Waterjetting 33c - More on enhanced cutting performance

Frontal photographs taken of waterjets, regardless of pressure, show that the jet spray widens as the jet moves away from the nozzle. Yet, because of the erosion of the outer layers of the jet by the surrounding fluid, the central core of effective jet pressure reduces as that distance grows. The normal way in which this can be seen is in the taper of the cut as a jet penetrates into a target material.

Yet in some cases better results can be obtained if the jet makes a series of passes to cut through that target layer. There is, however, a little problem, and this can be shown by the use of a curve, showing the depth of cut as a function of the number of passes made over the surface.

Figure 1. Depth of cut in two materials, as a function of the traverse speed. (After Hashish).

The graph shows the decline in the cutting ability of the jet with increasing number of passes, and the inability of the jet, at the highest speed, to penetrate through the mild steel plate.

One of the reasons to make passes at a higher speed is to improve the quality of the edge cut, since if the jet makes the pass with the particles of abrasive only cutting on the target one time (rather than the multiple cuts made by a particle at slower speeds, where it bounces down the cut.)

Momber has pointed out the decrease in performance with increased pass number, as well as noting the difference in the amount of energy required to cut through a target as a function of the speed and number of passes.

Figures 2 and 3. Effect of the number of passes on the depth achieved (lhs) and the relative amount of energy required to penetrate material as a function of traverse speed (rhs) (after Momber)

The slight loss in cutting power as the jet cuts deeper in secondary passes comes in part because the jet is constrained by, and thus cuts back into, the walls of the pre-existing cut.

In an earlier post on cutting I pointed out that because of the highly efficient way in which a plain waterjet cuts into material, Chinese investigators have shown that one can achieve a much improved volume removal rate by oscillating the jet perpendicular to the line of travel.

The optimal speed for cutting with an abrasive jet is, however, much slower than that of a plain waterjet, by a couple of orders of magnitude, so that the large scale oscillation that is effective with plain jets will not be similarly so with an AWJ. However the concept remains valid, and has been the subject of significant investigation, particularly in Australia, in the past few years.

The benefits of such oscillation, even over very short angles, can be illustrated with reference to a figure.

Figure 4. Oscillation of a jet perpendicular to the line of travel. The nozzle advances to the grey outline on the subsequent pass. The two lines indicate the range of oscillation. (Motion exaggerated relative to the current discussion)

If the nozzle is oscillated so that the jet moves over the relatively narrow range shown in Figure 4, then after a pass, when the nozzle advances to make the second pass (and it does not have to be in the part to do this) then the jet does not make contact with the target until the back of the previous cut. Thus there is much less energy loss in traversing the jet to the new surface, and cutting performance is improved. If the oscillation is kept small the walls of the cut will still act to confine the cutting ability of the jet, and improve depth-cutting capability.

Shunli Xu looked at oscillating a jet at angles below 10 degrees, while cutting half-inch thick 87% alumina plates. A simple visual correlation showed the relative benefit of oscillation when cutting the plate with a 45 ksi jet, with an AFR of 1.2 lb/min, at a speed of 3.1 inches/min.

Figure 5, Cuts made into a ceramic plate, without (lhs) and with (rhs) a nozzle oscillation of 8 degrees at 10 Hz. (after Shunli Xu)

The study also looked at the effect of changing the oscillation parameters on the surface roughness of the cut achieved, finding that this is controlled by the angle of oscillation, the frequency and the speed of traverse, as well as jet pressure and standoff distance (not shown). The study found that, under optimal conditions, surface roughness could be reduced around 11% relative to linear cutting.

Figure 6. Effect of change in oscillation parameters on the surface quality of cut in a ceramic target (after Shunli Xu).

The study found that the parameters which control the depth of cut gain were a little more complicated to disentangle, given that the density of particles striking an individual area of the target is controlled by both the jet residence time, and the parameters of the jet itself (AFR, pressure, traverse speed).

As a result the optimum value for oscillation angle and frequency varied depending on the jet parameters, but overall it was concluded that an optimal angle of oscillation would lie between 4 and 6 degrees, with higher oscillation frequencies giving better results. An average improvement with oscillation lay on the order of 23% over conventional non-oscillation at the same parameters.

