Abrasive waterjet cutting

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

There are a number of different abrasives that can be supplied by different sources, and the market for the small grains that are used in abrasive waterjet cutting extends considerably beyond just the waterjet business. All abrasives are not created equal; some work better in one condition, others in another. As with other tools that the waterjet cutter or cleaner will use, first you should decide what the need for the abrasive is and run a small series of tests to find out which is the best set of cutting conditions for that particular job.

The first item on the list should be the material that has to be cut. (Although abrasives are also used in cleaning, that will be covered in a later post). There are, simplifying greatly, two classes of material that have to be cut. One class responds in a brittle way (think glass) and the other responds in a ductile or yielding manner (think metal). Because of these different responses when the particles hit the surface, the way in which cuts are best made will vary between the two. Some years ago, Ives and Ruff shot abrasive particles at different targets and found that there was a difference in the amount of material removed, but the best angle at which the particles should be aimed changed with the material.

Angle Effect

Figure 1. The Effect of change in impact angle on erosion rate for ductile and brittle targets. (Ives and Ruff, Wear, 1978, pp 149 – 162).

Some work at MS&T just before I retired indicated that the shape of these curves changed a little, depending on the size of the abrasive that is used. There are also some changes with abrasive shape. And this is because of the entirely different way in which an abrasive particle cuts into the two different materials. In this post we’ll discuss only the ductile target.

If a relatively smooth particle is shot into a ductile material at an angle perpendicular to the surface, then when it hits the surface, the target material will flow out from underneath but not be removed. As the following micro-photograph shows, the particles can become embedded in the material – and even add to the weight of the piece on rare occasion.

Embedded particle

Figure 2. Microphotograph showing a sand particle buried in the surface of an aluminum target.

There is very little material removed in this case – as the black curve shows in Figure 1 – when the impact angle approaches 90 degrees. Consider that if you take a knife and push it down into butter you don’t remove any butter. But if you drag the knife over the butter surface you will peel off a layer.

So it is with abrasive hitting a ductile metal. If the abrasive is brought in at an angle, (optimized in the figure at 15 degrees) then the abrasive has a cutting energy along the surface and this will peel up, and remove small pieces of the surface. By taking a microphotograph along the edge of an abrasive cut, we were able to show the action of individual particles in cutting into the metal.

Surface abrasive cleaned

Figure 3. Individual particle impacts on an aluminum surface, showing the cutting and plowing action of the particles.

Where the surface is plowed up, but not removed, another particle has to hit that point to remove the relatively fragile lip. However, if the particle is a copper slag, or other relatively weak material, it can shatter during the cutting process, and the breaking pieces can break off that lip, so that – again in the right material – the slag may give a better performance than a more expensive alternative.

But if we are to cut metal in this way, what does that say about the shape of the particles that we need to use. Obviously, if they were round such as a steel or glass shot, then there would be no sharp edges to cut into and peel off the material. Thus a steel or glass grit will cut better, though each particle needs a certain thickness in all dimensions so that there will be enough energy to both cut into the material, and plow along it.

Glass beads re grit

Figure 4. Difference in cut depth achieved with broken glass fragments over glass beads when cutting metal.

A relatively round particle with sharp corners, and garnet is usually such a particle, can often work well in cutting a range of different ductile materials.

Particle cutting

Figure 5. Schematic of how a particle of different shapes might cut into material.

Now that is fine when a high-pressure abrasive waterjet (AWJ) is starting to cut into the surface, but as the jet cuts down into the surface, the angle of the cut will change. Yet even if the jet is pointing directly down into the target and moving along to cut through it, the cut surface is not usually a straight line down through the material.

Cut through one inch glass

Figure 6. Cutting through glass, note the curved path of the jet through the one-inch material.

Cuts into Plexiglas and other clear materials have allowed research scientists to monitor the cut path through the target as a function of time. It is not a constant shape, but, as Dr. Henning showed at the 18th International Conference, the edge of the cut changes with time. You can see the results of this in cuts that are made through metal where the paths of the cut, particularly lower in the cut, curve around and back towards the start of the cut.

