Waterjet Technology – Starting to make water jet cut hole

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

This small sequence of posts describes the initial milliseconds during which a high-pressure waterjet penetrates into a target material. Because this work was largely developed using rock targets, most of the illustrations will be with that material, but the concept applies, to a degree, also with abrasive laden jets penetrating into materials such as glass.

For this post, I am going to discuss just what happens with the jet being fired down onto the target surface without either the nozzle or the target moving. Much of this work was carried out in the 1960’s in the UK though I will begin with some tests that Dr. Bill Cooley carried out using his modification to a Russian hydraulic cannon that he redesigned so that it was capable of firing at 500,000 psi – and yes I have seen it fired at that pressure (I took the photo).

The Cooley Cannon ready to fire at 500,000 psi in an underground mine

Figure 1. The Cooley Cannon ready to fire at 500,000 psi in an underground mine.

In order to see how effective different processes were in cutting into different materials, the international scientific community that was developing waterjet technology at the time needed a method to compare the different approaches. The metric that was used was to define the Specific Energy as the amount of energy that it took to remove a unit volume of the target material. (And in time that will be subject of some more specific posts).

Bill’s cannon used stored gas that was suddenly released as a way of driving the water at the desired pressure and measured the pressure indirectly by timing the break of two pencil leads in front of the nozzle. This gave the jet velocity, and pressure could be back-calculated from that value.

Dr. Cooley took results from his work and from other scientists working with similar devices, to produce the following graph.

Specific energy as a function of the impacting jet length, measured in nozzle diameters

Figure 2. Specific energy as a function of the impacting jet length, measured in nozzle diameters. (Cooley, W.C., “Correlation of Data on Erosion and Breakage of Rock by High Pressure Water Jets,” Chapter 33, Dynamic Rock Mechanics, ed., G.B. Clark, 12th Symposium on Rock Mechanics, University of Missouri-Rolla, November, 1970, pp. 653 – 665.)

For those running a conventional cutting table, the water orifice is around 10 thousandths of an inch in diameter. So what this graph is saying is that once the first thousand diameters of length (1000 x 0.001= 10 inches) has hit the surface, then the process starts to become significantly less efficient. If the jet is moving at 2,000 ft/sec, that length arrives in around 0.0005 seconds. Why this rising inefficiency after that time, and how do we get around it?

Earlier in this series I mentioned that one of the tests to find the pressure at which a waterjet penetrates a target is to note the point at which, instead of the water hitting the surface and flowing along it, it changed direction to flow back towards the nozzle. This is because as the jet penetrates, it makes a hole, and the only way out of that hole is back along the way the jet came. Unfortunately, there is more water still coming down into the hole, and so the water leaving the hole (at the same volume flow rate) meets the water coming into the hole. The rapidly moving water going out is moving about as fast as that coming in, and so, as the hole gets deeper, the pressure at the bottom of the hole gets less. This has been measured by a number of folk, but Dr. Stan Leach was the first, and produced this plot:

Depth at the bottom of a hole, as a function of the incoming jet pressure

Figure 3. Depth at the bottom of a hole, as a function of the incoming jet pressure. (Leach, S.J., and Walker, G.L., “The Application of High Speed Liquid Jets to Cutting,” Philosophical Transactions, Royal Society (London), Vol. 260 A, 1966,pp. 295 – 308.)

Because the holes were preformed of metal (to hold the transducer) and were sized to the nozzle diameter, this is not as it turns out totally accurate although it illustrates the problem.

It isn’t totally accurate because, as the illustration from the last two posts showed, the erosion occurs initially around the edge of the jet rather than under it, and thus the hole created is about twice to three times the jet diameter rather than being of the same size.

Damage pattern around the impact point of a 10,000 psi pressure, 0.04 inch diameter jet on aluminum, target close to the nozzle

Figure 4. Damage pattern around the impact point of a 10,000 psi pressure, 0.04 inch diameter jet on aluminum, target close to the nozzle.

Nevertheless, as the hole deepens, the pressure at the bottom of the hole gets less, and after a while the jet penetration slows to almost a halt.

Penetration as a function of time

Figure 5. Penetration as a function of time (My Dissertation)

The sides of the hole, however, continue to erode, but from the bottom upwards so that after a short while, the narrower entry hole starts to constrict the flow out, and pressure begins to build-up in the hole.

Remember that a waterjet works by growing existing cracks in the material. So that if there is a natural crack in the rock, which may be as small as a grain boundary, or the scratch made by an abrasive particle as it moves back out of a hole in glass, then the water entering that small crevice will pressurize the walls and cause the crack to grow. Often there is more than one, and the result can be, in rock:

Rock breakage around the jet impact point on a 1-ft block of sandstone

Figure 6. Rock breakage around the jet impact point on a 1-ft block of sandstone (after Moodie and Artingstall Moodie, K., Artingstall, G., “Some Experiments in the Application of High Pressure Water Jets for Mineral Excavation,” paper E3, 1st International Symp on Jet Cutting Technology, Coventry U.K., April, 1972, pp. E3 25 – E3 44.)

