Waterjetting 37e - Using Cavitation to disintegrate rock

In most mines the main objective is to recover as much of the valuable minerals contained within the host (or gangue rock) while minimizing cost. When miners have to go underground and haul the ore to the surface before the minerals are recovered then there is a considerable expense both in hauling all the rock, including the large otherwise valueless host rock, to the surface, and then crushing it to a small size so as to liberate and separate the valuable components.

The work at MS&T, for a number of years, has looked at ways in which rock can be disintegrated, as it is mined, so that the different components are separated as they are freed from the vein. While this work has progressed significantly since it started, this video (of poor quality for which I apologize, but it was what was available at the time) describes where we started the work.

Figure 1. Tom Fort explains the work on cavitation disintegration of rock

  The work was carried on in a number of ways after that, some of which has been described in an earlier post.

Perhaps most relevant to the video at the top of the piece, we were able to develop a more continuous mining process where the material would be mined from the solid in the mine, rather with small hand samples in the lab. While the technology could be easily developed from existing machines now used for hydro-demolition, a more telling picture is to show, by running the product from a test on a sample of dolomite hosting a vein of galena, where the product was run over a Wilfey table.

Figure 2. Mined sample run on a Wilfey table.

The result shows that clear fragmentation of the galena particles and their liberation so that they form a separate (silver) stream on the table from the darker dolomite particles that lie closer to the riffles. It is not quite as easy to see the larger particles of galena which were also separated, but would be more easily recovered perhaps with a screen, since they are not quite as easily streamed from the dolomite.

Waterjetting 37d - Underground Drilling with Waterjets

The last post showed some of the experiments that we carried out as we developed a tool for drilling "around corners", demonstrating the ability of a high-pressure water jetting system to turn from a vertical well bore and drill out a lateral well within a turning radius of about 9 inches. This was not our earliest work on drilling, and that earlier effort had been directed to the reaming of geothermal wells.

The earlier project was also funded initially by Sandia Labs, although in this case the objective was slightly different. Overall we set out to show that one could go down a vertical well bore, some 9-inches in diameter, and ream a cavity 6-feet across down in the "hot dry rock" of a geothermal deposit.

When we began we weren't sure that we could drill in stressed rock, or what those effects on drilling rate would be. So we were fortunate to get some help from Doe Run and work in one of their mines, as the following video shows.

Figure 1. Early video on Underground Waterjet Drilling

  It was this series of tests that showed how relatively quiet water jets could be in drilling, unlike the competition at the time. Below the fold I will add the second part of this program, where we demonstrated the technique we planned on using for the reaming program.

Figure 2. Reaming a Geothermal Well Demonstration

Although this program moved on to focus on other aspects of the problem (we also demonstrated that this was practical in cutting granite, which led on to the Stonehenge and Millennium Arch projects) it was not until we were tasked with reaming large scale rocket motors, some years later, that we returned to this program, and developed the tools further.

Waterjetting 37c - A Drilling Diversion

I was asked some questions about waterjet drilling of holes the other day, and it is amusing to remember where it all started, more than 30-years ago. My apologies again for the quality of the video, which has been transferred from 16 mm film, to half-inch tape and thence via a DVD to its current form, losing a little in each transition.

Figure 1. The Cygnet Project part 1.

Perhaps one of the more memorable parts of this was that we arrived on site on the Monday, and spent the afternoon setting up. Since there really wasn't that much to the rig we were done quite quickly, and were then faced with an hour before the end of the day. The overall object was to drill a hole 50-ft long, and we had arranged the camera crew to come for the filming on Thursday, so that we could work out all the snags first. But we had a pleasant surprise.

We put our first test nozzle on the lance, and started drilling - it drilled the full length with no problems, we added a second length - same result, and third . . . and then the fourth and within the hour we had achieved the goal for the week. (And run out of drill lengths).

We had the same sort of experience some years later in trials I helped with that were run by the University of Queensland in Australia, although this time we were self-propelling the drill head, so that when we ran out of the outer rigid frame we attached a length of hose, and it kept on drilling, until we had run through that also. Problem was that the drill was then pulling itself forward without any advance rate control, and if it went too fast it did not drill a large enough hole for some of the following structure (we had backward pointing jets for propulsion and hole cleaning).

So somewhere I have a photo of three of us holding the hose back to slow the advance as the drill moved forward, so that it would maintain the 15-cm diameter IIRC. And again we accomplished the goal well before we had expected, and without the need for a lot of sophistication in the design.

Waterjetting 37b - How safe is it?

We began looking into the use of water jets to deal with energetic materials (a group that includes, but is not limited to, explosives) at the beginning of the 1980's. One of the earliest questions dealt with the need to find the jet pressures at which the explosive materials would react to the impact of the water.

We produced a short video of that initial work, and with apologies for the degradation that occurs with time and the transfer of the material through different media, here is what I believe was the second report we made, after we had been told that just showing that jets could remove material without reaction was not sufficient.

Figure 1. Looking for the initiation of explosive.

Mrs. Vicki Snelson did the initial commentary with Dr. Paul Worsey developing the technology that we used to see how the explosive was reacting.

Part of the problem, we discovered, was the the jet could start the explosive burning (deflagration) without causing it to detonate, but that the succeeding flow of water would wash that burning material away from the surface. Looking at the surface afterwards did not, as a result, show any reaction signs, and we could not always fully see the contact area. As a result the design was changed, so that the explosive could be held in a small chamber flooded with inert gas. The driving system was changed, so that only water entered the chamber, and so we could determine, by examining the chemical composition of the gases in the chamber after the test, if any reaction had, in fact taken place. Paul explains the modifications.

Figure 2. The first modifications to the test equipment.

Waterjetting 37a - Removing Explosives

One of the major efforts carried out at the High Pressure Waterjet Lab at Missouri S&T during my tenure related to removing explosives and other energetic materials from different casings. The casings ranged in size from very small anti-personnel mines to the large rocket motors that carry objects into space. The first part of that program dealt with the factors relating to the removal of explosives from warheads. Initially we had to show that this was relatively safe, and we initially thought that we were done when we showed that the explosive did not react as we went up to and beyond the pressures needed to wash the explosives from the case.

"Oh, no!" we were told, "That is not enough, we need to know how safe it is!" Which meant we had to determine, for each explosive we were likely to find, the waterjet pressure at which it reacted. (i.e. went BANG!) This meant that we had to fire water jets at pressures that ultimately reached 10 million psi (at which point all the explosives tested got seriously annoyed) to cover the field of response.

Once that safety level had been established then we could determine the pressures required to wash out the munitions, and from that design a tool to carry out the washout. We called the first one, WOMBAT, and here is a short video showing it in being used to clean out a SPARROW warhead.

Figure 1. Description of a Washout

The program examined a number of different approaches for different munitions, and we worked with Wilkes University in PA to build a full scale washout facility for shells, that was installed at the Navy facility in Crane, IN - but I'll show that, and movies of the development of a capability to cut into sealed munitions, in later posts.

Waterjetting 36d - Going through more complex walls.

