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 Cutting – Beginning to cut

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 few posts I have been discussing what happens under a waterjet as it first hits, and then penetrates into a target material. In many cases it is recommended that the nozzle move slightly relative to the target during this piercing process so that the water escaping from the developing hole does not have to fight its way past the succeeding slug of water entering the hole.

Now it might be thought that this problem would go away if the nozzle starts at the side of the target and then cuts into it. But this depends on a number of different factors, one of the more critical being as to whether the jet is cutting all the way through the material, or is only cutting a slot part of the way through it. As with a number of other topics, I am going to illustrate some of the concerns using granite as the target material, since it makes it easier to demonstrate some of the points I want to make.

If you were to look at one of the many statues that have been carved from granite over the thousands of years since the rock was first shaped into an art form, the rock usually appears as a relatively homogeneous material. That means (to those who don’t work with rock) that the rock has the same properties regardless of which direction you test them in.

Vermont carver granite

Figure 1. The Italian Carver’s Memorial, Dente Park, Barre, VT (From the Barre Granite Association via State Symbols USA)

However, if you were to ask a skilled quarry man, he would tell you differently. Because of the way that granite cools from the molten state in which it is injected up into the ground, it picks up an orientation to the crystals as they are formed. One of these orientations is roughly horizontal and called the Lift or grain of the rock. A second is perpendicular to this and vertical and is known as the Rift. The third plane, orthogonal to the other two, is called the Hard-Way because it is generally more difficult to work. These names relate to the ways in which the grains of the rock and the cracks around them align. They are virtually impossible for a lay person to detect, and a quarry man may need to feel the rock to tell you which way they lie. But they are used in splitting out the major blocks from the granite massif and come into play in breaking the large blocks down into handle-able sized pieces.

Granite bedding orientations

Figure 2. The A) Hard-Way B) Rift, and C) Lift planes of crystal orientation in granite

If, however, you were to shoot a short slug of water at high-pressure at a piece of granite (and we used the granite from Elberton in Georgia for this) then, depending on which direction the pulse came from relative to the three planes, the amount of rock that would break around the impact point would change.

In an earlier post discussing the splitting that occurs when pressure builds up within the cavity under a jet, I mentioned that the pressure would grow cracks that already existed. And it is for this reason that, when the jet impacts perpendicular to the existing crack planes, the volume of material broken out is greater than it is where the jet fires along the cracks. This can be shown using the cavity profiles from oriented samples into which the jets were fired.

Cavity profiles

Figure 3. Profiles from the cavities created around the impact points where the jet impacted granite blocks at different orientations.

One can use this information if, for example, one wanted to cut a thin line in granite, where the cut should be made in the direction of the crystals, i.e. making cuts along the lines shown in the A plane of figure 2.

Linear cuts on A plane

Figure 4. Linear cuts into granite along the lines shown in Figure 2.

In this set of cuts, the jet is cutting along the favored orientation of the crystals, and the rock only spalls when two jet paths approach each other in the lower right of the block.

If, however, the cuts are made in a direction perpendicular to the orientations, i.e. in the B and C planes, then the results are quite different.

Cratering on linear B

Figure 5. Cratering along the linear passes in cutting granite perpendicular to the Rift plane.

Where the jet strikes perpendicular to the Rift or Lift, then the pressurization under the jet is enough to cause those cracks to grow out to the surface and cause spallation along the cut. In many cases, this removes all the rock between two adjacent passes, even if they are more than an inch apart.

If one is to use the high-pressure waterjet system for slotting granite in a quarry, for example, then this can be a very useful tool, since by merely putting two jets on either side of the desired slot the spalling will remove the material between them without any further jet action. If the jets attack in the perpendicular plane, then the jet has to be rotated over the cut to get the same material removal rate.

In most cases, when cutting in a quarry, because the rock does vary in structure and grain size, it is better to ensure that all the rock is removed before the nozzles move into the cut by rotation. However, in smaller applications – such as where the excess rock is being removed around a planned sculpture –enhancing the spall around the impact point can lower the time and amount of energy required in removing unwanted rock.

That is, however, a relatively specialized application, and in most cases it is desirable that the cut be clean and smooth. This requires the use of abrasive in the waterjet stream, and so this will be the topic of the next few posts.

Cutting through one inch thick glass

Figure 6. Cutting through one inch thick glass, showing the cut through the side of the glass.

Waterjet Cutting – Deepening a hole and cautions with glass

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

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

The last three posts have described what happens when a jet of water first arrives on a surface and then starts to penetrate into the material. At a close stand-off distance, the erosion starts around the edge of the jet and continues to widen the hole as it gets deeper, until a point where the pressure at the bottom of the hole falls and the jet stops going deeper. The lateral flow away from the bottom of the jet continues to erode material, however, and so the hole gets a little wider at the bottom. This creates a small chamber under the entrance hole and this can build up enough pressure that it can cause the material around the hole to break.

