Waterjetting 30b - An opportunity missed, and a question raised

In the last post I wrote about the benefits of cutting a deep slot around the edges of a tunnel, and made reference to the work done here by Dr. El-Saie back in the mid-1970’s as part of his Doctoral Dissertation.

One of the concerns that we had to address was that the waterjet had to be able to penetrate all the different rock types that it might encounter, and at the same time, since the jet would only cut a short depth on each pass we also had to find a way of cutting the slot wide enough that the nozzle assembly could enter and deepen the slot over consecutive passes around the edge. Given that the available pressures in those days were limited, for us, to 30,000 psi and this pressure was insufficient, by itself, to cut through all the rocks we might encounter, Dr. El-Saie looked at several different ways of enhancing performance. These included adding abrasive to a high pressure jet stream, inducing cavitation into the jet stream and the potential for using the break-up of the jet into droplets to enhance cutting using the impact water hammer effect.

Because of other operating conditions it was not considered practical to try and develop the droplet impact idea for this program, and the work concentrated on examining the potential differences between abrasive waterjet injection and cavitation. To simplify the comparison the same basic nozzle design was used for the tests that were then run, although the shroud fitted to create the secondary (vacuum) chamber was modified to either allow abrasive entrainment, through ports, or to create cavitation. The presence of the ports did, however, allow the strength of the vacuum generated in the chamber to be measured as the jet passed through.

Figure 1. Nozzle designs used by Dr. El-Saie. Note that the upper design has ports leading into the vacuum chamber, so that abrasive can be drawn in by the jet passage. In the lower design there are no ports, and cavitation will be induced in the chamber by the jet passage, with the bubbles then drawn into the exiting jet.

One of the advantages of cavitating the jets is that the cavitation bubble collapse will spread out over a larger area on the target surface, so that the slot generated can be quite a bit larger than the originating jet. This can be shown in two pictures of a block of dolomite exposed to the same cavitating jet, at a pressure of 6,000 psi but one with the jet traversed along the block in a minute, while in the second case the jet is moved at a slower speed, taking five minutes to cross the block, which allows the jet to exploit the cracks generated by the cavitation. A fuller description of the process is given here.

Figure 2. Cavitation damage pattern on a block of dolomite showing the initial width of the jet (red lines), and the zone of damage that is being created around the traverse path.

Figure 3. Cavitation damage on a block of dolomite at a slower traverse speed, showing the width of the damage track that can be created. The slot is about half-an-inch deep.

In the course of the test program different shroud shapes were tested, but in all cases the comparison between an abrasive-laden jet and one containing cavitation bubbles was made with shroud shapes of the same overall dimensions.

The ratio of the exit diameter (discharge) from the shroud (D2) to that of the initial jet orifice (D1) was first changed to one of four different ratios, though the diameter of the initial jet was kept at 0.04 inches (1 mm). Of the different sizes tested the greatest vacuum in the chamber was measured with the smallest of the discharge diameters was tested.

Figure 4. The effect of increasing the throat length of the shroud on the vacuum pilled in the chamber, at different pressures.

If the discharge diameter was increased to 6.35 mm then the jet pressure had to be increased to 12,500 psi to obtain the same levels of vacuum achieved otherwise at 7,500 psi.

Figure 5. Vacuum pressures measured with a larger discharge diameter from the shroud, for different lengths and jet pressures.

Impact force measurements from the jet hitting a target at varying distances from the orifice, with and without the shroud showed relatively little difference in the overall total impact force (not considering abrasive) out to a distance of 6 inches. There was thus no apparent effect due to jet disintegration from the use of the shroud over these distances.

In order to compare the performance of the abrasive-laden jet with that of a cavitating jet, a new nozzle design was developed, and small samples of granite were rotated in front of each nozzle assembly, for 20 seconds. Because the jet had to be brought up to pressure for each test, and shut down afterwards, a steel shutter plate was placed between the nozzle and the target. One of the irritants in doing the tests was that the jets kept cutting through this shutter plate.

