Waterjetting 28b - More on abrasive use

The costs of running a high pressure waterjet table divide into two parts, one that covers the basic costs of the system, whether it is running or not, such as building rent, while the second covers those costs that are a part of the actual work. Of the latter costs it is the cost of the abrasive that is often the most significant. This comes about in two ways, since the abrasive must first be purchased for use, and then, after it has been used it must be disposed of. Depending on the materials that were cut, this disposal cost can be significantly higher than the original purchase price. In some work carried out at the High Pressure Waterjet Lab (HPWL) at Missouri University of Science and Technology (MST) in the past we have seen disposal costs that were more than three times the cost of the original abrasive. And one should bear in mind that, as a research lab, the table was used much less than a comparable conventional table in an industrial cutting environment. But we also did not have a cutting operational budget, and so the cost of abrasive was something that we examined, to see if it could be reduced.

The first idea was that we would just recycle the abrasive. The particles of the target materials that were cut are generally much smaller than the abrasive particles themselves, and so it should be relatively easy to remove them from the mix. However, as we looked into the process in more detail, it was clear that it would not be quite as simple as it might, at first, appear. Marian Mazurkiewicz (retired) and Greg Galecki (who now runs the HPWL) carried out studies on the behavior of the particles as they moved through the mixing chamber and were accelerated down onto the target material. They found, as noted in an earlier post, that most of the abrasive was crushed to a smaller size when it passed through the cutting head, and a mix that started out with a particle size of 210 microns as it was fed into the system, was leaving the focusing tube with an average size of 140 microns.

Figure 1. Percentage of abrasive at different sizes after it has passed through a mixing chamber (and before it has hit the target). (After Galecki).

The reason that this is a concern is that, as the particles become smaller, so a point is reached where, depending on the target material, the abrasive no longer has sufficient energy to effectively cut into the target. When cutting into metals such as titanium and steel, our targets of choice in the study, this cut-off grade was at around 100 microns.

Figure 2. Effect of particle size on the cutting performance of an abrasive jet in cutting steel. (The tests were part of a factorial experiment and are thus averages over a number of different test runs at differing abrasive feed rates (AFR and pressures).

Roughly 25% of the mix in the example shown in figure 1 lies below 100 micron at it leaves the chamber. After impacting the target this value increases to more than 50%. Obviously recycling this fine material and re-using it in the cutting process is going to be less effective than removing it from the mix. Generally alluvial garnets will break up more rapidly than mined garnet, because of the structure of the abrasive particles, and thus the percentage that leave the focusing tube at the larger and more effective diameters are lower with the alluvial mix. The results were, we found, confirmed in the cutting results, with alluvial garnet producing a generally shallower depth of cut that would be achieved, other things being equal, in the cutting tests.

A quick word of explanation of the tests we ran, which are described in more detail here. The tests are run at a standard pressure and nozzle size, and at a constant traverse rate, with the depth that the jet cuts into a standard steel at a fixed speed measured over a 4-inch traverse length.

The results of the tests showed that, because of the particle crushing during the cutting process, the abrasive would have to be screened, and for most effective re-use only the larger fraction (on average less than 40%) should be recycled. The rest would be too fine for effective re-use in the operations we were developing. (Although finer abrasive has use in other applications, it would have to be screened and stored). It was interesting to note, and perhaps logical in retrospect, that once the particles had been used once and the larger ones separated out, then the percentage that survived and could be reused a second and third time increased significantly. This is mainly because those particles that had some form of weakness crack (either from weathering or from the mining process) were broken during the first impact, and the particles that survived did not have these cracks, and would therefore inherently be more prone to survive multiple times.

For our purpose, therefore, given that there was a high cost in purchasing the abrasive, and an even higher one in disposing of the contaminated material after cutting (because of the contamination by the target material) there was a potential economic advantage in recycling the abrasive. There were several ways in which the particles can be separated, but a simple screening process, if carried out properly, is quite time consuming, since the particles are required to “sit” on a vibrating screen for several minutes to ensure accurate separation, and this can be labor intensive if it is carried out as a batch process. We tried a number of different ways, including using a counter-flow fluid column that worked well for low feed rates, but the most efficient unit for one operation (we build virtually all of ours, and extensively modified them over time) may not be the best in other cases. (The one that survived the longest was a Wilfey table (though not this one).

In conventional AWJ cutting the abrasive has also to be dried before it can be re-used, and that can also add power and labor costs to the process. Thus, as with many choices that must be made when developing an efficient cutting operation, the best answer is to carry out a series of tests yourself, and run the numbers to decide whether, in the long run, recycling would, or would not, be an effective choice.

