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-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 of a Waterjet Cut

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 three posts I have been discussing the quantity of heat that is created when machine tools are used in the cutting of rock, metals and other materials. The amount that the temperature of both the cutting tool and work piece material will increase, and the effect that this has on the cutting tool and the finished part can, as I have shown, be reduced if a quite small stream of high-pressure water is directed into the small zone where the cutting is taking place. But what happens if the cutting process doesn’t use the large scale typical mechanical cutting tools, but instead uses the very small particles embedded within the jet stream itself as part of an abrasive waterjet cutting system? For many years the evidence, after the cut was over, indicated that there was very little heat build-up in the part, and the process appeared to be a “cold cut,” but there was no immediate evidence, because of the rapidity with which the cut was made. However, with advances in technology that limitation was removed, and research scientists at the University of Hannover have now been able to make temperature measurements during cutting. (A Thermographical Map of Tool and Workpiece During the Cutting Process by Plain Waterjet and Abrasive Waterjet up to 900 MPa, H. Louis, A. Schenk, F. Pude and M. Mohamed, 17th International Conference on Waterjet Cutting Technology). The group used an infra-red camera connected into a computer to capture images as an abrasive waterjet cut into a target work sample. The instrument had been calibrated to show the color temperatures that the image revealed.

Figure 1. Temperatures read through an infrared camera as an abrasive jet cuts into a target plate.

Figure 1. Temperatures read through an infrared camera as an abrasive jet cuts into a target plate.

The arrangement by which the images were obtained was relatively simple:

Figure 2. Experimental arrangement allowing capture of the temperature build-up in the cutting head, the abrasive jet and the work piece during an AWJ cut

Figure 2. Experimental arrangement allowing capture of the temperature build-up in the cutting head, the abrasive jet and the work piece during an AWJ cut

During the course of the experiment the size of the cutting jet and the pressure were changed to find how these controlled the temperatures that were generated in the different parts of the operation. The work first examined the results when only a plain waterjet, without abrasive particles, was used in cutting.

Figure 3. Temperature build-up when plain waterjets (at 125,000 psi) are being used to cut a piece.

Figure 3. Temperature build-up when plain waterjets (at 125,000 psi) are being used to cut a piece.

Note that there is not a large amount of heat generated in the part, in this case a temperature rise to 126 Deg F was measured, though the temperature rise in the nozzle holder was similar in range. When the effects of jet flow and pressure were plotted, the role that an increase in pressure played in raising the part temperature around the cutting zone is clear. Note, in Figure 3, the region over which the temperature has been raised in the work piece.

Figure 4. Temperature rise in the nozzle holder as a function of jet pressure

Figure 4. Temperature rise in the nozzle holder as a function of jet pressure

Note that at pressures of up to 100,000 psi (700 MPa) the temperature rise is only up to 86 deg F, much less than that in conventional mechanical cutting. When abrasive is added to the jet stream, then the temperatures generated, as Figure 1 indicated, are higher in the nozzle holder, because of the impact of the particles with the focusing tube as part of the particle acceleration. The piece was moved under the jet at 1.2 inches/minute, with an abrasive feed of 0.06 lb/minute, with jet pressures varied from 42,000 psi to 115,000 psi. (300 to 800 MPa). The target was a metal alloy. Not surprisingly as the pressure in the jet increased, so did the temperature in the focusing tube.

Figure 5. Temperature increase in the focusing tube, as a function of jet pressure

Figure 5. Temperature increase in the focusing tube, as a function of jet pressure

Temperatures were measured at the top, middle and bottom of the cut which the AWJ made through the target material, and these are shown in the following plot:

Figure 6. Temperature build-up in the work piece during the cutting operation

Figure 6. Temperature build-up in the work piece during the cutting operation

The graph shows that the temperature build-up is greatest in the middle of the cut, although this difference is small, and begins to disappear as the jet pressure increases. At 100,000 psi the temperature can rise to 150 deg F. In most cutting work that temperature rise would not be enough to cause any damage to the part being cut. Where very temperature sensitive materials have been cut with the jet at lower pressures and higher speeds at MS&T the zone of influence of the cutting operation was measured in microns. It is in living tissue, which can be more sensitive to temperature, where this can be a problem. The University in Hannover is internationally recognized for the work that it has been carrying out in to the use of high pressure waterjets in medical applications. While this is a subject for another day (or several since the range of applications continues to grow from year to year) the caution comes from work on cutting bone and reported at the 18th International Jet Cutting Conference in Gdansk by Biskup et al “Temperature measurement during abrasive water jet cutting of cortical bone measured by thermocouples”). Bear in mind, however, that one of the problems that the technology is seeking to address in these bone cutting experiments is to achieve a better quality cut than can be achieved with a hand saw, which has often been the tool used by a surgeon when dissecting bone, and the required edge quality is sometimes more difficult to achieve with that tool.

Figure 7. Temperature build-up in bone under varying conditions and for two bone thicknesses, as a function of residence time.

Figure 7. Temperature build-up in bone under varying conditions and for two bone thicknesses, as a function of residence time.

It can be seen that a thicker bone sample does become vulnerable to too high a temperature if there is a significant exposure time before the part is pierced. However, with an appropriate selection of parameters the temperature can be kept down in a range where the tissue does not die, and the considerable advantages to jet use can therefore be used. Keeping the parts being cut cool is important in very delicate and precise work, where thermal distortion of the metal, particularly in thin but deep cuts, can otherwise lead to unacceptable failures to maintain tolerance.