Figure 6. Steel shutter plate cut through in 6 seconds during system start-up.

The shroud, made of stainless steel, was also wearing out within a few minutes. Unfortunately we did not recognize that this was demonstrating that abrasive waterjets were an effective method for cutting metal – that commercial development had to wait for the more perspicacious Dr. Hashish to work with Flow Research and bring the technology to the market in 1980.

Part of the reason for our lack of interest was because of a different conclusion that Dr. El-Saie drew from his work, based on the following two curves. The first comparison of different jet results occurred with a jet pressure of 7,000 psi.

Figure 7. Volume of material removed from granite samples, as a function of distance, for four different jet conditions at a jet pressure of 7,000 psi.

Note that the three water jets do not have much significant effect on the granite at this distance and jet pressure (we had to learn some later lessons to make them more productive at this pressure). But even at this pressure the abrasive waterjet was effectively cutting the granite.

But it was the change in the relative position of these curves, as the pressure was then increased to 20,000 psi that caught our attention. (The intermediate plots are not given here).

Figure 8. Volume of material removed from granite samples, as a function of distance, for four different jet conditions at a jet pressure of 20,000 psi.

The water feed was not useful, since the power required to accelerate that volume drew heavily from that available through the jet.

The plain jet, without a shroud will cut granite at this pressure, particularly when moved over the surface. (And we later used this system to carve the Millennium Arch_ – as well as the Missouri Stonehenge). But the performance at that time was not that impressive.

Opening the ports on the shroud, without feeding anything into the jet caused, we believed a greater jet breakup and thus some additional droplet impact effects that improved cutting performance over that of the plain, more coherent jet, in part because the jet was spread over a larger contact surface.

Closing the jets induced cavitation in the stream, and this gave the best performance of the four – including abrasive injection. Again this was, in part because of the larger area of damage that the cavitation generated on the target, over the narrower slot of the abrasive-laden and plain jets.

In comparison the abrasive waterjet did rather poorly. In retrospect this is perhaps more of a surprise – but it should be born in mind that there was little attempt at optimizing the feed condition (which later research shows has a dramatic effect on performance) or the chamber geometry. Further the slot cut was much narrower than that created by the cavitating jet.

But it certainly caused us, in that time interval, to look more at cavitation, and to totally miss the implications of the AWJ result.

Most of these illustrations come from the Doctoral Dissertation by Dr. A. A. El-Saie “Investigation of Rock Slotting by High Pressure Waterjet for Use in Tunneling”, Mining Engineering Department, Missouri University of Science and Technology, (Then University of Missouri-Rolla), 1977.

Waterjetting 28a - Cutting rock on a table

In previous posts I have written about the use of lower pressure water (around 10,000 psi) as a way of cutting through rock. From the time that we first made a hole through nine-inches of granite while I was a graduate student some 50-odd years ago the way that we have recommended that rock be cut, in a mining situation, has been to use lower pressure and higher volumes of water. This is so that as many natural fractures around the individual grains and crystals can be developed at one time as possible. However the result of this is that the cut progresses along the grain boundaries of the infrastructure of the rock, that a roughly edged cut is made, rather than a smooth cut surface. In mining applications this isn’t necessarily a bad thing, but when cutting counter tops and other ornamental structures for marble and granite surfaces inside a building then a rough surface is definitely not often required.

So how can a relatively smooth surface finish be created along the cut? One step,that works with softer rocks (such as some pink granites) is just to increase the jet pressure, while at the same time reducing the volume of the stream flow. Once one reaches the ultra-high pressure regime (which is, for this article, considered above 35,000 psi) and with jet diameters on the order of 0.01 inches or less, the jet stream is more typically going to cut through a crystal within the rock than to just work on the cracks that lie at the edges of the crystal.