Triangle depth cut

Figure 7. Cut into steel, with the face piece of metal removed to show the cut surface.

This path confirms an explanation first proposed by Dr Lars Ohlsson in his doctorate at Lulea in Sweden. He pointed out that the change in the surface of the cut is caused by the sequence of actions that a particle sees as it comes down onto the surface.

First it comes in almost vertically, with no lateral energy, and it cuts in the smooth, upper part of the cut. Then it rebounds out of the cut, but into the jet stream that gives it a little more energy and directs it along the cut to a second point where it will cut a little bit more of the metal. But during the first rebound the particle does not bounce perfectly along the cut but deviates to one side or the other. This means that when it makes the second cut, it will now cut more into one side of the wall or the other. Thus, where the second bounce occurs, the surface gets a little rougher.

jet bounce in cut

Figure 8. Frames from a high speed video showing abrasive waterjet cutting of glass, with the jet cutting, rebounding down the cut and then cutting again. (Lars Ohlsson “The Theory and Practice of Abrasive Water Jet Cutting”, Doctoral Thesis, Division of Materials Processing, Lulea University of Technology, 1995)

By the time of the third cut and rebound, the jet will now be coming into the opposing side of the cut with an even greater lateral portion of its energy, and so the cut will get a little rougher. Remember also that each cut is made up of the impacts of very many particles. So that succeeding particles also rebound along the curve cut by the preceding particle, and this also will exacerbate the roughness of the cut.

We’ll talk a little about reducing this effect in the next post.

Waterjetting – The Effect of Standoff Distance

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

One of the problems with relying on photographs is that they are sometimes not of the quality that one would wish. This has happened with today’s topic, where the pictures are old, smaller and in poorer condition than I had remembered. However, with your indulgence, I am going to step through them. I do apologize for their poor quality, however.

The topic is the way in which a waterjet first attacks a target. I have mentioned different parts of this process in the past. But in this post I want to show that it matters where the target is, relative to the nozzle, because the structure of the jet itself changes with that distance, which I call the standoff distance between the jet orifice and the initial target surface.

The break-up pattern of a waterjet

Figure 1. The break-up pattern of a waterjet (Yanaida K. “Flow Characteristics of Waterjets,” 2nd BHRA Conf. 1974, paper A2.)

As I mentioned last time, when the target is close to the nozzle, then the erosion pattern can, in the first few seconds of contact, be seen to be like a butterfly in pattern. The central part of the target under the jet is not eroded but there is severe erosion around the edges of the jet diameter, where a grain will see high differential pressures across its width and will be subject to high lateral jet flows.

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

Figure 2. Damage pattern around the impact point of a 10,000 psi pressure, 0.04 inch diameter jet on aluminum, target close to the nozzle

As the nozzle is moved away from the target surface, however, that pattern of erosion changes. As the jet structure picture shows, the central zone at the initial pressure reduces in radius, and there is an intermediate zone of rapidly diminishing pressure, with an outer shroud of fine droplets. The effect on the impacted target is that there continues to be a small zone with no erosion in the center, and that erosion is still concentrated around this zone, in that of high differential pressure, which now encroaches on that central sector.

Erosion of an aluminum target with the nozzle 2-inches above the surface

Figure 3. Erosion of an aluminum target with the nozzle 2-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice

That central small plateau is reduced to a very small point by the 3-inch standoff, which is where the jet reaches the end of the distance where the pressure remains constant over the central section. Thus, by a 4-inch standoff, the central section, though still present, is being eroded.

Erosion of an aluminum target with the nozzle 4-inches above the surface

Figure 4. Erosion of an aluminum target with the nozzle 4-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice

As the nozzle is moved further back from the surface, that central promontory disappears at around a six-inch standoff. It is interesting to note that at this point, the cavity is starting to get noticeably deeper.

