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 30a - Why cut slots in rock

Looking back over the period where we first started coming together to discuss high-pressure water jets, some 40-odd years ago I was reminded of the work of one of my then graduate students (and subsequent faculty member in Egypt) Dr. Ahmed El-Saie. He obtained his doctorate in 1977 and looking back on that work it is interesting to see how long it took for some of the ideas he worked on to come to fruition.

His dissertation focused on using a waterjet system (which I will discuss in a later post) as a way of cutting a slot around the edge of a tunnel before excavating the contained volume. Early in his dissertation, for example, he discussed the use of impact breakers as a method for improving the economics of tunnel driving over conventional drill and blast techniques. He felt that this would be particularly useful where the volume of overbreak around the tunnel beyond the desired size could be controlled by cutting a perimeter slot.

Apart from the benefits that come from mechanical excavation over blasting (workers don’t have to leave the working area during a blast, for example) other benefits can be shown by contrasting the damage to a block of Plexiglas when a detonator is fired in a small central hole in Plexiglas, with and without that perimeter cut.

Figure 1. Damage to a block of Plexiglas from a detonator fired in a central hole.

If, however, a relieving slot is first cut around the perimeter of the anticipated damage zone (we used the distance to some of the longest cracks) then a different result is obtained.

Figure 2. Effect when the experiment is repeated with a pre-cut slot around the perimeter.

As the photos clearly showed with the free outer surface the central core of material is broken out in pieces, there is a nice relatively flat front surface to the excavated hole, which lies at the back of the drilled hole. (This is a relatively important point in driving tunnels, since often the last third of the blast-hole length is not effectively broken out of the solid, and has to be re-drilled).

It is also important to notice that the cracks from the detonator explosion did not grow out beyond the edge of the slot, so that the tunnel wall would be stable and, because there would be no overbreak, the cost of tunnel support would be reduced considerably.

However, in the larger scale the depth that this slot would have to be cut is around 7-ft. This would require that the jets cut a slot wide enough for the nozzles to advance into the slot, and this required a considerably higher volume of rock to be removed by the waterjets.

Tests of such a device in a German coal mine used two different methods for cutting the slot. Initial trials at Rossenray Colliery in Germany used a waterjet assisted mechanical set of tools to to cut the slot to the desired width. The head, seen moving along the slot at the edge of the tunnel, had to make a number of passes to reach the depth needed.

Figure 3. Tunnel profiling in Germany using a combination of waterjets and metal tools to cut to the perimeter of a tunnel (after Bauman and Koppers)

Subsequent trials replaced the mechanical cutter with a set of waterjet nozzles alone, and this reduced even further the cutting forces required to make the slot (and would make the machine smaller and lighter as a result). Although the trial was successfully concluded the tool did not move into production, perhaps in part because of the change in the mining economy at that time.

To prevent the cracks from growing into the wall, however, a wide slot is not needed, and even a continuous crack around the edge of the hole can be effective. But how to control crack growth to a single direction from the borehole?

The answer came as part of the Master’s degree of another student, Steve McGroarty. If one drills a hole into a block of Plexiglas, and then notches the side, fills the hole with water, and fires an air rifle pellet into the hole, then the pressurized water will flow into the notches and cause the cracks to grow. These are a little difficult to see in the following picture, but the cracks grow at the bottom of the hole and from the edges of the v-cuts (made at the time with a saw).

Figure 4. Individual cracks growing out from 3 bored holes in plexiglas

The above test showed that energy could be focused into cracks if they could be properly aligned. (We could break off a corner of the block in a single piece, using a single notched hole). This work was then in the field by Steve in comparing results when he used explosives to drive a short tunnel underground.

In Steve’s case he drilled holes around the edge of the tunnel, and then notched some of these with a waterjet system. (Others were left un-notched to provide a comparison). The lance used had two jet nozzles and was fairly simple to insert, and the lance was run to the back of the hole, and the two opposing jets aligned to the proposed tunnel wall, raised to pressure and pulled back out of the hole, notching the walls. This is a fairly fast process, and used relatively little water.

The holes were then charged with a small amount of powder and fired just before the rest of the blasting round, which was distributed around the rest of the core rock, in order to break it into fragments.

Figure 5. Tunnel wall after the round had been cleared showing the clear break at the back of the holes, (the next round has been drilled along the edge) and parts of the drilled hole still evident in the wall of the tunnel. (after McGroarty)

The role of the waterjets was much smaller than if a complete slot had been cut, and this significantly reduced the cost financially, in energy and in time, and produced much the same desired result.

In later work we used the same notched borehole idea to break out large pieces of rock as we excavated the Omnimax Theater under the Gateway Arch in St. Louis, but that is a story for another day.

Next time I will discuss some of the ways that Dr. El-Saie used to cut the slot.

