Friday, May 29, 2015

Waterjetting 33c - More on enhanced cutting performance

Frontal photographs taken of waterjets, regardless of pressure, show that the jet spray widens as the jet moves away from the nozzle. Yet, because of the erosion of the outer layers of the jet by the surrounding fluid, the central core of effective jet pressure reduces as that distance grows. The normal way in which this can be seen is in the taper of the cut as a jet penetrates into a target material.

Yet in some cases better results can be obtained if the jet makes a series of passes to cut through that target layer. There is, however, a little problem, and this can be shown by the use of a curve, showing the depth of cut as a function of the number of passes made over the surface.

Figure 1. Depth of cut in two materials, as a function of the traverse speed. (After Hashish).

The graph shows the decline in the cutting ability of the jet with increasing number of passes, and the inability of the jet, at the highest speed, to penetrate through the mild steel plate.

One of the reasons to make passes at a higher speed is to improve the quality of the edge cut, since if the jet makes the pass with the particles of abrasive only cutting on the target one time (rather than the multiple cuts made by a particle at slower speeds, where it bounces down the cut.)

Momber has pointed out the decrease in performance with increased pass number, as well as noting the difference in the amount of energy required to cut through a target as a function of the speed and number of passes.

Figures 2 and 3. Effect of the number of passes on the depth achieved (lhs) and the relative amount of energy required to penetrate material as a function of traverse speed (rhs) (after Momber)

The slight loss in cutting power as the jet cuts deeper in secondary passes comes in part because the jet is constrained by, and thus cuts back into, the walls of the pre-existing cut.

In an earlier post on cutting I pointed out that because of the highly efficient way in which a plain waterjet cuts into material, Chinese investigators have shown that one can achieve a much improved volume removal rate by oscillating the jet perpendicular to the line of travel.

The optimal speed for cutting with an abrasive jet is, however, much slower than that of a plain waterjet, by a couple of orders of magnitude, so that the large scale oscillation that is effective with plain jets will not be similarly so with an AWJ. However the concept remains valid, and has been the subject of significant investigation, particularly in Australia, in the past few years.

The benefits of such oscillation, even over very short angles, can be illustrated with reference to a figure.

Figure 4. Oscillation of a jet perpendicular to the line of travel. The nozzle advances to the grey outline on the subsequent pass. The two lines indicate the range of oscillation. (Motion exaggerated relative to the current discussion)

If the nozzle is oscillated so that the jet moves over the relatively narrow range shown in Figure 4, then after a pass, when the nozzle advances to make the second pass (and it does not have to be in the part to do this) then the jet does not make contact with the target until the back of the previous cut. Thus there is much less energy loss in traversing the jet to the new surface, and cutting performance is improved. If the oscillation is kept small the walls of the cut will still act to confine the cutting ability of the jet, and improve depth-cutting capability.

Shunli Xu looked at oscillating a jet at angles below 10 degrees, while cutting half-inch thick 87% alumina plates. A simple visual correlation showed the relative benefit of oscillation when cutting the plate with a 45 ksi jet, with an AFR of 1.2 lb/min, at a speed of 3.1 inches/min.

Figure 5, Cuts made into a ceramic plate, without (lhs) and with (rhs) a nozzle oscillation of 8 degrees at 10 Hz. (after Shunli Xu)

The study also looked at the effect of changing the oscillation parameters on the surface roughness of the cut achieved, finding that this is controlled by the angle of oscillation, the frequency and the speed of traverse, as well as jet pressure and standoff distance (not shown). The study found that, under optimal conditions, surface roughness could be reduced around 11% relative to linear cutting.

Figure 6. Effect of change in oscillation parameters on the surface quality of cut in a ceramic target (after Shunli Xu).

The study found that the parameters which control the depth of cut gain were a little more complicated to disentangle, given that the density of particles striking an individual area of the target is controlled by both the jet residence time, and the parameters of the jet itself (AFR, pressure, traverse speed).

As a result the optimum value for oscillation angle and frequency varied depending on the jet parameters, but overall it was concluded that an optimal angle of oscillation would lie between 4 and 6 degrees, with higher oscillation frequencies giving better results. An average improvement with oscillation lay on the order of 23% over conventional non-oscillation at the same parameters.

Precision cutting is a task that has a number of complications. In many cases the cuts must follow intricate contours, rather than just making simple linear cuts than separate the material. Increasingly, also, pocket milling has become a valuable ability for this tool. Cut wall quality adequate for final surface finish is increasingly important in this case, and the ability of oscillation to improve that quality and enhance the depth over which a smooth cut was achieved was noted in the work. Similarly the taper of the cut was, on average, reduced 18% with greater improvement at higher oscillation frequencies and angles.

Secondary motions of the nozzle, beyond simple path following, are thus becoming a more important potential tool for the industry, and I will return to this topic again.

Hashish M. “A Modelling study of metal cutting with abrasive waterjets,” Journal of Engineering Materials and Technology, ASME, Vol 106, Jan 1984, pp. 88-100.

Momber A.W., Kovacevic R, Principles of Abrasive Waterjet Machining, Springer Science, p. 209

Shunli Xu Modelling the Cutting Process and Cutting Performance in Abrasive Waterjet Machining, PhD Thesis, Queensland University of Technology, 2005.