Waterjetting 27e - Borehole Back Pressure Effects

In the earlier posts on this chapter of waterjet technology I have dealt with the changes in cutting performance when a waterjet stream cuts in to material that is either under pressure, or contains internal stresses that may not be obvious at first glance. In this post I will focus, instead, on the changes in performance when the borehole becomes filled with water under pressure.

Figure 1. 12-inch cores of sandstone that have been drilled by the same jet drill, at the same speed, but at borehole pressures of 0, 500 psi, 1,000 psi, 1,500 psi and 2,000 psi. (Jet pump pressure 10,000 psi; 970 rpm; 40 inches/min ROP)

The water used in the test also contained a small amount of polyethylene oxide (Polyox) that, at the time, was the only polymer readily available to enhance jet performance under water, although there are now liquids such as Superwater that similarly help.

It can be seen that even the change in pressure to 500 psi is sufficient to dramatically shorten the distance that the jet cuts through the material on a single pass, and the range then only shortens a little as pressure further increases. But the hole drilled at 2,000 psi is barely large enough to let the high pressure lance and nozzle assembly pass.

First an explanation of the equipment that we used to run the tests. A triaxial cell was used as the basic vessel to hold the core. This is so-called since it allows pressure to be applied around the rock core, and also since the cap can slide within seals, axial pressure given the third of the orthogonal directions for loading.

Figure 2. Triaxial cell used for the drilling experiments.

A valve was fitted on the flow line of water out of the chamber (just above the pressure dial) and this controlled the fluid pressure in the cell. The diameter of the outer (reaming) jet was 0.04 inches, and the rapid decay in range with the increase in pressure led to a second experiment, to see how changing the diameter changed the results. The equipment was modified for this test, the feed pipe to the nozzle was bent, so that, as it made a single circuit over the underlying rock, it would trace out a circular path rather cut a single hole. Then the top of the sample was cut at an angle so that, with the rotation the distance from the jet to the target would vary and the range of the jet could be seen. (Figure 4).

Figure 3. Modified equipment to find the effective jet range against back pressure.

A simplified factorial experiment was run with three nozzle diameters and five back pressures, measuring the depth of cut into the sandstone in each case.

Figure 4. The resulting cut when a 0.03 inch diameter jet was rotated over sandstone with a 1,000 psi back pressure in the cell. The 10,000 psi jet was brought up to pressure with the jet at the greatest standoff (hole at the bottom) and the back pressure was set before making a single pass over the sample. The depth of cut was averaged over several readings made along its length.

The data was then plotted (with the curve smoothed here for simplicity in discussion).

Figure 5. A plot of range of jet cutting ability as a function of hole back pressure for three different nozzle diameters.

The graph shows that, for this set of conditions, the larger the jet the better, and that the first 500 psi of back pressure has an immediate effect on jet cutting effectiveness. Jet size should be at least 0.064 inches when drilling against back pressure in the hole. There was a significant improvement in cutting ability when the polymer (at 300 ppm) was subsequently added to the water, in a later series of tests. The small number of tests carried out, however, were too small a sample to provide more than guidance as to concentration since all three levels tested (100, 200 and 300 ppm) all showed considerably improved depths of cut (increasing to a depth of almost 2 inches against a back pressure of 2,500 psi) when contrasted with the performance levels shown above. The polymer tests were carried out with a jet nozzle diameter of 0.064 inches.

There are two parts to the effect of the borehole pressure. The first is simply one of increasing the resistance of the water to jet penetration, and lowering the effective jet pressure (since that is effectively the jet pressure less the borehole pressure).

It is important to recognize that it is not just the drop in effective pressure that causes the effect. To check that this was the case a hole was drilled with the same conditions otherwise as the left-hand rock sample in Figure 1, except that the jet pressure was dropped to 5,000 psi. Thus the differential pressure of the jet across the nozzle was less than that in the case of the other four rock samples shown in Figure 1. Yet the hole was of the same approximate irregular geometry as that shown by the left-hand core of Figure 1 even with the lower differential pressure with the prominent cone cut ahead of the bit that is not evident in the other cases.

Mike Hood has shown the effect of loss in cutting range by using back-lit shadow images of a jet at different back pressures.