A.A. El-Saie “Investigation of Rock Slotting by High Pressure Water Jet for use in Tunneling”, Doctoral Dissertation, Mining Engineering, University of Missouri-Rolla, 1977.
Bauman L. and Koppers M. “State of Investigation on High Pressure Waterjet Assisted Road Profile Cutting Technology,” BHRA 6th ISJCT, paper G2, pp. 283 – 300, 1982).
S.J. McGroarty “An Evaluation of the Fracture Control Blasting Technique for Drift Round Blasts in Dolomitic Rock”, M.S. Thesis, Mining Engineering, University of Missouri-Rolla, 1984.

Waterjetting 26d - Range, position and rewards for jet assisted cutting

In the earlier posts in this chapter I have discussed the problem of getting the nozzle of a waterjet system close enough to the tool:target contact that the jet retains enough power to be effective. At the same time the jet must strike within roughly 1/10th of an inch of that contact to be effective in helping with the cutting process. In the figure below, for example, the jet that comes from the nozzle ahead of the pick will initially strike in that region, while the jet at the back (right) of the pick box will not.

Figure 1. Potential positions for jet nozzles around a conical pick

There is one other consideration, perhaps more relevant in a rock cutting operation than in a metal cutting one, and that is the issue of tool wear. In the above situation while the rear jet can never hit the critical zone, the one at the front of the tool will lose effectiveness as the small carbide cutting cone wears and moves the crushing zone back under the pick shoulder. As an improvement consider the situation shown below:

Figure 2. Simplified schematic showing a high-pressure waterjet hitting the contact between a cutting tool and the underlying rock.

In this case when the tool is sharp then the jet is striking the rock just in front of the edge of the tool, and the performance is enhanced. Further, as the tool starts to wear, so the jet impact on the rock begins to move further forward of the tool contact. But because the face of the tool and the jet are almost parallel the slight change in distance is relatively insignificant.

By the same token, if the rearward jet in the first example had been moved so that the jet struck just under the back of the pick it would still have been able to remove the crushed rock, even as the bit wore. One way to improve the effect of the jet is to spread the water flow by making the jet into a fan or conic spray, this can be effective:

Figure 3. Reduction in thrust with lower pressure fan jets (after Hood)

Again the bit is cooled, keeping it sharper, but also even at the lower pressure if the rock is removed as soon as it is first fractured then it does not crush and then re-compact under the bit.

However higher pressures work better, both in terms of overall rate and in terms of the efficiency of cutting, based on British data.

Figure 4. Change in cutting performance with increasing jet pressure (after Morris 1985)

Given therefore the need to bring the jet to the crushing zone in as powerful a form as possible, one suggestion has been to bring the jet down through the center of the cutting pick.

Figure 5. Nozzle located above the contact point, but fed through the pick body. (After Fairhurst).

The problems with doing this are several. In the particular example shown the orifice is pointing the jet into the rock some quarter-of-an-inch above the crushing zone and this is too far away for the jet to achieve maximum benefit. Further as the tool will wear, so the contact surface will move back further away from the jet, further losing the assistance and failing to be able to remove any of the crushed rock as it is formed.

There are practical problems, however, when (as has been done in Russia) the orifice is brought closer to the tip of the tool. One of the difficulties is that whenever the tool is then used without the jet operating at pressure, then crushed rock will enter the nozzle and within a very short distance plug it with compacted fines.

It is then, frequently, not possible to use jet pressure to get that material out of the nozzle, (particularly when the pump is supplying several orifices on a cutting head). Without the water the tool rapidly erodes, because of another weakness in the design.

For when the orifice is placed within the lower tip of the tool, the volume of the orifice is removed from the bulk volume of the cutting bit, making it much more susceptible to wear.

As long as the jet is brought up to pressure first, and the tool only then brought into contact with the rock or other target, then the tool performs well. Unfortunately (as operators are human and thus prone to the occasional error) cutting heads have often been brought into contact with rock without the jets being at sufficient pressure, and the benefits of the jet assist are thus eliminated due to this loss in nozzle clearance.

There is a corollary to this, in that, as jets began to be used more frequently on cutting heads, the amount of water spraying into the working zone became both a source of irritation and a considerable unnecessary loss in power, given than the cutting head tool only makes contact with the rock for a small fraction of the rotation around the shaft axis.

Figure 6. Roadheader with jet assist working at the Middleton Mine in the UK

To reduce the volume of water, control valves were set into the flow channels so that water was directed at only those picks that were in contact with the rock. The problem with programming this is that, depending on where the head is around the profile of the tunnel, so the arc of the head that the picks are cutting on will change.

But the benefits, where all these different factors are considered in the design and operation of the machine are considerable. As a very rough statement, the cost of a machine will increase more than linearly as it’s weight is increased. In order to cut harder rock without jet assistance, the picks must be pushed harder into the rock, and this thrust must be resisted by the friction exerted between the floor of the tunnel and the base of the machine – usually treads. Thus harder rock requires that conventional machines be heavier. However, when jets are added to the machine that power cost is removed, as the thrust levels are reduced. Thus smaller (and more mobile) jet-assisted machines can cut more effectively than their conventional counterparts.

