Waterjet Cutting-Time and Materials

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

In the last Waterjet Weekly Blog, I wrote about the uses of waterjet technology. It is important to note this week that the savings in time that waterjet cutting brings to an operation underlines the old adage about operational costs – “Time and Materials.”  The time-savings become particularly true where the high-pressure cutting system is integrated into the modern cutting tables and both cutting and milling operations can be integrated under precise computer control.  It is that control (with the fine adjustment of cutting angles) that allows cuts with an edge alignment of one-thousandth of an inch through half-inch titanium at commercially viable rates – since achieving with speed control alone is often too expensive in time.

Test Block Cut

Figure 1. A test block cut at Missouri University Science & Technology showing ribs of less than 3 mm thickness between adjacent milled pockets

This is where the use of the better computer nesting programs becomes cost effective, where in the cutting of many parts from a single sheet, the move traverse time between cuts and the travel distances are minimized to the overall benefit of reduced cutting time.  It is, in this regard, also worth a comment over the waterjet nozzle choice and wear.

University Research of Waterjet Cutting Nozzles

Although it might appear that the cutting table in a University Research Center might not get much use, in fact, many years ago, the student design teams that build components for National and International competitions (the solar car, the solar house, the heavy lift vehicle, the concrete canoe, the strongest model bridge etc) discovered that we were willing to let them use it, in the evening hours. They could then design and make parts to the most efficient size to carry the loads needed, rather than having to go and buy the next largest commercially available piece, and be forced to design to that larger, heavier size.  As a result the vehicles they build are generally smaller and lighter – which means that they usually win or place.  (This has not been lost on the competition and places such as MIT now have several waterjet cutting tables in their shops too).  The result is that the water jet table runs much longer in the evening than during the day, but it also means that nozzle life and cutting efficiency over that life became important factors for us to assess.

Solar Car of the University of Missouri Science & Technology

Figure 2. Missouri University Science & Technology solar car

We therefore set out to compare nozzles, and to look at how they cut.  Our way of doing it isn’t likely to be the way that any other shop would do it, but in our case we prepared triangles of a mild steel, and cut them in the middle of the plane of the sheet.  I.e. we set the ¼-inch thick sheet on its edge, and cut down through the middle, starting at the sharp end of the triangle and cutting to the thick end, so that we could see how far along the triangle the jet would cut before it stopped cutting all the way through.  Then we cut off the metal on one side of the cut so that we could see what the quality of the cut was, and the average depth of the cut.  In our case we made the cuts at 40,000 psi at a cutting speed of 1.25 inches a minute, and, when we were doing the time effects we would run a “triangle” test after every hour of cutting other things.   We set a performance requirement for the cut so that when the cuts fell below a certain depth we would consider the nozzle to have worn out.  Our results varied, between nozzles, from 10 to 40-hours of operational life, since we had a fairly high standard of cut that was needed.

Test Cut - Steel Triangles

Figure 3. Two steel triangles made from ¼-inch ASTM-108 steel, cut along the middle of the plane and one side removed by milling along the edge of the cut.

Now this is not to say that you should follow the same path, but it is useful to know how well different nozzle designs cut your particular materials, and how long they last.  We were (despite being in “the business” for decades, quite surprised at the range of results we found.

One disadvantage in our case in working with so many work teams is that we rarely get to be there when they cut the parts out of stock, and teaching them to conserve materials by proper placing of the parts on the sheet is not always successful.  Yet the “Materials” part of the “Time and Materials” cost has, if anything become an even more controlling part of any operation that it has in the past.  Where once we could afford the small amounts of material each student would use, we now have to charge as prices of raw stock keep rising.  And this is true not only for us but for all shops. So what can be done.

Obviously the best nesting of parts on a sheet is one way of achieving this, but when there are large bits being removed from sheets of material, we can make some savings by cutting small parts out of the pieces of material that would otherwise be left as scrap from the internal cuts.  This is one advantage of waterjet cutting over conventional milling since, as an extreme example, we made a circular cut down through a 2.5-inch thick block of Hastalloy, removing that core for re-use, whereas with conventional cutting it would have come out as chips and have to be sold as just scrap.

Now these are all fairly mundane considerations, but I would like to close with a different opportunity, that is only just becoming more evident.  This is in the integration of this new tool in creating forms of art.  Vanessa Cutler has just come out with a new book “New Technologies in Glass” (http://www.amazon.co.uk/New-Technologies-Glass-Vanessa-Cutler/dp/1408139545) in which she shows some of the fascinating designs that can be achieved in using, among other tools, abrasive waterjet systems to cut glass.