In rock, that might not be such a bad thing since in many cases the intent is just to break the rock out of the way, so that a tunnel can be created that folk can walk or drive through. But in the case of glass and other such brittle materials, where the object is just to make a very fine cut with no side cracks, cracking the sheet is disastrous. This can be illustrated by the results when a jet was fired along the central axis of a 2-inch diameter core of granite. The escape of water into the cracks allowed the cycle to repeat several times, and the hole was, as a result, much deeper than it would have been if the cracking had not occurred.

2-inch diameter granite core that split when a short jet pulse was fired into the core, along the axis

Figure 7. 2-inch diameter granite core that split when a short jet pulse was fired into the core, along the axis.

And so, next time, I’ll write about some of the ways in which we can get around this problem.

Waterjet Glass Cutting and Piercing

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

Here’s a little demonstration you can carry out. Take a strip of paper, and cut a slit in it half way along the strip and half way through the paper. Now take both ends of the paper in your hands and pull them apart. This causes the cut (the crack) to grow through the paper and gives you two halves. If you do this a second time you should find that by stopping moving your hands you can stop the cut from growing all the way through the paper. Now repeat the process, but use a piece of paper that you have not cut a slot in. The amount of force you need to pull the paper apart is much higher, and I seriously doubt that once you get the tear (crack) to start that you can stop it before it goes all the way through the paper. (Remember this, and I’ll come back to it a bit later in time).

The idea of putting cracks on the edge of packages to lower the force you need to tear them open can be found on the edge of lots of candy bars, packs of peanuts and other goodies in stores. The serrated edge acts as a series of cuts or cracks, that concentrate the force applied when you pull on the edges of the packet so that the package tears at a much lower force, and you can control the tear so that you don’t end up throwing all the contents around the room.

Serrations and tear at the top of a packet of honey

Figure 1. Serrations and tear at the top of a packet of honey

Now at this point you might say that there aren’t any cracks in glass when we start to cut it. If the glass is very new, this is true. However, with all the chemicals in the air and the dust that is carried in the wind the surface actually contains a lot of very fine cracks although glass can look clear.

John Field, one of the earlier investigators of high-pressure waterjet impact, showed this in one of those brilliant yet simple demonstrations that, in this case, he carried out some forty-five-odd years ago. If waterjet impact grows surface cracks and glass acquires surface cracks from damage through being out in the air and if that surface layer is removed, then the underlying glass will have no cracks. So John took a glass slide, and etched off the surface of the lower half of the slide, by immersing it in acid. Then he fired a very high-speed droplet of water at the point on the slide where the acid etch stopped.

Impact of a high-speed droplet of water on glass

Figure 1. Impact of a high-speed droplet of water on glass. Above the dividing line the glass surface contains the micro-cracks and flaws that come with being exposed to the air over time. The lower section below the line has had these flaws removed. As can be seen the cracks only develop in the unetched part of the glass, where they grow pre-existing cracks, even into the side of the glass that was etched. (Field J.E. “Stress Waves, Deformation and Fracture Caused by Liquid Impact,” Phil. Trans. Royal Society, 260A, July 1966, pp. 86 – 93.)

In a single picture he captured the evidence that waterjets work by growing cracks (top half), and that without cracks there is no damage (bottom half). Understanding this opens up a whole vista of different applications, from the removal of soil from around pipelines underground (the new technology of hydro-excavation) to the removal of damaged concrete, while leaving healthy concrete in place (the developed field of hydro-demolition). These and other topics will be part of this series as it moves forward.

But as John showed, not all the cracks a jet will grow can be seen, and as Vanessa found, they don’t have to be at the surface to create problems. One of her early pieces was entitled “p1.” Within it are an uncountable series of holes, drilled deep into the glass.

Detail of the glass sculpture "p1", by Vanessa Cutler

Figure 2. Detail of the glass sculpture “p1″, by Vanessa Cutler

One of the skills Vanessa has learned is in controlling the quality of the pierce and its dimension, but initially, there had to be a period of learning.

Single cracks growing out from partial piercings in a test piece

Figure 3. Single cracks growing out from partial piercings in a test piece during development (Vanessa Cutler)

And so, in the next sequence of posts the simple idea of growing existing cracks will be explored. Mainly, in the beginning, this will focus on cracks that are already there, and how to usefully make them grow. But in some cases we don’t want all those cracks to grow, and that will also come up, as this series continues.

Waterjet Technology – Crack Growth and Granite Sculpture

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

The last post in this KMT Waterjet Blog 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.

9-inch thick block of granite drilled through by a 10,000 psi waterjet

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 than 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 blocks of that granite to form the MS&T Stonehenge that now sits on the University campus.

Stonehenge at Missouri University of Science and Technology

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.

The Millennium Arch at Missouri University of Science and Technology

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).

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).

Partial cut for one of the figures of the Millennium Arch

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:

Student name written with a high-pressure jet into plywood

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.