removed the cutting lance had two jets inclined outwards and the lance was rotated at around 90 rpm, as the lance made repeated passes over the surface, removing between a quarter and half-an-inch of rock on each pass, until it had penetrated through the rock. It took 22 hours of cutting to isolate the female figure from the host block. The slot width was around an inch, and there was some significant difficulty in cutting this slot as the quality of the rock changed within the blocks being cut. (The problem was solved by raising the cutting pressure).

Figure 4. Partial cut for one of the figures of the Millennium Arch, checking the depth.

This second sculpture illustrates both an advantage and a problem for the use of waterjets in cutting rock pieces. Use of the water gives a relatively natural look to the rock, although the vertical surfaces of the arch and the capstone were all actually “textured” to look natural using a hand-held lance at 15,000 psi. (The rock is a little harder than that from Georgia and most of the cutting took place at around 18,000 to 20,000 psi). But when the polished surfaces for the inside of the verticals and the isolated figures were prepared the rough initial surface required much more time to grind and polish flat, than a smoother initial cut would have needed.

Because water alone penetrates along crystal and grain boundaries in the rock the surface left is relatively rough. This gets to be even more of a problem if waterjets are used to cut wood. Here the “grain” boundaries are the fibers in the wood structure. Thus when a relatively low pressure jet (10,000 pai) cuts into the wood, it penetrates between the fibers and the cut quality is very poor. One of the first things I have asked students to do, when given the use of a high pressure lance for the first time, was to write their name on a piece of plywood. Here is an example:

Figure 5. Student name written with a high-pressure jet into plywood. Note that areas of the wood around the jet path are lifted by water getting into the ply beneath the surface layer, and that part of the top ply between cuts is removed in places.

I thought about having you guess the student name, Steve, but this is one of the more legible ones. (Female students generally cut the letters one at a time and were more legible, male students tried to write the whole name at once).

There are many similar examples that I could use to illustrate that, while there are tasks where waterjets alone work well, when it comes to precision cutting, then adding a form of sand to the jet stream to provide a much more limited range to the cutting zone can give a considerable advantage, and so the field of abrasive waterjet cutting was born, and discussion of that topic will lead, in time, to a whole series of posts.