Abrasive sizing

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

Over the past 30 years, abrasive waterjet cutting has become an increasingly useful tool for cutting a wide range of materials of varying thickness and strength. However, as the range of applications for the tool has grown, so the requirements for improved performance have also risen. Before being able to make a better quality cut, there had to be a better understanding of how abrasive waterjet cutting works so that the improvements could be made.

Parameters controlling AWJ

Figure 1. Some factors that affect the cutting performance of an abrasive waterjet (Hashish, Mohamed, “The effect of pressure on performance of Abrasive-Waterjet (AWJ) Machining”, Proceedings of Manufacturing International, April, 1988, Atlanta, GA, pp 255 – 263)

This understanding has not been easy to develop since there are many different factors that all affect how well the cutting process takes place. Consider, first of all, the process of getting the abrasive up to the fastest speed possible. And for the purpose of discussion, I am going to use a “generic” mixing chamber and focusing tube nozzle for the following discussion.

Simplified nozzle design

Figure 2. Simplified sketch of a mixing chamber and focusing tube nozzle used in adding abrasive to a high pressure waterjet

As high-pressure water flows through the small orifice (which in the sketch was historically made of sapphire), it enters a larger mixing chamber and creates a suction that will pull abrasive into the mixing chamber through the side passage. That side passage is connected through a tube to a form of abrasive feed mechanism that I will not discuss in detail today.

However, the abrasive does not flow into the mixing chamber by itself. Rather it is transported into the mixing chamber using a fluid carrier. In the some of the earliest models of abrasive waterjet systems, water was used as the carrier fluid to bring the abrasive into the mixing chamber. This, as a general rule, turned out to be a mistake.

The problem is that, within the mixing chamber, the energy that comes into the chamber with the high-pressure water has to mix not only with the abrasive but also with the fluid that carried the abrasive into the chamber. Water is heavier than air, and so if water is the carrier fluid, then it will absorb more of the energy that is available with the result that there is less for the abrasive, which – as a result – does not move as quickly and therefore does not cut as well. The principle was first discussed by John Griffiths at the 2nd U.S. Waterjet Conference, although he was discussing abrasive use in cleaning at the time.

Wet v dry feed

Figure 3. Difference in performance of water acting to carry the abrasive to the mixing chamber (wet feed) in contrast with the use of air as the carrier fluid. (Griffiths, J.J., “Abrasive Injection Usage in the United Kingdom,” 2nd U.S. Waterjet Conference, May, 1983, Rolla, MO, pp. 423 – 432.)

Note that this is not the same as directly mixing the abrasive into the waterjet stream under pressure – abrasive slurry jetting – which I will discuss in later posts.

The difference between the two ways of bringing the abrasive to the mixing chamber is clear enough that almost from the beginning, only air has been considered as the carrier to bring the abrasive into the mixing chamber. However, there is the question as to how much air is enough, how much abrasive should be added and how effectively the mixing process takes place.

In the earlier developments, the equipment available restricted the range of pressures and flow rates at which the high pressure water could be supplied, and these limits bounded early work on the subject.

One early observation, however, was that the size of the abrasive that was being fed into the mixing chamber was not the average size of the abrasive after cutting was over. (At that time steel was not normally used as a cutting abrasive). Because the fracture of the abrasive into smaller pieces might mean that the cutting process became less effective, Greg Galecki and Marian Mazurkiewicz began to measure particle sizes at different points in the process. (Galecki, G., Mazurkiewicz, M., Jordan, R., “Abrasive Grain Disintegration Effect During Jet Injection,” International Water Jet Symposium,Beijing, China, September, 1987, pp. 4-71 – 4-77.)

For example, by firing the abrasive-laden jet along the axis of a larger plastic tube (here opened to show the construction) the abrasive would, after leaving the nozzle, decelerate and settle into the bottom of the tube, without further break-up and without damage to the tube. Among other results, this allowed a measure of how fast the particles leave the nozzle, since the faster they were moving, the further they would carry down the pipe.

Green tube test

Figure 4. Test to examine particle size and travel distance, after leaving the AWJ nozzle at the left of the picture. The containing tube has divisions every foot, and small holes over blue containers, so that the amount caught in every foot could be collected and measured.

For one particular test, the abrasive going into the system was carefully screened to lie in the size range between 170 and 210 microns. It was then fed into a 30,000 psi waterjet at a feed rate of 0.6 lb/minute. The particles were captured after passing through the mixing chamber but before they could cut anything by using the tube shown in Figure 4. The size of the particles was then measured and plotted as a cumulative percentage adding the percentages found at each sieve size over the range to the 210 micron size of the starting particles.

After mixing sizing

Figure 5. Average size of particles after passing through a mixing chamber and exiting into a capture tube without further damaging impact

The horizontal line shows the point where 50% of the abrasive (by weight) had accumulated, and the vertical line shows that this is at a particle size of 140 microns. Thus, just in the mixing process alone, energy is lost in mixing the very fast moving water with the initially much slower moving abrasive.

And, as an aside, this is where the proper choice of abrasive becomes an important part of an effective cutting operation because the distribution of the curve shown in figure 5 will change with abrasive type, size, concentration added as well as the pressure and flow rate of the nozzle through which the water enters the mixing chamber.

I will have more to discuss on this in the next post but will leave you with the following result. After we had run the tests which I just mentioned, we collected the abrasive in the different size ranges. Then we used those different size ranges to see how well the abrasive cut. This was one of the results that we found.

Effect of particle size

Figure 6. The effect of the size of the feed particles into the abrasive cutting system on the depth of cut which the AWJ achieved

You will note that down to a size of around 100 microns the particle size did not make any significant difference, but that once the particle size falls below that range, then the cutting performance degrades considerably. (And if you go back to figure 5, you will note that about 30% of the abrasive fell into that size range, after the jet had left the mixing chamber).

