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 – Determining Angles of water blasting

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

How times change! I was reading a column in the British Farmer’s Weekly, and came upon this, where the author is discussing the need for a generator:

It will also be vital to keep the fuel flowing into the tractors, and power the pressure washer, and light the security lights, and all the other essentials of an average arable farm.

It is an indication of how far the use of pressurized water has come that it is now seen, at the lower end of its application, as a vital farming tool. Which is a good introduction to talk a little further about the use of cleaning streams and how to interact with differing target materials.

There was an initial first step when someone would send the lab a mystery block of material and asked – how do I cut it? Generally, the samples were small but we would find a flat surface on the material and carefully point a jet nozzle perpendicular to this surface. (In the early stages this was hand-held). When a jet strikes a surface but can’t penetrate it, then it will flow out laterally around the impact point under the driving force of the following water.

The test began with the jet at low pressure, and this was slowly raised until the point was reached when the pressure was high enough to just start cutting into the material. At this point, the jet had made a small hole in the target, and so the water flowing into that hole had to get out of the way of the water following. The sides of the hole stop it flowing laterally, and so it now shoots back along the original jet path. This spray can hit the lance operator if the nozzle is hand-held, but it is a fairly graphic way of determining the threshold pressure at which the material starts to cut (and I’ll get into what happens as the pressure continues to go up in a future series of posts).

But for the purpose of cleaning, the jet has to move over the surface, once it has made that initial hole, at pressure. But, in many materials, if the jet comes vertically down onto the target, then only the material directly under the jet will be removed. And so the jet has to be played on every square inch of the surface in order to ensure that it is cleaned or that the coating/layer is removed. In some sandstones, for example, two jet paths could be laid down, almost touching one another, and yet the rib of material between them would remain standing.

Adjacent jet passes in sandstone

Figure 1. Adjacent jet passes in sandstone; the cuts are about an inch deep, but note that even though the narrowest rib is about 1/8th of an inch wide, it is only when the cuts touch that the intervening material is removed.

Yet that rib of material was, in that case, so weak that it was easy to break it off with a finger. (This turns out to be a weakness in making delicate sculptures out of rock). To use the full pressure of the water can be a waste of energy if the material is very thick since it all must be eroded with such a direct attack.

Yet the minimum amount of material that needs to be removed is that that attaches the layer to the underlying material (the substrate concrete, steel etc) and this can be quite thin. Thus, in attacking a softer material, particularly one that can be cut with a fan jet, a shallow angle directed at the edge of the substrate can be more effective.

Round vs. fan cleaning from Hughes

Figure 2. Round vs. fan cleaning from Hughes (2nd US Waterjet Conference)

Because there is a balance between cutting down through the material to be removed and cutting along the edge to grow the separation crack between the materials, some practice is needed to find, for a given condition, what that angle would be.

Choice of angle from Hughes

Figure 3. Choice of angle from Hughes (2nd Waterjet Conference)

The more brittle the material, then the greater the angle to the surface, since rather than just erode the material, the jet may also shatter the layer into fragments that extend beyond the cut path. But otherwise using an angled jet to the surface can be more effective. Hughes (from whose paper at the 2nd Waterjet Conference I took these illustrations) has a simple test for orifice choice.

How target response influences nozzle selection

Figure 4. How target response influences nozzle selection. (Hughes 2nd Waterjet Conference)

Some of the more advanced cutting heads use a series of nozzles that spin within an outer protective cover, as they remove anything from layers of damaged concrete to thin layers of paint from ship hulls. Increasingly, these are connected to vacuum systems that will draw away the spent water and debris from within the contained space, without it entering the work space, and creating problems for the worker.

In order to reduce any collateral damage to the surroundings, these jets are often made very small (thousandths of an inch in diameter) so that their range is short, and they are inclined outward to cut to the edges of the confining shield.

We have had some success in turning those angles the other way, so that they cut into the shield, rather than away from the center, and also so that each jet is directed towards the path of the next jet around the circumference. The intent in this case is to allow the use of a slightly larger jet, with a greater cutting range. In this case the individual cleaning/cutting path is a little larger, but because the jet at the end of the cut moves into the range of the adjacent jet, then any remaining energy that it and the dislodged debris still have will not be enough to get through this second jet.

Inclined jet and shroud design

Figure 5. Inclined jet and shroud design.

The action of each jet then becomes not only to cut into and remove material, but also to contain the spent material from the other jets dispersed around the cutting arm, and to hold the debris in the center of the confinement for the very short time needed for it to be caught up in the vacuum line.

In all cases the choice of pressure, nozzle size, and operational factors such as angle of attack, come down to the target materials, those that have to be removed, and those that need to be left undamaged. And it is why it is useful, at the start of any new job, to take the time to do a little testing first, to make sure that the right choices of nozzle and angle have been made to get the job done quickly and efficiently.

