Waterjetting 13a - an introduction to milling

In contrast with the earlier use of high-pressure waterjets in material removal in civil engineering and mining, when industrial waterjet cutting first began it was used to make thin cuts through different materials (in the early days often paper and wood products). Through cutting, particularly in relatively thin stock, has a wide range of industrial uses, particularly when the pieces are cut “cold” and with edge qualities that are, even with the first cut acceptable as the final surface cut needed for the part.

Over time the advantages of this new cutting tool became more apparent, and the range of materials that the AWJ jet could viably cut was extended into metals and ceramics. Yet conventional machine tools do more than just cut the edges of parts, and so questions arose as to the best way to achieve the milling of internal pockets within different materials. Within relatively soft rock, and with pressurized water alone, it is possible to generate interesting shapes.

When we first started experimenting with cutting rock at Missouri University of Science and Technology (MO S&T) the support equipment that we had was very basic, and the budget similarly restricted. In order to achieve precise positioning and control of the speeds during the cutting process, we therefore mounted the nozzle and support lance on the traverse of a conventional lathe. The samples were mounted into the chuck, so that we could achieve controlled cutting speeds. To get a number of sample cuts in a single test we placed a sheet of metal, with slots cut into it, between the nozzle and the rock.

Figure 1. Rock rotates in a lathe while the nozzle traverses across the face.

The notches cut into the metal plate were cut wide enough to allow the jet to make a single pass over the rock surface as the rock rotated and the nozzle swept past the slot, and they were widely enough spaced that the cut made through one slot did not interfere with the adjacent cut made through another.

Figure 2. Slots cut through the mask into the rock target.

After a while we became a little more adventurous and realized that, by making the mask an interesting shape that we could leave part of the rock uncut, but mill out all the rest of the material exposed to the jet, by adjusting the feed rate of the nozzle relative to the rotational speed of the rock.

We thought at first that the feed of the nozzle (easy to set with the lathe) should be one jet diameter for each rotation of the rock, but the jet spreads as it moves away from the nozzle and this turned out to be a little too small a distance, and we ended up setting the feed at about 1.5 times the jet width. This “incremental distance” is going to vary between systems, as a function of nozzle design and size, jet pressure and the distance between the nozzle and the target. In this early work in the technology (this was back around 1972 IIRC) the nozzle stood back from the rock at about one inch standoff. In more modern applications that distance can be quite a bit less, and this changes the incremental distance. Also bear in mind that the speeds at which plain high-pressure waterjet cuts are most efficient are much higher, across the target surface, than the optimal speeds for AWJ work.

So, since there was a need to remind folk that waterjetting could be dangerous if proper care was not taken during its use, we used this idea and made a sculpture.

Figure 3. Skull figure carved out of sandstone.

For simple lettering and shapes such as that shown above, the practice was to cut the desired shape into a metal plate, using perhaps a cutting torch, and then attach this over the rock. The two locations for the retaining wire can be seen on the sides of the piece. This allowed the plate to rotate with the rock piece as the lathe turned, and did away with the stationary plate between the nozzle and the sample.

By adjusting the feed speed and the rotation speed of the piece a relatively smooth surface could be left in the excavated pocket. (See the depths of the eye sockets). The process is known as “Masked” milling, since the plate masks the sections of the rock that the jet should not be allowed to mill into.

This works well when the work piece allows the use of plain high-pressure water, since it is relatively simple to make the mask out of a material (in this case steel) that the jet would not erode significantly. Thus the same mask could be used repeatedly to make copies of the original (though I think, in this case we only made around three or four).

But what happens when the jet is an abrasive waterjet, and we want to make pockets in the same way as I have just described. Because the AWJ will cut through a thin mask it was not an optimal choice for the process.

One can, with precise control of the nozzle position, have the jet move back and forwards over the desired pocket geometry. With the more accurate controls available today it is possible to slow the nozzle as it reaches the end of the pocket, increment it over the desired distance, and then have it cut an adjacent path back along the material to the start side of the pocket. Here the process would be repeated, moving backwards and forwards until the desired pocket geometry had been covered.

The problem with this approach is that the depth of cut into the target is controlled, in part, by the length of time that the jet plays on any one point, or inversely as the speed with which the nozzle is moving over the surface. So moving the nozzle more slowly as it approached the edge of the pocket (which you have to do because the robotic arm driving the move can’t instantaneously stop, increment over, and reverse direction because of the inertia in the system) is problematic. This is true only however if the pocket has to have a smooth regular floor of a fixed depth but most, unfortunately, do. And slowing the nozzle at the end of the cuts means that the depth of the pocket would be deeper along the pocket profile, relative to the body of the cut.

And so, for lack initially of an alternative approach, for some time the industry used masks that would protect the sides of the pocket, and provide a space over which the nozzle could decelerate, increment over, and turn back. The mask would be eroded away, but in desirable parts (often expensive to make in the desired material) the ability of the abrasive waterjet to make the pocket in the first place allowed the expense of the mask to be written into the cost of making each part.

There is, however, at least one other way of doing this, and I will discuss that, next time.

