Waterjet Technology – Jet Assisted Mechanical Rock Cutting

This Waterjet Weekly is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

This Waterjet Weekly is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

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.

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

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.

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

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

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.

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).

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).

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

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.

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

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.

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)

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.

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

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.

Waterjet Technology – The Heat involved in Cutting Rock-Part A

Dr. Summers Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Series

As I was beginning to write these posts Bob Pedrazas, who is kind enough to transcribe these words over to the KMT Waterjet page, gave me some questions that had been asked about the technology of high-pressure and ultra-high pressure water jet cutting. At the time I gave an academic answer, pointing out that I would have to explain some background material, before the average reader might be able to follow the logic of some of my answers. Thus I did not plan to answer specific questions (though I would respond to comments) in the early days of the site.

It has been over one year since I first began putting these posts together on kmtwaterjet.com. Along the way I have tried to answer some of the questions on his list without specifically calling out the question – for example the answer to the question as to whether selecting the right system was important was, I hope, shown by a plot early in the series. I presented a comparative graph that showed that despite different systems having nominally the same power, water and abrasive values, when comparative cuts were made, in similar materials that there was a considerable difference between the depths of cut that could be achieved using the different designs. (So obviously selecting the best system has considerable benefit to the operator, over selecting another).

One question that was raised relates to the heat of the cutting process. And while it has a relatively simple short answer (waterjet cutting used to be sometimes called “cold cutting”) I am going to take a few posts to explain the answer in a little more detail. Part of the reason for this is that the information that I have comes from several sources, some in cutting rock, and some in cutting metal, and there are some different applications along the way that all fold into the general topic of heat in the cutting process.

Let me begin with the work of a friend of mine, Mike Hood, who was working in the South African gold mines at the time he decided to go for his doctoral degree (Hood, M. (1978) “A Study of Methods to Improve the Performance of Drag Bits Used to Cut Hard Rock,” Ph.D. thesis, University of Witwatersrand, R.S.A.”). To understand the problem that he addressed you should know that in some of the mines in South Africa the gold-bearing rock is contained in a very thin layer, within a surrounding host rock, which is a quartzite and very hard to cut.

Drilling the rock around a gold seam in South Africa

Drilling the rock around a gold seam in South Africa

However if miners are to get in and extract that thin vein (which is often only six-inches to a foot thick they have to drive passages that are big enough to work in (perhaps six-feet high). Thus rock on either side of the thin vein of gold-bearing material is drilled and then the entire rock face is blasted out using explosive. Now the gold ore is mixed in with all the other rock from the blast. This means that all that material must be lifted perhaps two miles to the surface, and then ground to a fine powder to release the gold. Both of these are very energy intensive operations.

Consider instead if, before the rock on either side of the vein was blasted out, the face could be cut with two slots, one above and one below the gold reef. That could then be removed, and the rock on either side could then be blasted, but instead of being hauled away it could be packed into the open space behind the working area, holding up the roof and saving a huge amount of the processing energy otherwise required. (There is less than half an ounce of gold in a ton of the reef ore, and when the rock on either side is included then this concentration becomes much less).

Design of cutting tool to carve a channel into the quartzite

Design of cutting tool to carve a channel into the quartzite

Mike was initially looking to used carbide cutting teeth to cut into the rock and make these slots. However, he rapidly discovered that as he dragged the bit across the rock, that even at relatively shallow cutting depths (about 2/10ths of an inch) the cutting tool was getting very hot very quickly, to the point that the carbide was starting to melt.

Temperatures building up on the carbide cutting tool

Temperatures building up on the carbide cutting tool

Obviously, if the bit could be kept cool, then the carbide would not soften, and thus remain sharp and able to cut better. Yet the temperatures that the bit was reaching very early in the cutting process meant that there was a lot of heat being generated during the cutting.

As a result he decided to run a series of tests in which he played water onto the leading edge of the bit to cool it. But because of the heat involved he wanted to get a fairly high flow rate to the bit, which was almost buried in the rock, and otherwise hard to get to. So to ensure the flow got to the right place he used higher-pressure waterjets that flowed through small nozzles, mounted to shoot the jets at different points on the carbide:rock cutting face.