Unfortunately there are sufficient cracks and crystal boundaries within the rock that it is not possible to ensure that at some point as the jet cuts down through the rock, and along the desired path, that it won’t find a crack at a critical length and alignment that the crack will break out a larger chip. This is less likely to happen within the cut, since the confinement of the surrounding rock acts to reduce excessive crack growth, but can quite often occur at the rock surface, particularly where there has been some earlier damage that has left larger cracks within that surface. (This includes heat treatment).

That, however, is a specialized case, and in the more typical situation an increase in jet pressure to 50 ksi will not, by itself, produce the clean edge needed. Part of the reason for this comes from the striation planes within marbles, which can offer an easier path for the jet to penetrate, as the cuts get deeper, rather than having the hole continue forward along the jet axis. To overcome these problems it is easier, with the ubiquity of abrasive waterjet systems, to instead change to add an abrasive to the waterjet.

Dimension stone (the trade name for the decorative rocks such as marble and granite) is generally through cut with slab depths that are less than an inch-and-a-half thick, although greater depths can be specially prepared. Often the slabs are polished before they are finally cut to shape. We found that preferable, since when doing the final polish with successively finer grinding wheels (used for example in creating the Millennium Arch) the edge stress that can be generated by the wheels themselves can cause chipping along the edge of the work. This, in turn, either requires a regrind down to remove the chip, or some form of repair, which we found it difficult to make invisible given the complex structure of the granite. This is particularly true when relatively narrow ribs of material are being cut. As an example, consider the cartoonish mining figure that was made some years ago.

Figure 1. Toon miner carved from 3-inch thick granite.

The front and back surfaces were polished before the figure was cut from the slab, given the extreme fragility of the edges of the pick, for example, which failed under very little pressure in several samples before one survived.

One problem with this approach is that the edges of the cut, while relatively smooth, do not have the polished look that the flat surfaces have. Apart from making the cut relatively slowly, in order to remove as many striations along the cut path as possible, one answer has been to use a spray on the rock surface which then gives the impression of having a polished surface, and as long as the object is kept inside the coating will likely remain. (When we tried this with pieces that ended up outside weathering removed that coating within a short number of years).

The problem with hand polishing large flat surfaces is that it becomes very difficult to maintain a truly flat surface over the entire block, and while the surface may end up smooth and polished, it will likely have some small undulations within it. It is therefore more productive (and, we found, cheaper) to have large flat surfaces machine polished before they were cut. One example of this was the sign that we made for the State Geological Survey. It was made in two parts, the lower part was a Missouri Granite, which held an upper half, carved from Missouri Marble, which was cut to the shape of the state.

Figure 2. Sign cut for the State Geological Survey

The lower granite slab was inset into two vertical grooves that were cut into the supporting blocks. The granite slab was cut to shape on our cutting table, with the inset cut out to hold the “toe” of the state. Because the granite was first machine-polished the lettering was etched into the surface using a reduced pressure for the cutting jet, and removing a thin layer of the surface, which was replaced with the black fill material to highlight the letters.

Figure 3. The Agency name was etched into the granite slab.

When it came time to cut the shape of the state in the marble, the block was first trimmed at the top (to help it fit into the table). A piece of plywood was placed under the rock before cutting to prevent any rebounding abrasive from hitting the under side of the slab and removing the polish from the surface.

Figure 4. The first cut across the marble, showing the supporting plywood.

The rest of the state had a contour cut along each surface, and when these were completed the slab was ready for mounting.

Figure 5. The finished slab, showing the state outline.

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 15a - Carving a Stonehenge

This post is a marriage of two different themes that have recently appeared at Bit Tooth. The first, under the Waterjetting title, has contained a discussion of the different aspects of high-pressure waterjet use. The second, more recent theme discussed both the original Stonehenge and then how the MS&T Stonehenge, which is a working calendar, functions. In this post I am going to talk about how the MS&T megalith was built, and since it involves the use of high-pressure water it seems appropriate to include it in the Waterjetting Series.

