Waterjet Cutting – The Instant of Contact: Cutting Aluminum

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

Plain high-pressure waterjets penetrate into material in a different way than that which occurs when abrasive is used to make cutting easier. And even with abrasive, there are different ways in which the target will react depending on how brittle it is. In this next segment, I will write just about the stages that occur as water alone cuts into a target.

In its simplest form consider first a spherical drop of water, moving at very high speed, which suddenly strikes a flat surface.

Waterjet droplet striking a flat surface

Figure 1. Waterjet droplet striking a flat surface

As the droplet impacts the surface but can’t penetrate it, so the water that comes into contact with the surface tries to flow away along the surface to get out of the way of the volume of water striking the surface behind it.

But in the early stages of the impact (see inset), the edges of the droplet ahead of that lateral flow are coming down onto the surface faster than the water can move that is trying to escape. In this range of activity, the distance that the edge of the droplet must travel, L, remains smaller than the distance, D, that the water must move to escape.

This instantly traps the water and with confinement comes a very rapid increase in pressure along the edge of the drop. This pressure also acts on the target surface, so that it is pushed down a little. This pressure was first measured by John Field at the Cavendish Lab in Cambridge, UK, who found that it could exceed three times the water hammer pressure that the water might otherwise exert.

For those not that familiar with the term, the water hammer pressure is also sometimes called the hydraulic shock pressure, and it can occur when a valve is suddenly closed in a feed line, and this sends a shock or pressure wave back up the line. (This is what can sometimes cause banging in feed pipes). Often there is a small air cushion built into water lines to act as a sponge, when such a shock occurs since otherwise the repetitive shocks can cause parts to fail.

This becomes more of a problem with higher pressures because the equation for the pressure that is generated is given by the equation:

Pressure = fluid density x impact velocity x sound speed in the fluid

Compare this with the impact pressure when a shock is not generated:

Pressure = 0.5 x fluid density x (impact velocity)^2

As a very rough rule of thumb, the speed of sound in water is roughly 4,800 ft per second.

If a waterjet is driven out of a nozzle at a pressure of 40,000 psi, then the speed at which it is moving can be roughly calculated as:

Jet velocity (ft/sec) = 12 x Square root (Pressure)

The jet velocity, to a first approximation, is thus 12 x 200 = 2,400 ft/sec.

The Water Hammer Pressure is thus 2 x (4,800/2,400) = 2 x 2 = 4 times the pressure exerted by the water more conventionally. Since that driving pressure was, in this case, 40,000 psi, the water hammer pressure would be 160,000 psi. With the multiplier that Dr. Field found, this can take that pressure up to around 500,000 psi for that instant of contact.

It is, however, only applied to the target at that instant of impact, and where there is the spherical end of the drop to cause the pressure accumulation across the face.

It does, however, cause a very high lateral jet to shoot out of the jet at about the point that the droplet curvature no longer provides confinement (at about 1/3 of the droplet diameter measured radially from the center of contact).

John Brunton, also at the Cavendish, has provided photographs of the damage done in that instant of contact.

Droplet impact damage on a sheet of Plexiglas

Figure 2. Droplet impact damage on a sheet of Plexiglas (Brunton “High Speed Liquid Impact” Proc Royal Soc London, 1965. P 79 – 85.)

Part of the damage comes from the high lateral velocity of the released water running into the wall of material not compressed under the generated pressure. Mike Rochester found that the diameter of this ring crack closely followed the diameter of the nozzle from which the droplet was released.

Relative size of the ring crack to that of the originating nozzle

Figure 3. Relative size of the ring crack to that of the originating nozzle (jet head) ( M.C. Rochester, J.H.Brunton “High Speed Impact of Liquid Jets on Solids” First BHRA symp Jet Cutting Tech, April `972, Coventry UK, paper A1.)

In our case, however, the jet is not a single droplet, but rather, at least close to the nozzle, a steady stream with the pressure constant across the diameter.

Thus, in the microseconds after the first impact as the jet continues to flow down onto the target, it is flowing out across the damaged zone created by that first impact. The resulting pattern of erosion, which we captured in aluminum, changes as the target moves away from the nozzle. Close to the nozzle the wear pattern looks like this:

Damage pattern around the impact point of a jet on aluminum, target close to the nozzle

Figure 4. Damage pattern around the impact point of a jet on aluminum, target close to the nozzle.

