Waterjet Technology-An Introduction to Waterjet Milling

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 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.

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

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

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.

Waterjet Technology – The Heat involved in Cutting Rock-Part A

Dr. Summers Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Series

As I was beginning to write these posts Bob Pedrazas, who is kind enough to transcribe these words over to the KMT Waterjet page, gave me some questions that had been asked about the technology of high-pressure and ultra-high pressure water jet cutting. At the time I gave an academic answer, pointing out that I would have to explain some background material, before the average reader might be able to follow the logic of some of my answers. Thus I did not plan to answer specific questions (though I would respond to comments) in the early days of the site.

It has been over one year since I first began putting these posts together on kmtwaterjet.com. Along the way I have tried to answer some of the questions on his list without specifically calling out the question – for example the answer to the question as to whether selecting the right system was important was, I hope, shown by a plot early in the series. I presented a comparative graph that showed that despite different systems having nominally the same power, water and abrasive values, when comparative cuts were made, in similar materials that there was a considerable difference between the depths of cut that could be achieved using the different designs. (So obviously selecting the best system has considerable benefit to the operator, over selecting another).

One question that was raised relates to the heat of the cutting process. And while it has a relatively simple short answer (waterjet cutting used to be sometimes called “cold cutting”) I am going to take a few posts to explain the answer in a little more detail. Part of the reason for this is that the information that I have comes from several sources, some in cutting rock, and some in cutting metal, and there are some different applications along the way that all fold into the general topic of heat in the cutting process.

Let me begin with the work of a friend of mine, Mike Hood, who was working in the South African gold mines at the time he decided to go for his doctoral degree (Hood, M. (1978) “A Study of Methods to Improve the Performance of Drag Bits Used to Cut Hard Rock,” Ph.D. thesis, University of Witwatersrand, R.S.A.”). To understand the problem that he addressed you should know that in some of the mines in South Africa the gold-bearing rock is contained in a very thin layer, within a surrounding host rock, which is a quartzite and very hard to cut.

Drilling the rock around a gold seam in South Africa

Drilling the rock around a gold seam in South Africa

However if miners are to get in and extract that thin vein (which is often only six-inches to a foot thick they have to drive passages that are big enough to work in (perhaps six-feet high). Thus rock on either side of the thin vein of gold-bearing material is drilled and then the entire rock face is blasted out using explosive. Now the gold ore is mixed in with all the other rock from the blast. This means that all that material must be lifted perhaps two miles to the surface, and then ground to a fine powder to release the gold. Both of these are very energy intensive operations.

Consider instead if, before the rock on either side of the vein was blasted out, the face could be cut with two slots, one above and one below the gold reef. That could then be removed, and the rock on either side could then be blasted, but instead of being hauled away it could be packed into the open space behind the working area, holding up the roof and saving a huge amount of the processing energy otherwise required. (There is less than half an ounce of gold in a ton of the reef ore, and when the rock on either side is included then this concentration becomes much less).

Design of cutting tool to carve a channel into the quartzite

Design of cutting tool to carve a channel into the quartzite

Mike was initially looking to used carbide cutting teeth to cut into the rock and make these slots. However, he rapidly discovered that as he dragged the bit across the rock, that even at relatively shallow cutting depths (about 2/10ths of an inch) the cutting tool was getting very hot very quickly, to the point that the carbide was starting to melt.

Temperatures building up on the carbide cutting tool

Temperatures building up on the carbide cutting tool

Obviously, if the bit could be kept cool, then the carbide would not soften, and thus remain sharp and able to cut better. Yet the temperatures that the bit was reaching very early in the cutting process meant that there was a lot of heat being generated during the cutting.

As a result he decided to run a series of tests in which he played water onto the leading edge of the bit to cool it. But because of the heat involved he wanted to get a fairly high flow rate to the bit, which was almost buried in the rock, and otherwise hard to get to. So to ensure the flow got to the right place he used higher-pressure waterjets that flowed through small nozzles, mounted to shoot the jets at different points on the carbide:rock cutting face.

Location of the jets on the carbide bits in the initial tests carried out by Dr Hood.

Location of the jets on the carbide bits in the initial tests carried out by Dr Hood.

There are two forces needed to make the drag bit cut into the rock as it moves forward. The first of these is the Thrust Force, which is the force which pushed the tool into the rock, so that it will cut to the required depth. The second is the drag or Cutting Force that is used to pull the bit along the face at the required depth.

Without the waterjets on the bit, the load on the machine was exceeded when the drag bit was cutting to about 5 mm deep (2/10ths of an inch) into the rock. But when the waterjets were added to the bit, not only did the bit stay cool, but it was able to cut more than twice as deep, at lower forces onto the bit, and thus with a lower power demand on the machine.

 

Change in thrust force when waterjets are placed in front of the drag bit

Change in thrust force when waterjets are placed in front of the drag bit

Change in cutting force when waterjets are placed in front of the drag bit

Change in cutting force when waterjets are placed in front of the drag bit

This result has since been repeated by a number of different laboratories around the world, and led on to the development of mining machines, and other applications.

In the next post I will explain why what happens does, and why adding such jets to a cutting tool can, in the right place, save considerable amounts of money and time.

 

Waterjet Cutting-Abrasive water jet and cut taper

Dr. Summers Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Series

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.