Precision cutting is a task that has a number of complications. In many cases the cuts must follow intricate contours, rather than just making simple linear cuts than separate the material. Increasingly, also, pocket milling has become a valuable ability for this tool. Cut wall quality adequate for final surface finish is increasingly important in this case, and the ability of oscillation to improve that quality and enhance the depth over which a smooth cut was achieved was noted in the work. Similarly the taper of the cut was, on average, reduced 18% with greater improvement at higher oscillation frequencies and angles.

Secondary motions of the nozzle, beyond simple path following, are thus becoming a more important potential tool for the industry, and I will return to this topic again.

Hashish M. “A Modelling study of metal cutting with abrasive waterjets,” Journal of Engineering Materials and Technology, ASME, Vol 106, Jan 1984, pp. 88-100.

Momber A.W., Kovacevic R, Principles of Abrasive Waterjet Machining, Springer Science, p. 209

Shunli Xu Modelling the Cutting Process and Cutting Performance in Abrasive Waterjet Machining, PhD Thesis, Queensland University of Technology, 2005.

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 23c - Abrasive Slurry cutting range

In an early section of these notes on high-pressure water and its uses, there was a review of some of the ways in which jet power could be assessed. For the most part the best way to see how changes in a system alter the way the jets cut is to run a simple cutting test and in many cases it will give the answer that is needed. However, sometimes it is important to go beyond the simple assessment of whether condition (a) is better than condition (b) and to try and explain why it is.

One of the early discoveries in trying to explain abrasive jet behavior was made by my colleagues Marian Mazurkiewicz and Greg Galecki who showed that, when creating an abrasive waterjet system in the conventional way, that a large amount of the abrasive was being fragmented in the mixing chamber of the nozzle.

Figure 1. Size distribution of garnet particles after being fed through the mixing chamber of an abrasive waterjet nozzle, AFR 0.6 lb/min, at 30,000 psi.

Before entering the chamber the particles had been screened to be very close to an average 200 micron size. After going through the chamber the average particle size (50%) was 140 microns, with roughly 25% of the particles being smaller than 100 microns, at which point we had found that the cutting performance gets significantly poorer.

The way in which we found the particle size, (and also assessed how fast the particles were moving after they left the nozzle) was to set the nozzle horizontally, and then to fire the jet down the center of a large plastic tube.

Figure 2. Plastic tube set up to capture the abrasive particles from an abrasive waterjet nozzle. (The nozzle is on the left of the tube).

Barriers were placed at 1-ft intervals along the tube, so that as the particles lost energy and fell to the bottom of the pipe they could be collected into the small dark blue containers under the tube, and then dried, sized and weighed. During a test the top half of the pipe sections are replaced, so that the jet is contained over the pipe length, which was just over 20-ft, and this was long enough to capture, within the length, all the abrasives from all the tests of abrasive waterjet nozzles that we carried out (which included all those commercially available at the time).

It was based in part on this test that we were able to understand why some abrasive waterjet nozzle designs work better than others, and also to begin to understand more of the mechanisms that drive the cutting process.

As the abrasive slurry system (ASJ) started to enter the American market we were thus ready to test the way in which it worked and to see if we could find any improvement, as had been reported by those who first used the system.

When we set the system up so that the ASJ system fed a nozzle in the same arrangement as with the AWJ system we got a surprise. Much of the abrasive was collecting at the far end of the pipe, and it was starting to poke a hole through the end piece.

Over time we extended the tube, and eventually moved it outside to ensure that we could capture all the abrasive in the same way as earlier.

Figure 3. The test set-up needed to capture all the abrasive when using an abrasive slurry jet rather than an abrasive waterjet. (The pipe is roughly 50-ft long).

Even at lower pressures the ASJ was carrying the abrasive much further than was the case with the conventional AWJ system, showing that the particles were retaining more energy over greater distances. In retrospect this is not surprising, since there is sensibly no break-up of the abrasive particles with the ASJ system during the mixing and acceleration of the particles.

This is because the particles are mixed in with the water before the water accelerates, and when it does the abrasive is already mixed throughout the jet, rather than trying to force its way into the jet, while at relatively slow velocity relative to the water. There is, as a result, no break-up of the particles, and larger particles will lose energy more slowly than smaller ones. (One of the findings of the AWJ tests carried out earlier).