Figure 7. Introduction of heavier machines to mine harder rock, until the advent of the waterjet assisted machine in 1980 (after Morris)

The savings in the reduced cost of the machine (saving $500,000) more than covered the cost of the high-pressure waterjet equipment (around $100,000).

Hood, M., A Study of Methods to Improve the Performance of Drag Bits used to cut Hard Rock, Chamber of Mines of South Africa Research Organization, Project No. GT2 NO2, Research Report No. 35/77, August, 1977.
Morris, A.H., "The Development of Boom-Type Roadheaders," Seminar on Water Jet Assisted Roadheaders in Rock Excavation, Pittsburgh, PA., May, 1982.
Fairhurst, C.E., Contribution A L'amelioration De L'abbatage Mecanique De Roches Agressives: Le Pic Assiste Et Le Pic Vibrant, Doctoral Thesis, L'Ecole Superieure des Mines de Paris, October, 1987, 221 pages (in French).

Waterjetting 26c - Cutting tool shape

When it was first discovered that high-pressure waterjets could significantly improve the performance of mechanical cutting tools, whether in machining metal, or in cutting rock, it was anticipated that this would have a broad-ranging application. This has not been the case, and the reasons are varied, depending on the application, but quite often they relate to the way in which the mechanical tool was expected to work. The examples will, again, come from rock cutting, but also apply when cutting or machining other materials.

Figure 1. Three common types of picks used in cutting into stone for driving tunnels, or for cutting and mining coal.

The initial work of Mike Hood, in cutting quartzite, had used a relatively simple flat-faced bit that was dragged across the rock surface at a known depth of cut and directing s single, or pair of jets to cut along the line of contact between the rock and the carbide was relatively straightforward.

Figure 2. Locations of the jets for Mike Hood’s initial tests on improving jet performance. (Dr. Hood)

Getting the jets to cover the full zone of contact and rock crushing was critical to achieving the best results for the tests, and proved also effective when the cutting tools were tried in the field.

Figure 3. Relative normal forces on a cutting bit with change in the position of the jets assisting in the cutting of rock (Dr. Hood). Note that the machine stalled at 4 mm penetration without the jet assist (the black line).

The cutting picks that are more commonly used in softer rock, shown in figure 1, are not quite the same shape, nor have quite the same purpose. Early trials were with the forward attack pick, which through the early 1980’s was the most common design used.

Figure 4. Laboratory trials with a jet added to a forward attack pick

Rather than having a flat face, this pick has a wedge-shaped front face. This is so that, as the pick cuts into the rock, so the wedge shape pressing into that groove will put a high lateral load on the rock on either side of the cut, causing it to shear off the solid. Those chips can be seen to the front right of figure 4.

Where the jet cuts into and removes the crushed rock under the front of the bit, this allows the bit to make a deeper bite, and this, in turn, makes it more likely that the tool will make larger chips. This is not an unflawed benefit, particularly if the jetted slot now extends a little deeper than the tool.

Figure 5. Illustrating the wedging action of the tool in creating lateral chips beside the tool.

As the chips get larger so the force required to break them from the solid increases, and the actions do not occur symmetrically on each side of the tool. As a result the lateral loading on the tool becomes more significant, and because the process of chip forming and breakage is cyclic so there greater fluctuating forces make their way through the drive train back to the driving shaft and motors.

With most machine designs these fluctuating loads are, however, reduced in overall magnitude, because of the reduced forces needed to move the pick forward, without having to deal with the crushed material under the pick, which the water jet has removed, providing it is within about 1/10th of an inch of the cutting tool.

Achieving that positioning becomes a little more difficult with the transition to a radial pick, however there were additional problems with that intermediate design, particularly in harder rocks, where they wore out at rates as high as 7 picks per foot of advance. This led to the development of the point attack pick, as shown in figure 1.

This pick design has become the most popular for use in mining machines over the past 20-years. The round shape of the tool and shaft are designed so that, as the pick wears it will rotate in the holder, and this will spread the wear evenly around the tool, making it last longer – and in the case mentioned in the last paragraph a change to this design reduced pick costs to around 1 pick per foot of advance. But there are a couple of problems with adding waterjets to this tool.

Figure 6. Point attack tool geometry (Goktan and Gunes)

This geometry makes it very difficult to bring in a waterjet to hit the right point at the rock:tool contact, because of the double cone at the end of the tool. While the nozzle can be positioned so that it can direct a jet into the right point (for example by being at the point where α is in the figure) the problem arises with the size of the nozzle mounting block, and the small size of the jet, where a large number are being used to cover all the picks on the cutting head and total flow volume is limited. To fit the nozzle block means that it must be at a greater standoff distance from the point (perhaps four or five inches), while the small orifice size means that the effective range of the jet may be no more than an inch or two.

