Waterjet Technology – Making gift items by water jet cutting

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

There is a time which can come in late Winter and very early Spring when demand declines and there is some free time for the occasional home project. Although many of us now know and understand how well waterjets and abrasive waterjet streams can cut material, this is still not that widely recognized by the General Public. This slack time can help to remedy that problem.

Uninformed ignorance of jet capabilities was certainly true for many years on our campus and seems to become more so again as the years pass since I retired. Further, at Conferences, I often heard the complaint that the industry needs to get its message out more clearly to a wider audience. The vast majority of potential industrial users are unaware of how well waterjetting in one of its forms could help solve their problems.

Now there are lots of ways of solving that problem, but today I want to talk about just one, the one we used to help us with the problem. It had to be something that would be used by those we gave it to. It had to be small, relatively cheap and quick to make and yet demonstrate some of the capabilities we wanted to show off. The answer ended up as a business card holder.

Business Card Holder – Missouri Miner female figure

Figure 1. Business Card Holder – Missouri Miner female figure

University labs are generally cash strapped, and so the material had to be relatively cheap, so we used sheets of a light foam. This allowed us to cut out the figure parts using water alone (at around 20,000 psi) which significantly reduced the cost. Early in the design of the female figure (this was the third in a series where we cut a different shape each year) it was pointed out that relative body size was more critical with female figures, and so two different thicknesses of foam were used. The first was half-an-inch thick and used for the body and pins, while a quarter-inch sheet was used for the legs and arms.

Foam miner front view

Figure 2. Foam miner front view – showing the two thicknesses of material with waterjet cutting

Putting a small hole in the position of the eye allowed the model to show how precise and small a cut could be made through thicker material. The five pieces that made up the total were held together with two rectangular pins that were cut from the thicker stock and fitted through slots cut to their shape in the different parts.

One of the advantages of cutting these (and we cut parts for around 300 figures, and used virtually all of them each year) is that it was also possible, with relatively little trouble, to cut the campus identifier on a leg of the figure. With not a lot of space this was originally UMR and then changed to “S & T” when the campus changed its name.

A later model of the card holder with the campus ID cut into the leg by waterjet cutting

Figure 3. A later model of the card holder with the campus ID cut into the leg by waterjet cutting

For speed in cutting, we only cut the letters in half the legs, though you may note that in this later version we also cut the connecting pins as round rod, rather than rectangular. In this way the figure could be repositioned as the owner decided what they wanted to do with them.

Basically, however they served as card holders, and having passed them around, (and provided them to senior campus officials as place card holders for dinner meetings) it has been amusing to see how avidly they were sought and kept by some of those to whom they were given.

Now we did not get to these figures in one step. The initial idea was to carve something out of rock, since the overall department was known as The Rock Mechanics and Explosives Research Center. However, if you are making something out of rock, particularly a person’s shape, they need to be larger, because of the weak strength of the rock.

Comic-book Miner cut out of Missouri Granite with waterjet cutting

Figure 4. Comic-book Miner cut out of Missouri Granite with waterjet cutting

The cost was also high, since the cuts had to be made with abrasive, and the rock had to be polished before it was cut. (Trying to polish the pick points after cutting led to several breakages, and this is something that is either perfect or worthless).

There are several good ideas that individual companies have which help sell their name and capabilities where the gifts are of metal and can be used for opening bottles or of some other benefit. But we could not afford the cost to cut a lot of pieces using abrasive, and nothing that we tried in metal had the cachet of the small miners.

In this case, the mascot of the campus is the Missouri Miner, and while the first model that we cut followed along the shape of that cartoonish figure, many of our graduates were going into coal mining, which is also my background, and so the second and third versions had coal mining helmets, and as a further demonstration of capabilities, a small circular cut in the helmet allowed a yellow rod to be put into the helmet to illustrate the miner’s cap lamp.

Where we were asked to prepare small souvenirs for another event we did use the Missouri Granite, but had learned this time to buy tiles that were already polished. Then all we had to do was to cut the shape of the state into the tiles, and then put a University logo sticker on the piece and we had our memento for the guests.

Small memento of the state shape carved out of granite tile by cutting with waterjet technology

Figure 5. Small memento of the state shape carved out of granite tile by cutting with waterjet technology

This was for a specific occasion where the sponsor was willing to pay for both the cutting costs and the materials, but in order to keep costs down (since these were given away) the pieces had to be small. This particular run was one of the more difficult to keep inventory on, since several disappeared during the short time of the cutting runs (which we have found is an occupational hazard with “artistic” pieces where there are lots of temporary folk involved in our work).