Location of the jets on the carbide bits in the initial tests carried out by Dr Hood.

Location of the jets on the carbide bits in the initial tests carried out by Dr Hood.

There are two forces needed to make the drag bit cut into the rock as it moves forward. The first of these is the Thrust Force, which is the force which pushed the tool into the rock, so that it will cut to the required depth. The second is the drag or Cutting Force that is used to pull the bit along the face at the required depth.

Without the waterjets on the bit, the load on the machine was exceeded when the drag bit was cutting to about 5 mm deep (2/10ths of an inch) into the rock. But when the waterjets were added to the bit, not only did the bit stay cool, but it was able to cut more than twice as deep, at lower forces onto the bit, and thus with a lower power demand on the machine.

 

Change in thrust force when waterjets are placed in front of the drag bit

Change in thrust force when waterjets are placed in front of the drag bit

Change in cutting force when waterjets are placed in front of the drag bit

Change in cutting force when waterjets are placed in front of the drag bit

This result has since been repeated by a number of different laboratories around the world, and led on to the development of mining machines, and other applications.

In the next post I will explain why what happens does, and why adding such jets to a cutting tool can, in the right place, save considerable amounts of money and time.

 

Waterjet Cutting-Abrasive water jet and cut taper

Dr. Summers Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Series

I have spent some time in recent weeks discussing the use of abrasives in waterjet cutting, and particularly some of the issues that are involved in getting the abrasive distributed relatively evenly through the jet stream, and accelerated to as high a velocity as possible by the time the jet leaves the focusing tube. This issue has become more important as clients request more precise cuts, and edge quality and alignment become more critical. As the jet cuts along a surface the amount of material that is removed (i.e. the depth of cut simplistically) is controlled by the number of particles that impact along that axis. And that, to a degree, is controlled by where that axis lies, relative to the axial diameter of the jet that runs parallel to the direction of cut. Different conditions give different particle densities, but even within those conditions, the material under the center of the jet will see many more particle impacts than those on the side.

Particle distribution across two abrasive waterjet streams with the same focusing tube diameter, but different waterjet orifice diameters

Particle distribution across two abrasive waterjet streams with the same focusing tube diameter, but different waterjet orifice diameters

As the above figure shows, in order to achieve the best abrasive cutting the rate of abrasive feed must be tailored to the nozzle size and the jet parameters. The density of the abrasive in the resulting stream can be optimized for those conditions and, as discussed in earlier posts, adding too much abrasive to the system will end up being counter productive. A simple example can show this, in a test where we cut grooves in a block of granite, with the concentration of abrasive in the jet stream increasing with each pass.

Cuts made into a granite block, with abrasive feed rate increased as the cuts progressed from the left-side of the block to the right. Note that beyond a certain AFR the depth of cut begins to decrease.

Cuts made into a granite block, with abrasive feed rate increased as the cuts progressed from the left-side of the block to the right. Note that beyond a certain AFR the depth of cut begins to decrease.

Figure 2. Cuts made into a granite block, with abrasive feed rate increased as the cuts progressed from the left-side of the block to the right. Note that beyond a certain AFR the depth of cut begins to decrease. (Yazici, Sina, Abrasive Jet Cutting and Drilling of Rock, Ph.D. Dissertation in Mining Engineering, University of Missouri- Rolla, Rolla, Missouri, 1989, 203 pages.) There is, however, a second consequence to the concentration of particles across the jet, and that is that the material under the jet on either side of the center-line of the cut will see a smaller number of particles impacting the surface, than that at the center. As a result the material will not be cut as deeply, and as the slots shown in Figure 2 illustrate, the cut will, as a result taper in on both sides. In many applications, where the material to be cut is relatively thin, or where the exact alignment of the edge is not that critical this may not be important. However there are applications where edge alignment is required on the order of a thousandth of an inch or two over the part thickness, with the part being half-an-inch or more thick. One way to achieve that precision of cut is to slow the traverse speed down. If the jet is moving slowly enough then there will be enough particles hitting the material at the edge of the cut, that the edge will be cut vertically downwards.