This is a short post illustrating an early stage in our development of a light-weight tool that could be carried into building rubble, after a collapse, and drill down through it to provide access for tools to search, without the danders of destabilizing the pile.

We illustrated the lack of vibration by placing a full glass of water on top of the first concrete block to be drilled. As Mr. Dorle, the graduate student who built the equipment explains, the tool cut through all material it is likely to encounter, while using a pump available at the local hardware store, and in a total envelope that fits into the back of a pickup.

Figure 1. Mr. Dorle describing his design for a waterjet cutting tool.

in later work the tool was made even simpler, since the rotary mechanism was the heaviest and most difficult to move with the lance it proved possible to remove it, and I'll post a video of one of those demonstrations shortly.

Waterjetting 36c - Cutting walls

This is just a short video from back in the days when 1/2-inch tapes were still our way of recording, but before we reached the higher quality resolution of today.

We had a problem in that the basement of our building was partially covered with dirt, and concrete window wells held that back from the windows used to light light into the rooms. It would have been prohibitively expensive for us to pay to remove the concrete conventionally, but it turned out that with the construction of a set of simple tools (there were no really good high-pressure swivels available at the time - hence the orbiting action of the nozzle as it moved over the slot) we were able to cut the walls relatively simply and quickly.

Figure 1. Cutting the window well protecting the wall. (Over 30 years ago)

In this first video segment I mis-spoke when I spoke of the nozzle as rotating, it was actually being moved over the wall in an oval pattern, as will be more evident in the second segment (below the fold).

As noted in the video the support platform for the rig is a simple platform shop lifter and the rig is held on the platform with a couple of G-clamps. There is relatively little reaction force and so the rig can be made very simply out of available tools. The lesson we learned very early on was that the rebounding water carried the removed cement and aggregate particles, so that PPE was important, and keeping folk back even more so.

After the window wells were removed we had to cut an entrance through the main wall of the building - about 14-inches thick.

Figure 2. Cutting the main wall behind the window well.

Note that it was not necessary to remove the glass in the window until after the walls had been cut, and it was time to remove both window and wall.

Waterjetting 36b - Katrina anniversary and the power of water

When I began to write these posts, I wrote about the difference between overall jet force and the focussed effects of a very high-pressure but small diameter jet. At the time I made reference to the relatively low pressure, but huge flow rate effects when Katrina hit New Orleans.

I specifically wrote about the damage to the Lower Ninth Ward when the levee collapsed and a wall of water, carrying a barge, suddenly flooded into the neighborhood. Though there was little water pressure, perhaps 20 or 30 psi, the volume of flow and the fact that it applied that pressure over the full face of a building, resulted in sufficient force to destroy the buildings and leave only the concrete slab, if that.

I was part of a survey group that went through the area after the Hurricane and made several hours of video which I summarized in a short review. On the occasion of the anniversary it is, perhaps appropriate to put this up. This comes with the sound, as I recorded it at the time. It begins in the Lower Ninth Ward, and then goes back to the start of the trip.

After visiting one of the smaller breaches in the city, we drove down the Delta to the point where the boats went out to service the rigs, and where the hurricane had come ashore as a more powerful force. Yet it was the water damage that was prevalent, as can be seen from those buildings that had survived because they were on stilts and thus out of the storm water.

We then returned to the Lower Ninth Ward, mainly near the breach. This is where the barge went through the breach, obviously at some speed, since it was carried beyond the main channel of the flow and then dropped onto the school bus as the flood waters passed further into the ward.

As I mentioned there are hundreds of photos and hours of video that we shot, but this gives some sense of the disaster.

Waterjetting 36a - Jet stripping of tires

Stripping tires to recover the rubber has been one of the topics that has come up on a number of occasions, and through a number of different tests we have been able to demonstrate that water jets can remove the material in a range of sizes depending on the desired output.

The work has ranged from cutting steel-corded truck tires, for subsequent rubber stripping, through to the simplest removal of rubber to separate the cords and to then separate the rubber from the water and use that as a subsequent feed stock for a variety of uses. The control on the marketability of the product comes through a guarantee of the size range of the product, and while this can be achieved in part through secondary processing, it is an advantage if the rubber can be controlled in a certain fine-particle size range. Which is why, in this movie, you will see the nozzle assembly rotate as such a high speed.

Figure 1. Short video of a jet stripping rubber from a tire.

As the video notes, by changing the jet pressure between 10,000 and 13,000 psi one can control whether one is just removing the rubber, also removing the fiber cords, or going all the way through the tire and also removing the rubber material behind the cord.

The economics of the operation also involve the tire size, the speed of the operation, which is controlled by the relative rotation speed of the tire, relative to that of the jets, and the resulting product size of the crumb rubber that is removed - as well as its cleanliness. Those are subjects that are much more job specific than we need to get into at this stage, since the object of the video was rather just to show that, without a huge investment in equipment, it is possible to recover the rubber.

Waterjetting 35e - A low cost version of the soil sucker

Posting will run just a little slow for a few weeks, as I run-through and catalogue the some 200 hours of video that I have amassed over the decades showing our waterjet research. There are a number of different review reports that I will insert over the next few weeks, as I find the good copies and then convert them from the earlier form into a digitized version. But I ask for your patience as I do this, since the conversion has to be done in real time.

To end the current thought over the removal of material through the addition of vacuum to a waterjet soil dislodging system, here is a short segment that shows us uncovering some inert mines. As it was part of a Humanitarian Demining Effort we had to make the system as inexpensive and simple as possible.

The pump therefore is one that you can buy at the local hardware store - operating at about 2 gpm and 1,000 psi and we have used a simple Shop Vac to provide the suction. The rest of the parts also came from the store, apart from the very small fractional hp electric motors that we used to turn the head and traverse the head over the ground. The excavation unit was, as you can see, mounted on a camera tripod. The idea being that if the mine reacted then the part destroyed would be so simple and inexpensive that it could be replaced within a few minutes for relatively little money.

Video of a small unit being used to uncover inert mines and other objects.
 

Mines are relatively sensitive things and so we had to be sure that the very low thrusts that the jet would exert in disaggregating the soil would not set the mine off. This required a combination of pressure, jet size and rotation speed considerations. The effectiveness of the combination selected is shown by the pebble sticking out of the side of the excavation which is not disturbed, although with only a little finger pressure it could be lifted out of the side wall.

We would then cut the mine apart, through the fuze, using an abrasive laden jet and in another post I will show the pictures of how we cut detonators apart and other sensitive explosive-laden materials, without their reaction, as well as showing pictures of what happens when we got a waterjet to ignite explosive.

Waterjetting 35d - More video on hydro-excavation

In the evolution of the design of a waterjet/suction tool described in the last post I commented on the ability to balance the jets so that they did not spray material beyond the suction shroud. At the same time the shroud, to be most effective, has to be within a quarter–of-an-inch of the final surface, which means that the jets have to cut clearance for the head as it moves. Bearing in mind that the head will be manipulated around the excavation, this means that clearance has to be maintained on all sides.

