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 – Mixing abrasive with a water jet and differences in orifice types

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.

Sometimes I would get the feeling (particularly when talking to some of my students) that the mixing chamber of an abrasive waterjet (AWJ) cutting system, together with the feeds in and the focusing nozzle outlet, were considered to be some magical black box out of which a perfect cutting stream issues to cut the desired material. There are, in fact a variety of different chamber designs that can be purchased from different manufacturers. Some will tell you that all designs cut roughly the same, and that there is little difference between them. As I commented in one of the earliest posts in this series this is not true. Over the years we have run numerous comparative tests on different designs, using different abrasives, abrasive feed rates and target materials, and have found a broad range of results. For example, in cutting steel at a fixed traverse speed and other conditions, we found an average comparative performance as follows:

Comparative performance between 12 nominally similar abrasive waterjet cutting nozzles in cutting through steel at a standard speed, pump pressure, and abrasive concentration.

Comparative performance between 12 nominally similar abrasive waterjet cutting nozzles in cutting through steel at a standard speed, pump pressure, and abrasive concentration.

I described the actual test in another post, and it is clear from these averaged results that there is a wide difference in performance between the nominally similar tools. While I don’t think there is a lot of interest in going through the details of different designs it might be helpful to explain some of the factors that play a part in producing jets of greater or lesser performance. To return, first, to the basic construction of the mixing chamber and focusing tube assembly (AWJ nozzle), one starts by recognizing that the major cutting performance will be achieved by the particles which remove material when they hit the target. The energy that they have, however, comes from the water that is fed into the AWJ nozzle through a small jeweled orifice, or waterjet nozzle, at the top of the mixing chamber.

Basic waterjet nozzle design That waterjet stream is small and initially highly focused and fast moving.

Basic waterjet nozzle design That waterjet stream is small and initially highly focused and fast moving.

As it moves through the mixing chamber, as I have described in other posts, the outer edges of the jet slow down, and gradually the jet fans out and breaks up into fragments. There is no benefit in trying to inject the abrasive into the jet at the beginning of this passage, since, at that point, the outer layers of the jet have enough energy to knock away the particles before they can enter the fastest moving segment in the core. Rather it is better to inject the abrasive further down the chamber, so that the jet will have begun to break down into slugs, and the abrasive can be positioned so that it is impacted by a sequence of the individual slugs and accelerated to the desired velocity. There is an additional benefit to moving the abrasive feed line a little further down the chamber. When the jet stream is rapidly switched on and off, when for example, piercing a series of small holes in a part, then the driving pressure pushing water out of the waterjet orifice switches off and on. When it switches off there is a short period where the differential pressure will draw fluid from the chamber back through the waterjet nozzle. If there are small particles of abrasive in the vicinity (and with some designs there are) then these can be drawn back through the upper orifice, and then pushed back down by the succeeding water flow in the next pulse. This can rapidly erode softer jeweled orifices, so that they round or chip, not always evenly, and degrade the resulting waterjet as it flows into the chamber. This disruption can move the jet from being in the center of the chamber, and cause poor abrasive pick-up or accelerate wear of the chamber walls, and in the focusing tube. All of these degrade performance. (The solution, if you can achieve it, is to use a diamond upper orifice, since this is largely non-responsive to the passage of the abrasive back and forth, and retains its shape and the jet performance it was designed to produce much longer – providing a cost benefit to the change).

Wear on a ruby waterjet orifice inserted at the top of a mixing chamber after 15 minutes of use. (The dark particles are small particles of garnet)

Wear on a ruby waterjet orifice inserted at the top of a mixing chamber after 15 minutes of use. (The dark particles are small particles of garnet)

Chipping on the edges of sapphire and ruby orifices (after Powell 2007 WJTA Conference)

Chipping on the edges of sapphire and ruby orifices (after Powell 2007 WJTA Conference)

Lack of wear on a diamond insert nozzle after being in use for several hundred hours.

Lack of wear on a diamond insert nozzle after being in use for several hundred hours.

With the abrasive inlet channel and jet passage designed to get the abrasive into the jet where the water jet is broken up, yet still moving at high speed, there needs to be a sufficient distance for an optimal energy transfer to occur. Beyond that point, with the particle and the abrasive (which will have partially been broken in the contact between the jet and the particle, between particles striking one another, and between contact between the particle and the walls of the AWJ nozzle) the cutting jet has to be refocused into the narrow cutting stream that is required to give the finished cut surface desired. The refocusing of the mixed jet (air, water and abrasive) is achieved with a focusing tube, which is made up of a conic section which brings the jet back together, and then a straight section which allows further energy transfer between the three component parts of the jet, before the jet issues from the orifice aimed at the target. The passage of the particle down this tube is not always straight. Wear typically begins at the tip of the conic section as it feeds into the tube. The wear within the tube will often take up a pattern, as any irregularity in the flow causes it to bounce from one wall of the tube to the other creating a wear pattern along the walls that has a wave-like structure.

Wear at different points along the waterjet focusing tube

Wear at different points along the waterjet focusing tube

When this wear reaches the mouth of the focusing tube, then the downstream orifice is eroded out of a circular shape, and the jet that comes out no longer will cut cleanly, or to as great a depth. At that point the nozzle is worn out and should be replaced. The point at which that replacement occurs varies depending on the quality of the cut that is required. Obviously when the cut being made is at a precision of a thousandth of an inch over a cut depth of half-an-inch (as with some aircraft parts) the replacement point is reached a little earlier than if the cuts being made are a rough cut merely, for example, to separate two parts one from another, and provide a rough shape to the piece. I’ll continue this topic in the next segment of this section.