Waterjetting 18d - Abrasive considerations

It would be best if, before I ended this short session on abrasive, I mentioned some of the practical constraints that sometimes limit the options for choosing abrasive types. To give a simple example, we were, at one time, demonstrating the ability of a waterjet drill to penetrate limestone. In the demonstration that morning we had used garnet as the abrasive and had made a steady penetration down to about 70-ft but the contracting office on the project did not seem overly impressed. So, after lunch, I suggested that we switch to an aluminum oxide abrasive, since we knew it was more aggressive.

Unfortunately for the afternoon program we were using a DIAjet type of system, where the abrasive is added to the water under pressure just downstream of the nozzle, and upstream of the delivery nozzle. While that worked well with the garnet abrasive (which passed without significant damage through the swivel on the end of the drill) that was not the case with the aluminum oxide. This is a much sharper abrasive and less prone to damage in mixing. As a result once we had the rig back in operation we were immediately struck by the black color of the water coming out of the hole – as the aluminum oxide stripped the inner lining from the hose carrying it to the nozzle. We then watched as, in real time, the pressure gage on the driving pump slowly slid back from the 10 ksi initial pressure to about 2 ksi as the abrasive ate out the orifices of the nozzle. Needless to say, having pretty much destroyed the downstream equipment in about five minutes, the afternoon demonstration was a bit of a disaster.

I also remember the first time that we used steel shot to try and cut through some rock, without giving too much thought to encasing the cutting operation. Those small spheres retained a lot more energy than most particles, and we were dodging the equivalent of shotgun pellets which ricocheted around the lab as we raced to shut the system down.

Both abrasives are, in their place, very effective tools in cutting materials that might be more difficult or uneconomic to cut by other means, but the peculiarities of their nature require that special precautions be used when they are used to make sure that there are not unintended consequences.

Sometimes the choices are simply practical. When we were cutting the walls of the Omnimax theater under the Gateway Arch in St. Louis, where we had to cut straight down (within half-an-inch either way over 15-ft of cut depth) through dolomite and chert it took less than a day to realize that the cost of using garnet to achieve the 12 – 15-inch individual cut depths was going to drive us out of economic reality within a week. Changing to a blasting sand (which we bought by the ton) did not change the cutting performance by much, but had a remarkable effect on overall costs.

Figure 1. Effect of abrasive type, size and feed rate on the depth of cut and optimal cutting condition when cutting rock. (after Yazici*)

Abrasive type and abrasive size both effect the depth of cut, and thus the economics of a cutting operation. Yet it is not possible to draw absolute rules since the different abrasives have different relative cutting efficiencies in different materials. For example, in the above plot boiler slag was relatively ineffective in cutting rock. On the other hand, with the right type of slag and steel Faber and Oweinah** have reported that slag can cut steel more than three times as efficiently as garnet. (This is partly because the slag shatters on impact and the fragments go on to scour the uplifted edges of the cavities generated by the initial impact of the particle.)

And while the British Welding Institute use smaller particles to cut softer materials, they have found it critical to use larger particles to get viable performance as the target material gets harder. In cutting steel I had mentioned in an earlier post, that garnet becomes less effective at a particle size below 100 micron. Yet in cutting aluminum (which is softer) the particles can be smaller and yet still effective.

Figure 2. The effect of particle size when cutting aluminum using corundum particles (after Faber and Oweinah ibid)

Yet, as discussed at the beginning, the cost of the abrasive must not only be set off against the potential for improving the cutting rate, one has to also look and see if there is an increase in the operating cost of the system when a harder, and thus often more effective cutting abrasive is used. Zaring et al showed this with a plot that they published at the 6th American Waterjet Conference***.

Figure 3. Relative benefits and costs of changing abrasive type (after Zaring et al***)

All things are, however, relative, and in some small cutting operations we have found it more economic to sacrifice the nozzle over the cutting time required in order to achieve a cut that could not be effectively achieved any other way.

As with many things in the waterjet business, while there are general rules that can be laid down to guide operations, when it comes to specific cases then it is often worth running a small series of tests on the projected target material, using different abrasives, at varying size ranges and feed rates, before calculating (usually using a normalized cost in dollars or gms per area of cut) the most effective abrasive for a given operation.

*Yazici, Sina, Abrasive Jet Cutting and Drilling of Rock, Ph.D. Dissertation Mining Engineering, Univ. of Missouri- Rolla, Rolla, MO, 1989, 203 pp.
**Faber, K., Oweinah, H., "Influence of Process Parameters on Blasting Performance with the Abrasive Jet," paper 25, 10th International Symp Jet Cutting Technology, Amsterdam, Oct, 1990, pp. 365 - 384.
***Zaring, K., Erichsen, G., Burnham, C., "Procedure Optimization and Hardware Improvements in Abrasive Waterjet Cutting Systems," 6th American Water Jet Conf, Houston, TX, Aug, 1991, pp. 237 - 248.