Figure 6. Illustration of the effect of fluid back pressure, the shadow image of the jet shows how back pressure reduces the range.

As mentioned above, the effects extend beyond reducing the jet range, and lowering the jet differential pressure. The increased confinement on the rock will compress the grains of the rock more tightly together, making it more difficult for the pressurized water to penetrate into the rock structure. This combines with the higher pressure required to grow the cracks to effectively reduce the ability of the jet to penetrate into the rock.

At the same time, if you listen as the back pressure is increased (we used a Lichtarowicz Cell the increasing pitch of the sound shows (as does the damage induced) that the collapse of the cavitation bubbles generated around the edges of the submerged jet is becoming more intense as the pressure increases. I have discussed how this can be used as a benefit in breaking up rock in an earlier post.

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 13a - an introduction to milling

In contrast with the earlier use of high-pressure waterjets in material removal in civil engineering and mining, when industrial waterjet cutting first began it was used to make thin cuts through different materials (in the early days often paper and wood products). Through cutting, particularly in relatively thin stock, has a wide range of industrial uses, particularly when the pieces are cut “cold” and with edge qualities that are, even with the first cut acceptable as the final surface cut needed for the part.

Over time the advantages of this new cutting tool became more apparent, and the range of materials that the AWJ jet could viably cut was extended into metals and ceramics. Yet conventional machine tools do more than just cut the edges of parts, and so questions arose as to the best way to achieve the milling of internal pockets within different materials. Within relatively soft rock, and with pressurized water alone, it is possible to generate interesting shapes.

When we first started experimenting with cutting rock at Missouri University of Science and Technology (MO S&T) the support equipment that we had was very basic, and the budget similarly restricted. In order to achieve precise positioning and control of the speeds during the cutting process, we therefore mounted the nozzle and support lance on the traverse of a conventional lathe. The samples were mounted into the chuck, so that we could achieve controlled cutting speeds. To get a number of sample cuts in a single test we placed a sheet of metal, with slots cut into it, between the nozzle and the rock.

Figure 1. Rock rotates in a lathe while the nozzle traverses across the face.

The notches cut into the metal plate were cut wide enough to allow the jet to make a single pass over the rock surface as the rock rotated and the nozzle swept past the slot, and they were widely enough spaced that the cut made through one slot did not interfere with the adjacent cut made through another.

Figure 2. Slots cut through the mask into the rock target.

After a while we became a little more adventurous and realized that, by making the mask an interesting shape that we could leave part of the rock uncut, but mill out all the rest of the material exposed to the jet, by adjusting the feed rate of the nozzle relative to the rotational speed of the rock.

We thought at first that the feed of the nozzle (easy to set with the lathe) should be one jet diameter for each rotation of the rock, but the jet spreads as it moves away from the nozzle and this turned out to be a little too small a distance, and we ended up setting the feed at about 1.5 times the jet width. This “incremental distance” is going to vary between systems, as a function of nozzle design and size, jet pressure and the distance between the nozzle and the target. In this early work in the technology (this was back around 1972 IIRC) the nozzle stood back from the rock at about one inch standoff. In more modern applications that distance can be quite a bit less, and this changes the incremental distance. Also bear in mind that the speeds at which plain high-pressure waterjet cuts are most efficient are much higher, across the target surface, than the optimal speeds for AWJ work.

So, since there was a need to remind folk that waterjetting could be dangerous if proper care was not taken during its use, we used this idea and made a sculpture.

Figure 3. Skull figure carved out of sandstone.

For simple lettering and shapes such as that shown above, the practice was to cut the desired shape into a metal plate, using perhaps a cutting torch, and then attach this over the rock. The two locations for the retaining wire can be seen on the sides of the piece. This allowed the plate to rotate with the rock piece as the lathe turned, and did away with the stationary plate between the nozzle and the sample.

By adjusting the feed speed and the rotation speed of the piece a relatively smooth surface could be left in the excavated pocket. (See the depths of the eye sockets). The process is known as “Masked” milling, since the plate masks the sections of the rock that the jet should not be allowed to mill into.

