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.

Waterjettting 30c - why cavitation isn't being used in machining metal.

Most applications of high-pressure waterjets need the jets to make relatively precise cuts into a target surface. When examining different ways of improving jet cutting performance, the narrowness of the cut which is achieved is therefor often a critical factor in deciding how to make the cut.

In this short segment of the series I have been discussing some of the findings that Dr. El-Saie made in his Doctoral Dissertation, and at the end of the last post had shown that he had found that, under the right conditions, a cavitating jet would remove more material from a surface than would an abrasive-laden waterjet of equal power within the same time frame, and at the same operating jet pressure.

This work was carried out prior to the submission of his Dissertation in 1977, and – because most of the work on refined nozzle design had still to be done – the nozzle designs that were used in the study were considerably cruder than those that have been developed, by a number of companies, in later work. For this reason it is not realistically possible to compare the results achieved in the early studies with the current state-of-the-art, particularly since, while there has been a great deal of work on improving the design of AWJ nozzles there has been almost none focused on improving the cavitation destruction of a waterjet.

The important distinction to make is that while there has been considerable work on reducing the damage that a cavitating flow can achieve when it impacts on a surface, there is almost none that has been aimed at making that damage worse. Part of the problem that this lack of work has left us with is that, in most cavitating systems, part of the cavitating cloud will collapse within the nozzle assembly. While this may only be a small fraction of the total, within a relatively short time (and this has been measured in fractions of a minute in an intense erosion design) the nozzle is destroyed. It is therefore unlikely that the most efficient nozzle design for inducing cavitation erosion has yet been developed.

Part of the reason that it has not has to do with the requirement that is listed at the top of the page. Where abrasive particles are mixed within a very narrow jet of high-pressure water, the abrasive cutting is confined, so that the slots can be controlled to a high degree. We have shown, for example, that it is possible to make cuts through titanium within a tolerance of 0.001 inches of the design requirement, and with a smooth surface over the full surface of the cut. (This requires, in thicker materials, that the nozzle be slightly tilted so that the jet taper over the depth of the cut is compensated for over the desired edge cut).

Unfortunately this precision in cutting cannot be achieved (at least with the current levels of understanding of the process and controls) with a cavitating jet system. In part this is because of the omni-directional nature of the collapse of the cavitation bubbles over a surface. An abrasive particle has the great majority of its velocity aligned with the axis of the cutting jet (though this might slightly deviate if the particle cuts into the surface more than once over the depth of the cut). Cavitation attack can occur in a more omnidirectional way.

To illustrate the point consider an experiment where we fired a waterjet along the edge of a block of dolomite, in such a way that the jet did not contact the rock, were the test to be carried out in air. The jet was carried out underwater, with back pressure of the surrounding water adjusted to intensify cavitation along the edges between the waterjet and the surrounding water. The collapse of those bubbles against the side of the rock eroded the cavities shown.

Figure 1. Samples of dolomite attacked by a cavitating jet. The jet is, in both cases aimed parallel to the cut face (along the line of the red arrow) and just off the block surface.

The samples shown in Figure 1 were exposed to the jet for a minute, with the samples held under water in a cell where the back pressure (BP) could be adjusted.

At a pressure of 6,000 psi, with a back pressure of 60 psi and a nozzle diameter of 0.02 inches the damage to the rock is small. Although it should be noted that there is a hole that eats into the rock perpendicular to the direction of jet flow.

This is more immediately obvious with the sample shown on the right, which was cut with a 7,000 psi jet, against a back pressure of 35 psi, and with a jet diameter of 0.030 inches.

It is also worth noting that the hole generated is not consistent in diameter as the hole deepens. The penetration of the jet into the wall, perpendicular to the main jet flow, is caused by the individual collapse of cavitation bubbles against the surface of the rock. And this is not a consistent phenomenon along the jet length, but for varying conditions it will occur at different distances from the nozzle. In this case the hole widens and deepens about half-an-inch below the rock top surface, eating further into the rock relatively consistently for the following few inches.

In a separate experiment the resulting hole can be seen to vary in depth over the length of the cut into the rock.

Figure 2. Hole cut into dolomite by a cavitating jet. Note that softer layers of the rock are excavated more deeply into the hole wall by the collapsing bubbles. The hole is roughly six inches deep.

The cut is also much broader than the originating jet, as can be seen from the cavity eaten into the rock surface perpendicular to the jet at the bottom of the sample. The cut is much more ragged than the precise line that would be cut by an AWJ, so that, although more volume is removed, the removal path is not as precisely defined as is needed in most applications. The irregularity of the slot shape can be seen where a cavitating jet is traversed over a 2-inch long block of dolomite over a period of five minutes.

Figure 3. Traverse path of a cavitating jet eroding a slot into dolomite. The block is roughly 2 inches long.

Clearly, at this stage in its development, there is little precision to the slot that is being generated in the rock, and it has little application in machining.

However the operating jet pressures of these cuts are within the range of those pumps that are available at relatively low cost at hardware stores, with the simplicity in use that this implies. And yet they are capable of disrupting, into the constituent grains of mineral, even the hardest of rocks that will be encountered. Gold ore, for example, can be relatively expensive to drill, because of its strength and toughness, and yet it can be penetrated in the same way, and with the constituent minerals separated, as was the dolomite.

Figure 4. Cavitation erosion of a sample of gold ore.

The problem comes in collecting the very fine grains of the mineral from the rest of the ore sample, once it is disaggregated.

Cavitation therefore, at present, is a tool better used in applications beyond those of the machine shop. It does not, however, have to stay limited to that restriction.

Waterjetting 9d - Deepening a hole and cautions with glass

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


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

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

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

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

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

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

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

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

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


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

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


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

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

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

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