Waterjet Cutting – Deepening a hole and cautions with glass

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

The last three posts have described what happens when a jet of water first arrives on a surface and then starts to penetrate into the material. At a close stand-off distance, the erosion starts around the edge of the jet and continues to widen the hole as it gets deeper, until a point where the pressure at the bottom of the hole falls and the jet stops going deeper. The lateral flow away from the bottom of the jet continues to erode material, however, and so the hole gets a little wider at the bottom. This creates a small chamber under the entrance hole and this can build up enough pressure that it can cause the material around the hole to break.

Progress in the high-pressure waterjet drilling of a hole in rock

Figure 1. Progress in the high-pressure waterjet drilling of a hole in rock.

In the last post I showed where this happened with a 1-ft cube of rock that had been broken with a single pulse, but this fracture of the target can occur when piercing glass or other brittle materials. So the question becomes how to stop the fracture if one is trying to cut glass. This applies when the job calls for making an internal cut in the glass, and not when cutting in from the side, although that also has some problems that I will address in a later post.

When starting an internal cut, it obviously means piercing a starter hole through the glass in a region that is going to be part of the scrap, if this is possible, as it would be, for example, when cutting a sculpture. A secondary reason for that location, apart from confining any small cracks that might happen during the pierce, is that these starter holes are larger in diameter (for the reason given above) than the cut line once the jet starts to move, and that hole section would appear as a flaw on a final cut line.

Vanessa Cutler, in New Technologies in Glass discusses the process of cutting in more detail but suggests that the starter hole be pierced at a lower pressure than that to be used in the cut. This is so that the pressure within the cavity will remain lower during the pierce, and insufficient to cause the glass to break. She suggests (and she has a vastly greater experience than I in this) that the piercing pressure be around 11,000 to 18,000 psi – this varies a bit with abrasive grit size, machine size and glass type.

Detail of the glass sculpture “p1″, by Vanessa Cutler

Figure 2. Detail of the glass sculpture “p1″, by Vanessa Cutler. (Note that these holes do not pierce all the way through the glass but all end at the same depth.)

She also recommends, when there are multiple cuts to be made on a sheet, that all the piercing holes be completed before any cutting begins. One of the reasons for this is to avoid constantly resetting the cutting pressure, which could be a problem if you forget to lower the pressure back down before starting the next pierce. (Would I as an Emeritus Professor ever be that absent-minded? Why else bring it up?)

You will notice, with abrasive cutting into glass, that there is not the belling at the bottom of the cut like with plain waterjet cutting and that the hole tapers with depth as the cutting effectiveness reduces with the fall in pressure with depth; and the jet is less able to cut into the side walls of the opening at these lower pressures.

Stepping back from the cutting of glass to the more general condition where the jet runs out of power at the bottom of the hole, the main reason for this is the conflict between the water in the fresh jet coming into the hole and the spent water trying to make it out of the hole at the same time.

One way of overcoming the problem is to interrupt the flow of water into the hole. Back in my grad student days, we tried doing this by breaking the jet into slugs, so that one slug would have enough time to travel to the bottom of the hole, cut a little, and then rebound out of the hole, before the next slug of water arrived. There was relatively little sophistication in the tool we designed to do this. Simply it was a disk, with holes drilled in it at an angle.

Interrupter disk placed in the path of a continuous jet

Figure 3. Interrupter disk placed in the path of a continuous jet. (My PhD Dissertation)

The reason for the angled holes was to make the disk self-propelling as it rotated under the jet, since the angled edges of the hole forced the disk to continue rotating once started. (On a minor note, the disk would rotate at several thousand rpm, and the noise that it made was loud enough that I was instructed to only carry out the tests after the staff had left for the evening).

The penetration of a waterjet into sandstone with the jet running continuously (black), with the jet interrupted (red) and with the jet rotated slightly off-axis (green)

Figure 4. The penetration of a waterjet into sandstone with the jet running continuously (black), with the jet interrupted (red) and with the jet rotated slightly off-axis (green). (Brook, N. and Summers, D.A., “The Penetration of Rock by High Speed Waterjets”, Intl. Journal Rock Mechanics and Mining Science, May, 1969)

As can be seen in figure 4, with the pulsating jet more energy was getting to the bottom of the hole without interference, and the hole continued to deepen over time. However, the interruption tool had a number of disadvantages, apart from the noise and that the disk would be very rapidly destroyed under an abrasive jet. It was wasting a significant portion of the energy: In a more optimized design that I won’t discuss further, the energy loss was about 50%.