The change in pick design and the difficulty in adding waterjets to the new tool therefore led to a discontinuation of the trials of the combined system. This was unfortunate since the forward attack picks initially cut better than the point attack, but wore out more rapidly – hence the change. But with the addition of the waterjets the tool lifetime, and sharpness, was increased in some cases more than five-fold times, while the other benefits – such as the ability to use smaller machines to carry out similar performance – made capital investment less. But these events occurred at the wrong time, as the coal market was entering one of its down cycles as the developments were being made, and the technology was therefore not adopted.

I will conclude this small chapter next time, by addressing one of the answers to the problem of getting water to the point attack tool.

Waterjetting 26b - Jet positions to help bit cutting

The addition of a high-pressure waterjet to the leading edge of a sharp tool can make a considerable difference to the performance of that tool. I have discussed this a little in two earlier posts, the first of which was an introduction to the topic, and the second highlighted the problems of getting the nozzle close to the active contact zone so that it can be effective.

In this post I will discuss the benefits that this jets can create in the performance of the machine. The discussion is largely focused on rock excavations, since that is where most of the basic and applied research was developed, but, as I also mentioned previously, this benefit can also be gained if the jets are added to machine tools that are cutting into metal – even metals that are otherwise hard to machine.

The idea of pushing a sharp(ish) tool into rock to break it out goes back to the deer antler picks used to pry flints from chalk some thousands of years ago. But it worked, although the picks are now made of metal and powered by machines. The shapes have also changed over the years.

To make an effective cut requires two different sets of forces be applied to the picks. The first of these is the one that pushes the pick into the rock and gives it the depth of cut that is needed. (I’ll call this the Normal or Thrust Force, since it acts perpendicular to the surface being cut). The second is the force required to pull the tool along the face, this is often referred to as the Cutting or Drag force. Neither are very constant in rock (as opposed to metal cutting) since the rock will chip around the but as it moves forward, which frees and blocks the passage of the tool as it moves.

As I mentioned last time, pushing the tool into the rock will cause the rock under the tool to crush, and then re-compact, if the particles aren’t removed. Thus the most effective time to remove them comes as the tool first breaks them free from the solid. This also saves the energy that would otherwise go into not only further crushing, but also re-compacting the particles. Once they are re-compacted and compressed they become harder to remove and help increase the friction on the tool that cause it to heat, and weaken.

But if the particles are effectively removed, then the region under the bit is washed free, there is less confinement on the remaining rock, and it becomes easier to break.

Figure 1. Crushed rock under an indenting tool. (Richard Gertsch)

Figure 2. Crushed rock under an indenting tool, with the tool removed and a 10,000 psi jet fired at the contact point after removal. Note that there is still some crushed rock that was not removed.

Figure 3. Crushed rock removed during crushing by a jet pointed under the bit as it indented the rock (basalt) (Richard Gertsch)

The impact on the forces that the bit sees can be dramatic. In the early tests of drag bits in cutting the quartzite rock that holds the gold veins in South Africa, Dr. Michael Hood took a tool that normally stalled out under full load, when it was cutting into the rock to a depth of 4mm.

Figure 4. Normal forces on a bit (in KN) without jet assistance (black) where the machine stalls at 4 mm penetration, and with jets at different locations along the cutting face.

Mike tried a number of different locations for the jets at varying points over the face of the drag bit. Initially he used higher pressure merely as a way of getting enough water to the bit to keep it cool, but quickly saw that the performance was greatly improved. As Figure 4 shows, the normal force pushing the bit into the rock was considerably lowered, even when the depth of cut into the rock was increased almost three-fold, with the best location for the jets showing that the machine retained considerably capacity for cutting.

Similar results were obtained with improvement in the cutting forces seen in pulling the bit down the face.

Figure 5. Change in cutting forces with high pressure jet applications to a cutting tool in basalt (Mike Hood)

Again the machine stalled with a depth of cut of 4 mm, without waterjet assistance, and cut to more than 11 mm depth with power to spare with waterjets in the optimal location. This was found to be at the corners of the cutting tool, since in this location the jets were confined by the uncut rock on either side of the tool, and thus rebounded to cover the entire line of contact between the bit and the rock.

Figure 6. Optimal location for the jets on the drag bit for cutting South African Quartzite. (Mike Hood)

For the jet to work most effectively the water must continue to remove all the crushed material from under the bit as it is created. Where the rock is already fractured (as it may be because of natural ground fractures or high stresses on the face because of the depth at which mining takes place) then the confinement of the space around the tool can be less and this reduces the ability of the water to spread along the face of the tool and remove all the crushed rock as it is formed.

Others have also looked at the position of the jet relative to the cutting face, and sometimes, especially in harder rock, where the jet can intersect broken rock above the cutting tool, it may be better to bring the jet into the crushed zone from behind the bit.

Figure 7. Changing bit performance with change in jet pressure at three jet positions relative to the bit. (After Ropchan, Wang and Walgamott).

