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 12c - Jet assisted metal cutting

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 )

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)

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)

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)

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)

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)

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.

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)

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.)