Waterjetting 34b - Cutting a hole.

There are several different aspects to be considered when planning a job entailing hole cutting, the accuracy needed for the hole(s) to be cut, both in shape and alignment, the quality of the wall and the speed of the operation. Not all are important in each case. But they are combined through the amount of energy and abrasive that they use into an overall cost of production.

As energy costs continue to increase, it is realistic to look at hole cutting in a little more detail. The smallest holes are usually those that are pierced by a single action of the jet. In other words the jet is brought up to a piercing pressure, and then exposed to the target for enough time to cut through, and the jet is then shut off. If the system is kept at pressure then this can be a fairly rapid way of cutting a large number of holes in a patterned manner through a piece, and I have seen examples where several hundred holes have been precisely located next to one another in making a precision part in this manner.

Figure 1. An array of 33 x 33 holes drilled by a 58 micron jet through glass (courtesy of Don Miller)

One advantage of a waterjet carrying abrasive is that it is not restricted to drilling vertical holes, and in one application the nozzle was inclined to the work-piece so that the holes were each precisely drilled at a shallow angle through the plate.

Where more precision is required a smaller jet and finer abrasive can be used to cut around the profile of a desired hole.

Figure 2. Perimeter cut to make a hole in a glass slide – as a reference scale, the coin is a penny.

Precision cutting of holes like this is not quite as easy as it may appear, and the above picture hides one of the problems, since the cut comes in from outside the hole itself.

In the more general case the hole is started and pierces through within the scrap material that will be cut from the part, and the jet then cuts into the hole profile, and follows it around, before exiting back into the center, so as to leave a smooth wall.

Figure 3. Illustrative path for a jet to cut the perimeter of a hole in the target.

The cut should come into, and leave the desired circle very close to tangent to the line, in order to sustain a smooth profile around the cut and give the precision required. With the proper programming of the path, it is not that difficult to cut holes of varying diameter through, for example, half-inch thick titanium.

Figure 4. Holes precisely cut through a half-inch titanium plate.

However, when cutting such holes it should be remembered that the jet path through the metal, particularly as it gets thicker, is not totally vertical. Thus, at the bottom of the hole, it is possible to get a small dimple at the location (which I have exaggerated in Figure 3, to make this point) that the jet enters and leaves the hole profile.

Looking at the underside of a plate, cut with similar parameters to those in Figure 4, one can see where, for different hole diameters, the cutting parameters were not adjusted properly, and such a dimple was left.

Figure 5. Detail from the bottom of a half-inch thick piece of titanium, with holes cut as for Figure 4. Note the small dimples left on the profile of the hole, where the parameters were not properly adjusted.

This dimple can be a considerable problem if, for example, the holes are then used to hold rivets that will be slid into the holes, but which will catch and be held if the dimple exists. In high precision parts the dimple size may not have to be that big for the piece to be out of compliance.

Unfortunately, as with many such problems, the best parameters to ensure that this is not a problem are specific to the job that requires the holes, in regard to material, thickness, hole size etc. However we have been able to hold required tolerance on such holes without a great deal of testing for the titanium pieces shown in the figures – the dimples were formed early in our program.

Small holes that are through pierced in relatively thin material allow a waterjet to practically cut around the central core of material, so that it can be recovered in a single piece, and in certain sizes that will allow the recovered stock to be used for a different part. Certainly the recovered material can be reclaimed at less cost than the scrap swarf that is the consequence of a conventional milling of the holes using a mechanical tool, and the edges of the hole have not been exposed to the heat that would pass into the part, were a conventional mechanical bit used.

This lack of heat and the sensible elimination of the Heat Affected Zone (HAZ) around the created opening has an additional benefit. With the lack of overall force which is also missing when abrasive-laden waterjets are used, support ribs can be cut to very thin dimension without distortion, and holes cut into islands left within the part, again without distortion, as this piece of titanium illustrates.

Figure 6. Four circular holes cut into a piece of titanium to show how thin the ribs can be cut, and that there is no distortion when the final, fourth hole is cut through the intersection island left from the first three cuts.

The combination of abrasive and an ultra-high speed waterjet has thus found a market (albeit one that has still many opportunities yet to exploit) where the ability to cut a thin slot around a shape creates the required geometry in the part, without heat distortion, and without the use of additional energy to grind up the unwanted material in the piece of material that is scrap to the current need, but which can be of benefit in future use. To date I have been discussing the cutting of small holes, but consider the case when the hole leaves, as re-useable material, the piece of Hastalloy shown in Figure 7.

Figure 7. Piece of Hastalloy removed from the core of a hole cut to generate a required cylinder.