Erosion of an aluminum target with the nozzle 6-inches above the surface

Figure 5. Erosion of an aluminum target with the nozzle 6-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice. (The lower of the two circular damage patterns was caused through experimental conditions and should be ignored). The presence of a central mound can barely be discerned.

By this time, the central section of the jet is beginning to break down into initially short strings that very rapidly break into droplets. The damage pattern that results shows a cavity that is slightly increasing both in diameter and depth.

Erosion of an aluminum target with the nozzle 8-inches above the surface

Figure 6. Erosion of an aluminum target with the nozzle 8-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice

By this time, the jet is continuing as a series of relatively large droplets, still holding a central structure, though surrounded by a rapidly decelerating cloud of mist.

Erosion of an aluminum target with the nozzle 10-inches above the surface

Figure 7. Erosion of an aluminum target with the nozzle 10-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice

It is one of the interesting oddities of the jet cutting business that the amount of material that is eroded from the target is a maximum at this distance.

However, and this was the subject of great debate back at the time that it was first presented, the ability to control the droplet size and condition as a function of distance and the reality that in most applications the target must be cut to depth meant that this has a very limited application. It can be used if the droplets are generated properly and used within the relatively narrow window that they exist to improve surface erosion of material.

However, as Mike Rochester found when he studied this, back at Cambridge in the early 1970’s, the presence of a layer of water on the surface, and as the hole deepens this is almost always there, rapidly diminishes the effect.

The effect of a layer of water in diminishing the “droplet impact” effect in erosion of a surface

Figure 8. The effect of a layer of water in diminishing the “droplet impact” effect in erosion of a surface. (After 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.)

There are ways of getting around this problem but the presence of water in the cavity that the jet has produced can also lead to problems, and these will be the topic of the next two posts.

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 Cutting: Folding Art!

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

While teaching Waterjet Technology courses at The University of Missouri-Science and Technology, each year I used to task every student in my classes to make a work of art as a final class project. Towards the end two new trends in that work were developing.

The first is that of “folding art,” by which I mean the ability to cut a complete shape in a thin sheet of steel, and then to fold it in such a way as to create a work of art. In the example below the initial cut and final “Missouri mule” are shown.

Missouri Mule - Sheet Metal (cutting shape)

Figure 1a. Missouri Mule – shape as cut. (Jason Zhao, 2009)

Missouri Mule - Sheet Metal

Figure 1b. Folded Missouri Mule (Jason Zhao, 2009)

The next step in this process is to treat the metal within the sculpture so that it reflects light in different ways. For a steel surface this “micro-etching” of the surface can be done at around 20,000 psi with plain water, and different light effects can be achieved with a three-tone picture created by running the jet once, twice or three times over the metal. (We have done this with bronze, aluminum and steel surfaces with good effect, as well as on polished samples of different rocks).

The surface indentation of the picture is quite insignificant, so that were this to be used more universally, for example to inset a bar code or small company logo, it would likely not affect the performance. However it can be used in combination with the “folding art” to texture different parts of, for example, our mule. On the other hand, one of my last students used a cleaning feature to the animal parts of the piece while leaving the rusted surface of the original metal for the fence, in making a country scene that begins to illustrate the potential that this new tool is likely going to be capable of in the future.

Of course the students we see have an engineering background.

As previously mentioned in last week’s blog, Vanessa Cutler, with her new book “New Technologies in Glass” (http://www.amazon.co.uk/New-Technologies-Glass-Vanessa-Cutler/dp/1408139545), works with world-class glass artists, so it will only be when we invite the art classes from local schools and colleges to participate that this new frontier of opportunity will begin to develop more fully. And it has the advantage (as Vanessa found with some of her art pieces) that, since it is all done under computer control, if something goes wrong with the original, it is not that difficult to make another one.

Rural Scene - Sheet Metal

Figure 2. Rural Scene (Kernan Shea, 2009)