In addition because there is no air in the ASJ jet stream there is no active component trying to disrupt the jet, and as a result the water remains coherent to a greater distance from the nozzle, and has, as a result, a much greater capability of transferring the energy to the particles to accelerate them to their full potential, given the design of the system.

There are various different ways in which this benefit can be illustrated, using lab data, but the clearest demonstration is to take a conventional waterjet system, and to run the triangle test and then, with the same amount of abrasive in the jet stream, and a roughly equivalent amount of water, to run the same test with an abrasive slurry system.

Figure 4. Comparison of the cuts achieved with an abrasive slurry system (upper) and an abrasive waterjet system (lower).

The picture shows that the two systems are cutting to sensibly the same depth, and the ASJ system is being operated at a quarter of the pressure of the equivalent AWJ system.

There have been many comparisons between the two systems in the time since this initial evaluation was made, and there is a rough consistency in the results that have been obtained, on the order of that shown in figure 4. The comparisons are not all equivalent to this, since the tests compare different attributes of the two systems, and, for example, there are additional advantages of the lower pressure ASJ system that can enhance the relative performance. One way, when one compares equivalent horsepower, is to increase the diameter of the ASJ nozzle. This allows use of a larger abrasive particle that, in turn, will give a further increase in achievable depth of cut.

Unfortunately the difficulties in achieving consistent performance with some ASJ systems, over long operating periods, has made it more difficult for this relatively new system to penetrate fully into the market place as yet.

Waterjetting 11c - Mixing abrasive with a water jet

Sometimes I would get the feeling (particularly when talking to some of my students) that the mixing chamber of an abrasive waterjet (AWJ) cutting system, together with the feeds in and the focusing nozzle outlet, were considered to be some magical black box out of which a perfect cutting stream issues to cut the desired material.

There are, in fact a variety of different chamber designs that can be purchased from different manufacturers. Some will tell you that all designs cut roughly the same, and that there is little difference between them. As I commented in one of the earliest posts in this series this is not true. Over the years we have run numerous comparative tests on different designs, using different abrasives, abrasive feed rates and target materials, and have found a broad range of results. For example, in cutting steel at a fixed traverse speed and other conditions, we found an average comparative performance as follows:

Figure 1. Comparative performance between 12 nominally similar abrasive waterjet cutting nozzles in cutting through steel at a standard speed, pump pressure, and abrasive concentration.

I described the actual test in another post, and it is clear from these averaged results that there is a wide difference in performance between the nominally similar tools.

While I don’t think there is a lot of interest in going through the details of different designs it might be helpful to explain some of the factors that play a part in producing jets of greater or lesser performance.

To return, first, to the basic construction of the mixing chamber and focusing tube assembly (AWJ nozzle), one starts by recognizing that the major cutting performance will be achieved by the particles which remove material when they hit the target. The energy that they have, however, comes from the water that is fed into the AWJ nozzle through a small jeweled orifice, or waterjet nozzle, at the top of the mixing chamber.

Figure 2. Basic nozzle design

That waterjet stream is small and initially highly focused and fast moving. As it moves through the mixing chamber, as I have described in other posts, the outer edges of the jet slow down, and gradually the jet fans out and breaks up into fragments.

There is no benefit in trying to inject the abrasive into the jet at the beginning of this passage, since, at that point, the outer layers of the jet have enough energy to knock away the particles before they can enter the fastest moving segment in the core. Rather it is better to inject the abrasive further down the chamber, so that the jet will have begun to break down into slugs, and the abrasive can be positioned so that it is impacted by a sequence of the individual slugs and accelerated to the desired velocity.

There is an additional benefit to moving the abrasive feed line a little further down the chamber. When the jet stream is rapidly switched on and off, when for example, piercing a series of small holes in a part, then the driving pressure pushing water out of the waterjet orifice switches off and on. When it switches off there is a short period where the differential pressure will draw fluid from the chamber back through the waterjet nozzle. If there are small particles of abrasive in the vicinity (and with some designs there are) then these can be drawn back through the upper orifice, and then pushed back down by the succeeding water flow in the next pulse. This can rapidly erode softer jeweled orifices, so that they round or chip, not always evenly, and degrade the resulting waterjet as it flows into the chamber. This disruption can move the jet from being in the center of the chamber, and cause poor abrasive pick-up or accelerate wear of the chamber walls, and in the focusing tube. All of these degrade performance. (The solution, if you can achieve it, is to use a diamond upper orifice, since this is largely non-responsive to the passage of the abrasive back and forth, and retains its shape and the jet performance it was designed to produce much longer – providing a cost benefit to the change).

