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 28a - Cutting rock on a table

In previous posts I have written about the use of lower pressure water (around 10,000 psi) as a way of cutting through rock. From the time that we first made a hole through nine-inches of granite while I was a graduate student some 50-odd years ago the way that we have recommended that rock be cut, in a mining situation, has been to use lower pressure and higher volumes of water. This is so that as many natural fractures around the individual grains and crystals can be developed at one time as possible. However the result of this is that the cut progresses along the grain boundaries of the infrastructure of the rock, that a roughly edged cut is made, rather than a smooth cut surface. In mining applications this isn’t necessarily a bad thing, but when cutting counter tops and other ornamental structures for marble and granite surfaces inside a building then a rough surface is definitely not often required.

So how can a relatively smooth surface finish be created along the cut? One step,that works with softer rocks (such as some pink granites) is just to increase the jet pressure, while at the same time reducing the volume of the stream flow. Once one reaches the ultra-high pressure regime (which is, for this article, considered above 35,000 psi) and with jet diameters on the order of 0.01 inches or less, the jet stream is more typically going to cut through a crystal within the rock than to just work on the cracks that lie at the edges of the crystal.

Unfortunately there are sufficient cracks and crystal boundaries within the rock that it is not possible to ensure that at some point as the jet cuts down through the rock, and along the desired path, that it won’t find a crack at a critical length and alignment that the crack will break out a larger chip. This is less likely to happen within the cut, since the confinement of the surrounding rock acts to reduce excessive crack growth, but can quite often occur at the rock surface, particularly where there has been some earlier damage that has left larger cracks within that surface. (This includes heat treatment).

That, however, is a specialized case, and in the more typical situation an increase in jet pressure to 50 ksi will not, by itself, produce the clean edge needed. Part of the reason for this comes from the striation planes within marbles, which can offer an easier path for the jet to penetrate, as the cuts get deeper, rather than having the hole continue forward along the jet axis. To overcome these problems it is easier, with the ubiquity of abrasive waterjet systems, to instead change to add an abrasive to the waterjet.

Dimension stone (the trade name for the decorative rocks such as marble and granite) is generally through cut with slab depths that are less than an inch-and-a-half thick, although greater depths can be specially prepared. Often the slabs are polished before they are finally cut to shape. We found that preferable, since when doing the final polish with successively finer grinding wheels (used for example in creating the Millennium Arch) the edge stress that can be generated by the wheels themselves can cause chipping along the edge of the work. This, in turn, either requires a regrind down to remove the chip, or some form of repair, which we found it difficult to make invisible given the complex structure of the granite. This is particularly true when relatively narrow ribs of material are being cut. As an example, consider the cartoonish mining figure that was made some years ago.

Figure 1. Toon miner carved from 3-inch thick granite.

The front and back surfaces were polished before the figure was cut from the slab, given the extreme fragility of the edges of the pick, for example, which failed under very little pressure in several samples before one survived.

One problem with this approach is that the edges of the cut, while relatively smooth, do not have the polished look that the flat surfaces have. Apart from making the cut relatively slowly, in order to remove as many striations along the cut path as possible, one answer has been to use a spray on the rock surface which then gives the impression of having a polished surface, and as long as the object is kept inside the coating will likely remain. (When we tried this with pieces that ended up outside weathering removed that coating within a short number of years).

The problem with hand polishing large flat surfaces is that it becomes very difficult to maintain a truly flat surface over the entire block, and while the surface may end up smooth and polished, it will likely have some small undulations within it. It is therefore more productive (and, we found, cheaper) to have large flat surfaces machine polished before they were cut. One example of this was the sign that we made for the State Geological Survey. It was made in two parts, the lower part was a Missouri Granite, which held an upper half, carved from Missouri Marble, which was cut to the shape of the state.

Figure 2. Sign cut for the State Geological Survey

The lower granite slab was inset into two vertical grooves that were cut into the supporting blocks. The granite slab was cut to shape on our cutting table, with the inset cut out to hold the “toe” of the state. Because the granite was first machine-polished the lettering was etched into the surface using a reduced pressure for the cutting jet, and removing a thin layer of the surface, which was replaced with the black fill material to highlight the letters.

