Waterjetting 33d - Nozzle oscillation along a contour

In the last post I reviewed, in part, Dr. Shunli Xu’s work on oscillating nozzles, and how they can, on occasion, almost double the penetration while also improving surface finish. The problem with that basic technique, however, is that the nozzle is oscillating perpendicular to the direction of the cut, which is fine when the nozzle is cutting a long straight pass, but becomes more of a problem when the cut is in the form of an intricate contour. Given that the nozzle assembly does not usually rotate to follow that contour, the motion that gives the correct direction of oscillation when the head is moving laterally won’t when the cut is being made into the depth of the piece, i.e. parallel to the oscillation direction.

The answer to this is to provide the nozzle with an orbiting motion similar to that which, for example, Clark Barker used to provide a circular motion to a cutting head back when we needed to cut a path ahead of a drill head at a time when high-pressure rotary couplings lasted about 14 minutes. (We knew this because we had bought a model of every one we could find in the country and run them to failure, and the best lasted that long, provided you kept it cool by playing a hose on it. That was some thirty-five years ago, and things have got a lot better, and cheaper in that time).

Figure 1. Mechanism for orbiting a nozzle (after Barker). In the above figure the outer sleeve rotates, driven by an external motor, and with a flexible connection to the nozzle, the nozzle moves in a circular path, without itself rotating as it moves, as it moves in an orbital path as the outer sleeve turns.

In the model for Clark Barker’s device the intent was to drill a hole in coal some 6-inches in diameter, and the tool worked well in being able to do this. (It was used in a tool that turned from a vertical well to drill out horizontally within a 9-inch turning radius. The device was proved in the field by Sandia Labs, who drilled out into a coal seam from a vertical well using the second generation of the tool that was developed.

In many ways the use of an orbiting mechanism for cutting is a lot simpler to develop, given that, as noted in the earlier post, the angle that the jets must move through is very small (around 8-degrees). With the nozzle constrained so that it remains pointing only slightly off-axis, there is no need for the more complex tool required to advance a drill tens of feet into a coal seam (and deal with all the debris that was flowing back out of the hole at the same time).

I have described John Shepherd’s Wobbler tool in an earlier post and it is worth returning to that design and our study for a little further analysis.

The object of our study was to examine how the tool could be used in milling pockets in material, and more specifically how best it could be used to create a relatively flat floor to the pocket, while maintaining relatively sharp corners to the pocket walls, a capability that conventional mechanical tool milling does not allow. (Unfortunately I can’t at the moment produce any of the figures from that work, though they can be found in the paper we gave at the 17th Waterjet Symposium in Mainz in 2004.)

When it came to the assessment of performance, it is perhaps of note that Dr. Zhang’s study found an optimal oscillation speed at around 8 Hz. It would appear from our study that the optimal oscillation to achieve greater depth was just below 8HZ, whereas that which gave the greater volume removal rate was at around 10 Hz, which would both lie close to the optimum suggested by Dr. Shunli Xu.

The assessments were admittedly for different overall phenomena, Dr. Xu was interested in achieving a greater and cleaner cut, while Dr. Zhang was more focused on achieving a milled surface, typically to be achieved with a single overall pass, nevertheless the relative agreement on an optimal parameter is significant.

Further, in order to achieve a smooth floor for the pocket, Dr. Zhang was incrementing the nozzle between passes as a function of the width swept out by the jet. The initial overlap of the jets provided an uneven floor to the pocket that was removed when the jets were further apart. (In most cases with a 120% spacing between the passes a smoother surface was achieved).

However, with the generalized conclusion being that the optimal basic operating parameter (oscillation/rotation speed) was in the same range for both studies, and with the angle that the jet swings through on the order of 6 degrees, again of similar range in both studies, would appear to validate the cross-transfer of information.

The path that the Wobbler makes is shown, in exaggerated form, in Figure 2, and a typical result for pockets cut in glass and steel are shown in figures 3 and 4.

Figure 2. John Shepherd's Wobbler and the path it drives the jet along as it moves over a target.

Figure 3. Pocket of varying depth and contour milled from glass using the Wobbler.

Figure 4. Lettering and the map of Missouri cut into metal using the Wobbler.

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.

Waterjetting 16b - Optimum Abrasive Feed Rate and Depth

The post that I wrote last week was focused on the misperception that you need to add more abrasive to an abrasive waterjet if you wish to cut through thicker material. This is wrong on a number of counts, but most particularly because a good operator will have tuned the nozzle to achieve the best cutting jet, based on pressure and abrasive feed rate (AFR) regardless of target material. What the operator may change is the operating pressure (which would change the optimum AFR) and the traverse speed since these control the depth and quality of the cut that the jet makes.

But, before leaving the topic, I would like to discuss, in a little more detail, the concept of the optimal amount of abrasive that one should use with a given jet, and what happens as that feed rate is changed. As I mentioned last time, because of differences in the shapes of the mixing chambers of the nozzles supplied by different manufacturers, the specific sizes and optimal flow rates will differ from nozzle to nozzle but the overall conclusions remain the same.