This works well when the work piece allows the use of plain high-pressure water, since it is relatively simple to make the mask out of a material (in this case steel) that the jet would not erode significantly. Thus the same mask could be used repeatedly to make copies of the original (though I think, in this case we only made around three or four).

But what happens when the jet is an abrasive waterjet, and we want to make pockets in the same way as I have just described. Because the AWJ will cut through a thin mask it was not an optimal choice for the process.

One can, with precise control of the nozzle position, have the jet move back and forwards over the desired pocket geometry. With the more accurate controls available today it is possible to slow the nozzle as it reaches the end of the pocket, increment it over the desired distance, and then have it cut an adjacent path back along the material to the start side of the pocket. Here the process would be repeated, moving backwards and forwards until the desired pocket geometry had been covered.

The problem with this approach is that the depth of cut into the target is controlled, in part, by the length of time that the jet plays on any one point, or inversely as the speed with which the nozzle is moving over the surface. So moving the nozzle more slowly as it approached the edge of the pocket (which you have to do because the robotic arm driving the move can’t instantaneously stop, increment over, and reverse direction because of the inertia in the system) is problematic. This is true only however if the pocket has to have a smooth regular floor of a fixed depth but most, unfortunately, do. And slowing the nozzle at the end of the cuts means that the depth of the pocket would be deeper along the pocket profile, relative to the body of the cut.

And so, for lack initially of an alternative approach, for some time the industry used masks that would protect the sides of the pocket, and provide a space over which the nozzle could decelerate, increment over, and turn back. The mask would be eroded away, but in desirable parts (often expensive to make in the desired material) the ability of the abrasive waterjet to make the pocket in the first place allowed the expense of the mask to be written into the cost of making each part.

There is, however, at least one other way of doing this, and I will discuss that, next time.

Waterjetting 8d - Choosing angles

How times change! I was reading a column in the British Farmer’s Weekly, and came upon this, where the author is discussing the need for a generator.:

It will also be vital to keep the fuel flowing into the tractors, and power the pressure washer, and light the security lights, and all the other essentials of an average arable farm.

It is an indication of how far the use of pressurized water has come, that it is now seen, at the lower end of its application, as a vital farming tool. Which is a good introduction to talk a little further about the use of cleaning streams, and how to interact with differing target materials.

There was an initial first step, when someone would send the lab a mystery block of material and asked – how do I cut it? Generally the samples were small, but we would find a flat surface on the material, and carefully point a jet nozzle perpendicular to this surface. (In the early stages this was hand-held). When a jet strikes a surface, but can’t penetrate it, then it will flow out laterally around the impact point, under the driving force of the following water.

The test began with the jet at low pressure, and this was slowly raised, until the point was reached when the pressure was high enough to just start cutting into the material. At this point the jet had made a small hole in the target, and so the water flowing into that hole had to get out of the way of the water following. The sides of the hole stop it flowing laterally, and so it now shoots back along the original jet path. This spray can hit the lance operator if the nozzle is hand-held, but it is a fairly graphic way of determining the threshold pressure at which the material starts to cut. (and I’ll get into what happens as the pressure continues to go up in a future series of posts).

But for the purpose of cleaning, the jet has to move over the surface, once it has made that initial hole, at pressure. But, in many materials, if the jet comes vertically down onto the target, then only the material directly under the jet will be removed. And so the jet has to be played on every square inch of the surface in order to ensure that it is cleaned, or that the coating/layer is removed. In some sandstones, for example, two jet paths could be laid down, almost touching one another, and yet the rib of material between them would remain standing.


Figure 1. Adjacent jet passes in sandstone, the cuts are about an inch deep, but note that even though the narrowest rib is about 1/8th of an inch wide, it is only when the cuts touch that the intervening material is removed.

Yet that rib of material was, in that case, so weak that it was easy to break it off with a finger. (This turns out to be a weakness in making delicate sculptures out of rock). To use the full pressure of the water can be a waste of energy, if the material is very thick, since it all must be eroded with such a direct attack.

Yet the minimum amount of material that needs to be removed is that that attaches the layer to the underlying material (the substrate concrete, steel etc) and this can be quite thin. Thus, in attacking a softer material, particularly one that can be cut with a fan jet, a shallow angle directed at the edge of the substrate can be more effective.