Figure 3. The Agency name was etched into the granite slab.

When it came time to cut the shape of the state in the marble, the block was first trimmed at the top (to help it fit into the table). A piece of plywood was placed under the rock before cutting to prevent any rebounding abrasive from hitting the under side of the slab and removing the polish from the surface.

Figure 4. The first cut across the marble, showing the supporting plywood.

The rest of the state had a contour cut along each surface, and when these were completed the slab was ready for mounting.

Figure 5. The finished slab, showing the state outline.

Waterjetting 13d - Pocket milling considerations

If I were to hand you a cake and suggest you take some, you would likely take a knife, and make two cuts to define a wedge of the cake, which you could then easily remove with your fingers, and easily eat.

Figure 1. Conventional cutting of a piece of cake.

However if one takes the approach used in many machine shops, and in many of the processes that are used to excavate soil and rock, then the process and product are a lot different. For, instead of just making two cuts that make it easy to remove and handle the product, instead the operator takes the knife and chops up the piece of cake into small particles.

Figure 2. Milling pockets into a piece of cake, with a pile of crumbs.

Now while it is true that this might produce nice clean pockets in the cake, it does leave the extracted cake in small pieces that cost a lot more energy to produce, are a lot more difficult to collect and handle, and are relatively worthless without being reprocessed back into a more useful form.

In some cases, where the cuts are made to the edge of the piece, it is not that difficult to adapt waterjet technology to emulate the knife in the above example. Cuts can be made along the edges of the pocket in such a way that they intersect within the piece to define a block of material that can be removed.

Figure 3. Piece of metal trimmed from a block in a form that allows it to be used for something else.

The savings can be considerable and are not restricted to small pieces such as this. Consider if you wish to create a tube, 2.5-inches long in Hastelloy. One could mill out the central volume to give the required shape:

Figure 4. Tube made from a block of Hastelloy

Or one could merely cut around the desired shape in the block, using a high-pressure abrasive waterjet, which would produce the central block as a single piece that could then be used as feed stock for another part.

Figure 5. Removed block of material from the piece shown in Figure 4. (For this initial demonstration the quality of the side cut was not considered important, it can be significantly improved for both internal and external surfaces, as discussed below).

Life becomes a little more complicated when the pocket must be generated without through cutting the part, and where the depths of the individual cuts must be precise, at the same time as the wall quality must my produced to final surface specifications.

There are a number of techniques that can be used in this case, depending on the material being cut, and on the depth that the pocket must be cut to. As mentioned in a previous piece, when higher quality surfaces are required then the cut must be made only with particles that have not cut, bounced and recut the work piece. Rather they must leave the cutting zone after the first cut, without further damage.

When the abrasive cutting conditions are controlled accurately, then the precision control in the depth of cut can be high, so that, for example, channels can be precisely cut into engine parts as needed.

Figure 6. Milled channels into a surface (note that the channels were deepened with three passes of the cutting head). (Courtesy Ormond)

In the above case the channels were narrow enough to be removed by the full width of the cutting jet, however, when larger pockets must be cut then this precision must be maintained over the full depth.

A problem that can then arise comes from the shape of the cut as it gets deeper. If the nozzle is not advanced into the cut (and this is not normally possible given the relative widths of the two) then there will normally be a slight taper into the cut as it deepens. With relatively thin work structure this taper will often be slight enough that it does not fall outside the tolerance required for the cut. Quality control can be achieved, therefore, with a series of rapidly moving nozzle, shallow individual cut passes. Omertz, for example, showed that a much better control of cut depth and wall quality can be achieved by twelve passes at a higher rate than a single pass over the surface with the same residence time on the part. However the optimal condition will depend on the tolerance on asperities in the floor, and the total depth of cut to be achieved, as well as the material being cut. In deeper parts the sides of the wall can narrow to interfere with the jet structure and create ripples that take the deeper wall surfaces beyond this acceptable limit.

Figure 7. Schematic of the jet structure as it moves away from the nozzle.