Waterjetting 14c - Intensifying Cavitation

In this short section discussing cavitation, I have, to date, described what it is and the normal method for determining cavitation resistance, and some of the damage that can come from unexpected cavitation in civil construction. Most efforts are directed at suppressing cavitation, since it can do a lot of damage and cause machines to fail. However, for those of us who work in excavation and cutting, finding ways to exploit cavitation and enhance the damage it causes can be productive.

The initial paper, that I am aware of, written with this suggestion was presented at the First International Symposium on Jet Cutting Technology (ISJCT) by Johnson, Kohl, Thiruvengadam and Conn “Tunneling, Fracturing, Drilling and Mining with High-Speed Waterjets Utilizing Cavitation Damage.” The paper was given in 1972, and one of the demonstrations that came out of this was of a small jet, at relatively low pressure, drilling a hole through a brick. The penetration rates reported in that paper were relatively low in comparison with conventional tools, but with a jet pressure of 2,000 psi and diameters of up to 1/8th inch, the hole could not be made without cavitation assistance.

The early nozzle designs were designed to induce cavitation bubbles in the center of the jet stream.

Figure 1. Cavitation induction in a nozzle (Johnson et al ibid)

Cavitation is created by developing shear or tensile forces in the water. In the left-hand part of figure 1 the jet is spun so that a vortex is generated in the flow as it enters the narrowest part of the jet. This creates the bubbles that are then carried down-stream in the center of the jet flow, and hopefully collapse at the target surface.

Perhaps the most advanced versions of this concept were later used by PetroPhysics, a company based in the Bay area of California. The Dickinson brothers used the aggressive, but spreading, jet coming from the orifice to clean oilwells and to allow them to drill holes into rock, without needing to rotate the nozzle assembly. The nozzle was a little more advanced than the initial design, but the jet had to drill a larger diameter hole, if the nozzle assembly was to follow the jet into the rock.

Figure 2. A detail of the Petrophysics nozzle

The great advantage of this tool was that, because it did not rotate, it could be used to drill long-lateral wells from the bottom of an existing oilwell out into the oil-bearing rock. This will significantly improve the penetration rate in many wells. (On a word of caution - there are other companies that claim to be able to do this, but some of them have less credibility than others, since the use of hose as a feed pipe means that the hose can fold up in the main well, suggesting that the bit is drilling, when it is not – there should be lots of cuttings and fine rock in the hole (making it more difficult for the drill to advance) if the drill is actually working).

Figure 3. Schematic of the PetroPhysics drilling concept.

Part of the problem with this system lies in the difficulty in ensuring that the hole diameter is of consistent size. When, in other work, we used a waterjet to drill a long horizontal hole, several hundred yards long, we found that the jet tends to preferentially cut at the bottom of the hole. If this is not built into the design (by tilting the nozzle up an adjustable amount) then it becomes very difficult to ensure that the hole runs straight, instead of curving increasingly downwards (and thus out of the target rock). Yet (though I never saw it work) it was reported that the drill was able to drill quite successfully in granite in California.

This design gave a broad path for cutting, and we were looking to provide a highly focused cutting stream, so the design suggested in the right-side of figure 1 seemed more attractive, and perhaps easier to construct. One of the thoughts in choosing this was that, as the jet hits a target, the reverse flow on rebound will protect the surface from some of the bubbles, whereas if the bubbles are in the center of the jet, then that lateral flow will force collapse against the surface.

Figure 4. The rebounding jet can protect the target from the collapse of cavitation bubbles, or focus their collapse if they are in the center of the jet.

But if a small, flat-ended probe is placed in the right place within the nozzle, then the flow of water past it will induce cavitation in the center of the stream, which is the object.

Figure 5. Sketch showing the location of the centerbody within the nozzle.

Note that the centerbody has to be back from the front of the orifice. The reason for this is that if it is not, then the vacuum pulled on the tip will be sufficient to drag air through the jet, so that all the probe will do is break the jet into droplets. (And we can prove that isn’t the case otherwise, as I will describe next time).

Figure 6. Relative change in erosion power of the cavitation as a function of the distance from the front of the tip to the face of the nozzle.

Even without this central probe in the nozzle it is possible for cavitation to occur, and bubbles to collapse, within the nozzle body. Doug Wright of StoneAge has shown that this can be quite severe, degrading nozzle performance significantly.

Figure 7. Cavitation damage within the throat of two nozzle channels (Doug Wright)

The simple designs that we used, pins held the parts together for alignment, allowed us to cavitate a waterjet at pressures up to 20,000 psi, at which the resulting stream drilled through a piece of alumina held close to the nozzle, and was able to drill (at a pressure of 6,000 psi) out more than 18-inches from the nozzle into dolomite.

Figure 8. Titanium probes showing the flow passages and the probe itself. Note the progression of cavitation damage at points on the probe. These caused the tip to break off after runs of several minutes.

Although there is a great potential for further development, this area of development fell into disuse, partly because of the power that can be generated and used for other purposes, and I will discuss those, next time.