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.)