Fig 3. Wear on a ruby waterjet orifice inserted at the top of a mixing chamber after 15 minutes of use. (The dark particles are small particles of garnet)

Figure 4. Chipping on the edges of sapphire and ruby orifices (after Powell 2007 WJTA Conference)

Fig 5. Lack of wear on a diamond insert nozzle after being in use for several hundred hours.

With the abrasive inlet channel and jet passage designed to get the abrasive into the jet where the water jet is broken up, yet still moving at high speed, there needs to be a sufficient distance for an optimal energy transfer to occur. Beyond that point, with the particle and the abrasive (which will have partially been broken in the contact between the jet and the particle, between particles striking one another, and between contact between the particle and the walls of the AWJ nozzle) the cutting jet has to be refocused into the narrow cutting stream that is required to give the finished cut surface desired.

The refocusing of the mixed jet (air, water and abrasive) is achieved with a focusing tube, which is made up of a conic section which brings the jet back together, and then a straight section which allows further energy transfer between the three component parts of the jet, before the jet issues from the orifice aimed at the target.

The passage of the particle down this tube is not always straight. Wear typically begins at the tip of the conic section as it feeds into the tube. The wear within the tube will often take up a pattern, as any irregularity in the flow causes it to bounce from one wall of the tube to the other creating a wear pattern along the walls that has a wave-like structure.

Figure 6. Wear at different points along the focusing tube

When this wear reaches the mouth of the focusing tube, then the downstream orifice is eroded out of a circular shape, and the jet that comes out no longer will cut cleanly, or to as great a depth. At that point the nozzle is worn out and should be replaced. The point at which that replacement occurs varies depending on the quality of the cut that is required. Obviously when the cut being made is at a precision of a thousandth of an inch over a cut depth of half-an-inch (as with some aircraft parts) the replacement point is reached a little earlier than if the cuts being made are a rough cut merely, for example, to separate two parts one from another, and provide a rough shape to the piece.

I’ll continue this topic in the next segment of this section.

Waterjetting 10b - Introduction to Abrasive use

In recent articles in this series I have written about the processes that occur as a high-pressure waterjet impacts on a surface and then begins to penetrate and cut into it. However, as I noted in the last post, one of the problems with using plain water as the cutting medium is that it can pressurize within the cut and exploit any surrounding cracks, to the point that the edges of the cut are cracked and fractured, often back up to the top surface of the material.


Figure 1. High-pressure waterjet cut along a sheet of Plexiglas, note the fracturing along the sides of the cut.

This is not usually desirable, and what is needed is a way of cutting into these materials, so that the cut edges remain smooth, and the risk of shattering around the cut line is much diminished. The way that is usually used for this is to add small amounts of a fine cutting abrasive into the waterjet stream, and use this to cut the slots in the material, with the water there to add cutting power.


Figure 2. Abrasive waterjet (AWJ) cuts through safety glass. Note that there are two sheets of glass with a thin plastic sheet attached between the two.

This can be of particular advantage if you are faced with trimming, for example, safety glass (as shown in Figure 2). Cutting and shaping this glass used to be a significant problem in the industry, since the presence of the plastic sheet, between the two glass layers meant that it was not always possible to get both to break to the same plane if scribed with a glass cutter. Failure rates of up to 30% were described, to the author, as common when the technology switch to AWJ took place. And with the abrasive in the water, the jet cuts through both layers without really seeing that there was a problem. (And complex contours can also be cut).

The combination of abrasive and high-pressure water has many advantages over existing tools. Among other things it removes the majority of the heat from the cut zone, so that in almost all cases the Heat Affected Zone (HAZ) along the edges of the cut disappears and the quality of the cut surface becomes, when properly cut, sufficient to require no further processing. This can lead to a significant savings in certain forms of fabrication.

