Tuesday, July 23, 2013

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.

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