Thursday, December 11, 2014

Waterjetting 28a - Cutting rock on a table

In previous posts I have written about the use of lower pressure water (around 10,000 psi) as a way of cutting through rock. From the time that we first made a hole through nine-inches of granite while I was a graduate student some 50-odd years ago the way that we have recommended that rock be cut, in a mining situation, has been to use lower pressure and higher volumes of water. This is so that as many natural fractures around the individual grains and crystals can be developed at one time as possible. However the result of this is that the cut progresses along the grain boundaries of the infrastructure of the rock, that a roughly edged cut is made, rather than a smooth cut surface. In mining applications this isn’t necessarily a bad thing, but when cutting counter tops and other ornamental structures for marble and granite surfaces inside a building then a rough surface is definitely not often required.

So how can a relatively smooth surface finish be created along the cut? One step,that works with softer rocks (such as some pink granites) is just to increase the jet pressure, while at the same time reducing the volume of the stream flow. Once one reaches the ultra-high pressure regime (which is, for this article, considered above 35,000 psi) and with jet diameters on the order of 0.01 inches or less, the jet stream is more typically going to cut through a crystal within the rock than to just work on the cracks that lie at the edges of the crystal.

Unfortunately there are sufficient cracks and crystal boundaries within the rock that it is not possible to ensure that at some point as the jet cuts down through the rock, and along the desired path, that it won’t find a crack at a critical length and alignment that the crack will break out a larger chip. This is less likely to happen within the cut, since the confinement of the surrounding rock acts to reduce excessive crack growth, but can quite often occur at the rock surface, particularly where there has been some earlier damage that has left larger cracks within that surface. (This includes heat treatment).

That, however, is a specialized case, and in the more typical situation an increase in jet pressure to 50 ksi will not, by itself, produce the clean edge needed. Part of the reason for this comes from the striation planes within marbles, which can offer an easier path for the jet to penetrate, as the cuts get deeper, rather than having the hole continue forward along the jet axis. To overcome these problems it is easier, with the ubiquity of abrasive waterjet systems, to instead change to add an abrasive to the waterjet.

Dimension stone (the trade name for the decorative rocks such as marble and granite) is generally through cut with slab depths that are less than an inch-and-a-half thick, although greater depths can be specially prepared. Often the slabs are polished before they are finally cut to shape. We found that preferable, since when doing the final polish with successively finer grinding wheels (used for example in creating the Millennium Arch) the edge stress that can be generated by the wheels themselves can cause chipping along the edge of the work. This, in turn, either requires a regrind down to remove the chip, or some form of repair, which we found it difficult to make invisible given the complex structure of the granite. This is particularly true when relatively narrow ribs of material are being cut. As an example, consider the cartoonish mining figure that was made some years ago.

Figure 1. Toon miner carved from 3-inch thick granite.

The front and back surfaces were polished before the figure was cut from the slab, given the extreme fragility of the edges of the pick, for example, which failed under very little pressure in several samples before one survived.

One problem with this approach is that the edges of the cut, while relatively smooth, do not have the polished look that the flat surfaces have. Apart from making the cut relatively slowly, in order to remove as many striations along the cut path as possible, one answer has been to use a spray on the rock surface which then gives the impression of having a polished surface, and as long as the object is kept inside the coating will likely remain. (When we tried this with pieces that ended up outside weathering removed that coating within a short number of years).

The problem with hand polishing large flat surfaces is that it becomes very difficult to maintain a truly flat surface over the entire block, and while the surface may end up smooth and polished, it will likely have some small undulations within it. It is therefore more productive (and, we found, cheaper) to have large flat surfaces machine polished before they were cut. One example of this was the sign that we made for the State Geological Survey. It was made in two parts, the lower part was a Missouri Granite, which held an upper half, carved from Missouri Marble, which was cut to the shape of the state.

Figure 2. Sign cut for the State Geological Survey

The lower granite slab was inset into two vertical grooves that were cut into the supporting blocks. The granite slab was cut to shape on our cutting table, with the inset cut out to hold the “toe” of the state. Because the granite was first machine-polished the lettering was etched into the surface using a reduced pressure for the cutting jet, and removing a thin layer of the surface, which was replaced with the black fill material to highlight the letters.

Figure 3. The Agency name was etched into the granite slab.

When it came time to cut the shape of the state in the marble, the block was first trimmed at the top (to help it fit into the table). A piece of plywood was placed under the rock before cutting to prevent any rebounding abrasive from hitting the under side of the slab and removing the polish from the surface.

Figure 4. The first cut across the marble, showing the supporting plywood.

The rest of the state had a contour cut along each surface, and when these were completed the slab was ready for mounting.

Figure 5. The finished slab, showing the state outline.


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