One solution to this is to return to the technology that we demonstrated with John Shepherd’s Wobbler namely to move the nozzle in such a way that the jet oscillates over the surface, not only making multiple passes along the floor of the desired cut at a high enough speed to achieve good depth control, but also by slightly widening the cut there is less chance of undesirable bounces from particles on the edge of the jet from causing out-of-tolerance rippling in the lower parts of the cut.

Figure 8. Exaggerated movement schematic to show how oscillating the head widens the cut, allowing the jet better access to the bottom of the cut without sidewall interference.

This particular aspect of nozzle motion has also been studied in Australia and Italy. The Australian results, for example, (Patel, Chen and Siores, 6th Pacific Rim Conference, Brisbane, 2000) looked at oscillation both along the cut and transverse to it. They found that surface quality was improved three-fold on the walls of the cut, and that, depending on oscillation angle and frequency, the smooth zone cut depth could be extended further into the work piece. The optimal angle of oscillation was found to be three degrees.

Figure 9. The effect of nozzle oscillation frequency on the depth of smooth cutting in the walls of the piece. (Patel, Chen and Siores, 6th Pacific Rim Conference, Brisbane, 2000)

How one breaks out the floor of the pocket, after defining the walls remains a work in progress.

Waterjetting 13c - On Milling and bas reliefs

In the last two posts I have been discussing how, either with the use of masks, or with an orbiting nozzle tool, it is possible to mill the material from a confined space within a surface, thereby creating a pocket.

There are a number of advantages to the latter technique, albeit it does require a special tool, rather than using masks that can be made from material already available at a shop.

Figure 1. Using a steel plate to provide a mask, while cutting a square pocket in glass (Courtesy of Dr. Cutler)

Figure 2. Detail of the corner of the pocket (Dr. Cutler)

With the oscillating tool, which can go deeper into the part to keep the distance from the nozzle to the work surface short, the corners can’t be as sharp as they are with the mask, since the outer radius of the focusing tube provides a limiting bound, once it moves into the cut. However, for shallower pockets where the nozzle can be further away, then the limiting corner radius sensibly becomes the orbiting radius of the nozzle.

Figure 3. A milled pocket in glass made using the Wobbler.

Note that the floor is relatively even in both cases, though in the masked case the view is taken only after the first pass over the glass. With the orbiting head it is possible to slightly tilt the head (it only requires a couple of degrees – depending on the other operational parameters) to ensure that the walls are being cut to as tight a tolerance to spec as desired (give or take a thou).

Dr. Hashish has noted, from some of the early work that he carried out, that it is possible to mill materials so that very thin skins (around 0.02 inches) can be left at the bottom of the pocket. As I will note in more detail next time, it is also possible to mill using abrasive waterjets in such a way as to leave intervening walls between adjacent pockets that are only that thick. If you have never had to do this in a conventional machine shop, you should know that as the wall of the pocket gets this thin, particularly at significant milling tool depth, the heat from the milling process, and the forces on the metal under the cutter are such that the wall will likely have some permanent deformation after the milling is over. Such is not the case where an abrasive waterjet system, of either variety, is used to cut the pocket.

Depths of cut uniformity can be held to a thousandth of an inch, though this requires some careful selection of both the abrasive size, and feed rate as a function of the other operational parameters of the system. As I mentioned last time, and Dr. Hashish demonstrated, as increasing precision is required in creating the floor of the pocket, so the abrasive being used must become finer and more precisely sieved to keep the wear pattern consistent.

. Figure 4. The effect of change in abrasive size on the smoothness of the pocket floor (Hashish M.: An investigation of milling with abrasive-waterjets, Trans. ASME, Journal of Engineering for Industry, Vol. 111, No. 2, 1989, pp. 158–166.)

There is an interesting niche market waiting to be developed in sculpting, I believe, based on putting some of these factors together. It was Professor Borkowski of the Unconventional HydroJetting Technology Center at Koszalin University of Technology* who first demonstrated that, by controlling the jet feed rate over the target, that the depth of cut into the material (and thus the depth to the floor of the pocket) could be controlled.

