Abrasive sizing

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

Over the past 30 years, abrasive waterjet cutting has become an increasingly useful tool for cutting a wide range of materials of varying thickness and strength. However, as the range of applications for the tool has grown, so the requirements for improved performance have also risen. Before being able to make a better quality cut, there had to be a better understanding of how abrasive waterjet cutting works so that the improvements could be made.

Parameters controlling AWJ

Figure 1. Some factors that affect the cutting performance of an abrasive waterjet (Hashish, Mohamed, “The effect of pressure on performance of Abrasive-Waterjet (AWJ) Machining”, Proceedings of Manufacturing International, April, 1988, Atlanta, GA, pp 255 – 263)

This understanding has not been easy to develop since there are many different factors that all affect how well the cutting process takes place. Consider, first of all, the process of getting the abrasive up to the fastest speed possible. And for the purpose of discussion, I am going to use a “generic” mixing chamber and focusing tube nozzle for the following discussion.

Simplified nozzle design

Figure 2. Simplified sketch of a mixing chamber and focusing tube nozzle used in adding abrasive to a high pressure waterjet

As high-pressure water flows through the small orifice (which in the sketch was historically made of sapphire), it enters a larger mixing chamber and creates a suction that will pull abrasive into the mixing chamber through the side passage. That side passage is connected through a tube to a form of abrasive feed mechanism that I will not discuss in detail today.

However, the abrasive does not flow into the mixing chamber by itself. Rather it is transported into the mixing chamber using a fluid carrier. In the some of the earliest models of abrasive waterjet systems, water was used as the carrier fluid to bring the abrasive into the mixing chamber. This, as a general rule, turned out to be a mistake.

The problem is that, within the mixing chamber, the energy that comes into the chamber with the high-pressure water has to mix not only with the abrasive but also with the fluid that carried the abrasive into the chamber. Water is heavier than air, and so if water is the carrier fluid, then it will absorb more of the energy that is available with the result that there is less for the abrasive, which – as a result – does not move as quickly and therefore does not cut as well. The principle was first discussed by John Griffiths at the 2nd U.S. Waterjet Conference, although he was discussing abrasive use in cleaning at the time.

Wet v dry feed

Figure 3. Difference in performance of water acting to carry the abrasive to the mixing chamber (wet feed) in contrast with the use of air as the carrier fluid. (Griffiths, J.J., “Abrasive Injection Usage in the United Kingdom,” 2nd U.S. Waterjet Conference, May, 1983, Rolla, MO, pp. 423 – 432.)

Note that this is not the same as directly mixing the abrasive into the waterjet stream under pressure – abrasive slurry jetting – which I will discuss in later posts.

The difference between the two ways of bringing the abrasive to the mixing chamber is clear enough that almost from the beginning, only air has been considered as the carrier to bring the abrasive into the mixing chamber. However, there is the question as to how much air is enough, how much abrasive should be added and how effectively the mixing process takes place.

In the earlier developments, the equipment available restricted the range of pressures and flow rates at which the high pressure water could be supplied, and these limits bounded early work on the subject.

One early observation, however, was that the size of the abrasive that was being fed into the mixing chamber was not the average size of the abrasive after cutting was over. (At that time steel was not normally used as a cutting abrasive). Because the fracture of the abrasive into smaller pieces might mean that the cutting process became less effective, Greg Galecki and Marian Mazurkiewicz began to measure particle sizes at different points in the process. (Galecki, G., Mazurkiewicz, M., Jordan, R., “Abrasive Grain Disintegration Effect During Jet Injection,” International Water Jet Symposium,Beijing, China, September, 1987, pp. 4-71 – 4-77.)

For example, by firing the abrasive-laden jet along the axis of a larger plastic tube (here opened to show the construction) the abrasive would, after leaving the nozzle, decelerate and settle into the bottom of the tube, without further break-up and without damage to the tube. Among other results, this allowed a measure of how fast the particles leave the nozzle, since the faster they were moving, the further they would carry down the pipe.

