Waterjet Cutting – Beginning to cut

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

In the last few posts I have been discussing what happens under a waterjet as it first hits, and then penetrates into a target material. In many cases it is recommended that the nozzle move slightly relative to the target during this piercing process so that the water escaping from the developing hole does not have to fight its way past the succeeding slug of water entering the hole.

Now it might be thought that this problem would go away if the nozzle starts at the side of the target and then cuts into it. But this depends on a number of different factors, one of the more critical being as to whether the jet is cutting all the way through the material, or is only cutting a slot part of the way through it. As with a number of other topics, I am going to illustrate some of the concerns using granite as the target material, since it makes it easier to demonstrate some of the points I want to make.

If you were to look at one of the many statues that have been carved from granite over the thousands of years since the rock was first shaped into an art form, the rock usually appears as a relatively homogeneous material. That means (to those who don’t work with rock) that the rock has the same properties regardless of which direction you test them in.

Vermont carver granite

Figure 1. The Italian Carver’s Memorial, Dente Park, Barre, VT (From the Barre Granite Association via State Symbols USA)

However, if you were to ask a skilled quarry man, he would tell you differently. Because of the way that granite cools from the molten state in which it is injected up into the ground, it picks up an orientation to the crystals as they are formed. One of these orientations is roughly horizontal and called the Lift or grain of the rock. A second is perpendicular to this and vertical and is known as the Rift. The third plane, orthogonal to the other two, is called the Hard-Way because it is generally more difficult to work. These names relate to the ways in which the grains of the rock and the cracks around them align. They are virtually impossible for a lay person to detect, and a quarry man may need to feel the rock to tell you which way they lie. But they are used in splitting out the major blocks from the granite massif and come into play in breaking the large blocks down into handle-able sized pieces.

Granite bedding orientations

Figure 2. The A) Hard-Way B) Rift, and C) Lift planes of crystal orientation in granite

If, however, you were to shoot a short slug of water at high-pressure at a piece of granite (and we used the granite from Elberton in Georgia for this) then, depending on which direction the pulse came from relative to the three planes, the amount of rock that would break around the impact point would change.

In an earlier post discussing the splitting that occurs when pressure builds up within the cavity under a jet, I mentioned that the pressure would grow cracks that already existed. And it is for this reason that, when the jet impacts perpendicular to the existing crack planes, the volume of material broken out is greater than it is where the jet fires along the cracks. This can be shown using the cavity profiles from oriented samples into which the jets were fired.

Cavity profiles

Figure 3. Profiles from the cavities created around the impact points where the jet impacted granite blocks at different orientations.

One can use this information if, for example, one wanted to cut a thin line in granite, where the cut should be made in the direction of the crystals, i.e. making cuts along the lines shown in the A plane of figure 2.

Linear cuts on A plane

Figure 4. Linear cuts into granite along the lines shown in Figure 2.

In this set of cuts, the jet is cutting along the favored orientation of the crystals, and the rock only spalls when two jet paths approach each other in the lower right of the block.

If, however, the cuts are made in a direction perpendicular to the orientations, i.e. in the B and C planes, then the results are quite different.

Cratering on linear B

Figure 5. Cratering along the linear passes in cutting granite perpendicular to the Rift plane.

Where the jet strikes perpendicular to the Rift or Lift, then the pressurization under the jet is enough to cause those cracks to grow out to the surface and cause spallation along the cut. In many cases, this removes all the rock between two adjacent passes, even if they are more than an inch apart.

If one is to use the high-pressure waterjet system for slotting granite in a quarry, for example, then this can be a very useful tool, since by merely putting two jets on either side of the desired slot the spalling will remove the material between them without any further jet action. If the jets attack in the perpendicular plane, then the jet has to be rotated over the cut to get the same material removal rate.

In most cases, when cutting in a quarry, because the rock does vary in structure and grain size, it is better to ensure that all the rock is removed before the nozzles move into the cut by rotation. However, in smaller applications – such as where the excess rock is being removed around a planned sculpture –enhancing the spall around the impact point can lower the time and amount of energy required in removing unwanted rock.