A slightly different experiment was tried by French investigators who tried locating waterjets around the carbide inserts of a drilling bit. Part of the problem with such bits is to ensure not only that the nozzle is close enough to the crushing zone as to remove the rock, but also to make sure that the nozzle is close enough to the surface that the jet retains enough power. In this particular case, by drilling a small hole through the carbide tool, the investigators were able to bring the two tip jets to the point that they needed, with enough power to be effective. This is shown by the ability to achieve a rate of penetration (ROP) which was more than double that of a conventional bit, with only conventional cooling, for the same amount of thrust force.

Figure 8. Change in rate of penetration with change in jet location on a drill bit.

I’ll return to this topic next time.

Hood, M., A Study of Methods to Improve the Performance of Drag Bits used to cut Hard Rock, Chamber of Mines of South Africa Research Organization, Project No. GT2 NO2, Research Report No. 35/77, August, 1977.
Ropchan, D., Wang, F-D., Wolgamott, J., Application of Water Jet Assisted Drag Bit and Pick Cutter for the Cutting of Coal Measure Rocks, Final Technical Report on Department of Energy Contract ET-77-G-01-9082, Colorado School of Mines, April, 1980, DOE/FE/0982-1, 84 pages.

Waterjetting Technology - 26a More on waterjet assisted cutting

When mankind first began cutting out flints to make the tools and weapons that helped make primitive life more successful they often used either bone antlers or stones from the river as the tool to cut into the chalk or other host rock that held the flint. For thousands of years as rock was excavated for broader use, including making building stone, the rock continued to be cut manually, and it has only really been in the last hundred and fifty years that manual picks have been replaced with power driven machines. However, in great part, the machines have had to be made larger and heavier than they might need to be because, in large part, unlike the pick swung by a miner, the machine cannot selectively attack the rock that it is facing, but must cut along a foreordained path.

Figure 1. Conventional tool path in cutting concrete, the tool has to cut through both the hard aggregate pieces as well as the softer cement.

The tools that cut through the rock mechanically must, therefore, be able to cut through all the different materials that they are likely to encounter. Where the rock is like a concrete, with hard and soft parts, then the tool must be able to cut through the hard (aggregate) as easily and fast as it removes the soft (cement) phase if the machine is to maintain productivity. When I wrote about cutting concrete, I pointed out that this “brute force and ignorance” approach to getting through material was expensive in the time that it took to make a hole, and in the energy that had to be expended, both combining to make the overall process itself more expensive than it need be. That cost includes not just the costs of the process itself, but is also less obvious in that the machine itself has not only to be bigger, but because it also typically sees a wide range in force applied through the cutters to the drive mechanism, it also has a shorter operational life because of these fluctuations. (One shearer model saw such failures within six months of start-up).

There are a number of different ways in which the sensible application of high-pressure waterjets can improve cutting performance, lower machine size and cost and provide a win-win situation. But there is a need for caution, since the small size of the waterjets that are often used is much below the scale of many other parts of the machine, and, as a result, the precision with which the jets need to be applied can often be neglected.

In an earlier post on this topic, I discussed how a mechanical tool will crush the rock over which it passes during cutting. This crushed rock confines the bit, and is often re-compacted so that frictional forces rise, and the temperatures can be high enough to soften tungsten carbide.

Figure 2. Crushed rock under the impact of a mechanical pick. The size of the indentation relative to the size of the crushed rock is evident.

If the jet is to be effective it has to be directed into the cut at the point where the crushed rock is being created, so that the jet can remove the broken pieces as they are being formed. It is this critical location of the jet relative to the bit:rock contact that is often missed by those who have tried to apply this technology in the years since Dr. Mike Hood first demonstrated the benefit.

A number of experiments over the years showed that if the jet is more than about a tenth-of-an-inch (2.5 mm) from the point of the pick where it enters the rock (or the edge of the tool if it is a broader shape) then the jet will not be able to reach and remove the crushed material. This is particularly true when the rock being cut is, as in the above figure, a basalt, which the jet of water cannot normally penetrate at pressures of 10,000 psi. thus, if the jet does not reach the crushed material then the energy put into its creation has been wasted.

There are two parts to that last statement. They deal with all three planes in which the jet lies, relative to the point of pick contact. There is the relative position at which the jet hits the rock, where it is critical that it hits just where the rock is being crushed, and then there is the distance of the nozzle from that contact point. The latter point is one that I have also written about in earlier posts, but which can also be neglected when engineers are designing systems. There are a number of papers (which seemed to be at a peak at the 8th International Waterjet Symposium in Durham, UK in 1986) where this distance was set incorrectly (values up to 0.3 inches and above were reported) and it is not, therefore, surprising that some investigators found that the results were not as good as expected.

Figure 3. Jet cutting at the front edge of a pick (Front cover of the 8th International Symposium on Jet Cutting Technology, BHRA, Durham, UK, Sept. 1986)

If the nozzle is too far from the rock contact, then the pressure of the jet will have fallen to a pressure that is too low to be effective. This has been a less obvious problem to overcome, since to many observers the jet seems coherent with distance, but, given that jet flow is often divided between a number of different nozzles on the cutting head, the individual orifice sizes can be quite small (perhaps 0.01 inches in diameter). If the effective jet throw distance is 125 diameters, then the range of the jet is 1.25 inches. Yet in a number of applications, because of difficulties in fitting the nozzle in place, the orifice can be placed more than 2.5 inches from the rock contact. Again the result of this is to make the jet sensibly ineffective.