Figure 2. Round v fan cleaning from Hughes (2nd US Waterjet Conference)

Because there is a balance between cutting down through the material to be removed, and cutting along the edge to grow the separation crack between the materials, some practice is needed to find, for a given condition, what that angle would be.


Figure 3. Choice of angle from Hughes (2nd Waterjet Conference)

The more brittle the material, then the greater the angle to the surface, since rather than just erode the material, the jet may also shatter the layer into fragments that extend beyond the cut path. But otherwise using an angled jet to the surface can be more effective. Hughes (from whose paper at the 2nd Waterjet Conference I took these illustrations) has a simple test for orifice choice.


Figure 4. How target response influences nozzle selection. (Hughes 2nd Waterjet Conference)

Some of the more advanced cutting heads use a series of nozzles that spin within an outer protective cover, as they remove anything from layers of damaged concrete to thin layers of paint from ship hulls. Increasingly these are connected to vacuum systems that will draw away the spent water and debris from within the contained space, without it entering the work space, and creating problems for the worker.

In order to reduce any collateral damage to the surroundings these jets are often made very small (thousandths of an inch in diameter) so that their range is short, and they are inclined outward to cut to the edges of the confining shield.

We have had some success in turning those angles the other way, so that they cut into the shield, rather than away from the center, and also so that each jet is directed towards the path of the next jet around the circumference. The intent in this case is to allow the use of a slightly larger jet, with a greater cutting range. In this case the individual cleaning/cutting path is a little larger, but because the jet at then end of the cut moves into the range of the adjacent jet, then any remaining energy that it and the dislodged debris still have, will not be enough to get through this second jet.


Figure 5. Inclined jet and shroud design.

The action of each jet then becomes not only to cut into and remove material, but also to contain the spent material from the other jets dispersed around the cutting arm, and to hold the debris in the center of the confinement for the very short time needed for it to be caught up in the vacuum line.

In all cases the choice of pressure, nozzle size, and operational factors such as angle of attack, come down to the target materials, those that have to be removed, and those that need to be left undamaged. And it is why it is useful, at the start of any new job, to take the time to do a little testing first, to make sure that the right choices of nozzle and angle have been made to get the job done quickly and efficiently.

Incidentally the idea behind the test of effective pressure, that the jet flows laterally when it hits something it can’t cut, can help, for example in easing the meat from the bone when a jet cuts a deer leg.


Figure 6. Cut across a deer leg, note how the jet has cleaned off the meat from the bone, undercutting the flesh.

Waterjetting 8b - Repairing concrete

Some years ago we were on a bridge in Michigan, working on a demonstration of the ability of high-pressure jets to remove damaged concrete from the surface of the bridge. Before the demonstration began the state bridge inspector walked over the bridge armed with a length of chain. He would drop the lower links of the chain against the concrete at regular intervals, and depending on the sound made by the contact, would decide if the concrete was good, or not. He then marked out the damaged zones on the concrete, and suggested that we get to work and remove those patches.


Figure 1. Automated removal of damaged concrete from a bridge in Michigan

The change in the sound that he heard, and used to find the bad patches in the 1concrete, was caused by the growth of cracks in that concrete. It was these longer cracks, and delaminations in the concrete that made it sound “drummy” and which identified it as bad concrete.

Now here is the initial advantage that a high-pressure waterjet has in such a case. The water will penetrate into these cracks. As I mentioned in an earlier post, water removes material by growing existing cracks until they intersect, and pieces of the surface are removed. The bigger the cracks in the surface, the lower the pressure that is needed to cause them to grow. This is because the water fills the crack, and pressurizes the water, the longer the crack, the greater the resulting force, and thus the greater the ease in removing material.

At an operating waterjet pressure of between 11,000 and 12,500 psi, for a normal bridge-deck concrete, the cracks that are long enough for an inspector to call the bridge “damaged” will grow and cause the damaged material to break off. The pressure is low enough, however, that it will not grow the smaller cracks in “good” concrete, which is therefore left in place.


Figure 2. Damaged area of bridge after jet passes.