That is, however, a relatively specialized application, and in most cases it is desirable that the cut be clean and smooth. This requires the use of abrasive in the waterjet stream, and so this will be the topic of the next few posts.

Cutting through one inch thick glass

Figure 6. Cutting through one inch thick glass, showing the cut through the side of the glass.

Waterjet Technology – Testing Waterjet Nozzle wear by cutting foam

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

Last week’s post discussed a simple test which helps to show not only how to compare the effect of different operating conditions (varying abrasive type, nozzle design, AFR etc) as a way of finding a possibly better and cheaper cut. It is also often handy to know when a nozzle is starting to wear out, so that different cutting operations might be scheduled to allow the nozzle to continue to work, without threatening the quality of a critical product.

Change in the cutting depth of a jet stream at 50,000 psi when traversed over ASTM A108 steel

Figure 1. Change in the cutting depth of a jet stream at 50,000 psi when traversed over ASTM A108 steel as a function of the time that the nozzle had been in use

While we have found that nozzles from a given manufacturer roughly agree in cutting performance and times before they wear out, the pattern of wear and performance change differs from one nozzle design to another. Also there is some variation in performance between nozzles even of the same design and under the same conditions.

There are also times when cuts are made without abrasive or when the cutting/cleaning jet is hand-held – what to do in those cases? Mainly we have used foam as the cutting target, set up so that the jet won’t cut all the way down through the foam all the way along the cut, so that, as with the steel, some idea of not only cutting depth but also cut quality can be seen.

Cuts through thick stiff packing foam

Figure 2. Cuts through thick stiff packing foam. Note the rough edge at the bottom of the extracted pieces, but the good initial quality of cut that was achievable for some 14-inches.

There is a caution in cutting foam in that some of the softer varieties are going to fold into the cut and give a slightly inaccurate measure of true performance; although for a quick comparison to see how a nozzle is lasting that is not a real issue. When cutting thicker material and also when going for higher quality cuts that is, however, something that should be borne in mind.

The white expanded foam that is used as a packing material is also very easy to cut, even with the pressures that can be found with a pressure washer type of system. Thus, if you are going to clean a deck or other surface, it helps to check by swiping the jet across such a piece of material to be sure that you have a good nozzle on the end of your lance before you start.

This may seem fairly logical; after all you just went to the hardware store and bought a new packet of nozzles. Well, as with the other nozzles we have looked at, quality is only assured after testing. In this particular case, we ran as many different varieties of fan nozzles as we could to see how they would perform when cutting across a piece of packing foam. It is not hard to cut packing foam with a high pressure jet. And since domestic cleaning is usually carried out at either 1,000 psi or 2,000 psi, we ran tests at both levels.

Results from a good, top, and a poor nozzle with cuts at 1,000 and 2,000 psi and with the foam moved through the jet at a distance of 3 inches

Figure 3. Results from a good, top, and a poor nozzle with cuts at 1,000 and 2,000 psi and with the foam moved through the jet at a distance of 3 inches. The number identifies the nozzle. Note that at 3 inches, number 18 could barely remove the top of the foam.

A fan jet is defined by the amount of water that it will allow to pass at a set pressure and by the angle of the cone with which the jet spreads out from the orifice. In passing, we found that the cone angle that the jet actually spread at was a little larger than that designated on the package.

The worst nozzle design that we found had difficulty in cutting into the foam even at a very close range:

On the other hand, the best nozzle was still able to cut the material with the nozzle held some nine inches from the foam.

Cutting result with the good nozzle held at nine inches above the foam target

Figure 4. Cutting result with the good nozzle held at nine inches above the foam target. At this distance the jet is removing as much material as the poor jet did at a 3-inch standoff.

A very typical result would have the jet fail to cut into the foam much beyond four inches from the nozzle. (I’ll use some photographs in a couple of weeks to explain in more detail why that is). And as a short editorial comment to those of you who clean around your house with a domestic unit, how many of you hold the nozzle that close to the surface? (Or at the car wash?) If you don’t you are losing most of the power that you are paying for and you are in the company of most of the students that I ran this demonstration with in my classes).