Figure 1. Pass of a cleaning head over a 2-inch sand layer sitting on a set of concrete blocks that are not confined. The video shows the removal of the sand, without water escape.

I apologize for the quality of the tape, but these were research records that we were making of the experiments, merely to get certain data from them and they were not intended for transmission when made.

The second point I wanted to include was that of the ability to use the same design to cut a trench in harder material, again without the spreading of water beyond the trench. The material is a relatively weak cement.

Figure 2. Four passes over a weak cement to show that all the material removed can be aspirated at the time of excavation.

The tapes show how one can cut trenches in either soil or light rock fairly quickly and without making much disturbance outside the slot. Obviously the material removed can be collected in a vacuum truck and poured back into the trench after the trench work is complete.

In a later post I will show how this can also be used as part of a tool we developed to find, expose and then neutralize landmines.

Waterjetting 35c - Developing a waste removal shroud - video.

The short videotapes in this segment show the evolution of a combination of a waterjet and a suction line as a way of easily removing soil or sand relatively quickly. It is a subject covered in an earlier post. These video clips show some of the tests that helped us to develop that design.

As mentioned in that earlier post the central tube connects to a vacuum line which removes the loosened debris and water. An earlier series of tests had shown that the suction nozzle had to be within quarter of an inch of the surface for the suction to be most effective. The jets had, therefore, to clear a way for this nozzle by cutting down through the material and pushing it into the mouth of the tube, before the tube arrived.

For the first test a single nozzle (out of the three on the head) was used at relatively low pressure.

Figure 1. Clip showing a single jet cleaning through 2-inches of sand.

However if the jet pressure is raised to cut harder material, then the jet has enough power to wash the material under and past the suction tube so that only a small part of the solid is picked up and the path fills back up with the washed sand.

Figure 2. A higher-pressure cutting jet does not give the debris time to be sucked out of the tank.

If three jets are used, but with the jets directed so that the paths hit each other within the suction zone this stopping each jet going further for a long enough time that the suction can remove both water and debris. 

 Figure 3. A three-jet combination where the jets are held within the shroud, leaving a clean path.

For those unable to see the video the configuration of the jets meant that they met under the shroud as shown.

Figure 4 The jet configuration around the shroud.

When this is combined with a protective (flexible) outer shroud the final result was a tool that removes material without over-spraying into the surrounding sand and destabilizing it. Leaving a clean channel.

Figure 5. Larger head design removing a 2-inch thick layer of sand.

In a subsequent post I will include (when I can find it among the 200-odd hours of material) a video of a similar (though smaller) tool cutting a clean channel into a soft cement, and leaving a clean path behind it, as shown in the earlier post. For those interested the parts for these cleaning heads were assembled from plumbing supplies from our local hardware store at a cost, per head of around a hundred dollars or so. (back in 1995 when we ran the tests).

Waterjetting 35b - Cutting the Missouri Stonehenge video

The video that I posted last time did not fare as well as had been hoped, in making the trip from my computer to the blogger post, and so this week, to see if there are other ways of peeling the apple, I have also posted a copy of the video to Youtube, to see if this works better.

The video is of the making of the Missouri Stonehenge for which, as I have mentioned in a previous post, we used a jet pressure of around 15,000 psi with a flow rate of 10 gpm.

The video makes the point that a high-pressure jet system can, with relatively little support, cut a straight edge down the side of a block, even if there is only a very thin layer of rock to remove. It is normally very difficult to do this with a conventional cutting saw, or similar tool, which requires more material on the free side to stop the blade from being deflected away from the cut line.

If this new posting works as I hope, then I will be posting a number of different videos that have been collected over the years, but on Youtube initially, though I will provide the link as I have above.

Waterjetting 35a - an overview on video

This post is by way of an introduction to an occasional new feature of the site, where I will videos to different posts to help with understanding. Adding videos is not in my skill set, so this first is just a general overview of some of the ways in which waterjets have evolved over the years. I have tried adding one before, but this is the start of a little more of a concerted effort to use this medium. The overall video runs some 15 min 39 seconds.

 It is a compilation of different tapes made over the years, some of which are now otherwise unavailable. I’m going to let the overall video speak for itself with this first piece, but will then come back in subsequent posts to discuss some of the parts of the video. I hope you find it interesting.

As I mentioned I will break this into segments and discuss those in more detail later.

Waterjetting 34e - Hole completions and core removal

When I began writing about hole cutting and drilling, a month ago, I was intending to talk just about the relative efficiencies of cutting the core into larger pieces, rather than designing a cutting pattern that would completely cover the surface of the excavation, milling and removing the core in fine particles. Other topics intruded, however, and it is only now that I am going to conclude this theme by discussing that particular point.

Earlier posts have discussed how, by inclining two jet paths so that they come into close proximity within the target (or intersect in some cases) a much larger volume can be removed with no increase in input energy to the process. The gain in production comes from working out which are the best angles to set the cutting jets at, relative to the overall work piece.

Figure 1. Intersection of two jets in cutting clay. The cuts were made with the sample lying horizontally – in the actual operation the jets cut up and down vertically and the included wedge would normally fall out.

There is no universal rule for selecting the best angle for this, or for selecting the best relative depth for the intersection. It depends on the material being removed, and on the logistics and relative sizes of the hole being driven, and the components that fit into it. The material responses depend very much on the strength and structure of the material. The paths of both jets will have to intersect to remove materials such as steel, whereas with clays, and many rocks the two paths need only be relatively close at their lower end for the intervening rib to separate.

The problems are not just constrained to the removal of that core. One of the significant problems that exists even when using conventional techniques to drive tunnels and other large holes is that of making sure that the diameter that is being cut remains the same size as the tunnel advances. Because it is easier to break the rock within the tunnel wall, because of the release of the surrounding rock pressure, it requires additional effort to cut out beyond the projected perimeter in order to give enough space for the tunnel. With explosive blasting of the tunnel this means that the perimeter holes are drilled out beyond the projected tunnel line, and in cutting with a waterjet a similar strategy is required.

This problem is not normally that severe, since the size of the nozzles and support equipment are not that much larger than the jet that does the cutting, but, when the jet is cutting at the edge of the excavation the jet will need to be inclined outward by an angle of somewhere around 20 - 25 degrees in order to cut clearance.

An additional problem arises with the need to break the pieces being removed from the solid into small enough fragments so that these can be moved out of the way and into a transport line, so that the cutting head can continue to advance. In the case of the Soil Saw (for which Figure 1 showed one of the earlier test cuts) the nature of the clay was such that, once it was broken from the solid it disintegrated relatively easily and could be moved. Had this not been the case the cutting tool could not have passed without cutting the piece into smaller chunks. And in this regard, once a piece has been broken from the solid and is floating in a suspension within the cut, it becomes much more difficult to cut, since it can be deflected away from the jet before the full force of the jet can cut into it.

Further most materials are not as friable as clay – particularly those that are manufactured , and the pattern of cuts has thus to be designed so that the fragments are positively cut to the right size, to make sure they fit through the various gaps and feeds. For most of our work this meant that the pieces should be smaller than walnuts, and usually of around half-an-inch in size.