The pattern close to the nozzle shows that directly under the jet, the pressure is relatively even on the surface of the metal. With no differential pressure across the grain boundaries in that region, the metal is uniformly compressed, and suffers no erosion. At the edges of the jet, however, there is not only the original ring crack damage created on the instant of impact, but also there is a differential pressure along the edges of the jet, which helps to dislodge those initial grains and provide crack loci for the water to exploit and remove material as it moves away from the original contact surface. The greatest portion of the damage at this point lies outside the edges of the impacting jet as the laterally flowing jet erodes material as the jet continues to flow.

As the target is moved further from the nozzle, the pressure profile changes from one with a constant pressure over the jet, to one where the central constant pressure region starts to decline in size. Rehbinder calculated the two components of the pressure in the target at the beginning of this erosion process at that point and provided the following mathematical plot.

Impact pressures calculated for the pressure into the target and that along it, during waterjet flow

Figure 5. Impact pressures calculated for the pressure into the target and that along it, during waterjet flow. (Rehbinder, G., “Erosion Resistance of Rock,” paper E1, 4th International Symposium on Jet Cutting Technology, Canterbury, UK, April, 1978, pp. E1-1 – E1-10.)

The result of this change in the pressure profile of the jet as it moves away from the nozzle can be seen in the change in the erosion patterns of the jet as it strikes an aluminum target, and that will be the topic for the next post.

Waterjet Technology – An intro to water jet structure

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

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.

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

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.

The break-up pattern of a waterjet

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 that follow were taken in Baxter Springs, KS, which has been recognized as the Birthplace of Waterjet Cutting.

Baxter Springs, Kansas. Birthplace of Waterjet Cutting

Figure 3. Baxter Springs, Kansas. Birthplace of Waterjet Cutting

In the early 1970’s, we used what was then a McCartney Manufacturing waterjet intensifier (today KMT Waterjet Systems) 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.

Arrangement for taking photographs of a high-speed jet

Figure 4. 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.

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

Figure 5. 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.

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

Figure 6. 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. Jacques Zakin and I tested a wide range of different polymers to see which would 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.

Jets with an orifice diameter of 0.01 inches at a pressure of 20,000 psi

Figure 7. 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:

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

Figure 8. 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 8 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.

Waterjet Technology – Cutting Plywood and Pork, and water jet safety

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

In the last two posts I have tried to show that there is a benefit to running an occasional calibration test on equipment to ensure that it is giving the best performance. This does not mean that the nozzle needs to be tested every day, although some of the cheaper pressure washer nozzles, for example, will wear out in less than an hour. An operator will learn over time about how long a nozzle will last and can, after a while, tell when it is starting to lose performance. But in working on a number of different jobs in succession, that sense of the performance may be missed, and it can be handy to have a standard target that a jet can be pointed at and that it should be able to cut in a known time.

One simple target is plywood, and, to continue the saga of nozzle comparisons through a slightly different approach, Mike Woodward used plywood sheets to compare different nozzles in one of the earliest comparisons of performance. We since duplicated his test equipment and ran tests with a more modern selection of nozzles but the basic results and conclusions remain the same.

In its simplest form, the idea is to build a holding frame that will hold small squares of plywood at fixed distances from the nozzle. In the frame shown below, the plywood pieces are set at one-foot distances apart with the nozzle held at a fixed point at the end of the test frame. Tests showed that it takes around 2,700 psi to cut through the plywood.

A simple frame to hold plywood samples

Figure 1. A simple frame to hold plywood samples

The initial tests that Dr. Woodward ran were run on nozzles that were run at 10,000 psi with a nominal flow rate of 10 gpm. The nozzles that were used cost in the range from $10.00 to $250 a piece. (And these costs were reported in 1985 at the 3rd American Waterjet Conference). Tests such as this are simple to run. Plywood pieces are set into the frame, the nozzle is placed at the end of the frame and the jet run for ten seconds. Over that time, the jet will cut through any of the pieces of plywood that it reaches with enough power to cut through, and generally, the jet will punch a hole through several pieces.

The different designs of nozzle that Mike Woodward tested in 1985

Figure 2. The different designs of nozzle that Mike Woodward tested in 1985

The profiles show that there was only one of the common nozzles at the time that fitted smoothly onto the end of the feed pipe. In the other cases, there is a small gap between the nozzle piece and the feed tube so that turbulence would be generated just as water entered the acceleration section of the nozzle.

The hole size in each plate was then measured and that width plotted as a function of the distance from the nozzle so that a profile of the jet cutting path could be drawn.

Profiles cut into the different pieces of wood showing the cutting power of the different jets

Figure 3. Profiles cut into the different pieces of wood showing the cutting power of the different jets as a function of distance and the actual amount of water flow as measured

As an additional part of the testing, a rough measure was kept of the effective nozzle life. Some other performance parameters for the different nozzles can be put into a table.