However there is one other feature to the photographs of the cuts that I would point out. Fan jets distribute the water over a diverging fan shape. But the results of the design fell into two different types, one where most of the water still concentrated in the middle of the jet, (as in Figure 4) and those where it was focused more on the side.

Cutting pattern with the jet streams more at the side of the flow

Figure 5. Cutting pattern with the jet streams more at the side of the flow (arrow points). Note that the two pressure cuts are on the other sides of the sample here.

The benefit of using foam is that it allows this picture of the jet structure to be easily seen, with very little time taken to swipe the nozzle over a test piece of material at the start of work, to make sure that the jet is still working correctly.

This is both an advantage and a disadvantage. Because the foam is relatively easy for a jet to cut even at a lower pressure, this means that the cut can become more ragged with depth where deep cutting is required.

One of the programs that we ran, some years ago, looked at how deeply you could cut into the stiff packing foam that is used in some industrial plants, where the item being packed needs to be held firmly yet will be released easily when needed. This requires that the foam be cut to a very tight tolerance, and at the time, pieces were still being cut by hand and then glued together. (Figure 2 above)

We found that we could cut up to about a foot of material before the small cut particles became sufficiently caught up in the cutting jet that the edge quality of the cut fell below specification. But in order to get to that depth we did have to add a small amount of a polymer to the cutting water. This helped to hold the jet more coherent over a greater distance and also reduced the amount of particulate that got caught up in the jet allowing the greater cutting depth.

Foam works as a simple sample to give some sense of the jet shape where the pressures are lower. When they are higher, a stiffer material is needed though it should still be cut by water without the need for abrasive. Plywood is a useful target in this case, and I will write about those tests next time.

Waterjet Technology – The Triangle Cut Comparison Test

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

This post is being written in Missouri, and while the old saying about “I’m from Missouri, you’re going to have to show me,” has a different origin than most folk recognize*, it is a saying that has served well over the years. We did some work once for the Navy, who were concerned that shooting high-pressure waterjets at pieces of explosive might set them off as we worked to remove the explosive from the casing. We ran tests under a wide range of conditions and said, in effect, “see it didn’t go off – it’s bound to be safe!” “No,” they replied, “we need to know what pressure causes it go off at, and then we can calculate the safety factor.” And so we built different devices that fired waterjets at pressure of up to 10 million psi, and at that pressure (and usually a fair bit below it) all the different explosives reacted. And it turned out that one of the pressures that had been tested earlier was not that far below the sensitivity pressure of one of the explosives.

That is, perhaps a little clumsily, a lead in to explain why simple answers such as “yes I can clean this,” or “yes I can cut that” often are not the best answers. One can throw a piece of steel, for example, on a cutting table and cut out a desired shape at a variety of pressures, abrasive feed rates (AFR) and cutting speeds. If the first attempt worked, then this might well be the set of cutting conditions that become part of the lore of the shop. After a while it becomes “but we’ve always done it that way,” and the fact that it could be done a lot faster with a cleaner cut, less abrasive use and at a lower cost is something that rarely gets revisited.

So how does one go about a simple set of tests to find those answers? For many years, we worked on cutting steel. Our tests were therefore designed around cutting steel samples because that gave us the most relevant information, but if your business mainly cuts aluminum, or titanium or some other material, then the test design can be modified for that reason.

The test that we use is called a “triangle” test because that is what we use. And because we did a lot of them, we bought several strips of 0.25-inch thick, 4-inch wide ASTM A108 steel so that we would have a consistent target. (Both quarter and three-eighths thick pieces have been used, depending on what was available). The dimensions aren’t that important, though the basic shape that we then cut the strips into has some advantage as I’ll explain. (It later turned out that we could have used samples only 3-inches wide, but customs die hard, and with higher pressures the original size continues to work).

Basic Triangle Shape for waterjet test cutting

Figure 1. Basic Triangle Shape for waterjet test cutting

The choice to make the sample 6-inches long is also somewhat arbitrary. We preferred to make a cutting run of about 3 minutes so that the system was relatively stable, and we had a good distance over which to make measurements, but if you have some scrap pieces that can give several triangular samples of roughly the same shape, then use those.