Ensuring that these cuts intersect in materials of varying properties will often require that the jets be designed to overcut in more favorable conditions, which wastes considerable energy. The alternative is to use the jets to cut relieving slots into the target, but to ensure that all the material is removed to the required depth on a pass by also including a mechanical component to the cut surface.

A cutting head designed by Rogaland Research shows the type of design required to achieve this, illustrating the angles of the two jets that will cut into the target as the head moves around the hole. In this case the design is to fit into a large diameter drill pipe to create a larger overall hole size.

Figure 2. The cutting head design developed by Rogaland Research (Vestavik, O.M., Abrasive Water-Jet Drilling Experiments, Progress Report, Rogaland Research, Stavanger, Norway, May, 1991.)

In this case the ribs of rock that are isolated by the jet cuts are removed by the action of the mechanical cutters in the second part of the bit.

Figure 3. Location of the jetted slots in the face of the drill-hole using the Rogaland tool. (After Vestavik)

In many cases the combination of a waterjet action to provide a free surface for the material to break to, and to relieve some of the confining stress on the material within the hole can significantly lower the mechanical forces required to break out the material. In such cases it makes much more sense to combine a waterjet action with that of a second removal device (which can be mechanical or thermal in some cases) to obtain a much more efficient combined system than that which would otherwise be the case. Where such systems have been used they have been shown to be more efficient in a number of cases than either of the component systems alone.

Unfortunately combining two systems to achieve optimal performance is not as easy as just merging the two sets of components, since there are additional benefits that come where the combination is further optimized so that the two parts work synergistically together. (One of the factors not included in the Rogaland design). I have written of this in the past, and will again soon.

Waterjetting 34d - Drilling small holes through steel and concrete

In the earlier posts in this section I have concentrated more on cutting the profiles of a hole, and the different ways in which this can be done efficiently. However there are many cases where the hole has to travel into the work piece to a depth greater than can be easily achieved without the nozzle entering the hole, or where the hole is being drilled into something with no realistic “other side.”

In these cases, choice of the best strategies to remove the central core of material from the hole varies, depending in part on how large the hole is, and in what sort of material. For relatively small diameter holes, then the jet itself can drill a hole large enough for the head to enter the target. (In part this is because the diameter of the nozzle and supporting pipe can either be made quite small, or the jet can be made more diffuse.) In other cases the cutting head has to be designed to remove that central volume in a way that allows the fragments to pass the nozzle assembly as it feeds down the hole.

To illustrate the first approach consider some work that took place at the University of Alabama-Huntsville. The tool that was needed had to drill down through the reinforced concrete of bridge abutments and foundations to give access to tools that would evaluate their condition, and that of the surrounding ground. However putting large holes in such structures is, in itself, a potentially weakening process and can provide a path for subsequent corrosion. The tool developed therefore was reduced in size to around 1/4th inch in diameter, capable of drilling holes of around 3/8th of an inch in size through the reinforced concrete, using a Direct Injection of Abrasion or suspension jet, operated at 5,000 psi.

Figure 1. Portable abrasive suspension injection system. Operates at 5,000 psi (pump is on the left of the platform). This unit fits in the bed of a pickup truck and cost less than $10,000 for the MS&T HPWL lab to build. The device built in Alabama cost roughly $14,000). The tank at the front mixes the abrasive with polymer and water before feeding it into the two supply tanks that alternately supply the feed line to the nozzle. The polymer is used to keep the abrasive in suspension and not to settle back out of the suspension.

The University team coined the name Multi-Intrusive Testing, (MIT) for the method and showed that it was capable of driving holes more than 3-ft deep, and of the required small size. In this case the use of a suspension jet, where the abrasive is added to the high-pressure water upstream of the nozzle assembly means that there is only a single feed line going down-hole. This is one of the features that allows the operational size of the tool to be reduced.

As existing infrastructure (bridges, roads etc) exists through the changes of typical yearly weather cycles the concrete and surrounding materials will slowly deteriorate, and erosion over time can remove the support for a bridge, corrode the internal components and wear away critical parts of the overall assembly. The, fortunately relatively infrequent, collapse of major bridges shows the risks of leaving this damage without repair, yet the very massive nature of many of the structures makes it difficult to detect this damage before it reaches critical size. Hence the need for the tool.

Laboratory tests showed that the rig could drill holes up to 3-ft deep, with the drill, which was rotated at speeds between 60 and 120 rpm, being capable of drilling through not only the concrete, but also the rebar embedded within it.

Figure 2. Hole drilled through reinforced concrete showing the ability to penetrate the steel reinforcing within the concrete. (after Graettinger et al ibid).

Within the geometry of the very small holes of this type, which are required to minimize damage to the structure, it is difficult to offset the nozzle to a sufficient angle that the hole is large enough for the nozzle to pass. In most cases this needs to be at an angle greater than 12 degrees, since shallower angles tend to have the abrasive and jet rebound into the center of the core without fully cutting into the wall to a depth to ensure that the hole diameter is maintained.

One way of overcoming this problem is to force the jet to diverge as it leaves the nozzle, since the induced spreading (typically 15 degrees or more) not only ensures that all the material ahead of the jet is removed, but also allows the wall diameter to be maintained.

The problem with the design, as with many similar drilling applications for waterjet and abrasive waterjet systems, is that the jet cuts to the required diameter at some distance ahead of the nozzle body itself. This can be illustrated with a picture of a diffused waterjet nozzle, that had been used, at MS&T, to drill through a sheet of steel (representing a borehole cased section), a layer of concrete (which would act as the sealing element in wells drilled to recover oil and natural gas for example) and then out into the sandstone rock beyond.

Figure 3. Dispersed abrasive jet (not rotated) used to drill through the simulated borehole wall. Note the two black lines that define the edges of the hole as it cuts into the steel plate.

Although the hole diameter was acceptable for passage of the tool once the steel was penetrated, as with the rebar in the earlier example, the hole diameter through the steel was too small to allow the head to progress. The answer, as has been discussed in earlier posts, is to advance the shroud (the outer cover over the nozzle) forward until it touches the edge of the cutting jet diameter at the required size. At this point, should the jet not initially cut the hole to the required size, then the drill will stop advancing, but the jet will be in contact with the obstructing material – and in a short interval will penetrate through it allowing the jet to drill further forward.

Difffusing the jet and removing the material will work for hole diameters up to about four inches, and we have used such a technique to drill through gravel and similar beds of loose material (with care, since while the jets have little disturbing force on the material around the hole much vibration can cause that material to destabilize and continually collapse into the hole being drilled – which ends up defeating the object of the exercise). I’ll talk about approaches for larger holes next time.

Waterjetting 34c - Holes, pressure and delamination

If you ever go to an Old-Time Miners celebration, you may watch a group of competitors drilling holes through rock by hand with a cold chisel and a hammer. (You can see an example here). In the competition the contestant has 5 minutes to drill either a ¾” or 1-inch diameter hole as deep as possible typically using a 4-lb hammer. The best results will reach around 8-inches deep in that time.