Performance of the different nozzles

Figure 4: Performance of the different nozzles

Clearly, just going out and buying the most expensive nozzle on the block is not necessarily the best idea. But it also depends on the use to which the nozzle is going to be applied. There are two different applications: that of cleaning a surface and that of cutting into it. The broader path achieved by nozzle 1, for example, which also removed the largest volume of wood per horsepower, makes it a good selection for cleaning and for reaching further from the nozzle as would be needed if one were cleaning the pipes of a heat exchanger bundle.

On the other hand, the more coherent flow through nozzle 2, which gave a narrower cut, might be a more effective tool in a cutting operation. In other cleaning operations, where the nozzle is being operated very close to the surface, then nozzle 3, which has a wider path, might be a better choice, though that is lost if the target surface is further away. And though there was not a great deal of difference in performance between nozzles 1 and 5, there is a considerable difference in price.

A smaller, lighter nozzle may be a beneficial trade-off if the nozzle body is fitting on the end of a lance that will be operated manually for several hours at a time.

There is an alternate way of using plywood as a target that I have also used in teaching class. The student is using a manually operated high-pressure cleaning gun at 10,000 psi and is to swing the gun horizontally so that the jet cuts into a piece of plywood that is set almost parallel with the jet path, but with the stream hitting the wood from the side initially further from the operator. But as the swing completes the jet cuts up where the nozzle almost touches it and then sweeps on past.

The result is that, over the distance, the jet can cut into the wood and a groove is carved into it.

Horizontal cuts into plywood

Figure 5. Horizontal cuts into plywood. There were about half-a-dozen students who had swiped the nozzle so that it just cleared the left edge of this 4-ft wide piece of plywood, and you may note that the cuts extend roughly ¾ of the way along the surface

Once the students had seen this cut, I would ask them how far away they thought, based on that measurement, the jet would cut into a person. Typically they said about three feet, and then, as a precaution, I suggested they add a foot or so more.

Then I took them over to a metal frame where we had hung a piece of pork. We carefully measured off the “safe” distance from the end of the nozzle to the pork.

“Now assume that is you”, I would say, “swing the jet as fast as you can, so that it barely has time to hit “your arm”, and we’ll just check that distance is correct.”

Piece of pork that has been traversed by a 10,000 psi jet several times

Figure 6. Piece of pork that has been traversed by a 10,000 psi jet several times, with a typical stand-off distance from the nozzle of more than four feet.

Invariably we got the result shown in Figure 6. The jet would cut into the meat to a typical depth of around two inches and groove the underlying bone. It was a salutary way of getting their attention about the safe use of waterjet technology, and I noticed that the staff also got a bit more cautious after we ran this class every year.

Waterjet Technology – The Triangle Cut Comparison Test

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 post is being written in Missouri, and while the old saying about “I’m from Missouri, you’re going to have to show me,” has a different origin than most folk recognize*, it is a saying that has served well over the years. We did some work once for the Navy, who were concerned that shooting high-pressure waterjets at pieces of explosive might set them off as we worked to remove the explosive from the casing. We ran tests under a wide range of conditions and said, in effect, “see it didn’t go off – it’s bound to be safe!” “No,” they replied, “we need to know what pressure causes it go off at, and then we can calculate the safety factor.” And so we built different devices that fired waterjets at pressure of up to 10 million psi, and at that pressure (and usually a fair bit below it) all the different explosives reacted. And it turned out that one of the pressures that had been tested earlier was not that far below the sensitivity pressure of one of the explosives.

That is, perhaps a little clumsily, a lead in to explain why simple answers such as “yes I can clean this,” or “yes I can cut that” often are not the best answers. One can throw a piece of steel, for example, on a cutting table and cut out a desired shape at a variety of pressures, abrasive feed rates (AFR) and cutting speeds. If the first attempt worked, then this might well be the set of cutting conditions that become part of the lore of the shop. After a while it becomes “but we’ve always done it that way,” and the fact that it could be done a lot faster with a cleaner cut, less abrasive use and at a lower cost is something that rarely gets revisited.

So how does one go about a simple set of tests to find those answers? For many years, we worked on cutting steel. Our tests were therefore designed around cutting steel samples because that gave us the most relevant information, but if your business mainly cuts aluminum, or titanium or some other material, then the test design can be modified for that reason.

The test that we use is called a “triangle” test because that is what we use. And because we did a lot of them, we bought several strips of 0.25-inch thick, 4-inch wide ASTM A108 steel so that we would have a consistent target. (Both quarter and three-eighths thick pieces have been used, depending on what was available). The dimensions aren’t that important, though the basic shape that we then cut the strips into has some advantage as I’ll explain. (It later turned out that we could have used samples only 3-inches wide, but customs die hard, and with higher pressures the original size continues to work).