So it would be best if the jet was moved slightly over the surface, and in these early tests, the easy way to do this was to have the target rotate with the jet hitting the rock just offset from the axis of rotation. (At the time high-pressure swivels weren’t yet available). This gave the upper curve in figure 4, and a much more rapid penetration of the target.

In more modern times, the nozzle is moved either by causing it to move slightly around the hole axis or by causing a slight oscillation or “dither” in the nozzle while the pierce is taking place. This is generally a feature of the control software that drives the cutting table. But the reason for the movement is to get the water flowing in such a way that the water going out of the hole does not interfere with that going in, and so there is a reduced risk of pressure build-up in the hole, with the consequent cracking that this would cause.

Waterjet Technology – Crack Growth and Granite Sculpture

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

The last post in this KMT Waterjet Blog series showed that the main way in which waterjets penetrate into materials is by growing cracks that already exist within the material, and I used glass as an example to show that this was true.

It is this process during which water penetrates into cracks, and then comes under pressure, either by the impact of more falling water (say under a waterfall in nature) or because the water freezes and then thaws, that causes the cracks in the rock to grow under natural attack and the rock to slowly erode. As this happens, the cracks slowly grow and extend to the point that they meet one another, separating small pieces of rock from the solid.

Within the body of a piece of rock, the largest cracks that exist are normally at the boundaries of the grains of different minerals that make up the bulk of the rock. (Back in 1961, Bill Brace showed that the strength of a rock reduced as the square root of the increase in the grain size of that rock ).( Brace, W. F. (1961): Dependence of fracture strength of rocks on grain size. Bulletin of the Mineral Industries Experiment Station, Mining Engineering Series. Rock Mech. 76, 99± 103.) More recently, though still back in 1970, my second grad student, John Corwine, showed that it was possible to predict the strength of a block of granite knowing the size of its crystals.

Which makes a good time to tell a little anecdote. Back when I was doing my own doctorate at the University of Leeds (UK), we were looking at how waterjets drilled through rock and how that might be used to make a drill. We had already run some tests of different rocks that we placed under a nozzle, and gradually raised the pressure of the jet to see what pressure it took to make a hole in the rock. Tests on granite had shown that the jet (with a maximum pressure of just under 10,000 psi) would not drill a hole into those rock samples, and so the granite had been set aside. But, with the equipment just finished and yet having to go to lunch, I asked Dennis Flaxington, the lab technician helping me, to put a new sample into the rig so that we could run a test in the afternoon. When I came back I found that he had used a piece of granite. I made several disparaging remarks, at which point he noted that, having spent some significant time putting the rock in the apparatus, I should just go ahead and run the test (which normally took about 5 minutes) rather than being an unmentionable. And so we did, and, as I posted earlier, this is the resulting hole in the rock, which we were now able to drill right through in a process that took about half-an-hour.

9-inch thick block of granite drilled through by a 10,000 psi waterjet

Figure 1. 9-inch thick block of granite drilled through by a 10,000 psi waterjet at Leeds University. It took over 30 minutes. (Summers, D.A., Disintegration of Rock by High Pressure Jets, Ph.D. Thesis, Mining Engineering, University of Leeds, U.K., 1968.)

How could this now work, when a single jet clearly did not penetrate into the granite in the earlier tests? The answer is that as we moved the rock under the nozzle (we were slowly spinning the rock under the nozzle, and then raising the rock, since at the time there were no high-pressure swivels available for us to use) the jet passed successively over the edges of the different crystals in the granite. As it entered and pressurized these small fractures, the pressure in the crack was enough to grow the crack and remove individual crystals along the jet path. By starting at the center and taking successive passes around the axis, a large depression was cut into the surface, and the rock could then be raised, and a second smaller layer removed. Repeating this slowly removed the rock in front of the nozzle, and at the end of the test we had drilled through 9 inches of granite.