Waterjet Technology: More than Cutting Metals

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

Wandering the aisles at the IMTS and Fabtech trade shows that bring the latest tools to the attention of the manufacturing industry, there is always a group of folk standing watching the cutting demonstrations. The waterjet cutting booths in particular have always seemed to attract more of a crowd over the course of the day, and the relative quiet where you can still talk as you stand by the waterjet table is, perhaps, one of the benefits that waterjet technology has brought as it grows increasingly popular. It similarly does not hurt when the demonstration involves cutting those show trinkets which can, after a quick rinse, be given as a souvenir and reminder of the tool capabilities.

Waterjet Cutting Foam

Figure 1. Small foam give-away of a female Missouri Miner

The introduction of high-pressure waterjet jets into the industry was not quite that high tech, since it had been discovered quite early in the technical history that waterjets at a pressure of around 5,000psi/350 bar were quite effective in removing the burrs left when parts are machined conventionally. (Robert Burns has since written that the test of jet effectiveness is that it can be used when a 0.5 mm lead in a mechanical pencil will move the burr, rather than breaking the lead). (http://www.processcleaning.com/articles/water-jet-deburring-for-complex-metal-parts). The water jet stream had the advantage, over mechanical probes, that they could also bounce around corners in the parts, thereby reaching regions more difficult to clear. The de-burring step can be eliminated in many processes today by integrating a high-speed waterjetting in with the cutting tool in conventional machining. However tool design and integration has proceeded much further in the larger-scale mining and excavation industry than in metal cutting, where it remains still as more of a demonstration in a University laboratory than a tool that has become established in the market-place, even though it has been shown to make it possible to machine even difficult materials).

Waterjet assisting PCD cutter

Figure 2. Schematic showing the location of a high-pressure waterjet acting as a waterjet assist to a mechanical tool in machining rock and metal.

Yet the de-burring example shows one of the advantages that high-pressure water jet cutting has demonstrated in growing applications over the years. The highly focused force of the jet, when properly placed on the target cuts without significant damage along either edge of the cut surfaces left afterwards. The early examples of this (and one of the first commercial successes) was in the cutting of cardboard, where the jet will cut across the sheet without crimping the edges, which keeps the strength of the box edge that would otherwise be lost.

Waterjet cutting edge cardboard

Figure 3. Waterjet cut across cardboard showing the lack of a crimp

Waterjet System cutting Cardboard

Figure 4: First Commercial sold “water only’ waterjet system sold by Ingersoll Rand in 1971 (now KMT Waterjet Systems).

In a similar way, when abrasive is added to the waterjet stream so that it can cut through metals, glass and other ceramic materials (such as those used in computer chips and similar applications), the cut surfaces that are left are unchanged by the cutting process. The heat-affected-zone that is induced with thermal and mechanical tool cutting is removed, and the local stresses induced in thicker material as it is cut or milled are much reduced, so that there is less distortion around the cutting path, and in any narrow ribs of material left either in the part, or the material separating pieces in the initial sheet. This, again, is more critical in thicker parts, where an abrasive jet can be used to outline the final shape of a milled pocket, for example, before the mechanical tool removes the central material, so that, in this way, a very narrow rib around the pocket can be left undistorted by the stresses imposed by the mechanical tool.

And the lack of a de-burring step remains since, in contrast with more mechanical cutting processes, two parts can be located and fixed together in their final alignment, before cutting fastener holes between them. The fasteners (whether rivets, or bolts) can then be inserted and made tight without the need to take the pieces apart to remove the residual burrs that would otherwise develop on both pieces of metal.

There are, in short, a variety of different ways in which the opportunities for business that a high-pressure waterjet cutting table brings can be developed. Some of these opportunities come from learning what is already being done in other aspects of waterjet technology, and some are unique to this application. Some of these ideas are well known, and others less so. But to try and cover them in a single article is difficult, which is why I will be writing this KMT Waterjet Technology Blog posted on www.kmt-waterjet.com. It is meant to give, at about five hundred to a thousand words a week, a more detailed explanation and background to why some things work, and others don’t, as well as thoughts on where the waterjet industry can grow. I look forward to the KMT Waterjet Weekly Waterjet Blog Series.

Waterjet Weekly Blog-A New KMT Waterjet Series

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

I have been a Curators’ Professor at the University of Missouri Science and Technology since 1980. For most of my career I have led a team developing the uses of high pressure waterjets, and these have ranged from the removal of skin cancer, and precise material cutting at the millimeter level, up to the carving of statues (including a Stonehenge) some 5 meters high. Waterjet technology has been developed to cut open, disarm, and remove the explosive from munitions, and mines.