As I have noted in one of the earliest waterjetting posts we had learned, from Russian literature, back in 1966 that waterjets could be used to cut into granite. From results of an unplanned test, we had learned that the pressures needed to cut through granite need not be that high. Others had predicted that it would take a jet pressure of up to 30-times the rock compressive strength in order to penetrate rock efficiently. However both the Russians and ourselves had been able to drill through a 30,000 psi granite with a waterjet pressure of only around 10,000 psi, rather than the predicted 900,000 psi.

We had done this by moving the jet over the surface so that, as the jet passed across the cracks between grains, so it would penetrate and pressurize the crack, causing it to grow and remove the grain, without having enough pressure to cut through the grain itself.

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.)

We had drilled this block of rock back in Leeds and the nozzle was pointing vertically downwards, as we rotated the rock beneath it. After drilling a shallow central hole, we stepped the nozzle out a short distance and repeated the process, breaking the outer ring of rock to the central core. Then after widening the hole, we could come back to the center and advance the nozzle into the rock, and repeat the process. Because of the nozzle size we had to continually step the hole to a smaller diameter as the hole got deeper, and thus there is a rapid taper to the walls.

Moving forward to the early 1980’s when Chancellor Marchello asked Dr. Marian Mazurkiewicz and I to cut the rock for his Stonehenge, and we knew that we could cut the rock at a pressure below 15,000 psi (the size of the pump that we had at the time). But tests where we had made multiple passes over a rock had shown that, with a direct vertical cut to an edge, that repeated passes would taper the cut inward over time – naturally doing what we had done artificially at Leeds.

Figure 2. Early tests on granite cutting. Note that the lower cut was made with two jets diverging at about 5 degrees, and the one above it with the jets diverging at 11 degrees. (The nozzle was spinning and moved across the face of the rock several times).


These tests showed that we needed a higher angle to ensure that the sides did not taper, and while this could be achieved with the nozzles angled at 15 degrees to the perpendicular, because the jets had to cut a path wide enough for the nozzle to enter the slot, an angle of 45 degrees was used after a short series of experiments.

Our team at the Rock Mechanics and Explosives Research Center (RMERC) had been asked to carve the rock, since we had just prior to the commission, been down in Georgia demonstrating to the granite industry there that waterjets were able to economically cut granite.

Figure 3. Starting to cut a 1-inch wide slot in granite, pressure 14,000 psi, 90 rpm, linear cutting speed around 9 ft/min, areal cutting rate around 20 sq. ft./hour.( Raether, R.J., Robison, R.G., Summers, D.A., "Use of High Pressure Water Jets for Cutting Granite," 2nd US Water Jet Conference, Rolla, MO., April, 1983, pp. 203 - 209.)

Concurrently with showing that this could be economic we had also shown that the technique removed the respirable dust from the air that is generated with a flame-torch cut, and that the noise level would drop to industrially acceptable levels from the “jet engine roar” of a cutting torch.

Figure 4 Showing the flame at the bottom of the burner spalling its way through the granite.

Figure 5. Cutting granite with a flame-jet lance, Graniteville MO 1979. (Note the cloud of very fine particles of granite being blown out of the right side of the slot).

The first thing to do in arranging to cut several hundred tons of granite was to find a source of supply. Unfortunately, at that time the granite quarries in the South-East part of Missouri were closed and other sites in the state did not prove practical. But because we had done the work with the Elberton Granite Association in Georgia, we were able to arrange to purchase rock from one of the Quarries around Elberton, Ga. They themselves had recently constructed their own version of the standing stones, the Georgia Guidestones although these had been cut to shape using flame jets, rather than water.

Figure 6. The Georgia Guidestones, Elberton GA.

The granite blocks were roughly split to shape in the quarry, and then shipped to Rolla by train. The first block was sent by truck and this proved the benefits of rail, although the size of the cars limited the scale of the monument to half that of the original in the UK. Which meant that the blocks – over 11 ft tall – were one-eighth the weight of the originals.

Figure 7. Blocks of granite in the cutting frame. The cutting lance is the thin rod in the center of the picture.