The effect of traverse speed on the edge taper angle (in degrees) in cutting titanium.

The effect of traverse speed on the edge taper angle (in degrees) in cutting titanium.

Notice that, because the jet tends to flare out a little as it moves away from the nozzle, the taper angle goes negative if the speed falls to too low a value. In this particular case the nozzle was moving across the surface at a speed of about quarter-of-an-inch per minute. To get enough particles on the sides of the jet to cut a parallel slot edge, however, means that much of the abrasive in the center of the jet is not doing any work, but is rather being powered up and paid for to no real advantage. Thus, in most cases, (though not all) cutting very slowly to achieve precision on the residual edge of the cut is an overly expensive way of achieving the precision. Given the relatively small angle that the taper cuts it is usually more cost-effective (providing the table allows this) to slightly tilt the cutting head, so that at higher cutting speeds the taper is effectively removed on the edge that is left. Obviously the taper on the piece of material being removed is made worse, but if that removed piece is going to be cut later into a different shape for another purpose, then this excessive taper on the initial surface comes with no great cost. The taper angle and the speed relationship will vary both for the material being cut, as well as for the different parameters of the abrasive waterjet, and so – as with most cases where this sort of precision is required – a small test program to establish the best parameters for the cut will be needed. There are other ways of achieving this precision in cutting. One is to make multiple passes over the surface, with the jet removing only very small increments of material at one time. Again if this is carried out carefully and precisely the edge quality can be maintained, at the same time as the depth of cut can be well controlled allowing pockets of material to be removed from the work piece. However that gets into the whole issue of milling material from a target, and that is the topic for another day. It brings up the inter-relationship between traverse speed and depth of cut (which combine to give the area of cut surface, which can be used in some cases to optimize the cutting performance of a system, particularly where edge quality is not that rigid a requirement). And more particularly it brings up the quality of the walls and floor of the pockets created.

Factors to be considered in milling a pocket, illustrated by a multi-level pocket created in glass.

Factors to be considered in milling a pocket, illustrated by a multi-level pocket created in glass.

Figure 4. Factors to be considered in milling a pocket, illustrated by a multi-level pocket created in glass.

Labels:abrasive cutting,abrasive particles,cut taper,milling,particle distribution,traverse speed

Waterjet Technology – Mixing abrasive with a water jet and differences in orifice types

This Waterjet Weekly is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

This Waterjet Weekly is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

Sometimes I would get the feeling (particularly when talking to some of my students) that the mixing chamber of an abrasive waterjet (AWJ) cutting system, together with the feeds in and the focusing nozzle outlet, were considered to be some magical black box out of which a perfect cutting stream issues to cut the desired material. There are, in fact a variety of different chamber designs that can be purchased from different manufacturers. Some will tell you that all designs cut roughly the same, and that there is little difference between them. As I commented in one of the earliest posts in this series this is not true. Over the years we have run numerous comparative tests on different designs, using different abrasives, abrasive feed rates and target materials, and have found a broad range of results. For example, in cutting steel at a fixed traverse speed and other conditions, we found an average comparative performance as follows:

Comparative performance between 12 nominally similar abrasive waterjet cutting nozzles in cutting through steel at a standard speed, pump pressure, and abrasive concentration.

Comparative performance between 12 nominally similar abrasive waterjet cutting nozzles in cutting through steel at a standard speed, pump pressure, and abrasive concentration.

I described the actual test in another post, and it is clear from these averaged results that there is a wide difference in performance between the nominally similar tools. While I don’t think there is a lot of interest in going through the details of different designs it might be helpful to explain some of the factors that play a part in producing jets of greater or lesser performance. To return, first, to the basic construction of the mixing chamber and focusing tube assembly (AWJ nozzle), one starts by recognizing that the major cutting performance will be achieved by the particles which remove material when they hit the target. The energy that they have, however, comes from the water that is fed into the AWJ nozzle through a small jeweled orifice, or waterjet nozzle, at the top of the mixing chamber.