Waterjetting 11c - Mixing abrasive with a water jet

Sometimes I would get the feeling (particularly when talking to some of my students) that the mixing chamber of an abrasive waterjet (AWJ) cutting system, together with the feeds in and the focusing nozzle outlet, were considered to be some magical black box out of which a perfect cutting stream issues to cut the desired material.

There are, in fact a variety of different chamber designs that can be purchased from different manufacturers. Some will tell you that all designs cut roughly the same, and that there is little difference between them. As I commented in one of the earliest posts in this series this is not true. Over the years we have run numerous comparative tests on different designs, using different abrasives, abrasive feed rates and target materials, and have found a broad range of results. For example, in cutting steel at a fixed traverse speed and other conditions, we found an average comparative performance as follows:

Figure 1. Comparative performance between 12 nominally similar abrasive waterjet cutting nozzles in cutting through steel at a standard speed, pump pressure, and abrasive concentration.

I described the actual test in another post, and it is clear from these averaged results that there is a wide difference in performance between the nominally similar tools.

While I don’t think there is a lot of interest in going through the details of different designs it might be helpful to explain some of the factors that play a part in producing jets of greater or lesser performance.

To return, first, to the basic construction of the mixing chamber and focusing tube assembly (AWJ nozzle), one starts by recognizing that the major cutting performance will be achieved by the particles which remove material when they hit the target. The energy that they have, however, comes from the water that is fed into the AWJ nozzle through a small jeweled orifice, or waterjet nozzle, at the top of the mixing chamber.

Figure 2. Basic nozzle design

That waterjet stream is small and initially highly focused and fast moving. As it moves through the mixing chamber, as I have described in other posts, the outer edges of the jet slow down, and gradually the jet fans out and breaks up into fragments.

There is no benefit in trying to inject the abrasive into the jet at the beginning of this passage, since, at that point, the outer layers of the jet have enough energy to knock away the particles before they can enter the fastest moving segment in the core. Rather it is better to inject the abrasive further down the chamber, so that the jet will have begun to break down into slugs, and the abrasive can be positioned so that it is impacted by a sequence of the individual slugs and accelerated to the desired velocity.

There is an additional benefit to moving the abrasive feed line a little further down the chamber. When the jet stream is rapidly switched on and off, when for example, piercing a series of small holes in a part, then the driving pressure pushing water out of the waterjet orifice switches off and on. When it switches off there is a short period where the differential pressure will draw fluid from the chamber back through the waterjet nozzle. If there are small particles of abrasive in the vicinity (and with some designs there are) then these can be drawn back through the upper orifice, and then pushed back down by the succeeding water flow in the next pulse. This can rapidly erode softer jeweled orifices, so that they round or chip, not always evenly, and degrade the resulting waterjet as it flows into the chamber. This disruption can move the jet from being in the center of the chamber, and cause poor abrasive pick-up or accelerate wear of the chamber walls, and in the focusing tube. All of these degrade performance. (The solution, if you can achieve it, is to use a diamond upper orifice, since this is largely non-responsive to the passage of the abrasive back and forth, and retains its shape and the jet performance it was designed to produce much longer – providing a cost benefit to the change).

Fig 3. Wear on a ruby waterjet orifice inserted at the top of a mixing chamber after 15 minutes of use. (The dark particles are small particles of garnet)

Figure 4. Chipping on the edges of sapphire and ruby orifices (after Powell 2007 WJTA Conference)

Fig 5. Lack of wear on a diamond insert nozzle after being in use for several hundred hours.

With the abrasive inlet channel and jet passage designed to get the abrasive into the jet where the water jet is broken up, yet still moving at high speed, there needs to be a sufficient distance for an optimal energy transfer to occur. Beyond that point, with the particle and the abrasive (which will have partially been broken in the contact between the jet and the particle, between particles striking one another, and between contact between the particle and the walls of the AWJ nozzle) the cutting jet has to be refocused into the narrow cutting stream that is required to give the finished cut surface desired.

The refocusing of the mixed jet (air, water and abrasive) is achieved with a focusing tube, which is made up of a conic section which brings the jet back together, and then a straight section which allows further energy transfer between the three component parts of the jet, before the jet issues from the orifice aimed at the target.

The passage of the particle down this tube is not always straight. Wear typically begins at the tip of the conic section as it feeds into the tube. The wear within the tube will often take up a pattern, as any irregularity in the flow causes it to bounce from one wall of the tube to the other creating a wear pattern along the walls that has a wave-like structure.

Figure 6. Wear at different points along the focusing tube

When this wear reaches the mouth of the focusing tube, then the downstream orifice is eroded out of a circular shape, and the jet that comes out no longer will cut cleanly, or to as great a depth. At that point the nozzle is worn out and should be replaced. The point at which that replacement occurs varies depending on the quality of the cut that is required. Obviously when the cut being made is at a precision of a thousandth of an inch over a cut depth of half-an-inch (as with some aircraft parts) the replacement point is reached a little earlier than if the cuts being made are a rough cut merely, for example, to separate two parts one from another, and provide a rough shape to the piece.