It was the way that miners, and others, have driven holes into rock for millennia, but the skill that gives the highest penetration rate isn’t based on the person with the largest strength and fastest striker arm. No, rather it is the driller that controls the twist of the chisel correctly between successive blows, turning it just enough that the rock between the new strike and the old is chipped off by the impact.

By indexing the drill around the hole (the distance varies a little with rock type) the volume of rock removed by crushing under the chisel impact is magnified several-fold by the chip that is broken off to the side.

Figure 1. Relative volume of rock crushed, and chipped by lateral wedging to the next cut over.

Obviously the chipping makes much better use of the energy than would be the case if the driller just tried to completely crush all the rock using the chisel. However the chisel has to crush some of the rock in order to penetrate below the surface and get a better purchase for the chipping to be effective.

This makes sense in many other cases as well. And in order to make the best use of a cutting or drilling tool you need to understand how it works, how the target material responds – and how these two factors can be combined to give the best performance.

However, the use of a waterjet cutting tool brings a little extra to the table, since as the jet cuts down into the material, it will not, in the first few milliseconds of penetration, put any great lateral pressure on the sides of the hole, but will only focus on removing material in front and to an extent to the immediate side of the jet path.

The change and growth of the lateral pressure in the walls around the hole, and the widening of the bottom of the cut, occurs as it becomes more difficult for the spent water to escape from the cutting region, and the increasing turbulence of the water at the bottom of the cut starts to eat into the walls of the slot.

Figure 2. Widening of a slot at the bottom as the pressure distribution at the bottom of the cut changes. (Cuts were made at different pressures and AFR into granite, at a constant traverse speed) The view is of the end of the block showing the lengths of the cuts made down into the black as the nozzle traversed on the top of the block and towards the camera.

This build up of pressure at the bottom of the cut can become a problem. As the resistance to the water flowing away increases, so the water can penetrate into any larger cracks, or layers in the material, and apply that higher pressure to the plane of weakness. This can, in turn, lead to delamination of the part, or in some rock types it can cause some severe spalling around the impact hole, which may not be the intended result. (Or the sample may split.)

Figure 3. Spalling around an impact point as a jet penetrated into a block of rock.

The way to minimize this build-up is to make sure that the parameters of cutting (the traverse speed and pressure particularly) are chosen so that this does not occur (lower pressure, faster speed). Where this choice of parameters means that the jet won’t cut all the way through the part on a single pass, then it is usually better to plan on making a series of passes along the cutting path, keeping a relatively smooth wall to the cut, and reducing the chances of getting delamination.

This also holds true when cutting glass, although one has also to consider the size of the abrasive in this case since that will control the size of the cracks that are made in the sides of the cut, and the smaller these are, then the higher the pressure required before they will grow.

Figure 4. The effect of particle size on the crack lengths generated on the sides of a cut into glass. (The cuts were made from left to right with particles of SS-70 (0.0117 in diameter); SS-230 (0.0278 in diameter); SS-110 (0.0139 in diameter). (Shotpeener gives size ranges)

As a result in borderline cases it may be helpful to use a finer mesh abrasive to reduce crack size on the interface, where there is a chance of pressure buildup in the bottom of the cut.

Incidentally modern machines allow considerable precision in making multiple cuts – so that repeated passes can be achieved with relatively consistent precision. Perhaps I can illustrate this with a slightly out-of-focus picture of the insert cut from a counter-sunk hole using two passes of a jet in comparison with the pile of chips that resulted from the conventional removal.

Figure 5. Single piece insert removed from a counter-sunk hole cut with a chamfered edge, and removed as a single piece, in contrast with conventional chips.

However there are occasions where the ability to use the down-hole pressure to penetrate and break out the central core of material can be an advantage. One such occurs in mining applications where the rock is held under confinement. Where the jet first cuts a slot around the outside perimeter of the hole, this relieves the ground stress on the material in the core of the hole. That expands a little, opening the cracks in its structure. (In some cases, where the ground stress is high, this stress relief alone is sufficient to either break the core material into disks or to pulverize it into small pieces, but in these cases the ground is often sufficiently close to breaking already that most sensible folk would not be there).

I will return to talk about the break-up of such cores next time.

Waterjetting 34b - Cutting a hole.

There are several different aspects to be considered when planning a job entailing hole cutting, the accuracy needed for the hole(s) to be cut, both in shape and alignment, the quality of the wall and the speed of the operation. Not all are important in each case. But they are combined through the amount of energy and abrasive that they use into an overall cost of production.

As energy costs continue to increase, it is realistic to look at hole cutting in a little more detail. The smallest holes are usually those that are pierced by a single action of the jet. In other words the jet is brought up to a piercing pressure, and then exposed to the target for enough time to cut through, and the jet is then shut off. If the system is kept at pressure then this can be a fairly rapid way of cutting a large number of holes in a patterned manner through a piece, and I have seen examples where several hundred holes have been precisely located next to one another in making a precision part in this manner.

Figure 1. An array of 33 x 33 holes drilled by a 58 micron jet through glass (courtesy of Don Miller)

One advantage of a waterjet carrying abrasive is that it is not restricted to drilling vertical holes, and in one application the nozzle was inclined to the work-piece so that the holes were each precisely drilled at a shallow angle through the plate.

Where more precision is required a smaller jet and finer abrasive can be used to cut around the profile of a desired hole.

Figure 2. Perimeter cut to make a hole in a glass slide – as a reference scale, the coin is a penny.

Precision cutting of holes like this is not quite as easy as it may appear, and the above picture hides one of the problems, since the cut comes in from outside the hole itself.

In the more general case the hole is started and pierces through within the scrap material that will be cut from the part, and the jet then cuts into the hole profile, and follows it around, before exiting back into the center, so as to leave a smooth wall.

Figure 3. Illustrative path for a jet to cut the perimeter of a hole in the target.

The cut should come into, and leave the desired circle very close to tangent to the line, in order to sustain a smooth profile around the cut and give the precision required. With the proper programming of the path, it is not that difficult to cut holes of varying diameter through, for example, half-inch thick titanium.

Figure 4. Holes precisely cut through a half-inch titanium plate.

However, when cutting such holes it should be remembered that the jet path through the metal, particularly as it gets thicker, is not totally vertical. Thus, at the bottom of the hole, it is possible to get a small dimple at the location (which I have exaggerated in Figure 3, to make this point) that the jet enters and leaves the hole profile.

Looking at the underside of a plate, cut with similar parameters to those in Figure 4, one can see where, for different hole diameters, the cutting parameters were not adjusted properly, and such a dimple was left.

Figure 5. Detail from the bottom of a half-inch thick piece of titanium, with holes cut as for Figure 4. Note the small dimples left on the profile of the hole, where the parameters were not properly adjusted.

This dimple can be a considerable problem if, for example, the holes are then used to hold rivets that will be slid into the holes, but which will catch and be held if the dimple exists. In high precision parts the dimple size may not have to be that big for the piece to be out of compliance.