In order to cover the bridge effectively and at a reasonable speed, six jets were directed down from the ends of a set of rotating crossheads, within a protective cover. The diameter of the path was around 2 feet, and the head was traversed over the bridge so that it took about a minute for the head to sweep the width of a traffic lane.


Figure 3. Scarifying jets, with the head raised above the deck so that their location can be seen. Normally the nozzles are positioned just above the deck, so that the rebounding material is caught in the shroud.

Unfortunately, while this means that the rotating waterjet head could distinguish between good and bad, and remove the latter while leaving the former, it could not read marks on concrete. So where the bridge inspector was not totally accurate, the jet removal did not follow his recommendations. It was, however, quite good at removing damaged concrete from reinforcing bar in the concrete, where the water migration along the rebar had also caused the metal to rust. And, since the pressure was low enough to remove the cement bonding, without digging out or breaking the small pebbles in the concrete, they remained partially anchored in the residual concrete. As a result when the new pour was made over the cleaned surface, the new cement could bond to the original pebbles, and this gave a rough non-laminar surface, which provided a much better bond than that left had the damaged material been removed mechanically with a grinding tool.


Figure 4. Rebar cleaned by the action of the jet as it removes the surrounding damaged concrete.

Waterjets had an additional advantage at this point in that, in contrast with the jackhammer that had previously been used to dig out the damaged region, but which vibrated the rebar when it was hit, so that damage spread along the bar outside the zone being repaired, with the jet action there was no similar force, so that the delamination was largely eliminated.

Now this ability to sense and remove all the damaged concrete is not an unmixed blessing. Consider that a bridge deck is typically several inches thick, and it is usually sufficient to remove damaged concrete to a point just below the top layer of the reinforcing rods. Once the damaged material is removed, then the new pour bonds to the underlying cement and the cleaned rebar. But the waterjets cannot read rulers either. So in early cases where the deck was more thoroughly damaged than the contractor knew at the time that the job began, the jet might remove all the damaged concrete, and this might mean the entire thickness of the bridge deck. And OOPS this could be very expensive in time and material to replace.

What was therefore needed was a tool that still retained some of the advantages of the existing waterjet system, that it cut through weakened concrete, and cleaned the rebar without vibration, but that it did so with a more limited range, so that the depth of material removal could be controlled.

There was an additional problem that also developed with the original concept. For though the jets removed damaged concrete well in this pressure range, the jets were characteristically quite large (about 0.04 inches or so). The damaged concrete is contaminated with grease and other deposits from the vehicles that passed over it. Thus any large volumes of cleaning water would also become contaminated, and, as a result will have to be collected and treated. That can be expensive, and so any way of reducing the water volume would be helpful.

The answer to both problems was to use smaller jets at higher pressures. Because of the smaller size, their range is limited, and at the same time the amount of water involved can be dramatically reduced. It does mean that the jet is no longer as discriminatory between “good” concrete and “bad.” This is not, however, a totally bad thing, since when working to clean around the reinforcing rods, there has to be a large enough passage for the new fill to be able to easily spread into all the gaps and establish a good bond.

Thus the vast majority of concrete removal tools that are currently in use are operated at higher pressures, and lower flow rates. This allows the floor to be relatively evenly removed down to a designated depth, and this makes the quantification of the amount of material to be used in repair to be better estimated, and the costs of disposal of the spent fluid and material to be minimized.


Figure 5. Scarified garage floor showing the rough underlying surface. This will give a good bond to the repair material, as will the cleaned rebar.

The higher pressure system has the incidental advantage of reducing the back thrust on the cutting heads, so that the overall size of the equipment can be reduced, allowing repair in more confined conditions.

Waterjetting 7a - An intro to jet structure

Once a waterjet starts to move out of the nozzle with any significant speed, as the pump pressure begins to build, it becomes more and more difficult to look at the stream of water and get any realistic idea of its structure. Mainly what is seen is the very fine mist that surrounds the main body of the jet, and while some idea of the structure can be obtained by making cuts through material, it can be quite expensive to actually see within that structure. Part of the problem is that though the mist is very fine, it is also moving at speeds in the range of a couple of thousand feet per second. The human eyeball isn’t quite that fast. But we can use a very high-speed flash (in this case it was on for two millionths of a second) which has the effect of “freezing” the motion.