Incidentally, the idea behind the test of effective pressure, which the jet flows laterally when it hits something it can’t cut, can help, for example, in easing the meat from the bone when a jet cuts a deer leg.

Cut across a deer leg

Figure 6. Cut across a deer leg, note how the jet has cleaned off the meat from the bone, undercutting the flesh.

Waterjet Technology – Removing Graffiti with Waterjet Blasting

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

When I began this series, I mentioned that the target material plays an important part in deciding which pressure and flow rate is best for a particular task. Sometimes, time also has a role and not always in the way of “faster is better”. I mention this because we made a mistake once. (Well we only made this mistake once, didn’t mean we haven’t made other mistakes). Almost thirty years ago, we carved the granite blocks that make up the University of Missouri S&T Stonehenge, a half-scale Americanized version of the British megalith. The campus Americanized it when Dr. Joe Senne, the Civil Engineering professor who designed it, incorporated an analemma based on the calendar developed by the Anesazi in New Mexico. This replaced the 19-stone inner bluestone ring of the original.

Missouri University Science and Technology Stonehenge replicated using waterjet technology

Figure 1. Missouri University Science and Technology Stonehenge replicated using waterjet technology.

The MS&T Stonehenge was chosen as one of the ten Outstanding Engineering Achievements of 1984, by the National Society of Professional Engineers, in part because the 160-ton 53-stone structure was carved from Georgia granite by high-pressure waterjets without the use of abrasive. It has, over the years, generated a lot of interest (even getting me onto the Tonight Show with Jay Leno) but that has also included the odd local “artist” who has adorned it with graffiti.

My initial response when this first happened was to go over to the monument immediately with a high-pressure pump and start to wash the paint off. And that was the mistake for two reasons. Firstly the paint was not totally dry, and secondly we had not protected the stone with an invisible protective coating to seal it. Thus, when we tried to wash the paint away, while we removed all the surface paint – and to a casual observer it remains clean – we had driven a small fraction of the still liquid part of the paint into the pores and grain boundaries of the granite. Thus, if you know where to look, there is still a slight discoloration where that first writing was removed.

Shortly after that, on the advice of the Georgia Granite Association, the campus found a coating that was applied to the rock sealing the pores and grain boundaries, and future cleaning was made a lot easier and more effective. However, it did not completely solve the problem since future cleaning had to be done in such a way as to remove the spray paint while leaving the protective coating.

And that reminds me of a funny story. Graffiti is a significant urban problem, and it costs cities like Albuquerque in New Mexico about $1.3 million a year in clean-up costs. Much of incentive for almost immediate removal is because it is a way for street gangs to mark their territory, and this motivates police to urge an aggressive treatment policy.

But what happens if it is art? There are street artists who have chosen to decorate generally abandoned buildings for free in various ways and not related to any gang activity. Perhaps the most famous of these is Banksy, who was making it to the news when a piece he painted on a London wall appeared in a Miami auction house. It was anticipated to be worth around $600,000 before being withdrawn from the auction.

Bansky “wall art” estimated to be worth up to $600,000

Figure 2. Bansky “wall art” estimated to be worth up to $600,000. (Banksy)

At one time, Albuquerque had a similar idea of hiring those who were spraying the town walls to instead create works of art on some of the otherwise blank concrete surfaces such as bridge abutments around town.

Unfortunately, this led to an awkward situation. One of the local artists had painted some of his art on a fly-over. Shortly thereafter, the city sent a crew out to cover up the remaining graffiti with a coat of whitewash. Unfortunately, the crew were not artistically trained, and so covered up the new work of art.

For some years, I had a photograph of a waterjetting crew working on that site. At first glance, they were removing graffiti, but in reality they were taking the white coating from the painting to re-expose it to public view.

And this is one of the advantages that waterjets possess in that they can, with care and training of the personnel, be used to preferentially remove individual layers of material, whether of dirt or paint, without doing any damage to the material underneath, the substrate of the surface.

This is important, for example, in removing paint from buildings where the underlying substrate may be a relatively weak wood surface, where any high jet pressure would be enough to eat away the softer parts of the wood turning a smooth wooden sill into an etched and rough surface far from the desired result. Thus, in these circumstances, there is a need for very fine pressure control if the desired result is to be obtained.

And sometimes, the material that is to be removed is not that easy to remove with water power alone at an acceptable rate because the pressure has to be lowered to the point that it only removes the paint at a slow and uneconomic rate. At that point, it is possible to add a relatively soft abrasive (something like a baking soda) that will not only be effective in removing the material but is soft enough that it will do relatively little damage to the surface. At the same time, many of these softer abrasives are also soluble, which means that the costs of clean-up can also be reduced.

In some cases the water pressure need be little more than tap pressure.

Low pressure waterjet graffiti removal using soluble abrasive

Figure 3. Low pressure waterjet graffiti removal using soluble abrasive.

For more information on waterblasting and industrial cleaning, visit the Aqua-Dyne Waterblasting website.