Unfortunately, as with many such problems, the best parameters to ensure that this is not a problem are specific to the job that requires the holes, in regard to material, thickness, hole size etc. However we have been able to hold required tolerance on such holes without a great deal of testing for the titanium pieces shown in the figures – the dimples were formed early in our program.

Small holes that are through pierced in relatively thin material allow a waterjet to practically cut around the central core of material, so that it can be recovered in a single piece, and in certain sizes that will allow the recovered stock to be used for a different part. Certainly the recovered material can be reclaimed at less cost than the scrap swarf that is the consequence of a conventional milling of the holes using a mechanical tool, and the edges of the hole have not been exposed to the heat that would pass into the part, were a conventional mechanical bit used.

This lack of heat and the sensible elimination of the Heat Affected Zone (HAZ) around the created opening has an additional benefit. With the lack of overall force which is also missing when abrasive-laden waterjets are used, support ribs can be cut to very thin dimension without distortion, and holes cut into islands left within the part, again without distortion, as this piece of titanium illustrates.

Figure 6. Four circular holes cut into a piece of titanium to show how thin the ribs can be cut, and that there is no distortion when the final, fourth hole is cut through the intersection island left from the first three cuts.

The combination of abrasive and an ultra-high speed waterjet has thus found a market (albeit one that has still many opportunities yet to exploit) where the ability to cut a thin slot around a shape creates the required geometry in the part, without heat distortion, and without the use of additional energy to grind up the unwanted material in the piece of material that is scrap to the current need, but which can be of benefit in future use. To date I have been discussing the cutting of small holes, but consider the case when the hole leaves, as re-useable material, the piece of Hastalloy shown in Figure 7.

Figure 7. Piece of Hastalloy removed from the core of a hole cut to generate a required cylinder.

Waterjetting 34a - Drilling holes with water jets

Looking back over the Waterjet Index I realized that while I have addressed different aspects of drilling holes with waterjets in various ways, I haven’t really brought it all together as a focused topic. So, herewith, and in the next few posts, that coalescence. It begins with a bit of a recap.

It was clear, early in the work on waterjet applications, that one of the key problems to be addressed was that of the ingoing water having to fight its way past the spent water already exiting the hole that had been created. This is particularly true when making an initial pierce through a target material, where there is very little relative movement between the nozzle and the target. Given that the interaction between the two flows occurs within about a hundredth of a second, the effect on cutting efficiency is relatively immediate.

So how to overcome the problem? One way is to pulse the jet, and in some early work at Leeds we built such a pulsating unit and spun it in front of the nozzle, chopping the jet into segments and allowing one segment to leave the hole, before the next arrived.

Figure 1. Pulsating disc to rotate ahead of nozzle and “pulse” the jet.

This was inefficient, because the energy put into the segments diverted by the disc was lost, and it was also extremely noisy, to the point that tests had to be carried out after everyone else left the building.

The alternative was to rotate the sample (this was in the days before high-pressure swivels and couplings were available) and align the jet just off the axis of rotation, so that the jet cut a hole somewhat wider than itself, as it passed over the target and thus provided an open path for the rebounding spent water that did not interfere with the path of the fresh jet travelling to the impact point.

A comparison of relative penetration rates showed that while a fixed nozzle and steady jet had sensibly stopped penetrating after about a second, the pulsed jet continued to drill a hole, as did the rotating jet, but the latter was able to drill faster and remove more material.

Figure 2. Comparison of a steady jet, a pulsating jet and a rotating jet as they penetrate into rock over time. All jets are at the same diameter, pressure and standoff distance.

By moving the nozzle out to a greater radius the hole created could be enlarged. This allowed the nozzle to move forward into the cavity created and the process could be repeated. In this way, over several iterations, a waterjet at a pressure of about 9,500 psi drilled though a block of granite, uniaxial compressive strength around 30,000 psi, and a new drilling tool had been demonstrated.

Obviously it is impractical to keep enlarging the hole by reaming it wider from the surface to allow the nozzle body to enter the hole and advance to the bottom. The jet must, from the beginning, drill a hole large enough for the nozzle to advance. And the easy way to do this is to incline the nozzle (at an optimal angle of around 20 deg, depending on the pressure, the type of waterjet and the target material properties). And while we still did not have a rotating swivel, we could turn and raise the target, while directing the jet through a small inclined orifice.

Figure 3. Inclined jet drilling a hole through a rock

Skip forward a couple of years, and we were drilling rock, at a rate of about 4-inches a minute, with a single jet. Then Jim Blaine, the RMERC machinist at the time, misunderstood a drawing, and added a central axially aligned orifice to the nozzle geometry. And within days we had increased the speed of drilling by two orders of magnitude. (Though by this time we also had a working rotating coupling to help rotate the nozzle).

Figure 4. Modified drill nozzle geometry

Figure 5. Drilling rate of advance as a function of hole diameter and rotation speed.

When one is using a single jet to cut the required profile of the hole, which must exceed the diameter of the nozzle holder if the drill is to advance into the hole, the rotation speed of the drill must be fast enough, relative to the rotation that the ribs of material left between adjacent passes of the jet along the hole wall are either non-existent (where the adjacent passes overlap) or are small enough that the mechanical impact of the nozzle body can break them off with very little force. In the latter case, however, this can put a mechanical load onto the drill string. The drill is often made up of only a length of high-pressure tubing, with the nozzle threaded on the end, so that any significant mechanical force can distort it and cause the drill to misalign and no longer drill a straight hole, so that this contact is discouraged.

On the other hand the rock through which the drill passes will likely change in strength and composition quite frequently, and so the depth to which the jet will cut will also change. This means that the hole diameter may reduce, so that the hole is no longer large enough to allow the nozzle body to pass. To stop this becoming a problem a small ring is mounted ahead of the nozzle, in the plane that the jet reaches the hole diameter required in this particular case. Now when the jet fails to cut to that diameter then the ring will stop the drill advancing, while the jet cuts along the line of contact and enlarges the hole to the required size, then allowing the drill to move forward. This is helped where the drill is spring-loaded so that the compression of the spring stops the drill advance, and the relaxing of the spring, as the obstacle is removed, allows the drill to move forward again.

Figure 6. Gaging ring on the front of a drilling nozzle.

That drill development was relatively straightforward, and was demonstrated in the late 1970’s. Subsequently commercial drilling systems were developed that used high-pressure water to drill holes in mine rock. They had the advantage over conventional tools in that the hose feeding the nozzle could maneuver in a much smaller space than a conventional drill, and thus longer holes could be more easily drilled from narrow working areas.

Unfortunately it still proved difficult to drill all rock with a plain waterjet, despite the use of ultra-high pressure equipment, and two different approaches were then tried, which I will discuss in the next posts.

Waterjetting 33d - Nozzle oscillation along a contour

In the last post I reviewed, in part, Dr. Shunli Xu’s work on oscillating nozzles, and how they can, on occasion, almost double the penetration while also improving surface finish. The problem with that basic technique, however, is that the nozzle is oscillating perpendicular to the direction of the cut, which is fine when the nozzle is cutting a long straight pass, but becomes more of a problem when the cut is in the form of an intricate contour. Given that the nozzle assembly does not usually rotate to follow that contour, the motion that gives the correct direction of oscillation when the head is moving laterally won’t when the cut is being made into the depth of the piece, i.e. parallel to the oscillation direction.