In October of 1965 I started work on a doctoral dissertation that would look at whether streams of high-pressure water could be used to mine minerals, rather than using the mechanical tools that my ancestors had been using for many generations to mine coal. Until the time of my great-grandfather the main tool had been a hand-swung pick, but he had made the transition to become a compressed-air-powered, coal-cutter operator, and worked with my grandfather on running that machine until the first World War, when his son joined the Northumberland Fusiliers and went off to Belgium.

When I started my work, after my mining engineer father had approved, I found that there was little information on hydraulic mining available in the technical literature. The Internet did not exist, and the few books and articles that I could obtain came through the Brotherton Library at Leeds University, where Inter-Library Loans would find a source, and even, on occasion, a translation, but in weeks not days.

Those conditions no longer hold true, and yet, while a computer now makes it easier to get instant access to all the world’s knowledge, filtering through that vast stack to find that needed bit of information still takes time. And there are other factors that have come into play, so that much of the knowledge that has been gained may soon be lost, forgotten or spread into so many distant places as to be effectively gone. And so I am going to put together a new series of posts about this technology. Since this is the first it is more of an explanation of the background, and as the posts continue so the structure and location may change, trying to better serve those who follow me in what has been the truly fascinating development of a new very broad-based industry.

Over the past decades high-pressure waterjets have found a wide variety of different uses around the world. From the small pumps that can be bought at the local hardware store and allow you to clean houses, furniture and cars through the thousands of horsepower used in pumps for the excavation and mining/petroleum industry a quiet revolution has occurred in many industries, beyond the sight of the general public. I was fortunate enough to be a part of the relatively small group of scientists/engineers/technicians who helped bring these changes about. Around the world there were perhaps 50 or so of us, and much of our interaction took place at conferences, where we learned more from each other in bars and restaurants than we did from the formal papers that we all gave. Some changes were fairly dramatic, the use of cleaning jets on oil platforms comes to mind, and were instantly adopted, others were a longer struggle, and yet these tools have yet to find more than 90% of their ultimate market, which will likely be in fields that most of us have not even thought of yet. And the tools that have already been developed are used in many more industries than the general public understands.

I retired a couple of years ago, and followed many of that original group in moving on to other interests. Before I left, however, I had written a book, and taught a class to senior undergraduates dealing with manufacturing use, as well as the earlier mining applications. The class is not taught now, and interest in the topic has also shrunk at the other major universities, as faculty have changed, and other topics bring the “research rain” that is needed to sustain the graduate classes of today.

This does not mean that waterjetting is less valuable, but instead is recognizes that the first flush of development is over, and that the really low-hanging fruit of application has been picked by folks such as myself. The range of applications remains immense, but the rewards are not now as easily obvious, and the research results now are not so dramatic.

The current plan is to begin the series with posts that come from my lecture set. But instead of being of the usual length they will be broken down into a set of sub-topics, so that there may be three or four posts that will cover the material of a single class. This will make each post of around a thousand words, and then the four sub-topics will be combined into a “class” version which will be posted as a pdf, and this will be down-loadable, and could be printed and put into what will, over time, become a somewhat updated version of that earlier book, but in a different format.

Since none of the anticipated readership is yet aware that this is happening, I expect that it will take some time for comments and questions to develop, but as these do they will be added into the mix.

Once the original posts have become established I hope to be joined by some of the folk that are still working in the field – as I said earlier it is one that is still continuing to grow at a fairly steady pace (one of the companies that makes equipment was on the national news recently because it could not get enough trained folk to help it meet demand). New ideas will turn up, and I look forward to giving my opinions. Sadly these may appear a bit negative at first, but there were many things we tried that looked good, but did not pan out. And those were usually not, therefore, the things that we wrote papers about. But knowing something won’t work, and more particularly why not, is also useful.

The early posts will also deal more with the history and general background, since these are likely to have a more general interest, and the more technical parts of the discussion will come later, though I will try and keep that at a relatively simple level for explanation.

I got my last patent this week, it dealt with drilling oilwells – the one before that dealt with treating skin cancer. I have worked on intercontinental ballistic missiles, at nuclear facilities, on land-mine clearance and in blocked caves. So perhaps I could claim to be a surgeon, rocket scientist, nuclear scientist, fire-fighting expert, and hazardous material specialist – and that neglects all the work on manufacturing and the evolution of tools that can cut through an inch of material within an accuracy of a thousandth of in inch.

All because of the power that comes when you push a pint of water through a tiny hole. Who’d a thunkit!!