The sample is then placed in a holder, clamped to a strut in the cutting table and set so that the 6-inch length is uppermost and the triangle is pointing downwards.

The holder for the sample triangle

Figure 2. The holder for the sample triangle

The nozzle is placed so that it will cut from the sharp end of the triangle along the center of the 0.25-inch thickness towards the 4-inch end of the piece. The piece is set with the top of the sample at the level of the water in the cutting table. The piece is then cut – at the pressure, AFR and at a speed of 1.25 inches per minute with the cut stopped before it reaches the far end of the piece, though the test should run for at least a minute after the jet has stopped cutting all the way through the sample.

The piece is then removed from the cutting table and, for a simple comparison, the point at which the jet stopped cutting all the way through the triangle is noted.

Showing the point at which the jet stopped cutting through various samples as a function of the age of the nozzle

Figure 3. Showing the point at which the jet stopped cutting through various samples as a function of the age of the nozzle – all other cutting conditions were the same (a softer nozzle material was being tested which is why the lifetime was so short). The view of the samples is from the underside (A in Fig 1.)

An abrasive jet cuts into material in a couple of different ways – the initial smooth section where the primary contact occurs between the jet and the piece and the rougher lower section where the particles have hit and bounced once on the target, and now widen and roughen the cut. Since some work requires the quality of the first depth, we take the steel samples, and mill one side of the sample, along the lower edge of the cut until the mill reaches the depth of the cut, and then we cut off that flap of material so that the cut can be exposed. Note that the depth is measured to the top of the section where the depth varies.

Typical example of a steel triangle that has been cut and then sectioned to show the quality of the cut

Figure 4. Typical example of a steel triangle that has been cut and then sectioned to show the quality of the cut

I mentioned in an earlier article that we had compared different designs from competing manufacturers. Under exactly the same pressure, water flow and abrasive feed rates, the difference between the cutting results differed more greatly than had been expected.

Sectioned views of six samples cut by different nozzle designs, but at the same pressure, water flow, AFR and cutting speed

Figure 5. Sectioned views of six samples cut by different nozzle designs, but at the same pressure, water flow, AFR and cutting speed

There was sufficient difference that we went and bought second and third copies of different nozzles and tested them to make sure that the results were valid, and they were confirmed with those additional tests. Over the years as other manufacturers produced new designs, these were tested and added into the table – this was the result after the initial number had doubled. (The blue are results from the first nozzle series tests shown above).

Comparative depths of cut using the same pressure and AFR but twelve different commercially available nozzle designs

Figure 6. Comparative depths of cut using the same pressure and AFR but twelve different commercially available nozzle designs

There were a number of reasons for the different results, and I will explain some of those reasons as this series continues, but I will close with a simple example from one of the early comparisons that we made. We ran what is known as a factorial test. In other words the pressure was set at one of three levels and the AFR was set at one of three levels. If each test ran at one of the combination of pressures and AFR values and each combination was run once then the nine results can be shown in a table.

Depths of cut resulting from cutting at jet pressures of 30,000 to 50,000 psi and AFR of 0.6, 1.0 and 1.5 lb/min

Figure 7. Depths of cut resulting from cutting at jet pressures of 30,000 to 50,000 psi and AFR of 0.6, 1.0 and 1.5 lb/min

The results show that there is no benefit from increasing the AFR above 1 lb/minute (and later testing showed that the best AFR for that particular combination of abrasive type, and water orifice and nozzle diameters was 0.8 lb/minute).

Now most of my cutting audience will already know that value and may well be using it but remember that these tests were carried out over fifteen years ago, and at that time, the ability to save 20% or more of the abrasive cost with no loss in cutting ability was a significant result. Bear also in mind that it only took 9 tests (cutting time of around 30 minutes) to find that out.

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* The reason that the “I’m from Missouri, you’ll have to show me,” story got started was that a number of miners migrated to Colorado from Missouri. When they reached the Rockies they found that, though the ways of mining were the same, the words that were used were different. (Each mining district has its own slang). Thus they asked to be shown what the Colorado miners meant, before they could understand what the words related to.