The blocks were brought to the RMERC and placed in position using a crane. Dr. Mazurkiewicz and his students had built this frame from wooden blocks, with the guide rails made from radio antenna mast. The lance moved on a cross-beam, also made from radio antenna mast. The low reaction force from the jets meant that the forces on the structure were very small. Thus the head itself could be pulled along the track using a bicycle chain, and small, fractional horse-power motors could be used to move the head and advance it into the slot. Although, by that time, self-rotating heads had been developed, it was decided that a better control of the cut edges could be achieved if the head was hydraulically rotated.

Figure 8. Detail of the cutting platform. The two hoses feed a hydraulic motor that gear-drives the rotation of the cutting lance. The high-pressure water feeds through the hose to a small swivel at the top of the lance. A small electric-motor driven screw behind the platform elevates and lowers it on the guide rails to advance the nozzle into the cut.

In order to keep the slot width as narrow as possible the nozzle holder was made as small as the feed pipe, with the two jets issuing from small carbide inserts within the holder.

Figure 9. Detail showing the nozzle holder and a nozzle orifice on the lance.

Experiments showed that an effective cutting rate of around 20-square feet an hour (depending on the direction of cut relative to the planes of the granite) could be achieved. The lance was rotated at 90 rpm, and moved down the cut at a speed of 9-ft per minute. The two jets, at a pressure of around 14,000 psi (there was some pressure loss in the system) would cut into the rock around 1/3rd of an inch on each pass, and the lance would be lowered this amount after the pass, and then the direction reversed and the jets would cut back along the rock. (This is somewhat faster than the hand-held stone flattening of the original Stonehenge rocks in England, although studies in Peru, where a similar technique was used to shape to blocks that build Machu Picchu showed that it is possible to flatten about 1 square foot an hour once you learn how to chip the rock). Professor Parker Pearson has also noted that the UK original had the rocks finally shaped after they had been erected).

Figure 10. Showing the jet arrangement, raised after a side had been trimmed so that the jets could be seen. Normally with the jet in the cut there is little to show the cutting action.

It took about a morning to cut one side of a block (or in later stages to cut one of the large blocks in half for the smaller stones). Once the second side had been cut, the block was turned and the rail aligned to cut the third and fourth sides. Overall, given that the operations had to be shut down during the winter where the temperatures were below freezing, the blocks were cut and completed over the course of two semesters, largely working with undergraduate student labor.

After the blocks were cut, they were taken to the site, where each was placed in position using a crane. Because of the precision required to align the blocks with the sun, this was a time-consuming operation. The major standing stones were then held in place with an additional pour of 18-inches of cement. (They stood on a cement platform).

Figure 11. Lowering a block into place.

The monument was dedicated at the Mid-summer solstice in 1984, with John Bevan, a Druid of the Gorsedd performed the dedication.

Figure 12. Speakers at the Dedication: Dr. Joe Senne – who designed the megalith; John Bevan – Druid; Dr. John Carlson – from the Center for Archaeoastronomy; Dr. Joseph Marcello – Chancellor.

The construction was sufficiently novel that it was awarded one of the ten Engineering Awards from the Society of Professional Engineers.

(Note there is a video of the construction available on DVD. This shows, in part, that the jets can trim an edge without any material on one side, something other tools find difficult, because the nozzle does not contact the rock). There are also other articles that I have written answering some questions and describing the site on the RMERC web page.)

Waterjetting 10a - Beginning to cut

In the last few posts I have been discussing what happens under a water jet as it first hits, and then penetrates into a target material. In many cases it is recommended that the nozzle move slightly relative to the target during this piercing process so that the water escaping from the developing hole does not have to fight its way past the succeeding slug of water entering the hole.

Now it might be thought that this problem would go away if the nozzle starts at the side of the target and then cuts into it. But this depends on a number of different factors, one of the more critical being as to whether the jet is cutting all the way through the material, or is only cutting a slot part of the way through it. As with a number of other topics, I am going to illustrate some of the concerns using granite as the target material, since it makes it easier to demonstrate some of the points I want to make.