Figure 1. 40,000 psi jet issuing from a 0.005 inch diameter orifice, front lit.

However this mist still hides the solid internal structure of the jet and does not change much in relative structure, even when the internal jet conditions can be quite different. Fundamentally the internal structure was described by Yanaida at the 1974 BHR Group Waterjet Conference, and his description has been validated by many studies since.


Figure 2. The break-up pattern of a waterjet (Yanaida K. “Flow Characteristics of Waterjets,” 2nd BHRA Conf. 1974, paper A2.)

This structure holds for jets across a wide range of pressure and flow volumes, but it is difficult to determine the exact transition points of that structure conventionally. And this can lead to very unfortunate results. I have twice seen people back a nozzle away and then move their hand in front of the jet to show that even high-pressure jets (these were being used to cut paper products and had no abrasive in them at the time) could be “safe.” If both cases the individuals were very lucky to escape injury (water can penetrate the pores of the skin and lacerate the internal parts without any surficial signs of injury, and, as I showed last time, if the nozzle is too close it will slice through flesh and bone). I thought to take today’s post to show, though the use of photographs, why that was such a stupid action.

The photos were taken down at Baxter Springs, KS in the early 1970’s and involved the use of what was then a MacCartney Manufacturing Co intensifier, to shoot jets of varying pressure, and nozzle diameter along a path, so that we could see how coherent the jets were. As I mentioned above, the problem with looking directly at the jet is that the internal structure is hidden by the surrounding mist. To overcome that part of the problem we shone the light along a ground glass screen (to diffuse it) that was placed behind the jet, so that we could see the outline of the internal structure.


Figure 3. Arrangement for taking photographs of a high-speed jet.

This more of the downstream mist from the photograph, and a much better idea of the internal structure of the jet, and where the solid section ended could be measured.


Figure 4. Backlit, 30,000 psi jet issuing from a 0.01 inch diameter nozzle, the distance across the photograph is 6 inches.

The benefit of the technique is perhaps more evident when nozzles at different pressures and diameters and different chemistry are compared. First consider the change with an increase in jet diameter. From the front-lit view there is little difference in the jets. From the backlit, it is clear that the smaller diameter jet only reaches 3-inches across the screen, while the larger jet barely reaches the end of the range.


Figure 5. The effect of doubling the orifice diameter at the same jet pressure on jet range, the photo length is 6 inches.

One of the parts of the study we were carrying out in 1974 was to examine the effect that adding different long-chain polymers had on jet structure. The ones that we were looking at include some that are now used in the oil and natural gas industry to make the “slick water” that is used in the fracking industry to improve production from shale reservoirs. But it also has an advantage in “binding” the jet together. And so, in the study, Dr. Jack Zakin and I tested a wide range of different polymers to see which would be give the best jet.

There were a number of different things we were looking for. In cutting paper, soft tissue and water sensitive material for example, the polymer can bind the water sufficiently well as to further lower wetting to the point where it doesn’t have an effect. It also can improve jet cutting under water – but I’ll cover those in a few post on polymer effects that will come to later in the series.

The effect of a polymer (in this case an AP273) is shown in two tests where the only change was to add the polymer to the water for the lower one.


Figure 6. Jets with an orifice diameter of 0.01 inches at a pressure of 20,000 psi, the range is 6 inches, and the lower jet has had the polymer AP273 added to the water.

The narrower stream in the lower frame is the effect that we were looking for. Putting change in diameter and the better polymers together gave, as an example, the following:


Figure 7. The effect of changing jet pressure, nozzle diameter and polymer content on jet cohesion.

It might be noted that the jet in the bottom frame has as much relative concentration (and power) at the end of the range as the top jet had at the beginning of the range.

Now it all depends on what you want the jet to do, as to which condition you wish to achieve. Inside abrasive mixing chambers the object is much different than it is when the object is to cut a foot or more of foam with high quality edges. And there have been some interesting developments with different polymers over the years, but I’ll save those stories for another day.

But bear in mind that those individuals who could slide their fingers under the jet in the top frame of figure 7 would have had them all cut off if the jet had been running instead under the conditions of the bottom two frames, and in all three cases, to the naked eye the jets looked the same.

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.