The jet has to be put into the right place, and with the correct amount of power, if it is to be of any use. Sometimes that can mean that the nozzle is placed behind the pick (so that it can be protected by the pick from the rock, yet can be brought close enough to the crushed zone that it can penetrate it from behind. This is a little more difficult to achieve, since the precision of location is a little more difficult.

Others have tried feeding the jet down through the pick, and I will explain some of the benefits and problems with this as I continue on this theme next time.

Waterjetting 22a - Mining horizontal coal

Over the last few posts I have discussed some of the problems that arise in dealing with the use of waterjets in mining coal, when the material mined has to be collected and transported away from the face where the coal is extracted. I thought I would follow on that thread in a few more posts, ending up, hopefully, where I began back in the process of removing thin layers of material (such as rust) from flat surfaces.

But to get there I am first going to go back to coal mining. One of the problems with adapting what we might call conventional hydraulic mining to coal is that many of the coal seams around the world are relatively flat – it is, after all, the way in which the vegetation that became coal was laid down. Thus the gravity that can be used in a steeply dipping seam as a way of carrying away the coal and the water together, is not initially that helpful.

There are several different ways that have been suggested over the years to solve the problem. Initially these were based on existing mining machines, and methods for mining the coal, but with the teeth of a conventional machine replaced with high-pressure waterjets. One such, as I have written earlier was the MS&T Hydrominer, where the cutting teeth along the edge of a coal plow were replaced with oscillating dual-orifice waterjets to cut a kerf around the coal being mined.

Figure 1. Artist’s impression of the initial Hydrominer, with jets cutting a slot one foot deep ahead of the wedge shape of the plow.

The water used was less than that conventionally used on a mining machine to suppress the dust generated as coal is mined from the solid, and the coal loads onto the armored face conveyor on which it rides down the face.

That particular design was based on an earlier mechanical machine, the Meco-Moore, which I had previously seen working on a longwall in the United Kingdom.

Figure 2. Meco-Moore mining machine set up to mine coal. The cutter jibs cut slots and the coal then collapses onto the transverse conveyor.

However this concept required a considerable investment in the supporting longwall equipment both to hold up the roof and to remove the coal. An alternative approach was to continue to conventional roof-and-pillar mining which is the most popular method of underground coal mining in the United States, but again replacing the cutting teeth with waterjets. The first of these was conceived by IIT Research Institute in Chicago, under Dr. Madan Singh.

Figure 3. A high-pressure waterjet continuous miner.

Unfortunately in this configuration the system did not work well. The jet pressures used were too high, and in consequence the volumes of the jets too low to achieve a deep penetration into the coal.

When the jets were replaced with a combination similar to that of the Hydrominer, and in a device we called RAPIERS, a slightly better performance was achieved, but the demand for innovation had, by that time passed for a spell, even though this particular machine was developed with considerable technical input and financial assistance from the Jet Propulsion Laboratory in Pasadena.

Figure 4. Progression of the RAPIERS machine in room-and-pillar mining.

Both of these machines required that a second set of machines sit behind the excavator and carry away the coal that had been mined, again at significant cost, and they also required machines to support the roof.

There is a different type of machine that is often used at the edge of the productive limit of surface mining. As seams near the surface get deeper so the cost of removing the overlying material becomes too expensive to justify continued mining. At that point companies may bring in an auger which can drill long holes into the coal, and remove the material as with conventional smaller augers that might be used for drilling in dirt (or even drilling holes in wood).

Figure 5. Conventional auger mining (Rosamine )

Because the auger drills a hole to the size of the following scroll, it is relatively easy to carry the coal back out of the horizontal hole, which might exceed 300 ft in depth. But there is a problem with the machine, in that the cutting force to push the auger teeth into the coal at the face of the machine has to be carried through the entire string of augers.

Because of the string of segments this becomes more difficult to control with longer depths, and in addition there is a friction loss due to the continual rubbing of the scrolls against the floor and sides of the hole. Together these act to limit the machine range, since there is little to steer the machine other than the direction of the hole, as it deepens.

Figure 6. Picks on the face of the auger, with early jets mounted in the center of the head to cut a central hole.

If, however, the picks on the face of the auger are largely replaced with waterjet nozzles, particularly at the outer edge of the auger, and with the flow directed there, rather than, as shown in Figure 6, towards the center, then an outer free face – up to a foot deep, can be cut ahead of the cutting head. With larger auger heads the nozzles can be placed across the face, to break the rib of coal, should it start to get too large – especially since the coal needs to be fragmented somewhat to feed down the auger.