Progress in the high-pressure waterjet drilling of a hole in rock

Figure 1. Progress in the high-pressure waterjet drilling of a hole in rock.

In the last post I showed where this happened with a 1-ft cube of rock that had been broken with a single pulse, but this fracture of the target can occur when piercing glass or other brittle materials. So the question becomes how to stop the fracture if one is trying to cut glass. This applies when the job calls for making an internal cut in the glass, and not when cutting in from the side, although that also has some problems that I will address in a later post.

When starting an internal cut, it obviously means piercing a starter hole through the glass in a region that is going to be part of the scrap, if this is possible, as it would be, for example, when cutting a sculpture. A secondary reason for that location, apart from confining any small cracks that might happen during the pierce, is that these starter holes are larger in diameter (for the reason given above) than the cut line once the jet starts to move, and that hole section would appear as a flaw on a final cut line.

Vanessa Cutler, in New Technologies in Glass discusses the process of cutting in more detail but suggests that the starter hole be pierced at a lower pressure than that to be used in the cut. This is so that the pressure within the cavity will remain lower during the pierce, and insufficient to cause the glass to break. She suggests (and she has a vastly greater experience than I in this) that the piercing pressure be around 11,000 to 18,000 psi – this varies a bit with abrasive grit size, machine size and glass type.

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

Figure 2. Detail of the glass sculpture “p1″, by Vanessa Cutler. (Note that these holes do not pierce all the way through the glass but all end at the same depth.)

She also recommends, when there are multiple cuts to be made on a sheet, that all the piercing holes be completed before any cutting begins. One of the reasons for this is to avoid constantly resetting the cutting pressure, which could be a problem if you forget to lower the pressure back down before starting the next pierce. (Would I as an Emeritus Professor ever be that absent-minded? Why else bring it up?)

You will notice, with abrasive cutting into glass, that there is not the belling at the bottom of the cut like with plain waterjet cutting and that the hole tapers with depth as the cutting effectiveness reduces with the fall in pressure with depth; and the jet is less able to cut into the side walls of the opening at these lower pressures.

Stepping back from the cutting of glass to the more general condition where the jet runs out of power at the bottom of the hole, the main reason for this is the conflict between the water in the fresh jet coming into the hole and the spent water trying to make it out of the hole at the same time.

One way of overcoming the problem is to interrupt the flow of water into the hole. Back in my grad student days, we tried doing this by breaking the jet into slugs, so that one slug would have enough time to travel to the bottom of the hole, cut a little, and then rebound out of the hole, before the next slug of water arrived. There was relatively little sophistication in the tool we designed to do this. Simply it was a disk, with holes drilled in it at an angle.

Interrupter disk placed in the path of a continuous jet

Figure 3. Interrupter disk placed in the path of a continuous jet. (My PhD Dissertation)

The reason for the angled holes was to make the disk self-propelling as it rotated under the jet, since the angled edges of the hole forced the disk to continue rotating once started. (On a minor note, the disk would rotate at several thousand rpm, and the noise that it made was loud enough that I was instructed to only carry out the tests after the staff had left for the evening).

The penetration of a waterjet into sandstone with the jet running continuously (black), with the jet interrupted (red) and with the jet rotated slightly off-axis (green)

Figure 4. The penetration of a waterjet into sandstone with the jet running continuously (black), with the jet interrupted (red) and with the jet rotated slightly off-axis (green). (Brook, N. and Summers, D.A., “The Penetration of Rock by High Speed Waterjets”, Intl. Journal Rock Mechanics and Mining Science, May, 1969)

As can be seen in figure 4, with the pulsating jet more energy was getting to the bottom of the hole without interference, and the hole continued to deepen over time. However, the interruption tool had a number of disadvantages, apart from the noise and that the disk would be very rapidly destroyed under an abrasive jet. It was wasting a significant portion of the energy: In a more optimized design that I won’t discuss further, the energy loss was about 50%.

So it would be best if the jet was moved slightly over the surface, and in these early tests, the easy way to do this was to have the target rotate with the jet hitting the rock just offset from the axis of rotation. (At the time high-pressure swivels weren’t yet available). This gave the upper curve in figure 4, and a much more rapid penetration of the target.

In more modern times, the nozzle is moved either by causing it to move slightly around the hole axis or by causing a slight oscillation or “dither” in the nozzle while the pierce is taking place. This is generally a feature of the control software that drives the cutting table. But the reason for the movement is to get the water flowing in such a way that the water going out of the hole does not interfere with that going in, and so there is a reduced risk of pressure build-up in the hole, with the consequent cracking that this would cause.

Waterjet Glass Cutting and Piercing

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

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

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

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

Serrations and tear at the top of a packet of honey

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

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

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

Impact of a high-speed droplet of water on glass

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

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

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

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

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

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

Single cracks growing out from partial piercings in a test piece

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

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