There are many different ways in which abrasive can be added to a high speed stream of water, and Dr. Hashish illustrated some of these in the introductory lecture he gave at an early WJTA Short Course, as follows:


Figure 3. Some different ways of introducing abrasive into the cutting stream of a high-pressure waterjet (After Hashish, WJTA Short Course Notes).

The top three (a, b, c) involve mixing the abrasive and the water streams at the nozzle, while the fourth (d) is a relatively uncommon design that is used in cleaning surface applications, and the fifth has never been very effective in any trial that we have run. The sixth (e) technique has become known by a number of different names, but for now, to distinguish it from the more widely used Abrasive Water Jet cutting (AWJ) I will give it the acronym ASJ, for Abrasive Slurry Jetting. It has a number of benefits in different circumstances, and I will write more about it in future posts. In more recent alternative designs to that shown by Dr. Hashish the flow to the abrasive holding tank is more commonly through a diverted fraction of the total flow from the pump or intensifier.


Figure 4. Very simplified illustration of the circuit where abrasive is added to the flow from the pump/intensifier before the nozzle. Obviously the abrasive is held in a pressurized holding vessel – the optimal design of which is not immediately obvious.

When fine abrasive is added to a narrow waterjet stream, and that jet is moving at thousands of feet a second, there are a number of considerations in the design of the mixing chamber, and those will be discussed in future posts. But one early conclusion is that, if the jet is going to be small, then the abrasive that will be mixed with it will also have to be quite small, though – as will be noted in a future post – not too small.


Figure 5. The simplified and generic components of a mixing chamber that mixes abrasive with high-pressure water in an AWJ system.

There were a number of problems with the early systems, such as that shown in Figure 5, at the time that systems first appeared on the market, and I will write about some of these as the next few posts continue to focus on this subject.

There have been a number of different abrasives used over the years, and it depends on the needs of the job as to which is the most suitable in a given case. In some cases discriminate cutting is required, and so an abrasive can be chosen that will cut the desired layer on the surface, but not the material behind it. In other cases the target material is extremely tough, and so abrasive may be selected that will rapidly erode the supply lines and nozzle, but which can still prove economically viable in certain cases.


Figure 6. Various types of abrasive, that can include (from bottom left going clockwise) blasting sand, copper slag, garnet and olivine.

There are many different properties of the cutting system, and the abrasive which control the quality and speed of the resulting cut. Some of these will be the topic of the next few posts, others will be discussed in further posts at a more distant time, when we discuss different cutting applications and the changes in a conventional system that might be made to get the best results in those cases.

Abrasive properties are not just a case of knowing what the material is. There is a difference, for example, in cutting ability between alluvially mined garnet and that mined from solid rock. There is a difference between different types of the nominally same abrasive when it comes from different parts of the world, and there are differences when the shapes of the abrasive differ. Glass beads and steel shot cut in a different way that glass and steel grit, for example. So there is plenty to discuss as we turn to a deeper discussion of abrasive waterjet cutting.


Figure 7. Parameters controlling the cutting by an abrasive waterjet system. (After Mazurkiewicz)

Waterjetting 1d - Not quite that simple!

When I first began the research on the applications of high-pressure water that was be one of the major parts of my professional life I must confess to a certain naïve innocence in regard to other folk’s work. One assumed that other folk had made similar mistakes to mine, and then corrected them, so that when different systems were compared that the early, obvious, mistakes had not been made.

One of the first times I found that this wasn’t the case was when we were asked to go and demonstrate that high-pressure waterjets could economically cut granite, in quarries located in the heart of the Granite industry, in Elberton, Georgia. We were working with Georgia Institute of Technology (Georgia Tech) at the time and were asked if we could, at very short notice, go down to a couple of quarries and run a demonstration.

Back during my graduate studies I had found that Russian claims were true that said that it was possible, with a 10,000 psi jet pressure to cut through a rock with a compressive strength of 30,000 psi. (I'll tell you how later)

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.)

Knowing this, and having a suitable pump at Rolla, our group ran some tests at the RMERC to get the angles right between the two jets that we were to use, and then, about a week later, we went down to Elberton and set up a system in the quarry.

Figure 2. 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.)