Green tube test

Figure 4. Test to examine particle size and travel distance, after leaving the AWJ nozzle at the left of the picture. The containing tube has divisions every foot, and small holes over blue containers, so that the amount caught in every foot could be collected and measured.

For one particular test, the abrasive going into the system was carefully screened to lie in the size range between 170 and 210 microns. It was then fed into a 30,000 psi waterjet at a feed rate of 0.6 lb/minute. The particles were captured after passing through the mixing chamber but before they could cut anything by using the tube shown in Figure 4. The size of the particles was then measured and plotted as a cumulative percentage adding the percentages found at each sieve size over the range to the 210 micron size of the starting particles.

After mixing sizing

Figure 5. Average size of particles after passing through a mixing chamber and exiting into a capture tube without further damaging impact

The horizontal line shows the point where 50% of the abrasive (by weight) had accumulated, and the vertical line shows that this is at a particle size of 140 microns. Thus, just in the mixing process alone, energy is lost in mixing the very fast moving water with the initially much slower moving abrasive.

And, as an aside, this is where the proper choice of abrasive becomes an important part of an effective cutting operation because the distribution of the curve shown in figure 5 will change with abrasive type, size, concentration added as well as the pressure and flow rate of the nozzle through which the water enters the mixing chamber.

I will have more to discuss on this in the next post but will leave you with the following result. After we had run the tests which I just mentioned, we collected the abrasive in the different size ranges. Then we used those different size ranges to see how well the abrasive cut. This was one of the results that we found.

Effect of particle size

Figure 6. The effect of the size of the feed particles into the abrasive cutting system on the depth of cut which the AWJ achieved

You will note that down to a size of around 100 microns the particle size did not make any significant difference, but that once the particle size falls below that range, then the cutting performance degrades considerably. (And if you go back to figure 5, you will note that about 30% of the abrasive fell into that size range, after the jet had left the mixing chamber).

Waterjet Cutting – Introduction to Abrasive Waterjet

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 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.

Cut along plexiglas

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.

Cut through safety glass

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 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:

Means of adding abrasive

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 (e) 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.

Abrasive Slurry Jetting Circuit

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.

Cutaway view of a cutting head for abrasive waterjet cutting

Figure 5. Cutaway view of a cutting head for abrasive waterjet cutting (ACTIVE AUTOLINE II by KMT Waterjet)

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.

Types of abrasive

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.

Parameters controlling Abrasive Waterjet Cutting

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

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 – Testing Waterjet Nozzle wear by cutting foam

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

Last week’s post discussed a simple test which helps to show not only how to compare the effect of different operating conditions (varying abrasive type, nozzle design, AFR etc) as a way of finding a possibly better and cheaper cut. It is also often handy to know when a nozzle is starting to wear out, so that different cutting operations might be scheduled to allow the nozzle to continue to work, without threatening the quality of a critical product.

Change in the cutting depth of a jet stream at 50,000 psi when traversed over ASTM A108 steel

Figure 1. Change in the cutting depth of a jet stream at 50,000 psi when traversed over ASTM A108 steel as a function of the time that the nozzle had been in use

While we have found that nozzles from a given manufacturer roughly agree in cutting performance and times before they wear out, the pattern of wear and performance change differs from one nozzle design to another. Also there is some variation in performance between nozzles even of the same design and under the same conditions.

There are also times when cuts are made without abrasive or when the cutting/cleaning jet is hand-held – what to do in those cases? Mainly we have used foam as the cutting target, set up so that the jet won’t cut all the way down through the foam all the way along the cut, so that, as with the steel, some idea of not only cutting depth but also cut quality can be seen.

Cuts through thick stiff packing foam

Figure 2. Cuts through thick stiff packing foam. Note the rough edge at the bottom of the extracted pieces, but the good initial quality of cut that was achievable for some 14-inches.