If now a photograph is scanned, so that the color of individual pixels along the photograph can be identified, then this color can be translated into a required depth By then setting the speed of the nozzle over that point on the target surface to give the required depth, then the jet will profile, from the color changes along the scanned path, the depth of cut on the milling path over the target. The details of the process are specified in the paper cited above, and the result has been the transfer of a 2-D image from a photograph to a 3-D bas relief cut into metal or other material surface. The depth control was well achievable using the rotational frequency of a stepping motor to drive the motion of the nozzle.

Figure 5. Outline of the process turning pictures into bas-relief (Dr. Borkowski).

The initial pictures that were obtained with the very first experiments were somewhat simple, though more than adequate to validate the concept. Where a smoother surface was required secondary passes could be made either in a parallel or orthogonal direction.

Figure 6. Early ball shape cut into metal to demonstrate speed control effect (Dr. Borkowski)

The next trial was with a ladies photograph:

Figure 7. Early trial of the technique to validate the effectiveness of the computer control (Dr. Borkowski)

More recently, as the process has been refined, much more detailed profiles have been demonstrated, as was seen, for example at the 2010 BHRA meeting in Graz.

Figure 8. Lizard bas-relief as shown at the 2010 waterjet meeting.

The concept of changing depth of cut, and thus being able to transfer photographs from the screen or paper onto metals or rock was an interesting academic challenge, that MS&T chose to address in a slightly different way.

Consider that the depth can be achieved by changing the speed of the nozzle on a single pass, so that the depth is controlled, or one can control the depth when only plain waterjets are used, by rapidly switching the jet on or off, as it makes sequential passes over the projected picture area.

The first image on steel led the subject in the first photo to mutter something along the lines of putting them on tombstones to remember those who had passed, so the next tests used photographs of my Grandfather and Dr. Clark, who founded the RMERC.

Figure 9. Images of my Grandfather and Dr. Clark transferred to basalt. (Dr. Zhao)

The technology advanced to the point that it was used to generate the plaque presented to me on my retirement from active academia.

Figure 10. My retirement plaque

Which seems to be a good point to close until next time.

*This University was kind enough to give me an honorary diploma.

Waterjetting 13b - Milling without a Mask

As abrasive waterjets have developed they have been used to both cut through materials, and, in more recent work, have been used to mill pockets within the internal part of the piece.

Figure 1. Milled Pocket in Glass

In the early parts of pocket milling simple linear cuts were made adjacent to one another across the space where the pocket needed to be created. However, with the need to slow the head down and reverse direction, the edges of the pocket were being cut deeper than the inside floor, and this could cause some problems with part life and utility.

The first step to overcome this problem was to provide a mask, cut to the size of the pocket to be cut, but made out of a harder material, such as steel. By placing the mask over the piece, and setting the machine so that the cuts were made at constant speed over the pocket, a flat floor could be cut. All the slowing and reversing of the head takes place over the mask, so that it is destroyed fairly quickly. But if it survives one milling, then for some parts this provides a process that cannot be achieved in other ways.

Consider, for example, the sheet of glass cut in figure 1. The corners of the pockets are relatively sharp and of consistent radius all the way down the wall, which is relatively straight. A conventional mechanical milling tool transmits high levels of force between the part being milled and tool holder. Therefore, to prevent vibrations, the tool diameter must be no less than a quarter of the tool length. This means that the radius of the pocket wall cannot be less than one-eighth of the pocket depth. That restriction does not exist with an abrasive waterjet milled pocket, where the radius can be much tighter.

This is a critical issue in the milling of parts, where the milling is to get weight out of the component. In many parts that are made for the aircraft industry the part can be designed so that much of the internal volume is not needed for strength, and can be removed to lower the weight. But with conventional tools there are limits to how much can come from a single pocket, not with the AWJ system.

As the above figure shows, and masking and other techniques allow, the radius of the corner can fall below a tenth of an inch even when milling pockets more than eight-inches deep.

There remain, however, a number of problems with the use of the masking technique. It takes time to make and install the mask, and it costs an additional expense that makes the process less competitive. One of the problems that arise with the use of masking comes with rebound of the abrasive from the mask. Dr. Hashish has illustrated this problem with a diagram.

