Waterjet Technology- Conventional Machining compared to Waterjet Tables cutting metals

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.

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 )

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)

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)


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)

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)

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)

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.

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)

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.)

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.