Which is, I suspect, an entry for the last piece of advice on making such gifts, and that is to plan on making more than you think you need and, if possible, be able to make more if needed. In a later post I will write about where you can get some artistic help for relatively little cost to help with ideas such as this.

Waterjetting Technology – Water Quality

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

One of the problems with taking a research team into the field is that you have to be able to provide answers and a path forward when things go wrong. So it was on a project we once had in Indiana, and it took about a year for me to live down the tale. We had set up a 350-hp high-pressure triplex for a project that involved washing explosives out of shells. Everything had been set up and was ready to go, and so we switched on the water to the pump, started the diesel engine and almost immediately noticed that we weren’t getting enough water downstream of the pump. What was the problem? We checked all the valves, couplings and hoses, and they all seemed to be OK. It was, however, a bitterly cold day with a howling wind around where we had the pump unit. And so I came up with the idea that it was the wind, chilling the pistons, which operated with their length exposed during part of the stroke. If the wind chill was cooling the pistons, then perhaps they weren’t displacing enough volume because they had shrunk. It became known as “The Wind Chill Factor” explanation and, as those of you who have done this sort of thing realize, it was bunkum! After a while, one of the team wandered back to the filter unit, pulled out the partially plugged filters, changed them to new ones and we were in business.

There are a couple of reasons that I tell this bit of history and they relate both to the quality, and the quantity of water that is being supplied at a site. I remember talking to Wally Walstad, who ran McCartney Manufacturing before it became KMT Waterjet Systems, about their second commercial installation and how the different water chemistry just a few hundred miles away had caused maintenance issues on the pumps that they had not expected.

Parts for a multi-piston high pressure pump

Figure 1. Parts for a multi-piston high pressure pump

It may seem obvious that a pump should be supplied with enough water so that it can work effectively. But the requirement, as one moves to higher-pressure pumps, becomes a little more rigorous than that. Consider that the water supplied must enter the piston and fill it completely during the time that the piston is pulling back within the cylinder. Because the piston is pulling back if the water flow into the cylinder is not moving in enough, then the piston will pull on the water. Water does not have any tensile strength, and so small bubbles of vacuum will form. When the piston then starts back to push the water out of the cylinder these bubbles, which are known as Cavitation, will collapse. In a later post I will tell you how to use cavitation to improve material removal rates. But the last place that you want it is in the high-pressure cylinder, since the bubble collapse causes very tiny high (around 1 million psi) micro-jets to form that will very rapidly eat out the cylinder walls, or chew up the end of the piston. (Happened to us once).

There is a Youtube video which shows the cavitation clouds forming in a pump (the white blotches) as the flow to the pump falls below that needed.

To avoid that happening there is a term called Net Positive Suction Head, NPSH. I am not going to go into the details of the calculations, though they are given in the citation. In most cases it is not necessary to make them (unless you are designing the pump). Where the unit being operated is a pressure washer, then the pressure that drives the water out of the tap and into the hose is usually sufficient to overcome any problems with the inlet pressure.

When flow rates run above 5 gpm, however, or when there is a relatively narrow fluid passage into the pump cylinders, or where the water reservoir is below the pump, then the normal system pressure may not be enough. There are two values for the NPSH which are critical – the NPSH-Required (NPSHR) and the NPSH-Available (NPSHA). Let me give a simple example of where one could get into trouble.

For example consider the change which occurs when a pump, normally rated at 400 rpm is driven at 500 rpm, for a 25% increase in output. At 400 rpm the NPSHR for a triplex pump supplied through a 1.25-inch diameter pipe from an open tank will be 8 psi. At 500 rpm, as the flow increases from 26.4 gpm to 33 gpm, the NPSHR rises to 9 psi, which is only a 12.5% change.

However, under the same conditions the NPSHA, which begins at 11.5 psi with a 26.4 gpm demand, falls to 7.8 psi at 33 gpm. When the required suction head is set against that available there was an initial surplus of 45% over that needed. But this changes to a shortfall of 12% when the pump is run at the higher speed. The pump will cavitate, inadequate flow will reach the nozzle to provide full pump performance and the equipment lifetime will be markedly reduced.