Figure 2. Abrasive rebound from a worn mask (Dr. Hashish)

If the mask is not shaped properly, or if it has been used before and is worn, then it may have a chamfered edge. When the abrasive waterjet stream strikes the curved surface it can be reflected back onto the work piece, giving an unwanted erosion shadow along the edge of the pocket.

Another problem can arise if the speed of the nozzle, and the distance that the nozzle moves between passes is not controlled to ensure a smooth and even cut over the pocket surface. As mentioned in an earlier post, the roughness of the cut increases if the abrasive particles are allowed to bounce and make a second cut within the piece. To ensure quality, as a result, the nozzle should be moved, relatively quickly, over the workpiece.

Yet the inertia of the cutting head, and the drive assembly in the table motion controller make this difficult to do at relatively high speed. John Shepherd at PIW Corp came up with an answer to this problem, that coincidentally did away with masking.

Figure 3. The Wobbler showing the nozzle motion.

The concept behind the device is that, by slightly oribiting the motion of the focusing tube around an axis, the jet will sweep out a circular path on the workpiece. Because it is only the end of the focusing tube that is moving the forces involved are small, and easily provided through a small motor on the device. The relative speed with which the nozzle moves over the surface is now much higher, while the speed of the main arm remains relatively low. The device was studied at MS& T and the parameters that controlled the depth and quality of cut were found by Dr. Shijin Zhang as part of his doctoral research.

As with the control of single passes of a non-oscillating nozzle, the distance between adjacent passes is critical to the satisfactory performance. If the distance is too great then ridges will be generated in the floor that are almost impossibly to remove using abrasive waterjets alone. Dr. Hashish, in an early paper on milling, for example, showed that if the upper layers of a pocket are aggressively milled with higher pressures and larger grit sizes, that this floor roughness cannot be later removed by using finer grit sizes. This is because the finer grit, while removing some surface asperities will still erode the surface relatively evenly, so that the roughness pattern shown in figure 4, cannot be later removed entirely.

Figure 4. Rough floor to the pocket where the distance between adjacent passes is too great. (Dr. Zhang)

On the other hand it is not always necessary to have a high quality surface for the pocket. For example MS&T have made a number of plaques where metal plates, cut and lettered with the AWJ are then inset into pockets in polished samples of marble or granite. Since these are not strength-bearing, and the plates are glued in place, the pocket floor does not have to be of that high a quality.

Figure 5. Milled pocket in the shape of the United States, Note the edge sharpness and the narrow cutting radii.

On the other hand, where a smooth surface is required then this can be equally well achieved through programming the path of the overall head movement, so that the nozzle sweeps the floor evenly. The glass plate in Figure 1 was also milled with the Wobbler.

Figure 6. Pocket cut into metal without a mask, using the Wobbler. Note the smooth floor.

I will come back to this topic next time.

Waterjetting 13a - an introduction to milling

In contrast with the earlier use of high-pressure waterjets in material removal in civil engineering and mining, when industrial waterjet cutting first began it was used to make thin cuts through different materials (in the early days often paper and wood products). Through cutting, particularly in relatively thin stock, has a wide range of industrial uses, particularly when the pieces are cut “cold” and with edge qualities that are, even with the first cut acceptable as the final surface cut needed for the part.

Over time the advantages of this new cutting tool became more apparent, and the range of materials that the AWJ jet could viably cut was extended into metals and ceramics. Yet conventional machine tools do more than just cut the edges of parts, and so questions arose as to the best way to achieve the milling of internal pockets within different materials. Within relatively soft rock, and with pressurized water alone, it is possible to generate interesting shapes.

When we first started experimenting with cutting rock at Missouri University of Science and Technology (MO S&T) the support equipment that we had was very basic, and the budget similarly restricted. In order to achieve precise positioning and control of the speeds during the cutting process, we therefore mounted the nozzle and support lance on the traverse of a conventional lathe. The samples were mounted into the chuck, so that we could achieve controlled cutting speeds. To get a number of sample cuts in a single test we placed a sheet of metal, with slots cut into it, between the nozzle and the rock.