Particle distribution across two abrasive waterjet streams with the same focusing tube diameter, but different waterjet orifice diameters

Particle distribution across two abrasive waterjet streams with the same focusing tube diameter, but different waterjet orifice diameters

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.

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.

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.

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.

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

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.

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

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

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

Labels:abrasive cutting,abrasive particles,cut taper,milling,particle distribution,traverse speed

Waterjet Technology – Abrasives and cutting depth

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.

Waterjet Technology – Abrasives and cutting depth

In the past few KMT Waterjet Blog posts I have been discussing the use of abrasive in waterjet cutting, and in this and the next two posts I would like to talk a little about the abrasive feed rate (AFR), abrasive size and the selections for the best cutting performance. As with my other posts I can only write in general terms about these because the combination of waterjet system, nozzle design and abrasive selection will change the best values to use, and the results on different systems will differ in some way or other from the results I will mention. However in all cases the overall principles remain the same, and can be applied as general rules.

In the last post I noted that cutting performance fell as the average size of the abrasive fed into the system dropped below 100 microns. As part of that study we looked at the amount of abrasive that survived going through the mixing chamber in that size range. A simplified average of the results obtained are shown in the following table:

Percentages of the initial feed that survive at larger than 100 microns, for differing feed conditions.

Percentages of the initial feed that survive at larger than 100 microns, for differing feed conditions.

In an earlier post I had mentioned the “Green Tube” test that was used at the University of Missouri S&T as a way of measuring the particle size and speed, after the particles had passed through the nozzle, but without hitting a target. Because the distance that the particles travel is a function of the energy they obtained during mixing, some idea of the overall particle energy can also be obtained.

KMT Waterjet Abrasive Transfer Feedline V

KMT Waterjet Abrasive Transfer Feedline V

However, when the particle sizes were analyzed at different distances from the nozzle we noted that there was a large percentage of small particles in the short distances from the nozzle, but that as the particles were collected at greater distances from the nozzle, so the average particle size grew larger.

After thinking about this for a short while, the reason became obvious, and – at the same time – made it a little more difficult to draw simple conclusions from the test.

The reason for the greater collection of smaller particles nearer the nozzle is that they are decelerated more rapidly than the larger particles, once they start traveling through the air. If we go back to the basic equation that we learned in school:

Force = mass x acceleration

For a given particle, the force to accelerate the particle in the mixing chamber will, simplistically, be the pressure exerted on the particle by the water, multiplied by the cross-sectional area of the particle. If the particle is a sphere, with a diameter d, then the area be π x(d/2)^2. But the mass of the particle is a function of the volume, which is related to the cube of the diameter. Thus the acceleration, for a given particle size and at constant fluid pressure, will vary inversely with the diameter of the particle. In other words the smaller particles will accelerate faster in the mixing chamber and focusing nozzle.

KMT Waterjet abrasive line feeding into the KMT cutting head

KMT Waterjet abrasive line feeding into the KMT cutting head

Once the particle leaves the nozzle, however, the acceleration from the water is replaced by a deceleration as the particle is now moving through air that is relatively stationary. Now the situation is reversed and it is the smaller particle that decelerates faster, and thus will have a shorter effective range than particles that survived the mixing process in a larger size. This was therefore the explanation for the results that we saw in our tests.

Unfortunately life becomes a little more complicated than this when the nozzle is held close to the target. This is because, while the air between the nozzle and the target may be relatively stationary, at greater distances, the small gap means that the surrounding air is also drawn into the slot and flows with the stream along the cut. There is thus less resistance to the particles, which retain their energy to a greater distance – improving cutting depth. However that also changes if the jet is cutting through layers where there may be water or air in the gaps between the layers.

This work was carried out initially by Dr. George Savanick during work carried out at the then US Bureau of Mines, on cutting rock. It applies in other cases, however, since there are often times when cuts are needed between two work pieces with a gap between them. (The example in mind is cutting through the different tubes that bring oil out of a well. This casing can be made up of several different diameters of pipe, ranging perhaps from a 20-inch diameter outer pipe to a 3.5-inch diameter inner one, with other tubes between). What Dr. Savanick showed was that if the gap between the layers was filled with some relatively soft material that provided little resistance to cutting, but held its shape and provided confining walls on either side of the jet, that the range of the jet could be extended beyond that where the jet was cutting water or air between the layers. These factors then play a part in determining how far an abrasive jet will cut through material.

Often it is not just the ability of the jet to cut through the material, but also the straightness of the cut and the quality of the edge that are important. If, for example, one can be sure that there are no burrs on the edges of a cut between two overlapping layers of material, then the parts may not have to be separated, cleaned and re-assembled before being fastened together. This elimination of several manufacturing steps can significantly lower the cost of assembling, for the sake of discussion, aircraft components. In turn this may then justify the use of the AWJ system as the better manufacturing tool, even when it does not seem that the initial cutting process is much cheaper than the alternative.

I mention these considerations, because as I go through the different applications of these tools I can only be somewhat general in discussion of overall effects. The way in which the abrasive mixes with the water, the amount of particle breakup and the different speeds of the abrasive leaving the nozzle vary with the nozzle design and operating conditions. They are also tailored to an extent by the particular job that has to be completed. Thus a recurring piece of advice in this series will be to find a test piece of material and test out a range of options before committing to the final cut. The series will try and suggest where that range might be, but in the final decision I do not have the equipment or other conditions that exist in your shop, and thus only you can be the final judge.