I’ll continue this topic in the next segment of this section.

Waterjetting 6a - An introduction to testing nozzle performance

In the next few posts I will be writing about some of the tests that you can run to see how a nozzle is performing. But before getting into the details of the different tests, you should recognize that this is where a little homework will be required if you are to get the most benefit from the topic.

The world that encompasses waterjet use has grown beyond the simple categories by which we used to define it. New techniques make it possible to cut materials that used to be more difficult and expensive to produce, and as practical operational pressures have increased so the scale, precision and economics of new opportunities have developed.

It is this range of application that makes it impractical for me to give specific advice for every situation, and instead, by explaining how to make comparisons, and what some benchmarks might be, to allow you to better understand your system, its capabilities and both the initial performance of nozzles, and then the evaluation to decide when they may best be replaced.

One lesson I learned early was that nozzles from different companies behaved in different ways, and that drawing conclusions on optimal performance, for example the selection for which pressure level and nozzle size was best, using one design would not necessarily hold with a competing design. Further there were nozzles that began their life on our system doing very well relative to others, but which quickly declined in performance. Thus, as part of an evaluation of different designs, we would test the nozzle cutting performance, against a standard requirement, at fixed time intervals so that we would know when it was wearing out and should be replaced.


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.

Both the shape of the curve and the effective lifetimes of different competing nozzle designs varied quite significantly. And obviously, since most folk don’t spend a lot of their time cutting through more than an inch of steel, the operational lifetimes of nozzles will vary with the requirements for the particular job. Nevertheless the relative ages at which nozzles can no longer reach that target can differ significantly,


Figure 2. Comparative effective nozzle life over which, operated at a pressure of 50,000 psi, a jet could cleanly cut a path through a 1.4 inch thick steel target at a traverse rate of 1.5 inches/minute.

As mentioned, the tests were carried out using nozzles from several manufacturers and, at the beginning of the test the longest lasting nozzle was not necessarily the one that produced the fastest cut, but consistently, over the interval, and for about twice as long as the competition, it was able to achieve the goal.


Figure 3. Depths of cut in steel after (top) 1,000 minutes of nozzle use, and (bottom) after 1,500 minutes of nozzle use.

In the particular case in which we made the comparison the major interest was in achieving a clean separation of the parts, and the edge quality was not as significant a factor. In many uses of this tool that edge quality will be important and would have given a different set of numbers (as Figure 3 would indicate) than the ones that were found for our application. As a result the judgment that the nozzle is worn out will change to a different time, and the relative ranking of the different nozzle designs may also change.

The only way in which anyone can make a rational decision on which is the best nozzle for an application, and how long it will be effective is by testing the nozzle against the stated requirement. When we began the test we anticipated that the difference between nozzles from different manufacturers, when fed with water at the same flow rate, and with the same quantity and quality of abrasive would not differ that much. As Figure 2 shows, we were wrong in that idea.

There are a number of different impacts that a change in nozzle design (i.e. in most cases buying a competing design over that initially used) can bring to a cutting operation. However these impacts are also governed by the pressure at which the work is being carried out, the amount of abrasive that is used, the relative nozzle diameters (if using a conventional abrasive waterjet system) and the speed at which the cut is made. But an initial assessment of relative merit should be carried out with equivalent parameters for the different designs.

In general, however, we ran tests at a number of pressures, and with varying abrasive feed rates, to ensure that the comparative evaluations were fair, and consistent. As a result we found that there were a number of different factors that came into play, which are not always recognized, and which could bias the results that we observed.

In the posts that follow this I will first cover some of the different tests that can be used, and then go on to explain some of the results and why they sometimes make it difficult to accept a simple comparison of results when, for example, the abrasive is not the same in both cases. To give a simple example of this, consider a conventional abrasive waterjet nozzle that is operated at increasing pressure.

Increasing the pressure will improve the cutting speed and/or the cut quality, as a general rule. It will reduce the amount of abrasive that is needed, but this is where the “yes, but’s . . . .” start to appear. As the pressure of the jet increases, so the amount of abrasive that is broken within the mixing chamber will also increase, so that the average size of the particle coming out of the nozzle will become smaller. The amount of this size reduction is a function of the quality of the abrasive that is being used, and a function of the initial size of that abrasive.

Within a certain size range, that reduction in the particle size does not significantly change the cutting performance, but if the mix contains too many small particles, particularly if the distance to the work piece is also significant, then the cutting performance can be reduced because of the particle break-up. Different nozzle designs produce different amounts of very fine material even from the same feed rate of the same abrasive into the nozzle. When the initial feed rate of the abrasive, or a different abrasive is used, then estimating which design and set of operating pressures is best becomes more difficult, as an abstract estimation.

This is why, in the posts that follow, the comparisons are made are based on actual measurements and why I recommend that everyone test their system using more than one design/set of operating parameters so that they can be confident that the combination that they are using will provide the best combination for the job to be done.