Sunday, June 14, 2015

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.

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