This supply pressure required should thus be checked with the manufacturer of the pump. In most cases where we have run pumps at 10,000 psi and higher, we have fed the water into the pump at the designated flow rate, but using a supply pump that ensures that the pressure on the inlet side of the pump valves is at least 60 psi.

One of the problems, as mentioned at the top of the piece, is that when going to a new site the immediate quality of the water is not known. There are two things that need to be done. The first of these, of particular importance at higher pressures, is to check the water chemistry. It is important to do this before going to the site since it usually takes some time to get the results, and if there are some chemicals in the water that may react with pump or system parts, it is good to know this ahead of time so that the threatened parts can be changed to something that won’t be damaged.

There is a specific problem that comes with cutting systems in this regard, since at 50,000 psi or higher, water quality becomes more important, even just in the nozzle passages. And I will deal with this in a few weeks when I talk about different nozzle designs.

And equally important is the cleanliness of the water. Particularly when tapping into a water line that hasn’t been used for a while (as we did), there is a certain amount of debris that can be carried down the line when it is first used. The smart thing to do is to run water through the line for a while to make sure that any of the debris is flushed out, before the system is connected up. The second is to ensure that there is more than one filter in the line between that supply and the pump.

Many years ago, when prices were much lower than they are today, Paddy Swan looked at the costs of increasingly dirty water on part costs. The costs are in dollars per hour for standard parts in a 10,000 psi system and the graph is from the 2nd Waterjet Conference held in Rolla in 1983.

costs for parts when increasingly dirty water is run through a pump

Figure 2. 1982 costs for parts when increasingly dirty water is run through a pump (S.P.D. Swan “Economic considerations in Water Jet Cleaning,” 2nd US Water Jet Conference, Rolla, MO 1983, pp 433 – 439.)

Oh, and the moral of the opening story became one of our sayings in the Center, not that we were original, William of Ockham first came up with it about seven hundred years ago. It’s known as Ockham’s Razor, and simply put it means that the simplest answer is most likely the right one, or don’t make things more complicated than they need be!

Waterjet Technology – Pumps, Intensifiers and Cannons

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

When we say a rock is hard it means something different, in terms of strength, to the meaning when we say that we want an egg hard boiled. Terms have to be – and usually are – defined through the way in which they are used. At the same time, each trade, industry or profession has certain terms that it adopts for its own with more specialized meanings than those which we, in the general public, are familiar.

Ask someone on the street what level of pressure they consider to be “high” and they might answer with numbers that range from 100 psi to perhaps 2-3,000 psi. And yet, within the industry those pressures are really quite low, relative to those most commonly used in cleaning and cutting. High-pressure waterjet systems are now available for water jet cutting metals, that will generate streams that run continuously at 90,000 psi, and the highest pressure jet that we generated in the MS&T Laboratories was at around 10 million psi.

Within that very broad range some simple divisions make it easier to group the ranges and applications of the different tools that are now common within different parts of the industry. At the same time, over the period of my professional life, the technology has moved forward a long way. Consider that when I wanted to run at test at 50,000 psi back around 1970, I had to use this particular set of equipment.

MS&T Water Cannon firing 12 gallons of water at 50,000psi

Figure 1. MS&T Water Cannon firing 12 gallons of water at 50,000 psi

The water cannon was made by cutting the end from a 90-mm howitzer and threading a one-inch nozzle on the end. Smaller orifices could then be attached beyond that to give different flow combinations. The pressure to drive the cannon was generated by putting 2,000 gm. of smokeless powder in a cartridge, and then loading and firing the cannon. We had been given the mount, which rotates around two axes by the then McDonnell Douglas (now Boeing), who had used it to hold and move the Gemini spacecraft while they were being inspected.

The pressure divisions which were debated and agreed by the Waterjet Technology Association back in the mid 1980’s broke the pressure range into three separate segments, which described the industry at the time.

The first range is that of the Pressure Washers. Operating pressures lie at and below 5,000 psi.

A small pressure washer being used to clean a drain

Figure 2. A small pressure washer being used to clean a drain (Mustang Water Jetters)

These are the types of unit which are often found in hardware stores for use in homes, and while I won’t get into this until some later posts on safety and on medical applications, it should be born in mind that it is possible to do serious injury even at these relatively low pressures. (Folk have been known to use the jets to clean off their shoes after work … need I say more – a waterjet cuts through skin at around 2,000 psi, and skin is tougher than the flesh underneath). At pressures below 2,000 psi, these are often electrically powered. A gasoline motor is often used to drive the portable units that operate above that pressure range.

