Wednesday, July 10, 2013

Waterjetting 11a - More thoughts on Abrasive

In the last post I mentioned that the abrasive particles, which are fed into a high-pressure waterjet stream to form the Abrasive WaterJet (AWJ) cutting tool, can be significantly crushed when mixing with the high-pressure waterjet, and before they leave the mixing chamber. Because of this - depending on the application - the choice of abrasive can play a significant role in how well the AWJ performs. I have mentioned a number of times that the Waterjet Lab is located at Missouri University of Science and Technology. That meant (apropos “show me”) that it was an appropriate place to run comparative tests between different abrasives to find which is the best.

It turns out that there is no one single answer to that question, since the abrasive that was the most economical and effective to use in one case does not necessarily give the best results in another. Which brings me to the first point in today’s post. It is relatively easy to get small samples of the different abrasives that might be used in a given job. Setting up a small series of test runs, in which the different abrasives being considered, are fed to the nozzle and use to cut standard cuts into test samples, is a relatively easy way to find out which is the best abrasive for that particular material and cutting path. However it is best not to use only a single test run, we would generally run a series with three different jet pressures and three different abrasive feed rates.

Figure 1. Table showing the change in optimal Abrasive Feed Rate (AFR) on cut depth at different pressures.

By bracketing the range that is likely to have the best concentration of abrasive for each pressure (which is not at the same abrasive feed rate, or AFR) the best result can be found for each different pressure value, and the most economical and effective choice for the task in hand can be quickly found. It is important to include economics in the evaluation, since there have been a number of cases we looked at where the most effective choice for abrasive in terms of giving the fastest clean cut was not that much more effective than the second place abrasive, and that alternative was sufficiently cheaper that it made more sense to use it.

The pricing of abrasive, however, is not something that it is easy to generalize over, since there are a number of different factors that come into play, depending where in the country you are located. As a rough guidance, however, we have found that garnet is a more universal cutting abrasive than most others, with less extraneous “issues”, and while it can be less effective than other selections in some conditions, in general it will cut more materials effectively and economically than its challengers. Further mined garnet, in general, performs better than alluvial garnet since it does not have the degree of damage within the particles that leads them to fragment more easily in the mixing chamber.

There are, however, more factors that just the abrasive type that have to be considered. There include the particle size, and range, and then, as noted in the table, there is the selection of the AFR to match the cutting conditions on the table.

One of the more neglected factors relates to the amount of air that is used to carry the abrasive from the hopper into the mixing chamber. The person who did more to shine a light into this corner of the technology was Tabitz, in France. (Tabitz, Schmidt, Parsy, Abriak, and Thery “Effect of Air on accceleration process in AWJ entrainment system, 12th ISJCT, Rouen, 1994 p 47 - 58.)

Because abrasive can cut into the parts of the flow meter, the equipment that they used included a trap between the hopper and the mixing chamber, where the particles could be collected, while the air passed forward to be measured and enter the mixing chamber.

Figure 2. Apparatus used by Tabitz in measuring the air flow to the cutting head and mixing chamber. (ibid)

The results from the measurement showed that as the jet pressure increased, so for that particular nozzle design, did the amount of air that was being drawn into the chamber – although you may note that it begins to reach a constant volume as the pressure approaches 280 MPa (40,000 psi).

Figure 3. Effect of increased jet pressure on the amount of air drawn into the nozzle, as a percentage of the total volume of the resulting jet. (Tabitz et al)

The problem that this relatively large volume of air presents is that it has to be accelerated at the same time as the energy in the jet is being transferred to the abrasive particles. The larger the amount of air in the mix, then the greater the amount of water energy that has to be diverted into accelerating the air. This leaves less energy available to accelerate the abrasive itself.

Tabitz modeled the result with a simulation in a computer program, which illustrates, for different abrasive feed rates, how the average abrasive particle velocity falls as the amount of air in the mix increases:

Figure 4. Simulated effect of an increase in air flow on the reduction in average abrasive particle velocity (after Tabitz et al).

Placing small instruments in front of abrasive-laden waterjets can lead to a relatively short life for those instruments, and measurements of actual particle velocities, though they have been made by a number of researchers, have not been as comprehensive as the above chart might indicate.

Nevertheless there is some indication that the above curves are accurate in principle, if not totally real. A jet with very little air might accelerate particles to 1,880 ft/sec, for example. However with 70% air in the mix, then the particle velocity might fall to 1,700 ft/sec, and with 95% of the jet made up of air, then the abrasive particle speed may fall to 1,200 ft/sec. Part of the difficulty in assessment is because of the very short time interval in which the abrasive particles are accelerated while in the mixing chamber. Because the rate of acceleration of the particles is inversely related to their size. Smaller particles are accelerated faster. And this is the counter to the point that was made in the last post about smaller particles cutter less efficiently than larger ones.

Part of the reason for this is that the smaller particles are decelerated faster in air than larger particles. The results of this in terms of cutting power is one of the areas that still requires more research. If, for example, smaller particles are used in an application (for example to achieve a finer detail in the surface cutting) then the effective range of the jet can become smaller than with larger particles. There are some caveats to that statement, and I will go into some of that explanation in the next post.

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