If you were to look at one of the many statues that have been carved from granite over the thousands of years since the rock was first shaped into an art form, the rock usually appears as a relatively homogeneous material. That means (to those who don’t work with rock) that the rock has the same properties regardless of which direction you test them in.


Figure 1. The Italian Carver’s Memorial, Dente Park, Barre, VT (From the Barre Granite Association via State Symbols USA)

However, if you were to ask a skilled quarry man he would tell you differently. Because of the way that granite cools from the molten state in which it is injected up into the ground, it picks up an orientation to the crystals, as they are formed. One of these orientations is roughly horizontal, and called the Lift or grain of the rock. A second is perpendicular to this, and vertical and is known as the Rift. The third plane, orthogonal to the other two is called the Hard-Way, because it is generally more difficult to work. These names relate to the ways in which the grains of the rock, and the cracks around them, align. They are virtually impossible for a lay person to detect, and a quarry man may need to feel the rock to tell you which way they lie. But they are used in splitting out the major blocks from the granite massif, and come into play in breaking the large blocks down into handle-able sized pieces.


Figure 2. The A) Hard-Way B) Rift, and C) Lift planes of crystal orientation in granite

If, however, you were to shoot a short slug of water at high-pressure at a piece of granite (and we used the granite from Elberton in Georgia for this) then, depending on which direction the pulse came from relative to the three planes, the amount of rock that would break around the impact point would change.

In an earlier post discussing the splitting that occurs when pressure builds up within the cavity under a jet, I mentioned that the pressure would grow cracks that already existed. And it is for this reason that when the jet impacts perpendicular to the existing crack planes, that the volume of material broken out is greater than it is where the jet fires along the cracks. This can be shown using the cavity profiles from oriented samples into which the jets were fired.


Figure 3. Profiles from the cavities created around the impact points where the jet impacted granite blocks at different orientations.

One can use this information if, for example, one wanted to cut a thin line in granite, where the cut should be made in the direction of the crystals, i.e. making cuts along the lines shown in the A plane of figure 2.


Figure 4. Linear cuts into granite along the lines shown in Figure 2.

In this set of cuts the jet is cutting along the favored orientation of the crystals, and the rock only spalls when two jet paths approach each other in the lower right of the block.

If, however, the cuts are made in a direction perpendicular to the orientations, i.e. in the B and C planes, then the results are quite different.


Figure 5. Cratering along the linear passes in cutting granite perpendicular to the Rift plane.

Where the jet strikes perpendicular to the Rift or Lift, then the pressurization under the jet is enough to cause those cracks to grow out to the surface, and cause spallation along the cut. In many cases this removes all the rock between two adjacent passes, even if they are more than an inch apart.

If one is to use the high-pressure waterjet system for slotting granite in a quarry, for example, then this can be a very useful tool, since by merely putting two jets on either side of the desired slot the spalling will remove the material between them, without any further jet action. If the jets attack in the perpendicular plane, then the jet has to be rotated over the cut to get the same material removal rate.

In most cases, when cutting in a quarry, because the rock does vary in structure, and grain size, it is better to ensure that all the rock is removed before the nozzles move into the cut, by rotation, but in smaller applications, such as where the excess rock is being removed around a planned sculpture, then enhancing the spall around the impact point can lower the time and amount of energy required in removing unwanted rock.

That is, however, a relatively specialized application, and in most cases it is desirable that the cut be clean, and smooth, and this requires the use of abrasive in the waterjet stream, and so this will be the topic of the next few posts.


Figure 6. Cutting through one inch thick glass, showing the cut through the side of the glass.

Waterjetting 5a - Making gift items

There is a time, which can come in late Winter and very early Spring, when demand declines and there is some free time for the occasional home project. Although many of us now know and understand how well waterjets and abrasive waterjet streams can cut material, this is still not that widely recognized by the General Public. This slack time can help to remedy that problem.

Uninformed ignorance of jet capabilities was certainly true for many years on our campus, and seems to become more so again as the years pass since I retired. Further, at Conferences, I often heard the complaint that the industry needs to get its message out more clearly to a wider audience. The vast majority of potential industrial users are unaware of how well waterjetting in one of its forms could help solve their problems.