High-pressure units are defined as those that operate in the pressure range from 5,000 psi to 35,000 psi. The drive motors are usually either electrical or use a diesel drive, and units of over 250 horsepower are now available. Many of these units are known as positive displacement pumps. That is taken to mean that the pump, being driven by a motor at a constant speed, will put out the same volume of water, regardless of the pressure that it is delivered at (up to the strength of the drive shaft).

To ensure that the pressure does not rise above the normal operating pressure, several safety devices are usually built into the flow circuit so that, should a nozzle block, for example, a safety valve would open allowing the flow to escape. Different flow volumes can be produced in larger units by placing a gear box between the pump and the motor. As the motor speed changes, for the same piston size in the pump, so the volume output changes also. However, because the pump can only deliver at a certain power the size of the pistons can also be changed so that, at higher delivery pressures, the same motor will produce a lower volume of water. I’ll go into that in a little more detail in a later piece.

Section through a high-pressure pump

Figure 3. Section through a high-pressure pump showing how the crankshaft drives the piston back and forth in the cylinder block, alternately drawing low pressure (LP) water in, and then discharging high pressure (HP) water out

Normally, there are a number of pistons connected at different points around the crankshaft so that, as it rotates, the pistons are at different points in their stroke. This evens the load on the crankshaft, and produces a relatively steady flow of water from the outlet. (Which, in itself, is a topic for further discussion).

As the need for higher pressures arose, the first pumps in the ultra-high pressure range (that above 35,000 psi) were intensifier pumps. These pumps are designed on the basic principal that the force exerted on a piston is equal to the pressure of the fluid multiplied by the area over which it is applied. Thus, a piston that is designed with two different diameters can produce pressures much higher than those supplied.

The basic elements of a waterjet intensifier

Figure 4. The basic elements of a waterjet intensifier

Fluid at a pressure of perhaps 5,000 psi is pumped into chamber C. As it flows in, the piston is pushed over to the left, drawing water into chamber B. At the same time the water in chamber D is being pushed out of the outlet channel, but because of the area ratio, the delivery pressure is much higher. If, for example, the ratio of the two areas is 12:1, then the pressure of the water leaving the pump will be at 12 x 5,000 = 60,000 psi.

Over the years, the materials that pumps are made from and the designs of the pumps themselves have changed considerably, so that pressure ranges are no longer as meaningful as they were some 25-years ago when we first set these definitions, but they continue to provide some guidance to the different sorts of equipment, and the range of uses of the tools within those divisions, so I will use these different pressure range definitions in the posts that follow.

Using Nature’s Crack System

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 this section of the series on Waterjetting, the focus is on the way in which high-pressure waterjets grow cracks in their target. As John Field showed, even the presence of microscopic cracks on a glass surface are enough to initiate the larger cracks that lead to failure. In many cases, however, the most useful growth can be achieved if the cracks only extend to the point that they remove a desired amount of material. This becomes important where there are weaknesses and flaws in the material – such as the layers between plies of wood, or even Kevlar – which should not be grown as the jet cuts down through the material. And in a later article, this topic will be a part of a discussion as exactly what happens as a jet drills a hole into a target. But, for today, I would like to talk about crack growths in rock and soil, both because it is one of the oldest ways in which water can penetrate into material, and also because it holds the potential to be one of the newest areas into which waterjetting is growing, and will likely further advance into a more significant business.

And to begin consider that, as water penetrates into the cracks in a rock and grows those cracks slowly under natural forces, rocks with minerals in them will see those mineral particles separately broken out. The classic example of this is with gold. One of the ways in which the Forty-Niners found the gold in California was by panning for the gold particles in the rivers and tracking the gold deposits back up-stream until they reached the original gold deposits of the Sierra Mountains. Not that this was the first time that water transport had helped in gold mining. One of my favorite stories to begin classes is to remind them of Jason and the Argonauts.