The answer to this is to provide the nozzle with an orbiting motion similar to that which, for example, Clark Barker used to provide a circular motion to a cutting head back when we needed to cut a path ahead of a drill head at a time when high-pressure rotary couplings lasted about 14 minutes. (We knew this because we had bought a model of every one we could find in the country and run them to failure, and the best lasted that long, provided you kept it cool by playing a hose on it. That was some thirty-five years ago, and things have got a lot better, and cheaper in that time).

Figure 1. Mechanism for orbiting a nozzle (after Barker). In the above figure the outer sleeve rotates, driven by an external motor, and with a flexible connection to the nozzle, the nozzle moves in a circular path, without itself rotating as it moves, as it moves in an orbital path as the outer sleeve turns.

In the model for Clark Barker’s device the intent was to drill a hole in coal some 6-inches in diameter, and the tool worked well in being able to do this. (It was used in a tool that turned from a vertical well to drill out horizontally within a 9-inch turning radius. The device was proved in the field by Sandia Labs, who drilled out into a coal seam from a vertical well using the second generation of the tool that was developed.

In many ways the use of an orbiting mechanism for cutting is a lot simpler to develop, given that, as noted in the earlier post, the angle that the jets must move through is very small (around 8-degrees). With the nozzle constrained so that it remains pointing only slightly off-axis, there is no need for the more complex tool required to advance a drill tens of feet into a coal seam (and deal with all the debris that was flowing back out of the hole at the same time).

I have described John Shepherd’s Wobbler tool in an earlier post and it is worth returning to that design and our study for a little further analysis.

The object of our study was to examine how the tool could be used in milling pockets in material, and more specifically how best it could be used to create a relatively flat floor to the pocket, while maintaining relatively sharp corners to the pocket walls, a capability that conventional mechanical tool milling does not allow. (Unfortunately I can’t at the moment produce any of the figures from that work, though they can be found in the paper we gave at the 17th Waterjet Symposium in Mainz in 2004.)

When it came to the assessment of performance, it is perhaps of note that Dr. Zhang’s study found an optimal oscillation speed at around 8 Hz. It would appear from our study that the optimal oscillation to achieve greater depth was just below 8HZ, whereas that which gave the greater volume removal rate was at around 10 Hz, which would both lie close to the optimum suggested by Dr. Shunli Xu.

The assessments were admittedly for different overall phenomena, Dr. Xu was interested in achieving a greater and cleaner cut, while Dr. Zhang was more focused on achieving a milled surface, typically to be achieved with a single overall pass, nevertheless the relative agreement on an optimal parameter is significant.

Further, in order to achieve a smooth floor for the pocket, Dr. Zhang was incrementing the nozzle between passes as a function of the width swept out by the jet. The initial overlap of the jets provided an uneven floor to the pocket that was removed when the jets were further apart. (In most cases with a 120% spacing between the passes a smoother surface was achieved).

However, with the generalized conclusion being that the optimal basic operating parameter (oscillation/rotation speed) was in the same range for both studies, and with the angle that the jet swings through on the order of 6 degrees, again of similar range in both studies, would appear to validate the cross-transfer of information.

The path that the Wobbler makes is shown, in exaggerated form, in Figure 2, and a typical result for pockets cut in glass and steel are shown in figures 3 and 4.

Figure 2. John Shepherd's Wobbler and the path it drives the jet along as it moves over a target.

Figure 3. Pocket of varying depth and contour milled from glass using the Wobbler.

Figure 4. Lettering and the map of Missouri cut into metal using the Wobbler.

Waterjetting 33c - More on enhanced cutting performance

Frontal photographs taken of waterjets, regardless of pressure, show that the jet spray widens as the jet moves away from the nozzle. Yet, because of the erosion of the outer layers of the jet by the surrounding fluid, the central core of effective jet pressure reduces as that distance grows. The normal way in which this can be seen is in the taper of the cut as a jet penetrates into a target material.

Yet in some cases better results can be obtained if the jet makes a series of passes to cut through that target layer. There is, however, a little problem, and this can be shown by the use of a curve, showing the depth of cut as a function of the number of passes made over the surface.

Figure 1. Depth of cut in two materials, as a function of the traverse speed. (After Hashish).

The graph shows the decline in the cutting ability of the jet with increasing number of passes, and the inability of the jet, at the highest speed, to penetrate through the mild steel plate.

One of the reasons to make passes at a higher speed is to improve the quality of the edge cut, since if the jet makes the pass with the particles of abrasive only cutting on the target one time (rather than the multiple cuts made by a particle at slower speeds, where it bounces down the cut.)

Momber has pointed out the decrease in performance with increased pass number, as well as noting the difference in the amount of energy required to cut through a target as a function of the speed and number of passes.

Figures 2 and 3. Effect of the number of passes on the depth achieved (lhs) and the relative amount of energy required to penetrate material as a function of traverse speed (rhs) (after Momber)

The slight loss in cutting power as the jet cuts deeper in secondary passes comes in part because the jet is constrained by, and thus cuts back into, the walls of the pre-existing cut.

In an earlier post on cutting I pointed out that because of the highly efficient way in which a plain waterjet cuts into material, Chinese investigators have shown that one can achieve a much improved volume removal rate by oscillating the jet perpendicular to the line of travel.

The optimal speed for cutting with an abrasive jet is, however, much slower than that of a plain waterjet, by a couple of orders of magnitude, so that the large scale oscillation that is effective with plain jets will not be similarly so with an AWJ. However the concept remains valid, and has been the subject of significant investigation, particularly in Australia, in the past few years.

The benefits of such oscillation, even over very short angles, can be illustrated with reference to a figure.

Figure 4. Oscillation of a jet perpendicular to the line of travel. The nozzle advances to the grey outline on the subsequent pass. The two lines indicate the range of oscillation. (Motion exaggerated relative to the current discussion)

If the nozzle is oscillated so that the jet moves over the relatively narrow range shown in Figure 4, then after a pass, when the nozzle advances to make the second pass (and it does not have to be in the part to do this) then the jet does not make contact with the target until the back of the previous cut. Thus there is much less energy loss in traversing the jet to the new surface, and cutting performance is improved. If the oscillation is kept small the walls of the cut will still act to confine the cutting ability of the jet, and improve depth-cutting capability.

Shunli Xu looked at oscillating a jet at angles below 10 degrees, while cutting half-inch thick 87% alumina plates. A simple visual correlation showed the relative benefit of oscillation when cutting the plate with a 45 ksi jet, with an AFR of 1.2 lb/min, at a speed of 3.1 inches/min.

Figure 5, Cuts made into a ceramic plate, without (lhs) and with (rhs) a nozzle oscillation of 8 degrees at 10 Hz. (after Shunli Xu)

The study also looked at the effect of changing the oscillation parameters on the surface roughness of the cut achieved, finding that this is controlled by the angle of oscillation, the frequency and the speed of traverse, as well as jet pressure and standoff distance (not shown). The study found that, under optimal conditions, surface roughness could be reduced around 11% relative to linear cutting.