Basic Triangle Shape for waterjet test cutting

Figure 1. Basic Triangle Shape for waterjet test cutting

The choice to make the sample 6-inches long is also somewhat arbitrary. We preferred to make a cutting run of about 3 minutes so that the system was relatively stable, and we had a good distance over which to make measurements, but if you have some scrap pieces that can give several triangular samples of roughly the same shape, then use those.

The sample is then placed in a holder, clamped to a strut in the cutting table and set so that the 6-inch length is uppermost and the triangle is pointing downwards.

The holder for the sample triangle

Figure 2. The holder for the sample triangle

The nozzle is placed so that it will cut from the sharp end of the triangle along the center of the 0.25-inch thickness towards the 4-inch end of the piece. The piece is set with the top of the sample at the level of the water in the cutting table. The piece is then cut – at the pressure, AFR and at a speed of 1.25 inches per minute with the cut stopped before it reaches the far end of the piece, though the test should run for at least a minute after the jet has stopped cutting all the way through the sample.

The piece is then removed from the cutting table and, for a simple comparison, the point at which the jet stopped cutting all the way through the triangle is noted.

Showing the point at which the jet stopped cutting through various samples as a function of the age of the nozzle

Figure 3. Showing the point at which the jet stopped cutting through various samples as a function of the age of the nozzle – all other cutting conditions were the same (a softer nozzle material was being tested which is why the lifetime was so short). The view of the samples is from the underside (A in Fig 1.)

An abrasive jet cuts into material in a couple of different ways – the initial smooth section where the primary contact occurs between the jet and the piece and the rougher lower section where the particles have hit and bounced once on the target, and now widen and roughen the cut. Since some work requires the quality of the first depth, we take the steel samples, and mill one side of the sample, along the lower edge of the cut until the mill reaches the depth of the cut, and then we cut off that flap of material so that the cut can be exposed. Note that the depth is measured to the top of the section where the depth varies.

Typical example of a steel triangle that has been cut and then sectioned to show the quality of the cut

Figure 4. Typical example of a steel triangle that has been cut and then sectioned to show the quality of the cut

I mentioned in an earlier article that we had compared different designs from competing manufacturers. Under exactly the same pressure, water flow and abrasive feed rates, the difference between the cutting results differed more greatly than had been expected.

Sectioned views of six samples cut by different nozzle designs, but at the same pressure, water flow, AFR and cutting speed

Figure 5. Sectioned views of six samples cut by different nozzle designs, but at the same pressure, water flow, AFR and cutting speed

There was sufficient difference that we went and bought second and third copies of different nozzles and tested them to make sure that the results were valid, and they were confirmed with those additional tests. Over the years as other manufacturers produced new designs, these were tested and added into the table – this was the result after the initial number had doubled. (The blue are results from the first nozzle series tests shown above).

Comparative depths of cut using the same pressure and AFR but twelve different commercially available nozzle designs

Figure 6. Comparative depths of cut using the same pressure and AFR but twelve different commercially available nozzle designs

There were a number of reasons for the different results, and I will explain some of those reasons as this series continues, but I will close with a simple example from one of the early comparisons that we made. We ran what is known as a factorial test. In other words the pressure was set at one of three levels and the AFR was set at one of three levels. If each test ran at one of the combination of pressures and AFR values and each combination was run once then the nine results can be shown in a table.

Depths of cut resulting from cutting at jet pressures of 30,000 to 50,000 psi and AFR of 0.6, 1.0 and 1.5 lb/min

Figure 7. Depths of cut resulting from cutting at jet pressures of 30,000 to 50,000 psi and AFR of 0.6, 1.0 and 1.5 lb/min

The results show that there is no benefit from increasing the AFR above 1 lb/minute (and later testing showed that the best AFR for that particular combination of abrasive type, and water orifice and nozzle diameters was 0.8 lb/minute).

Now most of my cutting audience will already know that value and may well be using it but remember that these tests were carried out over fifteen years ago, and at that time, the ability to save 20% or more of the abrasive cost with no loss in cutting ability was a significant result. Bear also in mind that it only took 9 tests (cutting time of around 30 minutes) to find that out.

__________________________________________

* The reason that the “I’m from Missouri, you’ll have to show me,” story got started was that a number of miners migrated to Colorado from Missouri. When they reached the Rockies they found that, though the ways of mining were the same, the words that were used were different. (Each mining district has its own slang). Thus they asked to be shown what the Colorado miners meant, before they could understand what the words related to.