The trials demonstrated that high-pressure water could cut granite at commercial rates, we cut a slot some 11 ft long and about 2-ft deep, and, after a couple of days of work, we went home. Georgia Tech then went to one of our competitors who set up to run a similar test. We had been done in 2 days, it took them two weeks to cut a slot about 2 ft long and 6-ft deep. They were running a jet system at 45,000 psi, roughly 3 times the pressure of our system. Why did they do so badly?

Well it turned out that they connected their pumps to the nozzle through a very narrow length of high-pressure tubing, and we calculated (as later did they) that of the 45,000 psi being supplied at the pump, some 35,000 psi had been lost in overcoming friction between the pump and the nozzle, As a result they were trying to cut the granite with jets at a pressure of 10,000 psi effective pressure, and it was much slower than our system which retained most of the 14,000 psi from the pump to the nozzle. (Hilaris, J.A., Bortz, S.A., "Quarrying Granite and Marble using High Pressure Water Jet," paper D3, 5th International Symposium on Jet Cutting Technology, Hanover, FRG, June, 1980, pp. 229 - 236.)

Now you may note that I said something about mistakes – it turns out that we had made an identical mistake a few years earlier and had added a second 10-ft length of narrow diameter tubing to the nozzle, and suddenly a system that had cut adequately with 10-ft of tubing did not work with 20-ft. The reason was the pressure loss in the tubing was too great at the longer length, and the pressure fell below that required to cut into the rock. (But at the shorter length we were drilling the hard sandstone at 12-ft/minute).

It is a very simple mistake, and many folk have made it over the years. The system has to be designed from one end to the other to ensure that all the parts are properly sized for the systems that are to be used. (And I will refer to other cases such as that above as we go through this series.)

It is not just the diameter of the feed lines that is important. In 1972 it took, on average, 150 man-hours and about $2,000 for the U.S. Navy to clean a single ship boiler using chemicals and mechanical scrubbing and cleaning. An enterprising company showed the Navy that it was possible to use waterjet lances to clean the tubes. In the demonstration they cleaned a boiler in 10 hours, and it cost around $700. This being Government work, the Navy then arranged a competition to find the most effective contractor. Based on the performance of the system that had been used in the first demonstration they asked 5 companies to compete in cleaning boilers. The operating equipment was designated as having to operate at 20 gpm, at a pressure of 10,000 psi. The results were not even close, even with systems nominally the same.

Figure 3. Relative cleaning efficiency in areal percentage cleaned, of five competing systems in cleaning heat exchanger tubes in Navy boilers. (Tursi, T.P. Jr., & Deleece, R.J. Jr, (1975) Development of Very High Pressure Waterjet for Cleaning Naval Boiler Tubes, Naval Ship Engineering Center, Philadelphia Division, Philadelphia, PA., 1975, pp. 18.)

One of the differences between the competing systems, you won’t be surprised to hear, was that some had smaller feed hoses than others.

There are many different reasons that the various systems performed as they did. One of the aims of this series is to ensure that, should you be asked to engage in such a competition, you will know enough to follow the path of company A, rather than company E.

As systems have become more sophisticated the different factors that control the performance of the jets have increased in number. As a simple example, when abrasive particles are mixed with high-pressure water in streams of abrasive-laden waterjets at pressures that can run up to 90,000 psi in pressure, for high precision cutting of material, the factors controlling performance now include not only the delivery system for the water, but also that for the abrasive, the type of abrasive and the configuration of the nozzle through which that final cutting jet is created.

Again, when we were asked to compare the performance of these different systems we set up nominally identical test conditions under which to determine which nozzle system would perform better. If I were honest I would tell you that before the tests began I expected that the variation in performance of the systems would vary by perhaps 10% between the best and the worst. We were quite surprised by the result.

Figure 4. Comparative performance between 12 nominally similar abrasive waterjet cutting nozzles in cutting through steel at a standard speed, pump pressure, and abrasive concentration.

I use these last two figures to show that all the details of a high-pressure waterjet system are important, when it comes to optimizing performance. One of the reasons to write this series is to ensure that folk that use these systems in the future do not make the mistakes that we made, as we learned how to tune the systems from getting poor performance to the commercially viable rates that are achieved today.

Unfortunately much of the early research and tests that are the basis for this knowledge were performed before the Internet existed. As a result I will have to use references to books and papers (as above) rather than using the electronic references that are the more common habit now.

This concludes the basic introduction to the series, which will now focus on more specific subjects.