Waterjet Technology-Milling and bas reliefs

This Waterjet Weekly is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

This Waterjet Weekly is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

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.

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

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

Detail of the corner of the pocket

Detail of the corner of the pocket

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.

A milled pocket in glass made using the Wobbler.

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.

The effect of change in abrasive size on the smoothness of the pocket floor

The effect of change in abrasive size on the smoothness of the pocket floor. Dr. M. Hashish

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.

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

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.

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

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

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

The next trial was with a ladies photograph:

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

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.

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

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.

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

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.

My retirement plaque

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.

Waterjet Technology – Milling without a Mask

Dr. Summers Waterjet Blog

KMT Waterjet Systems Waterjet Series

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.

Waterjet milled pocket in glass

Waterjet 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.

Abrasive rebound from a worn mask (Dr. Hashish)

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, (http://bittooth.blogspot.com/2013/06/waterjetting-10c-abrasive-waterjet.html)

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.

The Wobbler showing the nozzle motion.

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:

(http://books.google.com/books/about/Three_Dimensional_Milling_Using_an_Abras.html?id=-WjHuH7wQaAC)

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.

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

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.

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

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.

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

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

Note the smooth floor. I will come back to this topic next time.

 

Waterjet Technology-Abrasive waterjet cutting

Dr. Summers Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Series

There are a number of different abrasives that can be supplied by different sources, and the market for the small grains that are used in abrasive waterjet cutting extends considerably beyond just the waterjet business. All abrasives are not created equal, some work better in one condition, others in another. As with other tools that the waterjet cutter or cleaner will use, first you should decide what the need for the abrasive is, and run a small series of tests to find out which is the best set of cutting conditions for that particular job.

The first item on the list should be the material that has to be cut. (Although abrasives are also used in cleaning, that will be covered in a later post). There are, simplifying greatly, two classes of material that have to be cut. One class responds in a brittle way (think glass) and the other responds in a ductile or yielding manner (think metal). Because of these different responses, when the particles hit the surface, the way in which cuts are best made will vary between the two. Some years ago Ives and Ruff shot abrasive particles at different targets and found that there was a difference in the amount of material removed, but the best angle at which the particles should be aimed changed with the material.

The Effect of change in impact angle on erosion rate for ductile and brittle targets.

The Effect of change in impact angle on erosion rate for ductile and brittle targets.

Some work at The University of Missouri Science and Technology  just before I retired indicated that the shape of these curves changed a little, depending on the size of the abrasive that is used. There are also some changes with abrasive shape. And this is because of the entirely different way in which an abrasive particle cuts into the two different materials. In this post we’ll discuss only the ductile target.

If a relatively smooth particle is shot into a ductile material at an angle perpendicular to the surface, then when it hits the surface the target material will flow out from underneath, but not be removed. As the following micro-photograph shows the particles can become embedded in the material – and even add to the weight of the piece on rare occasion.

Microphotograph showing a sand particle buried in the surface of an aluminum target.

Microphotograph showing a sand particle buried in the surface of an aluminum target.

There is very little material removed in this case, as the black curve shows in Figure 1, when the impact angle approaches 90 degrees. Consider that if you take a knife and push it down into butter you don’t remove any butter. But if you drag the knife over the butter surface you will peel off a layer.

So it is with abrasive hitting a ductile metal. If the abrasive is brought in at an angle, (optimized in the figure at 15 degrees) then the abrasive has a cutting energy along the surface and this will peel up, and remove small pieces of the surface. By taking a microphotograph along the edge of an abrasive cut, we were able to show the action of individual particles in cutting into the metal.

Individual particle impacts on an aluminum surface, showing the cutting and plowing action of the particles.

Individual particle impacts on an aluminum surface, showing the cutting and plowing action of the particles.

Where the surface is plowed up, but not removed, another particle has to hit that point to remove the relatively fragile lip. However, if the particle is a copper slag, or other relatively weak material, it can shatter during the cutting process, and the breaking pieces can break off that lip, so that – again in the right material – the slag may give a better performance than a more expensive alternative.

But if we are to cut metal in this way, what does that say about the shape of the particles that we need to use. Obviously if they were round, such as a steel or glass shot, then there would be no sharp edges to cut into and peel off the material. Thus a steel or glass grit will cut better, though each particle needs a certain thickness in all dimensions so that there will be enough energy to both cut into the material, and plow along it.

