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 12A - The heat involved in cutting - Part A - Cutting Rock

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 is almost a year since I first began putting these posts together. 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.

Figure 1. Drilling the rock around a gold seam in South Africa (New Scientist )

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).

Figure 2. Design of cutting tool to carve a channel into the quartzite (Mike Hood )

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.

Figure 2. Temperatures building up on the carbide cutting tool (after Hood ibid)

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.

Figure 4. Location of the jets on the carbide bits in the initial tests carried out by Dr Hood. ( hood Dissertation )

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.

Figure 5. Change in thrust force when waterjets are placed in front of the drag bit (Hood Dissertation)

Figure 6. Change in cutting force when waterjets are placed in front of the drag bit (Hood Dissertation )

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.