Abrasive waterjet cutting

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

There are a number of different abrasives that can be supplied by different sources, and the market for the small grains that are used in abrasive waterjet cutting extends considerably beyond just the waterjet business. All abrasives are not created equal; some work better in one condition, others in another. As with other tools that the waterjet cutter or cleaner will use, first you should decide what the need for the abrasive is and run a small series of tests to find out which is the best set of cutting conditions for that particular job.

The first item on the list should be the material that has to be cut. (Although abrasives are also used in cleaning, that will be covered in a later post). There are, simplifying greatly, two classes of material that have to be cut. One class responds in a brittle way (think glass) and the other responds in a ductile or yielding manner (think metal). Because of these different responses when the particles hit the surface, the way in which cuts are best made will vary between the two. Some years ago, Ives and Ruff shot abrasive particles at different targets and found that there was a difference in the amount of material removed, but the best angle at which the particles should be aimed changed with the material.

Angle Effect

Figure 1. The Effect of change in impact angle on erosion rate for ductile and brittle targets. (Ives and Ruff, Wear, 1978, pp 149 – 162).

Some work at MS&T just before I retired indicated that the shape of these curves changed a little, depending on the size of the abrasive that is used. There are also some changes with abrasive shape. And this is because of the entirely different way in which an abrasive particle cuts into the two different materials. In this post we’ll discuss only the ductile target.

If a relatively smooth particle is shot into a ductile material at an angle perpendicular to the surface, then when it hits the surface, the target material will flow out from underneath but not be removed. As the following micro-photograph shows, the particles can become embedded in the material – and even add to the weight of the piece on rare occasion.

Embedded particle

Figure 2. Microphotograph showing a sand particle buried in the surface of an aluminum target.

There is very little material removed in this case – as the black curve shows in Figure 1 – when the impact angle approaches 90 degrees. Consider that if you take a knife and push it down into butter you don’t remove any butter. But if you drag the knife over the butter surface you will peel off a layer.

So it is with abrasive hitting a ductile metal. If the abrasive is brought in at an angle, (optimized in the figure at 15 degrees) then the abrasive has a cutting energy along the surface and this will peel up, and remove small pieces of the surface. By taking a microphotograph along the edge of an abrasive cut, we were able to show the action of individual particles in cutting into the metal.

Surface abrasive cleaned

Figure 3. Individual particle impacts on an aluminum surface, showing the cutting and plowing action of the particles.

Where the surface is plowed up, but not removed, another particle has to hit that point to remove the relatively fragile lip. However, if the particle is a copper slag, or other relatively weak material, it can shatter during the cutting process, and the breaking pieces can break off that lip, so that – again in the right material – the slag may give a better performance than a more expensive alternative.

But if we are to cut metal in this way, what does that say about the shape of the particles that we need to use. Obviously, if they were round such as a steel or glass shot, then there would be no sharp edges to cut into and peel off the material. Thus a steel or glass grit will cut better, though each particle needs a certain thickness in all dimensions so that there will be enough energy to both cut into the material, and plow along it.

Glass beads re grit

Figure 4. Difference in cut depth achieved with broken glass fragments over glass beads when cutting metal.

A relatively round particle with sharp corners, and garnet is usually such a particle, can often work well in cutting a range of different ductile materials.

Particle cutting

Figure 5. Schematic of how a particle of different shapes might cut into material.

Now that is fine when a high-pressure abrasive waterjet (AWJ) is starting to cut into the surface, but as the jet cuts down into the surface, the angle of the cut will change. Yet even if the jet is pointing directly down into the target and moving along to cut through it, the cut surface is not usually a straight line down through the material.

Cut through one inch glass

Figure 6. Cutting through glass, note the curved path of the jet through the one-inch material.

Cuts into Plexiglas and other clear materials have allowed research scientists to monitor the cut path through the target as a function of time. It is not a constant shape, but, as Dr. Henning showed at the 18th International Conference, the edge of the cut changes with time. You can see the results of this in cuts that are made through metal where the paths of the cut, particularly lower in the cut, curve around and back towards the start of the cut.

Triangle depth cut

Figure 7. Cut into steel, with the face piece of metal removed to show the cut surface.

This path confirms an explanation first proposed by Dr Lars Ohlsson in his doctorate at Lulea in Sweden. He pointed out that the change in the surface of the cut is caused by the sequence of actions that a particle sees as it comes down onto the surface.

First it comes in almost vertically, with no lateral energy, and it cuts in the smooth, upper part of the cut. Then it rebounds out of the cut, but into the jet stream that gives it a little more energy and directs it along the cut to a second point where it will cut a little bit more of the metal. But during the first rebound the particle does not bounce perfectly along the cut but deviates to one side or the other. This means that when it makes the second cut, it will now cut more into one side of the wall or the other. Thus, where the second bounce occurs, the surface gets a little rougher.

jet bounce in cut

Figure 8. Frames from a high speed video showing abrasive waterjet cutting of glass, with the jet cutting, rebounding down the cut and then cutting again. (Lars Ohlsson “The Theory and Practice of Abrasive Water Jet Cutting”, Doctoral Thesis, Division of Materials Processing, Lulea University of Technology, 1995)

By the time of the third cut and rebound, the jet will now be coming into the opposing side of the cut with an even greater lateral portion of its energy, and so the cut will get a little rougher. Remember also that each cut is made up of the impacts of very many particles. So that succeeding particles also rebound along the curve cut by the preceding particle, and this also will exacerbate the roughness of the cut.

We’ll talk a little about reducing this effect in the next post.