Now there are lots of ways of solving that problem, but today I want to talk about just one, the one we used to help us with the problem. It had to be something that would be used by those we gave it to. It had to be small, relatively cheap and quick to make, and yet demonstrate some of the capabilities we wanted to show off. The answer ended up as a business card holder.

Figure 1. Business Card Holder - Missouri Miner female figure.

University labs are generally cash strapped, and so the material had to be relatively cheap, so we used sheets of a light foam. This allowed us to cut out the figure parts using water alone (at around 20,000 psi) which significantly reduced the cost. Early in the design of the female figure (this was the third in a series , where we cut a different shape each year) it was pointed out that relative body size was more critical with female figures, and so two different thicknesses of foam were used. The first was half-an-inch thick and used for the body and pins, while a quarter-inch sheet was used for the legs and arms.

Figure 2. Foam miner front view – showing the two thicknesses of material

Putting a small hole in the position of the eye allowed the model to show how precise and small a cut could be made through thicker material. The five pieces that made up the total were held together with two rectangular pins that were cut from the thicker stock and fitted through slots cut to their shape in the different parts.

One of the advantages of cutting these (and we cut parts for around 300 figures, and used virtually all of them each year) is that it was also possible, with relatively little trouble, to cut the campus identifier on a leg of the figure. With not a lot of space this was originally UMR, and then changed to “S & T” when the campus changed its name.

Figure 3. A later model of the card holder with the campus ID cut into the leg.

For speed in cutting we only cut the letters in half the legs, though you may note that in this later version we also cut the connecting pins as round rod, rather than rectangular. In this way the figure could be repositioned, as the owner decided what they wanted to do with them.

Basically however they served as card holders, and having passed them around, (and provided them to senior campus officials as place card holders for dinner meetings) it has been amusing to see how avidly they were sought and kept by some of those to whom they were given.

Now we did not get to these figures in one step. The initial idea was to carve something out of rock, since the overall department was known as The Rock Mechanics and Explosives Research Center. However, if you are making something out of rock, particularly a person’s shape, they need to be larger, because of the weak strength of the rock.

Figure 4. Comic-book Miner cut out of Missouri Granite

The cost was also high, since the cuts had to be made with abrasive, and the rock had to be polished before it was cut. (Trying to polish the pick points after cutting led to several breakages, and this is something that is either perfect or worthless).

There are several good ideas that individual companies have, that help sell their name and capabilities where the gifts are of metal, and can be used for opening bottles or of some other benefit. But we could not afford the cost to cut a lot of pieces using abrasive, and nothing that we tried in metal had the cachet of the small miners.

In this case the mascot of the campus is the Missouri Miner, and while the first model that we cut followed along the shape of that cartoonish figure, many of our graduates were going into coal mining, which is also my background, and so the second and third versions had coal mining helmets, and as a further demonstration of capabilities, a small circular cut in the helmet allowed a yellow rod to be put into the helmet to illustrate the miner’s cap lamp.

Where we were asked to prepare small souvenirs for another event we did use the Missouri Granite, but had learned this time to buy tiles that were already polished. Then all we had to do was to cut the shape of the state into the tiles, and then put a University logo sticker on the piece and we had our memento for the guests.

Figure 5. Small memento of the state shape carved out of granite tile.

This was for a specific occasion where the sponsor was willing to pay for both the cutting costs and the materials, but in order to keep costs down (since these were given away) the pieces had to be small. This particular run was one of the more difficult to keep inventory on, since several disappeared during the short time of the cutting runs (which we have found is an occupational hazard with “artistic” pieces where there are lots of temporary folk involved in our work).

Which is, I suspect, an entry for the last piece of advice on making such gifts, and that is to plan on making more than you think you need, and, if possible, be able to make more if needed. In a later post I will write about where you can get some artistic help for relatively little cost to help with ideas such as this.