Waterjetting 11b - Abrasives and cutting depth

In the past few posts I have been discussing the use of abrasive in waterjet cutting, and in this and the next two posts I would like to talk a little about the abrasive feed rate (AFR), abrasive size and the selections for the best cutting performance. As with my other posts I can only write in general terms about these because the combination of waterjet system, nozzle design and abrasive selection will change the best values to use, and the results on different systems will differ in some way or other from the results I will mention. However in all cases the overall principles remain the same, and can be applied as general rules.

In the last post I noted that cutting performance fell as the average size of the abrasive fed into the system dropped below 100 microns. As part of that study we looked at the amount of abrasive that survived going through the mixing chamber in that size range. A simplified average of the results obtained are shown in the following table:

Figure 1. Percentages of the initial feed that survive at larger than 100 microns, for differing feed conditions.

In an earlier post I had mentioned the “Green Tube” test that was used at Missouri S&T as a way of measuring the particle size and speed, after the particles had passed through the nozzle, but without hitting a target. Because the distance that the particles travel is a function of the energy they obtained during mixing, some idea of the overall particle energy can also be obtained.

However, when the particle sizes were analyzed at different distances from the nozzle we noted that there was a large percentage of small particles in the short distances from the nozzle, but that as the particles were collected at greater distances from the nozzle, so the average particle size grew larger.

After thinking about this for a short while, the reason became obvious, and – at the same time – made it a little more difficult to draw simple conclusions from the test.

The reason for the greater collection of smaller particles nearer the nozzle is that they are decelerated more rapidly than the larger particles, once they start traveling through the air. If we go back to the basic equation that we learned in school:

Force = mass x acceleration

For a given particle, the force to accelerate the particle in the mixing chamber will, simplistically, be the pressure exerted on the particle by the water, multiplied by the cross-sectional area of the particle. If the particle is a sphere, with a diameter d, then the area be π x(d/2)^2. But the mass of the particle is a function of the volume, which is related to the cube of the diameter. Thus the acceleration, for a given particle size and at constant fluid pressure, will vary inversely with the diameter of the particle. In other words the smaller particles will accelerate faster in the mixing chamber and focusing nozzle.

Once the particle leaves the nozzle, however, the acceleration from the water is replaced by a deceleration as the particle is now moving through air that is relatively stationary. Now the situation is reversed and it is the smaller particle that decelerates faster, and thus will have a shorter effective range than particles that survived the mixing process in a larger size. This was therefore the explanation for the results that we saw in our tests.

Unfortunately life becomes a little more complicated than this when the nozzle is held close to the target. This is because, while the air between the nozzle and the target may be relatively stationary, at greater distances, the small gap means that the surrounding air is also drawn into the slot and flows with the stream along the cut. There is thus less resistance to the particles, which retain their energy to a greater distance – improving cutting depth. However that also changes if the jet is cutting through layers where there may be water or air in the gaps between the layers.

This work was carried out initially by Dr. George Savanick during work carried out at the then US Bureau of Mines, on cutting rock. It applies in other cases, however, since there are often times when cuts are needed between two work pieces with a gap between them. (The example in mind is cutting through the different tubes that bring oil out of a well. This casing can be made up of several different diameters of pipe, ranging perhaps from a 20-inch diameter outer pipe to a 3.5-inch diameter inner one, with other tubes between). What Dr. Savanick showed was that if the gap between the layers was filled with some relatively soft material that provided little resistance to cutting, but held its shape and provided confining walls on either side of the jet, that the range of the jet could be extended beyond that where the jet was cutting water or air between the layers. These factors then play a part in determining how far an abrasive jet will cut through material.

Often it is not just the ability of the jet to cut through the material, but also the straightness of the cut and the quality of the edge that are important. If, for example, one can be sure that there are no burrs on the edges of a cut between two overlapping layers of material, then the parts may not have to be separated, cleaned and re-assembled before being fastened together. This elimination of several manufacturing steps can significantly lower the cost of assembling, for the sake of discussion, aircraft components. In turn this may then justify the use of the AWJ system as the better manufacturing tool, even when it does not seem that the initial cutting process is much cheaper than the alternative.

I mention these considerations, because as I go through the different applications of these tools I can only be somewhat general in discussion of overall effects. The way in which the abrasive mixes with the water, the amount of particle breakup and the different speeds of the abrasive leaving the nozzle vary with the nozzle design and operating conditions. They are also tailored to an extent by the particular job that has to be completed. Thus a recurring piece of advice in this series will be to find a test piece of material and test out a range of options before committing to the final cut. The series will try and suggest where that range might be, but in the final decision I do not have the equipment or other conditions that exist in your shop, and thus only you can be the final judge.