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.

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 20b - cutting slots in coal

There are several ways in which a high-pressure waterjet can be used to interact with a surface or material. It can be aimed to make a high-precision cut into or through a material, it can be used to clean a surface, or it can be used to bulk remove material – to name but three applications. At the moment, in these posts, we are concentrating on the third of these, and last time I mentioned that, if working with soft material, such as clay or soil, that there was an advantage to using two simultaneous jets cutting over a surface, to improve the efficiency of material removal by a factor of perhaps more than ten-fold.

I want to revisit that topic this week, and stepping for a moment away from soil and into coal, which is a harder material, I want to illustrate that the point (of concurrent dual jet use) is still valid but there is a wrinkle, if you are cutting along the edge of an advancing mining machine.

Cutting coal with water jets is not new. But I am going to skip that historical review today, and rather continue on the theme of dual-jet use. When I was first taught to mine coal, there had not been a huge amount of new technology in the industry – and for that matter there still has not been the need for much advanced sophistication where the basic ideas still work.

If you are going to break a material from the solid, it really helps to have a second free surface (as well as the face that you are attacking through). Thus when miners used to work the coal they would first undercut the coal seam using a pick to swing across the surface ad successively chip out a strip of coal about a couple of inches wide at the bottom of the seam, and going back as far as they could reach (about two to three feet). The pattern that this leaves isn’t usually seen in coal mines (since they move on) but I have seen it in the salt mines of Wielicza, the underground rooms in the castle in Naples, and in the old workings of the quarries around Bath in the UK.

Figure 1. Grooved wall at Wielicza salt mine (Wielicza Salt Mine ) The grooves are formed by the successive swings of the pick in the cut that incrementally chip a deeper groove into and along the back of the slot.

Of course cutting the slot in thinner seam coal mines was a little less comfortable (this from the days when smoking was yet to be banned in mines).

Figure 2. Miner “corving” at Seaton Delaval mine (Beamish Collection)

When mechanized machines were first developed for use underground, it was logical to begin with a machine that would cut this slot (the most arduous of mining labor) and replace the miner. To do this the machine developed was, to a very large extent, a variation of what you would think of as a chain saw. Driven by either compressed air or electricity, a long cutter bar would (like the chain saw) drag the cutters along a path (in the mining case perhaps six feet deep) that would create the slot required as a second free surface into which to break down the coal. (You learn very early in the game that a slot less than about two inches high is fairly useless, since the pressure of the overlying ground will just squeeze too narrow a slot closed, and the effort to cut the slot is wasted.)

Once that slot has been made along the perhaps 200-yard long face, then small holes were drilled, at perhaps 4 – 6 ft intervals in the middle of the face, sticks of explosive were placed in those holes, and, at the end of the shift the explosive was fired, breaking down the coal into the immediately surrounding area, and ready for the coaling shift to come on and shovel the coal (in 15 yard intervals per miner) onto the conveyor. (My job at one time).

One of the early advances in mining machines was the Meco-Moore, a machine that cut a slot not only under the coal, but also at the top and back of the seam.

Figure 3. Meco-Moore Mining Machine

This worked fairly well as a concept, but the small cross conveyor that was put on the machine to move the coal from the back of the cut to the conveyor had been adapted from a farm conveyor, and coal is a lot heavier and more aggressive than wheat. As a result the conveyor, and hence the machine, was always breaking down, and so it was replaced with shearers and plows, and the world moved on.

But shearers generate a lot of dust and sparks from the picks that rotate through the coal and adjacent rock, and occasionally hit sandstone. This led to explosions that killed many miners, and so, in the early 1970’s we were asked to develop a new method of mining. The logical thought was to build on the success of the Meco-Moore as a slot cutting tool, and add a plow shape to move the central volume of coal over to the conveyor. Jets would replace the cutter bars at the top, back and bottom of the seam, as a way of freeing the central block.

Figure 4. Original concept for the Hydrominer

We quickly found that using a single jet to cut a slot in coal did not help as much as we had expected. If we cut it horizontally then, as I explained above, the slot would close before it could be effectively used. And if it were cut vertically then the movement of the machine forward meant that every cut had to start afresh and could not take advantage of the previous pass to cut deeper.

And so we came to the idea of using two adjacent jets to cut into the coal at the same time, spacing the jets about an inch apart, and, in this way, removing the rib of coal with the slot cutting, to give a passage into which the nozzle holder, and plow blade edge could advance.

But if the two jets were parallel then the forward movement of the nozzles during each pass would mean that the second oscillating pass would be cutting fresh coal along its length and thus the depth of cut achieved would be only a couple of inches.

So we (Clark Barker, Marian Mazurkiewicz and I) decided to put the two orifices one above the other in a single nozzle block, with the jets pointing out at about fifteen degrees to the line of advance, but divergent from one another.

Figure 5. One versus two jet arrangement

In this way the jets cut a slot about two-inches wide, but as the nozzle moved into this slot it moved into an air space, so that when the jets made the second pass along the surface they did not hit coal until the back end of the previous cut. Within a few passes the two jets were cutting over a foot ahead of the plow face, instead of a couple of inches. This additional leverage from the wedge head of the plow as it entered the cut now meant that the force on the plow was dramatically reduced, and the machine could plow off a strip of coal some 2-3 ft deep and perhaps 6 ft high at rates of between 10 and 20 ft a minute. Given that the jets infused the coal as they cut it, virtually eliminating coal dust from the air, and there are no sparks since cutting occurs by water, under water, so the technique is safer.

Figure 6. Slot cut by the two jet system (about 2 inches wide) and the leverage this gives in breaking off large pieces of coal shown in a surface test.

Unfortunately the world market at the time was only about ten machines a year, and so the design was dropped (after an underground test) – but that is another story.