Figure 1. Rock rotates in a lathe while the nozzle traverses across the face.

The notches cut into the metal plate were cut wide enough to allow the jet to make a single pass over the rock surface as the rock rotated and the nozzle swept past the slot, and they were widely enough spaced that the cut made through one slot did not interfere with the adjacent cut made through another.

Figure 2. Slots cut through the mask into the rock target.

After a while we became a little more adventurous and realized that, by making the mask an interesting shape that we could leave part of the rock uncut, but mill out all the rest of the material exposed to the jet, by adjusting the feed rate of the nozzle relative to the rotational speed of the rock.

We thought at first that the feed of the nozzle (easy to set with the lathe) should be one jet diameter for each rotation of the rock, but the jet spreads as it moves away from the nozzle and this turned out to be a little too small a distance, and we ended up setting the feed at about 1.5 times the jet width. This “incremental distance” is going to vary between systems, as a function of nozzle design and size, jet pressure and the distance between the nozzle and the target. In this early work in the technology (this was back around 1972 IIRC) the nozzle stood back from the rock at about one inch standoff. In more modern applications that distance can be quite a bit less, and this changes the incremental distance. Also bear in mind that the speeds at which plain high-pressure waterjet cuts are most efficient are much higher, across the target surface, than the optimal speeds for AWJ work.

So, since there was a need to remind folk that waterjetting could be dangerous if proper care was not taken during its use, we used this idea and made a sculpture.

Figure 3. Skull figure carved out of sandstone.

For simple lettering and shapes such as that shown above, the practice was to cut the desired shape into a metal plate, using perhaps a cutting torch, and then attach this over the rock. The two locations for the retaining wire can be seen on the sides of the piece. This allowed the plate to rotate with the rock piece as the lathe turned, and did away with the stationary plate between the nozzle and the sample.

By adjusting the feed speed and the rotation speed of the piece a relatively smooth surface could be left in the excavated pocket. (See the depths of the eye sockets). The process is known as “Masked” milling, since the plate masks the sections of the rock that the jet should not be allowed to mill into.

This works well when the work piece allows the use of plain high-pressure water, since it is relatively simple to make the mask out of a material (in this case steel) that the jet would not erode significantly. Thus the same mask could be used repeatedly to make copies of the original (though I think, in this case we only made around three or four).

But what happens when the jet is an abrasive waterjet, and we want to make pockets in the same way as I have just described. Because the AWJ will cut through a thin mask it was not an optimal choice for the process.

One can, with precise control of the nozzle position, have the jet move back and forwards over the desired pocket geometry. With the more accurate controls available today it is possible to slow the nozzle as it reaches the end of the pocket, increment it over the desired distance, and then have it cut an adjacent path back along the material to the start side of the pocket. Here the process would be repeated, moving backwards and forwards until the desired pocket geometry had been covered.

The problem with this approach is that the depth of cut into the target is controlled, in part, by the length of time that the jet plays on any one point, or inversely as the speed with which the nozzle is moving over the surface. So moving the nozzle more slowly as it approached the edge of the pocket (which you have to do because the robotic arm driving the move can’t instantaneously stop, increment over, and reverse direction because of the inertia in the system) is problematic. This is true only however if the pocket has to have a smooth regular floor of a fixed depth but most, unfortunately, do. And slowing the nozzle at the end of the cuts means that the depth of the pocket would be deeper along the pocket profile, relative to the body of the cut.

And so, for lack initially of an alternative approach, for some time the industry used masks that would protect the sides of the pocket, and provide a space over which the nozzle could decelerate, increment over, and turn back. The mask would be eroded away, but in desirable parts (often expensive to make in the desired material) the ability of the abrasive waterjet to make the pocket in the first place allowed the expense of the mask to be written into the cost of making each part.

There is, however, at least one other way of doing this, and I will discuss that, next time.

Waterjetting 11d - AWJ and cut taper

I have spent some time in recent weeks discussing the use of abrasives in waterjet cutting, and particularly some of the issues that are involved in getting the abrasive distributed relatively evenly through the jet stream, and accelerated to as high a velocity as possible by the time the jet leaves the focusing tube.