Difference in cut depth achieved with broken glass fragments over glass beads when cutting metal.

Difference in cut depth achieved with broken glass fragments over glass beads when cutting metal.

A relatively round particle with sharp corners, and garnet is usually such a particle, can often work well in cutting a range of different ductile materials.

Schematic of how a particle of different shapes might cut into material.

Schematic of how a particle of different shapes might cut into material.

Now that is fine when a high-pressure abrasive waterjet (AWJ) is starting to cut into the surface, but as the jet cuts down into the surface the angle of the cut will change. Yet even if the jet is pointing directly down into the target, and moving along to cut through it, the cut surface is not usually a straight line down through the material.

Cutting through glass, note the curved path of the jet through the one-inch material.

Cutting through glass, note the curved path of the jet through the one-inch material.

 Cuts into Plexiglas and other clear materials have allowed research scientists to monitor the cut path through the target, as a function of time. It is not a constant shape, but, as Dr. Henning showed at the 18th International Conference, the edge of the cut changes with time. You can see the results of this in cuts that are made through metal where the paths of the cut, particularly lower in the cut, curve around and back towards the start of the cut.

Cut into steel, with the face piece of metal removed to show the cut surface.

Cut into steel, with the face piece of metal removed to show the cut surface.

This path confirms an explanation first proposed by Dr Lars Ohlsson in his doctorate at Lulea in Sweden. He pointed out that the change in the surface of the cut is caused by the sequence of actions that a particle sees as it comes down onto the surface.

First it comes in almost vertically, with no lateral energy, and it cuts in the smooth, upper part of the cut. Then it rebounds out of the cut, but into the jet stream that gives it a little more energy, and directs it along the cut to a second point where it will cut a little bit more of the metal. But during the first rebound the particle does not bounce perfectly along the cut, but deviates to one side or the other. This means that when it makes the second cut, it will now cut more into one side of the wall or the other. Thus, where the second bounce occurs, so the surface gets a little rougher.

Frames from a high speed video showing abrasive waterjet cutting of glass, with the jet cutting, rebounding down the cut and then cutting again.

Frames from a high speed video showing abrasive waterjet cutting of glass, with the jet cutting, rebounding down the cut and then cutting again. (Lars Ohlsson “The Theory and Practice of Abrasive Water Jet Cutting”, Doctoral Thesis, Division of Materials Processing, Lulea University of Technology, 1995)

By the time of the third cut and rebound, the jet will now be coming into the opposing side of the cut with an even greater lateral portion of its energy, and so the cut will get a little rougher. Remember also that each cut is made up of the impacts of very many particles. So that succeeding particles also rebound along the curve cut by the preceding particle, and this also will exacerbate the roughness of the cut.

We’ll talk a little about reducing this effect in the next post.

 

Waterjet Cutting-Introduction to Abrasive Waterjet

This Waterjet Weekly is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

This Waterjet Weekly is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

In recent articles in this series I have written about the processes that occur as a high-pressure waterjet impacts on a surface and then begins to penetrate and cut into it. However, as I noted in the last post, one of the problems with using plain water as the cutting medium is that it can pressurize within the cut and exploit any surrounding cracks, to the point that the edges of the cut are cracked and fractured, often back up to the top surface of the material.

High-pressure waterjet cut along a sheet of Plexiglas, note the fracturing along the sides of the cut.

High-pressure waterjet cut along a sheet of Plexiglas, note the fracturing along the sides of the cut.

This is not usually desirable, and what is needed is a way of cutting into these materials, so that the cut edges remain smooth, and the risk of shattering around the cut line is much diminished. The way that is usually used for this is to add small amounts of a fine cutting abrasive into the waterjet stream, and use this to cut the slots in the material, with the water there to add cutting power.

Abrasive waterjet (AWJ) cuts through safety glass. Note that there are two sheets of glass with a thin plastic sheet attached between the two.

Abrasive waterjet (AWJ) cuts through safety glass. Note that there are two sheets of glass with a thin plastic sheet attached between the two.

