Friday, December 6, 2013

Waterjetting 16a - Abrasive Feed

I was talking with someone the other day who mentioned that it was necessary to increase the amount of abrasive feed to a waterjet whenever the material to be cut was thicker. This is actually a myth, or put another way, wrong! It is the equivalent of saying that you should use a duller knife if you want to cut thinner material.

Today’s topic therefore is about optimizing the abrasive feed rate into an abrasive waterjet cutting system, but you should remember that different manufacturers have different nozzle assembly designs. Thus the graphs and tables that I will use to illustrate the discussion relate to one particular nozzle design that was used at that time. Not all the nozzle designs were the same, but the results illustrate the points that I am going to make. But it underlies the recommendation that you should each run some standard tests with your system so that you have a baseline of performance and data to tell you where your system works best.

There are several reasons why different designs produce different cutting results, and I will point out some of them in what follows. To begin, however, consider again the basic elements of the waterjet mixing chamber.

Figure 1. Section through the mixing chamber of a conventional abrasive injection system.

A small high pressure stream of water enters the chamber through the upper nozzle, passes through the chamber, creating a vacuum that pulls abrasive into the chamber, and mixes with that abrasive before the resulting AWJ exits through the focusing tube.

One of the first things to understand is that, in the cutting jet that issues from the tube the actual cutting comes from the abrasive particles. From work that has been carried out at a number of places we know that the higher the velocity of the particles, the greater the damage that they will do on the target.

Figure 2. Relative mass loss when steel balls hit a mild steel plate (after Hutchings, I.M., Winter, R.E., Field, J.E., "Solid Particle Erosion of Metals;The Removal of Surface Material by Spherical Projectiles," Proceedings of the Royal Society, London, Vol. A348, 1976, pp. 379 - 392.)

Discussion in earlier posts pointed out that there are different ways in which waterjets attack ductile and brittle materials. However the relationship between an increase in impact velocity and damage occurs whether the targets are brittle or ductile. In figure 2 the target was a ductile steel, in more brittle material it is the coalescence of cracks that removes material, and higher velocities create larger cracks, as shown in figure 3.

Figure 3. Effect of impact velocity on crack length, (after Evans, A.G., "Impact Damage Mechanics: Solid Projectiles," in Erosion, Treatise on Materials Science and Technology, Vol. 16, ed C. Preece, Academic Press, 1979.).

And the same form holds true if steel balls are fired into sandstone.

Figure 4. Relative amount of Berea sandstone removed by the impact of steel balls of varying size (after Ripkin, J.F., Wetzel, J.M., A Study of the Fragmentation of Rock byImpingement with Water and Solid Impactors, Final Report on U.S. Bureau of Mines, Contract HO 210021, February, 1972.).

The above graphs show that it is more effective to have the abrasive moving faster in terms of the damage done by individual particles. Which brings up the first consideration in the design and use of an AWJ mixing chamber.

In order to get the particles moving as fast as possible they have to get their energy from the water jet entering the chamber. But the jet enters the chamber as a solid stream that then breaks into droplets (as shown in earlier pictures) as it passes down the chamber. If the jet remains in a solid stream all the way down, and out of the focusing tube, then the abrasive will find it difficult to penetrate into the center of the jet stream and pick up all the needed jet energy.

This can be illustrated by looking at the velocity and distribution of particles coming out of a nozzle with two different sizes of waterjet orifice but the same size of focusing tube diameter.

Figure 5. Relative particle distribution across a 40,000 psi jet with a focusing tube diameter of 2.3 mm (0.09 inches), and an AFR of 1 lb/min for two waterjet orifice sizes (after Mazurkiewicz, M., Olko, P., Jordan, R., "Abrasive Particle Distribution in a High Pressure Hydroabrasive Jet," International Water Jet Symposium, Beijing, China, September, 1987, pp. 4-1 - 4-10.)

The smaller waterjet breaks up fully within the chamber entraining and accelerating the abrasive particles and providing the desired cutting stream. The larger sized jet does not completely breakup, and fewer particles can mix into the center of the jet giving a more diffuse and less efficient cutting stream. In this case changing from a 0.005 inch waterjet orifice to a 0.013 inch diameter orifice (at roughly 7 times more power, because of the higher flow rate) produces a poorer result.

It is therefore important to ensure that there is an efficient energy transfer between the water and the particles. But the jet energy can only be diffused to a certain number of particles before it significantly begins to reduce in the amount of energy that it imparts to each particle. In other words if you put too much abrasive into the jet stream, then the amount of energy each particle gets is reduced, as it the overall cutting efficiency.

In an example I have used in class I noted that if I pick up a small child and run down the corridor, then I can carry the child at about my normal running speed, on the other hand if I pick up a couple of football players and try the same run I will be barely able to stagger. So the optimum carrying capacity of any jet can be determined for a given water flow rate, which is itself based on the waterjet orifice diameter and the pressure at which the water is supplied.

I will return to this topic next time, but you can see, in the concluding figure, that when a lower abrasive feed rate is fed to the nozzle, that the percentage of the abrasive moving in the higher velocity range rises to over 60% compared with only 20% of the particles when the abrasive feed is too high. And that means that the cutting performance will be less with the higher abrasive feed rates. (The numbers are a little high to reinforce the point).

Figure 7. Particle velocity distribution on leaving the focusing tube (after Isobe, T., Yoshida, H., Nishi, K., "Distribution of Abrasive Particles in Abrasive Water Jet and Acceleration Mechanism," paper E2, 9th International Symposium on Jet Cutting Technology, Sendai, Japan, Oct, 1988, pp. 217 - 238.)

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