Last time I pointed out that the driving waterjet had to break up within the mixing chamber in order to properly mix with the abrasive and to bring this up to a maximum speed before the mix left the focusing tube. Where the driving jet is too large then this breakup is not complete and the mixing is not efficient. As a result the jet that comes out of the end is more diffuse and the abrasive will not have reached the full velocity possible. However, if the incoming waterjet is made smaller for the same AFR and other mixing chamber geometries, then the cutting performance will decline.

Figure 1. Effect of increase in jet pressure when cutting aluminum with an AFR of 1.7 lb/minute (after Hashish, M., "Abrasive Jets," Section 4, in Fluid Jet Technology- Fundamentals and Applications, Waterjet Technology Association, St. Louis, MO, 1991.)

For a similar reason adding a polymer to the jet fluid should only be carried out with some care for the consequences. Long-chain polymers can give a jet increased cohesion and this can, at high enough concentrations, inhibit jet breakup in the mixing chamber thus reducing the effectiveness of mixing in the chamber.

Figure 2. The effect of changing cutting fluid on AWJ performance (after Dr Hashish ibid)

Polyox, (polyethylene oxide) is an extremely effective polymer for increasing jet performance by cohering the jet and reducing the friction losses between the pump and the nozzle. However, as the graph shows, adding it to some abrasive systems will reduce performance since the more coherent jet makes it more difficult for the abrasive to mix and accelerate to full velocity. At lower concentrations the polymer allows the jet to breakup, but keeps the slugs of water together making energy transfer more efficient. Higher velocity abrasive means that less is required to achieve the same cutting performance as Walters and Saunders showed.

Figure 3. Effect of adding polymer in reducing the amount of abrasive required to cut stainless steel (after Walters, C.L., Saunders, D.H., "DIAJET Cutting for Nuclear Decommissioning," Paper J2, 10th International Symposium on Jet Cutting Technology, Amsterdam, Netherlands, October, 1990, pp. 427 - 440.)

At low levels of abrasive feed Dr Hashish has shown that increasing the amount of abrasive in the feed increases cutting performance.

Figure 4. Effect of increase in AFR on depth of cut in mild steel at a feed rate of 6 inches/min (After Dr. Hashish ibid), waterjet diameter 0.01 inches.

However, as the abrasive flow rate continues to increase the cutting performance reaches a plateau and can decline, as Dr. Hashish illustrated. An AFR of 20 gm/sec is equivalent to a feed of 2.6 lb/minute.

Figure 5. The effect of higher AFR on cutting depth at 3 jet pressures on a mild steel target (after Dr. Hashish ibid)

Note that in this case the nozzle geometry was not optimized for operation at the highest jet pressure. More visibly we ran a series of cuts across a granite sample, where the only thing that changed between cuts was that we increased the abrasive feed rate in cuts from the left to the right. It can be seen that beyond a certain AFR the jet starts to cut to a shallower depth.

Figure 6. Successive cuts made into a granite block at increasing AFR from the left to the right.

Interestingly the optimum feed rate doesn’t just depend on the pressure and water flow rate (waterjet orifice size) of the system. Faber and Oweinah have shown that as the feed particle size gets larger, so the optimum AFR reduces.

Figure 7. Optimal Abrasive feed rate as a function of particle size (after Faber, K., Oweinah, H., "Influence of Process Parameters on Blasting Performance with the Abrasive Jet," paper 25, 10th International Symposium on Jet Cutting Technology, Amsterdam, October, 1990, pp. 365 - 384.)

The process of finding an optimal feed rate for a system is thus controlled by the design of the mixing chamber based on the relative position of the abrasive feed tube and the size of the waterjet orifice. This controls how well the abrasive that is fed into the system can mix with the jet and acquire the velocity that it needs for most effective cutting. Then, as the above plot shows, the optimal AFR is also influenced by the size of the particles that are being fed into the system, since as the particles become larger beyond a certain size, so the cutting effectiveness declines.

Part of the reason for this is that, as the AFR increases so there is an increased risk of particle to particle impact breaking the particles down into smaller sizes. (And an earlier post showed that smaller particles cut less effectively – as does figure 7 above). We screened the particles that came from several different designs of AWJ nozzle assemblies capturing them after they left the nozzle but without further impact, so that the size range is indicative of that which a target material would see,

The table is a summary of some of the results and it shows results for a feed that began at 250 microns giving the percentage of the particles that survived at larger than 100 microns.

Figure 8. Percentage of the 250 micron sized feed that survives at above 100 micron for differing jet conditions. (the numbers are averaged from several tests).

It can be seen that when the feed rate rises to 1.5 lb a minute that there is a drop in abrasive size at higher jet pressures, and this is likely to be due to the increased interaction with particles. Since cutting effectiveness is controlled by particle size, count and velocity the only slightly greater amount of particles that survive above 100 microns at 1.5 lb/minute relative to those that survive at 1 lb/minute suggest that spending the money to increase the AFR above the optimal value (in this case around 1 lb/min) is a wasted investment.

It is therefore important to tune the system to ensure that, for each jet pressure and nozzle design that is used, that the AFR has been optimized.