This can be of particular advantage if you are faced with trimming, for example, safety glass (as shown in Figure 2). Cutting and shaping this glass used to be a significant problem in the industry, since the presence of the plastic sheet, between the two glass layers meant that it was not always possible to get both to break to the same plane if scribed with a glass cutter. Failure rates of up to 30% were described, to the author, as common when the technology switch to AWJ took place. And with the abrasive in the water, the jet cuts through both layers without really seeing that there was a problem. (And complex contours can also be cut).

The combination of abrasive and high-pressure water has many advantages over existing tools. Among other things it removes the majority of the heat from the cut zone, so that in almost all cases the Heat Affected Zone (HAZ) along the edges of the cut disappears and the quality of the cut surface becomes, when properly cut, sufficient to require no further processing. This can lead to a significant savings in certain forms of fabrication.

There are many different ways in which abrasive can be added to a high speed stream of water, and Dr. Hashish illustrated some of these in the introductory lecture he gave at an early WJTA Short Course, as follows:

Some different ways of introducing abrasive into the cutting stream of a high-pressure waterjet (After Hashish, WJTA Short Course Notes).

Some different ways of introducing abrasive into the cutting stream of a high-pressure waterjet (After Hashish, WJTA Short Course Notes).

The top three (a, b, c) involve mixing the abrasive and the water streams at the nozzle, while the fourth (d) is a relatively uncommon design that is used in cleaning surface applications, and the fifth has never been very effective in any trial that we have run. The sixth (e) technique has become known by a number of different names, but for now, to distinguish it from the more widely used Abrasive Water Jet cutting (AWJ) I will give it the acronym ASJ, for Abrasive Slurry Jetting. It has a number of benefits in different circumstances, and I will write more about it in future posts. In more recent alternative designs to that shown by Dr. Hashish the flow to the abrasive holding tank is more commonly through a diverted fraction of the total flow from the pump or intensifier.

Very simplified illustration of the circuit where abrasive is added to the flow from the pump/intensifier before the nozzle. Obviously the abrasive is held in a pressurized holding vessel – the optimal design of which is not immediately obvious.

Very simplified illustration of the circuit where abrasive is added to the flow from the pump/intensifier before the nozzle. Obviously the abrasive is held in a pressurized holding vessel – the optimal design of which is not immediately obvious.

When fine abrasive is added to a narrow waterjet stream, and that jet is moving at thousands of feet a second, there are a number of considerations in the design of the mixing chamber, and those will be discussed in future posts. But one early conclusion is that, if the jet is going to be small, then the abrasive that will be mixed with it will also have to be quite small, though – as will be noted in a future post – not too small.

The simplified and generic components of a mixing chamber that mixes abrasive with high-pressure water in an AWJ system.

The simplified and generic components of a mixing chamber that mixes abrasive with high-pressure water in an AWJ system.

There were a number of problems with the early systems, such as that shown in Figure 5, at the time that systems first appeared on the market, and I will write about some of these as the next few posts continue to focus on this subject.

There have been a number of different abrasives used over the years, and it depends on the needs of the job as to which is the most suitable in a given case. In some cases discriminate cutting is required, and so an abrasive can be chosen that will cut the desired layer on the surface, but not the material behind it. In other cases the target material is extremely tough, and so abrasive may be selected that will rapidly erode the supply lines and nozzle, but which can still prove economically viable in certain cases.

Various types of abrasive, that can include (from bottom left going clockwise) blasting sand, copper slag, garnet and olivine.

Various types of abrasive, that can include (from bottom left going clockwise) blasting sand, copper slag, garnet and olivine.

There are many different properties of the cutting system, and the abrasive which control the quality and speed of the resulting cut. Some of these will be the topic of the next few posts, others will be discussed in further posts at a more distant time, when we discuss different cutting applications and the changes in a conventional system that might be made to get the best results in those cases.

Abrasive properties are not just a case of knowing what the material is. There is a difference, for example, in cutting ability between alluvially mined garnet and that mined from solid rock. There is a difference between different types of the nominally same abrasive when it comes from different parts of the world, and there are differences when the shapes of the abrasive differ. Glass beads and steel shot cut in a different way that glass and steel grit, for example. So there is plenty to discuss as we turn to a deeper discussion of abrasive waterjet cutting.

Parameters controlling the cutting by an abrasive waterjet system. (After Mazurkiewicz)

Parameters controlling the cutting by an abrasive waterjet system. (After Mazurkiewicz)