Figure 6. Effect of change in oscillation parameters on the surface quality of cut in a ceramic target (after Shunli Xu).

The study found that the parameters which control the depth of cut gain were a little more complicated to disentangle, given that the density of particles striking an individual area of the target is controlled by both the jet residence time, and the parameters of the jet itself (AFR, pressure, traverse speed).

As a result the optimum value for oscillation angle and frequency varied depending on the jet parameters, but overall it was concluded that an optimal angle of oscillation would lie between 4 and 6 degrees, with higher oscillation frequencies giving better results. An average improvement with oscillation lay on the order of 23% over conventional non-oscillation at the same parameters.

Precision cutting is a task that has a number of complications. In many cases the cuts must follow intricate contours, rather than just making simple linear cuts than separate the material. Increasingly, also, pocket milling has become a valuable ability for this tool. Cut wall quality adequate for final surface finish is increasingly important in this case, and the ability of oscillation to improve that quality and enhance the depth over which a smooth cut was achieved was noted in the work. Similarly the taper of the cut was, on average, reduced 18% with greater improvement at higher oscillation frequencies and angles.

Secondary motions of the nozzle, beyond simple path following, are thus becoming a more important potential tool for the industry, and I will return to this topic again.

Hashish M. “A Modelling study of metal cutting with abrasive waterjets,” Journal of Engineering Materials and Technology, ASME, Vol 106, Jan 1984, pp. 88-100.

Momber A.W., Kovacevic R, Principles of Abrasive Waterjet Machining, Springer Science, p. 209

Shunli Xu Modelling the Cutting Process and Cutting Performance in Abrasive Waterjet Machining, PhD Thesis, Queensland University of Technology, 2005.

Waterjetting 33b - More on flow inlet conditions

The structure of the jet flow from an orifice makes a tremendous difference to the ability that the jet then has in terms both of its range and its cutting ability. And one of the major factors that control the structure of the jet lies in the flow conditions just upstream of the orifice itself. From time to time, over the decades, we would go out and buy every nozzle that was available for a certain purpose, and run tests between them, trying to find which would, under otherwise similar conditions, provide the best performance. Rarely did the most expensive nozzle give the best result. And the performance of even the best nozzle was also controlled by the flow channel upstream of that nozzle. This, in turn, controlled the condition of the water entering the nozzle. To illustrate the point let me use the example of some tests we made with fan-jet nozzles. In this particular case to objective was to clean large surfaces, but the generalized conclusion also holds true of nozzles of different shapes and jets up to even the highest of pressures used (and we have gone up to 10 million psi).

Simple cleaning nozzles, of the sort that are used in most pressure washers, have historically produced fan-jets that spread in one plane away from the orifice. There are a large variety of these on the market, of varying flow rate and geometry, and it was an initial challenge to find a simple way of relatively ranking the jet quality. Our initial answer, for the first cut, was to take blocks of polystyrene foam and traverse these at a fixed speed under the jet at different pressures and distances from the nozzle. This foam is very easily cut by a jet. So the tests were carried out at 1,000 psi and 2,000 psi, which is the range of pressures of the electrically powered pressure washers found in most hardware stores these days. The difference between two nozzles that were nominally supposed to achieve the same performance was striking:

Figure 1. Comparison of performance between a “better” fan nozzle (top) and a “poor” one (lower sample) when cutting polystyrene packing foam at low pressures.

As you may note at 1,000 psi the poor design was barely able to remove the surface of the polystyrene, rather than cutting deeply into it, as was the case with most of the nozzles tested, and as exemplified in the top cuts.

My point today however, is not the inherent faults in the design of the nozzle shape itself, but rather to highlight the problems that the particular design had, as a result of the way that water was fed into the orifice.

For in this case, unlike many of the conventional nozzles, where the flow is directed directly at the orifice down a channel aligned with the orifice, the nozzle were small discs arrayed along a spray bar, of the type that is used for car and truck washing rigs where a single channel feeds a number of sprays.

The flow in this case is primarily along the distribution manifold, and, as such, perpendicular to the axis of the resulting jets. When the water, therefore, exits from the individual nozzles it retains a component of this lateral velocity, and this tears the jet apart relatively close to the nozzle. The results are evident in the cut made in the lower half of Figure 1.

It is surprisingly easy to remedy this. A short tube inserted behind the nozzle orifice, and protruding up into the manifold channel allows the water some chance to collimate in the direction of flow before it accelerates through the nozzle orifice, and the result, relative to the original cut is quite significantly better.

Not that short lengths of tube are completely effective, but they are a start. One of the more effective means of getting a water jet to move as a cylinder in short jets (such as those seen at Disneyworld and at Detroit Airport is to run the water from the supply pump through a small stabilizing chamber and then pass it into a collimating tube full of drinking straws (or their technical equivalent) which sit just behind the nozzle. Providing the geometries are properly selected you can get the very smooth cylinders of water that are a feature of the jumping streams.

A similar structure lies upstream of the the nozzle at the Gateway Geyser across the river from the Gateway Arch in St. Louis. The fountain shoots a jet of water to the same height (630 ft) as the Gateway Arch on the other side of the river, and to quote Wikipedia:

the Gateway Geyser was designed and constructed by St. Louis–based Hydro Dramatics. It was completed in 1995 at a cost of $4 million. Three 800-horsepower (600 kW) pumps power the fountain, discharging 8,000 U.S. gallons of water per minute (50 L/s) at a speed of 250 feet (76 m) per second. The fountain has an axial thrust of 103,000 pounds-force (460 kN); water is jetted out of the 6-foot (1.8 m)-tall aerated nozzle at a pressure of 550 pounds per square inch (3.8 MPa).

These are more complex flow straighteners than the simpler ones that are used in low pressure cleaning systems, and with considerable effect in controlling the jet flows from the monitors of hydraulic mining equipment. By channeling the water into a multitude (perhaps 200) small diameter channels and then recombining the water at the nozzle the resulting flow is laminar.

Where the water flow is much lower, such as when being used in an ultra-high pressure system, the flow can be stabilized by allowing a long straight run-up of the pipe leading into the nozzle. (Typically the rule of thumb was that the length should be around 125 pipe diameters, however work at the U.S. Bureau of Mines showed that the length of straight section did not need to be this long – a length of around 4-inches proved effective.)

Figure 2. The improvement in jet performance with a straight inlet section (after Kovsec et al*)

A similar improvement can also be seen when the flow conditions are correct when working with higher pressure jets.

As a general rule, however, such care is not taken in the construction and lead-in to the nozzles, and the jet will begin to taper and reduce in effective diameter from the time that it leaves the nozzle.

I’ll talk more about that, next time.

*Kovscek, P.D., Taylor, C.D. and Thimons, E.D., Techniques to Increase Water Pressure for Improved Water-Jet-Assisted Cutting, US Bureau of Mines RI 9201, Report of Investigations, 1988, pp 10.