From this experience, over time we went on to cut, for a University, a lot of granite. Obviously, to cut at a competitive rate we had to cut at a higher pressure than just 10,000 psi. But, after showing that we could cut Georgia granite at a competitive rate in tests run at 15,000 psi down in Elberton, Georgia, Dr. Marian Mazurkiewicz and I led a group of our students in cutting 53 blocks of that granite to form the MS&T Stonehenge that now sits on the University campus.

Stonehenge at Missouri University of Science and Technology

Figure 2. View of the Stonehenge at Missouri University of Science and Technology, the vertical blocks are some 11 ft tall. The entire sculpture was cut by high pressure water jets operating at between 12,500 and 15,000 psi. (MS&T RMERC ).

Cutting commercially is not quite as simple as it might appear, since larger blocks such as those shown in Figure 2 will contain rock that varies quite significantly in properties as the cuts progress. In the Stonehenge case the rock came from close to the top of the quarry, and the cracks in the rock were quite well defined. Some fifteen years later, we were fortunate enough to be asked to cut a second sculpture, but this time working with the internationally acclaimed artist, Edwina Sandys. Edwina had designed a sculpture for the campus, the Millennium Arch, which required that we cut two figures from blocks of Missouri granite, and polish them to create one group, while using the original pieces as part of an Arch that would stand some 50 ft away.

The Millennium Arch at Missouri University of Science and Technology

Figure 3. The Millennium Arch at Missouri University of Science and Technology. (Each vertical leg of the Arch is some 15 ft long, and the figures removed and in the background, are 11 ft tall).

The vertical legs were first cut to shape, and then the figures cut out from them. In order to contain the crack growth to limit the amount of material removed the cutting lance had two jets inclined outwards and the lance was rotated at around 90 rpm, as the lance made repeated passes over the surface, removing between a quarter and half-an-inch of rock on each pass, until it had penetrated through the rock. It took 22 hours of cutting to isolate the female figure from the host block. The slot width was around an inch, and there was some significant difficulty in cutting this slot as the quality of the rock changed within the blocks being cut. (The problem was solved by raising the cutting pressure).

Partial cut for one of the figures of the Millennium Arch

Figure 4. Partial cut for one of the figures of the Millennium Arch, checking the depth.

This second sculpture illustrates both an advantage and a problem for the use of waterjets in cutting rock pieces. Use of the water gives a relatively natural look to the rock, although the vertical surfaces of the arch and the capstone were all actually “textured” to look natural using a hand-held lance at 15,000 psi. (The rock is a little harder than that from Georgia and most of the cutting took place at around 18,000 to 20,000 psi). But when the polished surfaces for the inside of the verticals and the isolated figures were prepared, the rough initial surface required much more time to grind and polish flat than a smoother initial cut would have needed.

Because water alone penetrates along crystal and grain boundaries in the rock, the surface left is relatively rough. This gets to be even more of a problem if waterjets are used to cut wood. Here the “grain” boundaries are the fibers in the wood structure. Thus when a relatively low pressure jet (10,000 pai) cuts into the wood, it penetrates between the fibers and the cut quality is very poor. One of the first things I have asked students to do, when given the use of a high pressure lance for the first time, was to write their name on a piece of plywood. Here is an example:

Student name written with a high-pressure jet into plywood

Figure 5. Student name written with a high-pressure jet into plywood. Note that areas of the wood around the jet path are lifted by water getting into the ply beneath the surface layer, and that part of the top ply between cuts is removed in places.

I thought about having you guess the student name, Steve, but this is one of the more legible ones. (Female students generally cut the letters one at a time and were more legible, male students tried to write the whole name at once).

There are many similar examples that I could use to illustrate that, while there are tasks where waterjets alone work well, when it comes to precision cutting, then adding a form of sand to the jet stream to provide a much more limited range to the cutting zone can give a considerable advantage, and so the field of abrasive waterjet cutting was born, and discussion of that topic will lead, in time, to a whole series of posts.