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.