Waterjetting 15c - Surface quality

In the beginning, when the very name of waterjet cutting was, in itself enough to create an interest, the quality of the surface that was left after the jet passed was not a critical factor. The surface was, relative to other techniques, left in a cold condition without the imposition of additional stresses brought about by the cutting process. This, in itself brought some benefits to the use of this new tool, and when melded with the benefit of a continually sharpening blade, meant that there were a wide variety of applications where it had an immediate (in not so quickly recognized) benefit. One of these was in the cutting of food products. Here, where the aesthetic need for a sharp clean-cut surface could be joined with the reduced crushing along the cut line, provided an increased appeal both in immediate and longer-term to the use of a plain waterjet system.

Back in the days when we were all trying to find new ways of using this new tool, I remember colleagues from the National Research Council in Canada commenting about relative trials in cutting peaches. The problem with mechanical cutting of freestone peaches is that the knives occasionally hit the stone, distorting the cutting edge and, over time, making for a ragged cut. In looking for alternatives, the NRC compared laser cutting with that of a high-pressure waterjet system. With the laser there was this strong smell of burning, a relatively wide cut appeared on the surface, but the high water content of the flesh of the peach inhibited further cutting. With the waterjet, in contrast, the cut was cleanly and effectively achieved within the necessary time. The cut was clean, and without the compression of fibers that would, otherwise over time lead to discoloration and customer dislike. More recently this same result has led to the widespread use of waterjets in trimming, for example, the lengths of celery that are found in the grocery sections of many stores.

Cut edge quality is thus an important issue when it comes to selling the benefits of this new tool. But edge quality means different things to different customers. To aircraft manufacturers that we have worked with, being able to cut through half-inch thick titanium with a precision of 0.001 inches over the alignment of the cut was a critical problem. In cutting the walls of the Omnimax Theater under the Gateway Arch in St. Louis we were asked to maintain the wall alignment, over its fifteen-foot depth, within an inch of vertical. (Which turns out to be a precision of 0.005 inches per inch, not that much greater a tolerance than that we provided in the aircraft application). In cutting through the granite to make the Millennium Arch the concern was more to ensure that when the central figure was completely outlined that there would be no protrusion that would stop the two parts of the piece being separated.

In traditional processing, it is common to expect that, after the pieces of an assembly have been separately cut out, that the part edges will be finished separately, before the components are brought together in final assembly. Thus, as an example, after using a cutting device to cut the shapes of different parts of an assembly, secondary finishing is built into the cost and time schedule to allow for this additional step of cleaning up the edges of the parts to remove the cut imperfections that form as a part of conventional cutting.

But, in many cases, the use of an abrasive waterjet system does not require that second step. The quality of the initial surface cut is well within the bounds of precision and accuracy that are required to meet the final assembly part requirements. As a result there is at least one, and often more steps that can be eliminated from the assembly schedule, and the costs for those set against any additional costs that the waterjet cutting costs might have initially brought into the overall picture. In assembly costs time is money, and the elimination of steps in a process can be significantly greater than just those occurred in the process itself. It is an advantage that is not often fully recognized.

Consider, for example, the simple case where two surfaces are to be riveted together. In the older, conventional practice the two parts would be mated one to the other, while a machinist came along and drilled the rivet holes through the two parts. The parts would then be separated while the burrs and machining residue was removed from the two pieces separately, and then they would be relocated and re-aligned, and a riveter would then come along and drive in the rivets to hold the parts together.

In contrast, where an abrasive waterjet is used the tool can cut through both (or more) parts without burring on the edge, leaving holes of high enough edge quality that there is no burring or need for subsequent re-finishing. The rivets can be immediately installed in the appropriate holes, which are already aligned and ready, and two lengthy and costly process steps can be saved.

As a trivial issue, but one of increasing importance as the cost of raw materials increases, the component pieces that are cut from the solid to make the two parts can also be recovered as a single piece, with the opportunity for use in another application, rather than being rendered down into scrap that must first be reconstituted in to a solid piece.

To continue with the analogy (though it is admittedly a bit hard to find a secondary use for metal of this shape) the metal removed from the rivet hole can be recovered in a single piece.

Figure 1. Rivet hole metal removed from a sample as a single piece, compared with the conventional alternative.

There is, however, one caveat to this discussion, and that relates to surfaces that will be precision welded after cutting. One of the problems that abrasive waterjets can create, is the occasional embedment of the abrasive particles within the cut surface. (Figure 2).

Figure 2. SEM image showing abrasive particles embedded in a surface after cutting. (Dr. Galecki)

Where this is a possible problem then the simple answer, as my colleague Dr. Greg Galecki has demonstrated, is to repeat the cutting path but with a high-pressure waterjet along feeding over the cut surface (at a higher traverse speed). This removed the buried pieces and leaves the surface in the condition required.

Figure 3. SEM image of cleaned surface showing a representative site where the embedded particle has been removed.

Energy Summit - the second part (until 3 pm)

This is the second post on the contents of the Energy Summit held at the University of Missouri this week. The first post covered the keynote address by T. Boone Pickens, and that can now be seen, together with Chancellor Carney’s opening remarks, and those of Senators Bond and McCaskill as a video. (Warning it is a 1 hr 30 min video and some 219 MB). Mr. Pickens remarks were also picked up by the local Missourian. (The school of journalism was also holding briefings and interviews that ran concurrent with the summit). After the keynote, the Summit got underway with a brief review, by the chief research officers of the four campuses of the University of Missouri system, of the energy related research that each was carrying out. This was a fairly top level skim through a project range that covered fuel cells, hydrogen, a new way of storing gas on carbon bricks, and studies on a wide range of pathways to generate transportation fuels, and also prefaced a number of the papers, poster presentations and displays, the latter two of which were going on outside the main auditorium.