Figure 7. Waterjets across the face of an auger (courtesy W.A. Summers)

The reduction in the amount of force that this allows on moving the auger into the coal can be illustrated by example. In developing a version of the machine we built an artificial coal face, made up of coal pieces and cement. It is a little more resistive than conventional coal, however the student, Chris Cannon, had little difficulty pulling the machine into the face with a come-along, even though he only had one uninjured arm at the time of the test.

Figure 9. Chris pulling the 2-ft diameter auger into the artificial coal seam.

By confining the coal and water it was possible to recover both, so that the water could, if needed be recycled.

I’ll continue the thread next post.

Waterjetting 12c - Jet assisted metal cutting

The first two posts in this section described how, in cutting through rock, the tool and the rock would be compressed together so that temperatures could be created in and around the tool that would exceed 2,000 deg C. That temperature is sufficient to melt the cutting tool, and in other situations is hot enough that it can ignite pockets of gas in underground operations that can have fatal results. However, by adding a small flow (less than 1 gpm) of water to the cutting pick not only is this risk of gas ignition or pick melting significantly diminished, but the water acts to remove the fragments of the rock as they are broken under the bit. This has two beneficial effects, first it removes the small rock that would otherwise be re-crushed and rub against the bit, causing the temperature rise due to friction. The second is that by also keeping the tool cool and sharp it can penetrate much deeper into the rock under the same forces, improving the efficiency of the cutting.

When a cutting tool is used to cut metal instead, the processes are somewhat different. However, because the tool rubs against the metal and cuts and deforms the metal that will be removed as a chip heat will still build up around the cutting zone.

Figure 1. Temperatures around a cutting tool in metal (Gear Solutions Magazine )

If you look closely at the temperature contours you will see that the lines stretch beyond the point where the cut is being made, and both the chip and the machined surface of the metal heat up to 500 degC. This narrow strip of metal on the surface of the piece is referred to as the Heat Affected Zone or HAZ, since the metal in this region has had its properties changed by the heat and deformation. And while the impact is more severe with a thermal method of cutting (such as plasma) there is some affect with mechanical cutting.

This can be seen, for example, if a metal piece is machined without cooling of the interface between the bit and the chip. Depending on the material being cut, this can lead to chips that are thermally damaged, are long and can be dangerously hot.

Figure 2. Strips of metal milled without cooling (Dr. Galecki)

If the surface of the chips are examined then the amount of heat damage is evident.

Figure 3. Surface of the chip showing the damage from the heat during cutting. (Dr. Galecki)

However this problem with the heat generated during cutting has been widely recognized, and so it has become standard practice to play a cooling fluid over the cutting zone during machining. To be effective the water must pass into the passage along the tool face and down into the cutting zone. It thus acts both to lubricate the passage of the chip up the blade, and separating it from the cutting tool, while cooling the bit and keeping it sharp.

Figure 4. Insertion of the jet into the cutting zone. (Dr. Mazurkiewicz)

When this is properly placed, and as with the jet assisted cutting of rock the precision required in placing the jet is around 1.10th of an inch, then the chip and metal surface are cooled and the tool remains sharp.

However, with conventional, lower pressure cooling, while the chip length is reduced and the surface is somewhat improved, overall cutting forces do not change.

Figure 5. Chips formed with conventional cooling (note the poor edge quality). (Dr. Galecki)

When the waterjet pressure is increased to the ultra-high pressure range, so as to ensure that adequate water reaches the tool, then the cutting forces are reduced and the amount of damage to the metal is further reduced

The result can be seen in the form of the chips that are removed, which are now much shinier in appearance:

Figure 6. Chips from high-pressure jet assisted cutting (Dr. Galecki)

Note that the surface of the chips are shiny, and that they are relatively small in size. The shiny surface is similarly reflected in that left on the machined part.

Figure 7. Cut surface left after high-pressure jet assistance to the cutting tool.

The resulting reduction in damage to the machined surface, as well as the lower machine forces, and the consequent lowering of the potential for “chatter” during cutting gives a higher cut surface quality which, because of the reduced damage to the surface has a higher fatigue resistance.

The amount of modification required to the equipment is not necessarily large, since the high pressure water can be carried to the tool through relative small tubing that has a small footprint. The pump can be located elsewhere. Further, while conventional cooling requires additives to the water (which make it more costly to treat the scrap) the clean water used in the jet makes this less of a concern.

Figure 8. Arrangement with a jet added to the cutting tool on a lathe. There are also instruments on the platform. (Dr. Galecki)

These results show that the heat damage that can be anticipated with conventional machining of metal can be significantly reduced with the addition of high-pressure water. This becomes even more clear where abrasive is added to the jet stream, and fortunately, thanks to colleagues in Germany, we have thermal images of this, which I will share, next time.

(For further reading see Mazurkiewicz, M., Kabala, Z., And Chow, J., "Metal Machining With High Pressure Water Cooling Assistance - A New Possibility," ASME Journal of Engineering for Industry, Vol. 111, February, 1989.)