There is a caution in cutting foam in that some of the softer varieties are going to fold into the cut and give a slightly inaccurate measure of true performance; although for a quick comparison to see how a nozzle is lasting that is not a real issue. When cutting thicker material and also when going for higher quality cuts that is, however, something that should be borne in mind.

The white expanded foam that is used as a packing material is also very easy to cut, even with the pressures that can be found with a pressure washer type of system. Thus, if you are going to clean a deck or other surface, it helps to check by swiping the jet across such a piece of material to be sure that you have a good nozzle on the end of your lance before you start.

This may seem fairly logical; after all you just went to the hardware store and bought a new packet of nozzles. Well, as with the other nozzles we have looked at, quality is only assured after testing. In this particular case, we ran as many different varieties of fan nozzles as we could to see how they would perform when cutting across a piece of packing foam. It is not hard to cut packing foam with a high pressure jet. And since domestic cleaning is usually carried out at either 1,000 psi or 2,000 psi, we ran tests at both levels.

Results from a good, top, and a poor nozzle with cuts at 1,000 and 2,000 psi and with the foam moved through the jet at a distance of 3 inches

Figure 3. Results from a good, top, and a poor nozzle with cuts at 1,000 and 2,000 psi and with the foam moved through the jet at a distance of 3 inches. The number identifies the nozzle. Note that at 3 inches, number 18 could barely remove the top of the foam.

A fan jet is defined by the amount of water that it will allow to pass at a set pressure and by the angle of the cone with which the jet spreads out from the orifice. In passing, we found that the cone angle that the jet actually spread at was a little larger than that designated on the package.

The worst nozzle design that we found had difficulty in cutting into the foam even at a very close range:

On the other hand, the best nozzle was still able to cut the material with the nozzle held some nine inches from the foam.

Cutting result with the good nozzle held at nine inches above the foam target

Figure 4. Cutting result with the good nozzle held at nine inches above the foam target. At this distance the jet is removing as much material as the poor jet did at a 3-inch standoff.

A very typical result would have the jet fail to cut into the foam much beyond four inches from the nozzle. (I’ll use some photographs in a couple of weeks to explain in more detail why that is). And as a short editorial comment to those of you who clean around your house with a domestic unit, how many of you hold the nozzle that close to the surface? (Or at the car wash?) If you don’t you are losing most of the power that you are paying for and you are in the company of most of the students that I ran this demonstration with in my classes).

However there is one other feature to the photographs of the cuts that I would point out. Fan jets distribute the water over a diverging fan shape. But the results of the design fell into two different types, one where most of the water still concentrated in the middle of the jet, (as in Figure 4) and those where it was focused more on the side.

Cutting pattern with the jet streams more at the side of the flow

Figure 5. Cutting pattern with the jet streams more at the side of the flow (arrow points). Note that the two pressure cuts are on the other sides of the sample here.

The benefit of using foam is that it allows this picture of the jet structure to be easily seen, with very little time taken to swipe the nozzle over a test piece of material at the start of work, to make sure that the jet is still working correctly.

This is both an advantage and a disadvantage. Because the foam is relatively easy for a jet to cut even at a lower pressure, this means that the cut can become more ragged with depth where deep cutting is required.

One of the programs that we ran, some years ago, looked at how deeply you could cut into the stiff packing foam that is used in some industrial plants, where the item being packed needs to be held firmly yet will be released easily when needed. This requires that the foam be cut to a very tight tolerance, and at the time, pieces were still being cut by hand and then glued together. (Figure 2 above)

We found that we could cut up to about a foot of material before the small cut particles became sufficiently caught up in the cutting jet that the edge quality of the cut fell below specification. But in order to get to that depth we did have to add a small amount of a polymer to the cutting water. This helped to hold the jet more coherent over a greater distance and also reduced the amount of particulate that got caught up in the jet allowing the greater cutting depth.

Foam works as a simple sample to give some sense of the jet shape where the pressures are lower. When they are higher, a stiffer material is needed though it should still be cut by water without the need for abrasive. Plywood is a useful target in this case, and I will write about those tests next time.