Movie poster for Jason and the Argonauts

Figure 1. Movie poster for the 1963 film version of Jason and the Argonauts (IMDb)

It is a theme that has been made into a movie several times, and tells the story of how the Greek Prince Jason and a band of companions go in search of the Golden Fleece, and the adventures that he has along the way. Despite the mythical creatures the story is thought to be likely based on some measure of truth, with the voyage taking place some time before 1300 B.C. But our focus is on the fleece rather than the voyagers.

Suggested path that Jason followed to get to the River Rhion in Georgia

Figure 2. Suggested path that Jason followed to get to the River Rhion in Georgia.(Google Earth)

Within the Caususus mountains of Georgia lies the modern town of Mestia, which was thought in Roman times to be the site of Colchis, where Jason found the Golden Fleece. The reality is not quite as dramatic as the legend since as the Roman historian Strabo noted

“It is said that in the country of Colchis, gold is carried down by mountain torrents, and that the barbarians obtain it by means of perforated troughs and fleecy skins, and that this is the origin of the myth of the Golden Fleece”

The torrents of water in the Svaneti valley outside Mestia

Figure 3. The torrents of water in the Svaneti valley outside Mestia, (Nika Shmeleva Google Earth at 43deg02’29.74”N, 42deg42’25.13E)

It is thought that the miners of the time directed the streams so that they flowed over the veins of gold and eroded out the particles so that the gold was carried down to the valley. Here it was fed through the troughs that Strabo described, and the heavy gold particles were captured as they tangled in the wool of the fleece. To recover the gold the miners would then hang the fleeces in trees, so that they would dry, and the gold could be shaken loose. Unfortunately as the fleeces hung in the trees they provided a tempting target for Greek thieves. (In a later version that I will write about in the next post the sheep fleece was replaced with brush that could be dried and burned to release the gold).

Water was thus, in one of the earliest “automated” mining processes, used to both dislodge and then carry the valuable mineral from the mining site. The overall power of water to move soil has been used to wash away material for over a hundred years. In the 1973 War between Egypt and Israel, the Egyptian Army gained a significant advantage in the early hours of the war by using waterjet monitors to wash away the defensive barrier along the edges of the Suez Canal, rather than using conventional mechanical excavators.

To deal with the massive earthen ramparts, the Egyptians used water cannons fashioned from hoses attached to dredging pumps in the canal. Other methods involving explosives, artillery, and bulldozers were too costly in time and required nearly ideal working conditions. For example, sixty men, 600 pounds of explosives, and one bulldozer required five to six hours, uninterrupted by Israeli fire, to clear 1,500 cubic meters of sand.

The quoted Sunday Times report of the time suggested that the Israeli Army had anticipated that it would take 24-hours to remove the barriers giving time for their Army to mobilize and arrive. However, using a set of five pumps per breech site the Egyptian Army was able to make an opening in as short as a 2-hour time, with the mobilized water cannon opening 81 breeches, and removing 106 million cubic feet of material in that first day of the war. They were thus able to initially advance into the Sinai with relatively little resistance.

The pressure of the water does not have to be high to disaggregate the soil, but large volumes were needed in that application both to break the soil loose and to move it out of the way. Moving the debris out of the way is an important part of the operation, and while, in the above case it could be just pushed to one side, in many more localized jobs, particularly in cities, that is not an answer. However if the soil can be collected with the water, then the fluid can help to move the soil down a pipe away from the working area. And, more importantly, if the soil can be captured as it is being broken loose, then both can be collected before the water has had a chance to penetrate into the soil around the hole, and so the walls of the hole will not get wet and will remain stable and not fall in.

One way that we have achieved this is to rotate a pair of waterjets relatively rapidly (depending on the material the jet pressure can range from 2,000 psi to 10,000 psi) so that the surface layer is removed, and to immediately take this away by combining the jet action with a vacuum for removal. (In the initial trials we used a Shop Vac to remove both water and debris). This combination has become known as hydro-excavation, and will be the topic of a couple of posts in the future.

Similarly the use of high pressure to break an ore down into its different parts, so that the valuable mineral can be separated from the host rock at the mining machine, is become a new way to reduce the costs of transporting and processing the ore and make mining more efficient. As yet this latter is still more of a laboratory development, though it will develop for greater use in the future, and there will be additional posts on this too in the future. But, in both cases, the use of waterjets to effectively rely on extending pre-existing cracks makes the systems work. In the next post I’ll write about a couple of other ways of getting enough cracks into the rock as ways of making it easier to separate and remove valuable materials from underground.