 

Waterjet Technology- Conventional Machining compared to Waterjet Tables cutting metals

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.

The first two posts in this section described how, in cutting through rock, the tool and the rock would be compressed together so that temperatures could be created in and around the tool that would exceed 2,000 deg C. That temperature is sufficient to melt the cutting tool, and in other situations is hot enough that it can ignite pockets of gas in underground operations that can have fatal results. However, by adding a small flow (less than 1 gpm) of water to the cutting pick not only is this risk of gas ignition or pick melting significantly diminished, but the water acts to remove the fragments of the rock as they are broken under the bit. This has two beneficial effects, first it removes the small rock that would otherwise be re-crushed and rub against the bit, causing the temperature rise due to friction. The second is that by also keeping the tool cool and sharp it can penetrate much deeper into the rock under the same forces, improving the efficiency of the cutting. When a cutting tool is used to cut metal instead, the processes are somewhat different. However, because the tool rubs against the metal and cuts and deforms the metal that will be removed as a chip heat will still build up around the cutting zone.

Figure 1. Temperatures around a cutting tool in metal (Gear Solutions Magazine )

Figure 1. Temperatures around a cutting tool in metal (Gear Solutions Magazine )

If you look closely at the temperature contours you will see that the lines stretch beyond the point where the cut is being made, and both the chip and the machined surface of the metal heat up to 500 degC. This narrow strip of metal on the surface of the piece is referred to as the Heat Affected Zone or HAZ, since the metal in this region has had its properties changed by the heat and deformation. And while the impact is more severe with a thermal method of cutting (such as plasma) there is some affect with mechanical cutting. This can be seen, for example, if a metal piece is machined without cooling of the interface between the bit and the chip. Depending on the material being cut, this can lead to chips that are thermally damaged, are long and can be dangerously hot.

Figure 2. Strips of metal milled without cooling (Dr. Galecki)

Figure 2. Strips of metal milled without cooling (Dr. Galecki)

If the surface of the chips are examined then the amount of heat damage is evident.

 Figure 3. Surface of the chip showing the damage from the heat during cutting. (Dr. Galecki)


Figure 3. Surface of the chip showing the damage from the heat during cutting. (Dr. Galecki)

However this problem with the heat generated during cutting has been widely recognized, and so it has become standard practice to play a cooling fluid over the cutting zone during machining. To be effective the water must pass into the passage along the tool face and down into the cutting zone. It thus acts both to lubricate the passage of the chip up the blade, and separating it from the cutting tool, while cooling the bit and keeping it sharp.

Figure 4. Insertion of the jet into the cutting zone. (Dr. Mazurkiewicz)

Figure 4. Insertion of the jet into the cutting zone. (Dr. Mazurkiewicz)

When this is properly placed, and as with the jet assisted cutting of rock the precision required in placing the jet is around 1.10th of an inch, then the chip and metal surface are cooled and the tool remains sharp. However, with conventional, lower pressure cooling, while the chip length is reduced and the surface is somewhat improved, overall cutting forces do not change.

Figure 5. Chips formed with conventional cooling (note the poor edge quality). (Dr. Galecki)

Figure 5. Chips formed with conventional cooling (note the poor edge quality). (Dr. Galecki)

When the waterjet pressure is increased to the ultra-high pressure range, so as to ensure that adequate water reaches the tool, then the cutting forces are reduced and the amount of damage to the metal is further reduced The result can be seen in the form of the chips that are removed, which are now much shinier in appearance:

Figure 6. Chips from high-pressure jet assisted cutting (Dr. Galecki)

Figure 6. Chips from high-pressure jet assisted cutting (Dr. Galecki)

 

Note that the surface of the chips are shiny, and that they are relatively small in size. The shiny surface is similarly reflected in that left on the machined part.

Figure 7. Cut surface left after high-pressure jet assistance to the cutting tool.

Figure 7. Cut surface left after high-pressure jet assistance to the cutting tool.

The resulting reduction in damage to the machined surface, as well as the lower machine forces, and the consequent lowering of the potential for “chatter” during cutting gives a higher cut surface quality which, because of the reduced damage to the surface has a higher fatigue resistance. The amount of modification required to the equipment is not necessarily large, since the high pressure water can be carried to the tool through relative small tubing that has a small footprint. The pump can be located elsewhere. Further, while conventional cooling requires additives to the water (which make it more costly to treat the scrap) the clean water used in the jet makes this less of a concern.

Figure 8. Arrangement with a jet added to the cutting tool on a lathe. There are also instruments on the platform. (Dr. Galecki)

Figure 8. Arrangement with a jet added to the cutting tool on a lathe. There are also instruments on the platform. (Dr. Galecki)

These results show that the heat damage that can be anticipated with conventional machining of metal can be significantly reduced with the addition of high-pressure water. This becomes even more clear where abrasive is added to the jet stream, and fortunately, thanks to colleagues in Germany, we have thermal images of this, which I will share, next time. (For further reading see Mazurkiewicz, M., Kabala, Z., And Chow, J., “Metal Machining With High Pressure Water Cooling Assistance – A New Possibility,” ASME Journal of Engineering for Industry, Vol. 111, February, 1989.)