A little more on Jetting - surgery, art and diesel replacement

Being somewhat jet lagged today, after returning from Austria last night, I won't attempt anything that requires any great mental dexterity today, but rather continue on the theme about developments that are going on in my field that are likely not to be known to the more general public. One of the things that has been fascinating has been the way in which waterjets are being developed for use in the medical field. There was another illustration of this in a paper by Biskup et al, at Leibniz University in Hannover. When surgeons repair torn ligaments in the knee (an Anterior Cruciate Ligament Reconstruction) they use screws to hold the transplanted piece in place. Use of different materials for these screws has shown to have some long-term problems when they are made of metal. Recent work has shown that if these screws are themselves, however, made of bone, then, over time, the screw is integrated into the surrounding structure and becomes more stable. However it is hard to machine bone conventionally because if it gets heated, then the bone dies. If, however, it is shaped using an abrasive waterjet, to cut the thread, and the internal channel, which is also used to turn the screw, then temperatures can be controlled so that the bone is still viable. (Though it needs some chemical treatment to remove prions).

Back in 2006 the conference was held in Gdansk, and one of the interesting papers was on some work being done by Przemyslaw Borkowski at Koszalin University of Technology, in transferring photographs into inscribed pictures cut into metal. We took that idea and have moved it into a surface textural change that creates the picture as a 3-tone image in metal and rock (hence the "art") but there has been another development that has moved this into commercial availability. Nathan Webers and Carl Olsen of Omax Corporation, has modified the software on their cutting tables so that a controlled depth image can be inserted into a surface from an image. This, for example, is a lizard etched into aluminum.


The image was about 6 inches across and maybe half-an-inch deep.

It may make life a lot easier for those who have to carve images into stone, though removing a little of the artistic license with which sculptors apply their craft.

Incidentally Przemyslaw gave a talk at this conference on the use of waterjets in crushing coal. If you can take coal down to about 5 microns and mix it with water at 50% (roughly) GE have shown - by running a locomotive for more than 700 hours on the track - that it can replace diesel fuel. Looking at different available coals, it appears that brown coal is a little easier to break to the required size range than bituminous, which were the two varieties that he looked at. Using waterjets to do the crushing can make the process technically simpler and less energy intensive than other methods of getting the coal down to the required size.

The Waterjet Meeting in Graz

I was not planning on writing about the Conference that I have been at this week. It deals with the uses of high-pressure (typically about 55,000 psi) water jets. However there were a couple or three papers that had some relevance to the topic of energy, so I thought I would craft these into a small post.

The first one was a paper by Professor Soyama of Tohoko University in Japan, who has been looking at the events that occur during a cavitation cloud collapse. For those not familiar with what this is, when you adjust the flow pattern of a fluid so that forces try to stretch the fluid, the fluid ruptures into small bubbles. Instantaneously the bubbles have nothing in them, but they fill with vapor from the walls, over time. However there is rarely much time since, as the bubbles move into an area where there is a positive pressure in the water they collapse. But when they collapse it is not totally symmetrical. As a result tiny micro-jets (known in some circles as Munroe Jets) are formed, and these can generate impact pressures of up to a million psi (we proved this theoretical estimation, made originally by Al Ellis at UCSD). At the same time, as Professor Soyama notes, the temperatures that are locally generated can be very high, to the point where light can be generated in the 300 to 700 nm wavelength range. The combination of the two creates an condition where carbon dioxide, injected into the flow, can be (and in his laboratory was) converted into methane. This is a relatively new discovery, and at a scale that may likely be impractical to put to large-scale commercial development, but on the other hand . . . . . . .

The second paper was by Franz Trieb of BHDT GmbH who talked about the use of high pressure jets in helping the construction of prefabricated brick walls. At the rate of 400 sq. m. per day, for 3 workers. It uses the Redbloc system which glues well-formed brick courses together and gives the high quality wall. The water jets cut and trim shapes (such as door entries or windows) in the wall, which is then delivered to the construction site as a finished assembly, and can be rapidly assembled. The resulting house has a number of other benefits including lower cost.

The third item of interest was the description by attendees from KMT high-pressure systems who talked about the transition to gher pressures (up to 90,000 psi) where individual pieces can be cut about 50% faster rates than with conventional pressure. The overall result is a reduction in cutting costs, and lowering of the energy required to cut a part, and the water needed for the process.

Overall it has been a very interesting meeting, though likely my last. There was a mild incredulity when I brought up the subject of peak oil.

Views from the road

I am setting out on another trip, that will last some ten days, but am taking my laptop and so should be able to continue some sort of commentary as I go. But I also thought I would add the occasional view of where I am, since it may be of interest. To kick the idea off, this is one of the items that I helped with at the Millennium. It sits on our campus and, if you look carefully, you can see that the two back statues came out of the legs of the Arch in the foreground. The figures are just over 3 m high, and the legs are about 70 cm thick. Each leg weighed in around 30 tons before we cut them. (It is granite and we used only water in making the sculptures).

The Millennium Arch, by Edwina Sandys

10:55 pm I stopped by and took an up-to-date photo, rather than the stock one, and have changed them out. We drove off into a torrential thunderstorm with a tornado warning coming up the road behind us (and Beethoven's Violin on the radio).