Waterjetting 12b - Jet Assisted Mechanical Rock Cutting

Whenever material is cut using a conventional mechanical tool then heat is generated in the process. High-pressure waterjets can dramatically change this process, with considerable benefit. However, to explain some of the reasons requires a somewhat lengthy explanation. Which is why this topic is stretched over a number of posts.

In the last post I discussed how the heat generated proved to be a problem in the mechanical cutting of hard rock, where dragging a cutting tool across the surface caused a rapid temperature rise, to the point that the carbide began to melt.

Figure 1. New and used bit (Dr. Hood's Dissertation)

However, when the process was stopped before the bit was entirely eaten away, Mike discovered that the bit was not melting from the front, with all the flow of material dragging back under the bit. Rather the heat (which I profiled last time) was causing the bit to deform, so that it initially pushed the front of the bit slightly forward. This is visible in the bit and sketch of the location on the rock shown in Figure 2.

Figure 2. Deformation of the bit, as it starts to heat (Dr. Hood’s Dissertation)

Now why is this? Well the bit is being pushed into the rock so hard, in order to penetrate some 2/10ths of an inch or so that it is crushing the rock under the bit. But the process of breaking that rock occurs in stages. As the bit first starts to penetrate it cracks the rock, with cracks a little apart, but these intersect and break out pieces of rock, that can’t escape (being surrounded by rock and the bit). So the rock is crushed into very fine particles, which are then re-compacted and tightly fill all the space under the bit. I can show this with a picture from some experiments that Richard Gertsch carried out as part of his doctorate, at Missouri S&T.

Figure 3. Crushed rock under a cutting tool, the rock is basalt, and the white lines within the grey rock are the pulverized particles after the indentation. (Dr. Gertsch's Dissertation).

There are two things that a pair of high pressure jets of water can achieve if they are pointed at the bit:rock interface. But they have to hit at the point where the bit is breaking the rock. (And later work showed that they had to be within 3 mm – 1/8th of an inch – of hitting that point otherwise they won’t work).

Figure 4. Optimized location of the jets on the cutting tool. (Dr Hood's Dissertation)

The first, and anticipated advantage was that it would cool the bit, so that it would stay sharp. But the jets did more. If that jet (at a pressure of 10,000 psi) were played on the rock, after it had been crushed and re-compacted, then it would only be able to remove a small fraction of the crushed material, which would still resist the bit cutting.

Figure 5. Rock after indentation, with a jet cutting into the crushed material after it is crushed and re-compacted. (Dr. Gertsch's Dissertation)

But consider the case where the jet is playing onto the rock as those first cracks are made and the rock is still in larger pieces. The jet has enough energy to push that out of the way of the bit, and remove it all, as it is broken loose, without it being crushed to powder and without re-compaction. The jet thus cleans out the path ahead of the bit, so that it can penetrate deeper into the rock, at a lower force, as I showed in the force diagrams last time.

Figure 6. Rock cut with a tool that has jets playing on the face as the bit penetrates. (Note there is no crushed material and the cracks are flushed open and easy to grow much deeper into the rock). (Dr. Gertsch's Dissertation)

It is very important to understand, however, that it is the combined simultaneous action of the bit in breaking the rock, and the water in immediately flushing away the chips and keeping the bit cool that makes this work. And for that to happen the two processes have to occur at the same place. Placing the jet 1/3rd of an inch from the cutting face is too far. And this was sadly not understood by a number of those who studies the process around the world. However, in the UK, the Safety in Mines Research Establishment put a high-pressure pump on a tunneling machine.

The only underground mine that they could test it in was a limestone mine, and the machine they had available was only 25 tons and could not cut the limestone, so they built an artificial rock face out of sandstone to demonstrate that the idea would work. Problem was that they had so many visitors that they ran out of the demonstration rock.

Someone tried it on the mine limestone. Without the jets the head bounced around without cutting the rock. With the jets on it cut into the rock, so well that the mine asked that it be left there until it had drilled a tunnel for them.

Figure 7. Waterjet assisted road-header at the Middleton Mine in the UK.

The technology went on into commercial development, but a change in bit design from the flat cutters where the jets could reach the cutting zone, to a double conic pointed bit where it could not (easily) meant that the technology fell into abeyance, although investigators in Russia and ourselves developed some answers.

And so, from the cutting of the rock, we learned that when high-pressure waterjets were played into the crushing zone under a cutting bit, that they cooled the bit and removed the broken rock as it was produced. Thus a 25-ton machine (cost at the time $125,000) with a jet assist (say another $75,000) could outperform a 125-ton machine (cost $675,000) which did not have the assist. And as an incidental advantage since the rock is not totally crushed under the bit any longer, there is no respirable dust generated. But the way in which it worked would not work in cutting metal, where there shouldn’t therefore be the same advantage – yes?

Well actually no! When Dr. Marian Mazurkiewicz added waterjets to the cutting tool in a metal cutting operation in a machine shop, he found many of the same benefits. But we’ll talk about that, next time.