The first invited Speakers then came to the podium with Dale Klein, Chairman of the U.S. Nuclear Regulatory Commission leading off the presentations.

Mr Kein noted that his agency is a regulator, not an advocate, and that he is currently looking at 17 applications for 26 new nuclear plants, with another 3 applications for 5 plants being anticipated. (However this is likely to include the AmerenUE application for a second plant at their Fulton site. The company indefinitely suspended that request this morning, just after getting word that a bill that would allow it to charge for construction before it was finished was not going to happen).

Mr Klein walked us through the process of getting a permit, noting that it would take 30 months to review the application, 12 months to get public comment, and then it might take some 44 months to get the plant built. The current costs are in the range of $5 - $7 billion per plant, and they are licensed for 40 years initially, with a possible 20-year extension. (For comparison he noted that the USS Enterprise, the first nuclear powered aircraft carrier, was commissioned in 1960, and is coming to the end of its service life –within 3 – 5 years. He felt that the public needs more education on the benefits of nuclear power, and what not to be afraid of.

Coming next to the podium, Dr. Joan Woodard ( executive vice president at Sandia National Labs) mentioned her last visit to town, some 35 years ago as she drove out to Sandia to take up her first job there. She talked of nations climbing the energy ladder which takes nations from no power, through burning dung, and then to carbon fuels and to higher levels of consumption as society advances. However she noted that the US curve was flattening as is that of the European Union, and it is other nations, from Korea and Australia to China and India that are growing and seeing increased levels of energy demand as that growth continues. This is, in time, bound to strain the system, due to the demographics of a growing world population combined with growing standards of living, and thus individual energy demands.

She felt that Secretary Chu does a “wonderful job” in explaining the coming mandate that is Climate Change, and she noted the ever-shrinking condition of the Arctic ice cap. (Obviously she has not seen the latest ice data from the Arctic, which shows that the coverage is returning to the seasonal normal for the past 28 years, since she commented that the rate of shrinkage of the ice cap was accelerating, when, if you look at the plot, it obviously isn’t.) Nevertheless, in light of the mandate she felt that the energy enterprise must change to encompass not only the desire for economic prosperity (the ladder) and the regional environment, but also national security issues. In this she felt that while Global Trade can be good, it also creates tensions over such concerns as Russia, and now China buying up large quantities of the world reserves of a number of commodities. And in that regard we must consider that Chinese companies that are doing the purchasing are an extension of the state.

Droughts in Africa will drive migrations, leading to further conflict. Further the US is vulnerable to national disasters. Both of which threaten our security, although she then went on to mention more conventional threats. These include attacks by hackers into the control systems for our energy networks and the threats posed by global proliferation of nuclear knowledge. To protect against these threats we need a system that will, if it does fail, does so “gracefully” but which has high reliability and resilience against attack.

Daniel Cole senior vice president of Ameren then talked about his early job as a “pirate” down at Branson, MO. Here as part of the “tourist” entertainment he would regularly be pelted with bags filled with rock, but simulating gold. That job was excellent training for his current one with the utility. The company has 2.4 million electric customers, and a million natural gas customers. They produce some 16,600 MW which is nominally 61% coal, 30% natural gas fired. However because coal provides baseload and natural gas is for peaking demand supply, it turns out that 85% of actual production is coal-fired. It is also cheaper. But in the process last year, for example, it produced 70 million tons of carbon dioxide.

The nation produced some 6 billion tons. Now if the system goes to a cap and trade system and one might project growth to 6.2 billion tons generated in a couple of years, the cap might instead mandate total production is held to 5.5 billion tons. This amount would then be parceled out as a series of allocations. Each allocation would either be designated to a company at a price or subject to auction. The company could also offset some of its production with some alternate activity (such as paying for no-till farming for example).

They priced the cost that the company would face after the Lieberman Warner bill was proposed. It set a price of $50/allowance (1 ton of carbon) in 2015, rising to $100 by 2030. With the production of the company being 70 million tons, this will give an additional bill of $3.5 billion in 2015. This will mean, according to Mr Cole, the rapid disappearance of existing coal plants, but Missouri currently has the lowest electricity rates in the nation, and such a burden on their carbon production would have to be passed on as a very rapid increase in power costs per kWh to the customer to more than double that of today. The results when the requirements of the Waxman Markey bill were evaluated were even more severe.

Ameren is part of the Electric Power Research Institute (EPRI) which has examined different technologies (pdf) to see if, in fact, these targets are attainable, anticipating increases in efficiency of use, and a 0.1 to 0.7% growth in demand.

Their conclusion was that renewable sources will only act to stabilize carbon dioxide levels, and that while increased use of nuclear power can initiate a drop in levels, it will be a switch to advanced coal generation that will be required to make significant reductions. But to have a real impact the focus must look at coal, focus on adoption of new technology, and be international in application. But the answers will come as silver buckshot not as a silver bullet.

This is the third post on the Energy Summit
The second post covered the Keynote, and the first described the program.

The final speeches of the first day will be covered next.