This issue has become more important as clients request more precise cuts, and edge quality and alignment become more critical. As the jet cuts along a surface the amount of material that is removed (i.e. the depth of cut simplistically) is controlled by the number of particles that impact along that axis. And that, to a degree, is controlled by where that axis lies, relative to the axial diameter of the jet that runs parallel to the direction of cut. Different conditions give different particle densities, but even within those conditions, the material under the center of the jet will see many more particle impacts than those on the side.

Figure 1. Particle distribution across two abrasive waterjet streams with the same focusing tube diameter, but different waterjet orifice diameters (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.)

As the above figure shows, in order to achieve the best abrasive cutting the rate of abrasive feed must be tailored to the nozzle size and the jet parameters. The density of the abrasive in the resulting stream can be optimized for those conditions and, as discussed in earlier posts, adding too much abrasive to the system will end up being counter productive.

A simple example can show this, in a test where we cut grooves in a block of granite, with the concentration of abrasive in the jet stream increasing with each pass.

Figure 2. Cuts made into a granite block, with abrasive feed rate increased as the cuts progressed from the left-side of the block to the right. Note that beyond a certain AFR the depth of cut begins to decrease. (Yazici, Sina, Abrasive Jet Cutting and Drilling of Rock, Ph.D. Dissertation in Mining Engineering, University of Missouri- Rolla, Rolla, Missouri, 1989, 203 pages.)

There is, however, a second consequence to the concentration of particles across the jet, and that is that the material under the jet on either side of the center-line of the cut will see a smaller number of particles impacting the surface, than that at the center. As a result the material will not be cut as deeply, and as the slots shown in Figure 2 illustrate, the cut will, as a result taper in on both sides.

In many applications, where the material to be cut is relatively thin, or where the exact alignment of the edge is not that critical this may not be important. However there are applications where edge alignment is required on the order of a thousandth of an inch or two over the part thickness, with the part being half-an-inch or more thick.

One way to achieve that precision of cut is to slow the traverse speed down. If the jet is moving slowly enough then there will be enough particles hitting the material at the edge of the cut, that the edge will be cut vertically downwards.

Figure 3. The effect of traverse speed on the edge taper angle (in degrees) in cutting titanium.

Notice that, because the jet tends to flare out a little as it moves away from the nozzle, the taper angle goes negative if the speed falls to too low a value. In this particular case the nozzle was moving across the surface at a speed of about quarter-of-an-inch per minute.

To get enough particles on the sides of the jet to cut a parallel slot edge, however, means that much of the abrasive in the center of the jet is not doing any work, but is rather being powered up and paid for to no real advantage. Thus, in most cases, (though not all) cutting very slowly to achieve precision on the residual edge of the cut is an overly expensive way of achieving the precision.

Given the relatively small angle that the taper cuts it is usually more cost-effective (providing the table allows this) to slightly tilt the cutting head, so that at higher cutting speeds the taper is effectively removed on the edge that is left. Obviously the taper on the piece of material being removed is made worse, but if that removed piece is going to be cut later into a different shape for another purpose, then this excessive taper on the initial surface comes with no great cost.

The taper angle and the speed relationship will vary both for the material being cut, as well as for the different parameters of the abrasive waterjet, and so – as with most cases where this sort of precision is required – a small test program to establish the best parameters for the cut will be needed.

There are other ways of achieving this precision in cutting. One is to make multiple passes over the surface, with the jet removing only very small increments of material at one time. Again if this is carried out carefully and precisely the edge quality can be maintained, at the same time as the depth of cut can be well controlled allowing pockets of material to be removed from the work piece.

However that gets into the whole issue of milling material from a target, and that is the topic for another day. It brings up the inter-relationship between traverse speed and depth of cut (which combine to give the area of cut surface, which can be used in some cases to optimize the cutting performance of a system, particularly where edge quality is not that rigid a requirement). And more particularly it brings up the quality of the walls and floor of the pockets created.

Figure 4. Factors to be considered in milling a pocket, illustrated by a multi-level pocket created in glass.