Basic waterjet nozzle design That waterjet stream is small and initially highly focused and fast moving.

Basic waterjet nozzle design That waterjet stream is small and initially highly focused and fast moving.

As it moves through the mixing chamber, as I have described in other posts, the outer edges of the jet slow down, and gradually the jet fans out and breaks up into fragments. There is no benefit in trying to inject the abrasive into the jet at the beginning of this passage, since, at that point, the outer layers of the jet have enough energy to knock away the particles before they can enter the fastest moving segment in the core. Rather it is better to inject the abrasive further down the chamber, so that the jet will have begun to break down into slugs, and the abrasive can be positioned so that it is impacted by a sequence of the individual slugs and accelerated to the desired velocity. There is an additional benefit to moving the abrasive feed line a little further down the chamber. When the jet stream is rapidly switched on and off, when for example, piercing a series of small holes in a part, then the driving pressure pushing water out of the waterjet orifice switches off and on. When it switches off there is a short period where the differential pressure will draw fluid from the chamber back through the waterjet nozzle. If there are small particles of abrasive in the vicinity (and with some designs there are) then these can be drawn back through the upper orifice, and then pushed back down by the succeeding water flow in the next pulse. This can rapidly erode softer jeweled orifices, so that they round or chip, not always evenly, and degrade the resulting waterjet as it flows into the chamber. This disruption can move the jet from being in the center of the chamber, and cause poor abrasive pick-up or accelerate wear of the chamber walls, and in the focusing tube. All of these degrade performance. (The solution, if you can achieve it, is to use a diamond upper orifice, since this is largely non-responsive to the passage of the abrasive back and forth, and retains its shape and the jet performance it was designed to produce much longer – providing a cost benefit to the change).

Wear on a ruby waterjet orifice inserted at the top of a mixing chamber after 15 minutes of use. (The dark particles are small particles of garnet)

Wear on a ruby waterjet orifice inserted at the top of a mixing chamber after 15 minutes of use. (The dark particles are small particles of garnet)

Chipping on the edges of sapphire and ruby orifices (after Powell 2007 WJTA Conference)

Chipping on the edges of sapphire and ruby orifices (after Powell 2007 WJTA Conference)

Lack of wear on a diamond insert nozzle after being in use for several hundred hours.

Lack of wear on a diamond insert nozzle after being in use for several hundred hours.

With the abrasive inlet channel and jet passage designed to get the abrasive into the jet where the water jet is broken up, yet still moving at high speed, there needs to be a sufficient distance for an optimal energy transfer to occur. Beyond that point, with the particle and the abrasive (which will have partially been broken in the contact between the jet and the particle, between particles striking one another, and between contact between the particle and the walls of the AWJ nozzle) the cutting jet has to be refocused into the narrow cutting stream that is required to give the finished cut surface desired. The refocusing of the mixed jet (air, water and abrasive) is achieved with a focusing tube, which is made up of a conic section which brings the jet back together, and then a straight section which allows further energy transfer between the three component parts of the jet, before the jet issues from the orifice aimed at the target. The passage of the particle down this tube is not always straight. Wear typically begins at the tip of the conic section as it feeds into the tube. The wear within the tube will often take up a pattern, as any irregularity in the flow causes it to bounce from one wall of the tube to the other creating a wear pattern along the walls that has a wave-like structure.

Wear at different points along the waterjet focusing tube

Wear at different points along the waterjet focusing tube

When this wear reaches the mouth of the focusing tube, then the downstream orifice is eroded out of a circular shape, and the jet that comes out no longer will cut cleanly, or to as great a depth. At that point the nozzle is worn out and should be replaced. The point at which that replacement occurs varies depending on the quality of the cut that is required. Obviously when the cut being made is at a precision of a thousandth of an inch over a cut depth of half-an-inch (as with some aircraft parts) the replacement point is reached a little earlier than if the cuts being made are a rough cut merely, for example, to separate two parts one from another, and provide a rough shape to the piece. I’ll continue this topic in the next segment of this section.