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.