Waterjet Technology – Making gift items by water jet cutting

David Summers WWWaterjet Technology – Making gift items by water jet cutting

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.

Figure 1. 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.

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

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.

Waterjet cutitng A later model of the card holder with the campus ID cut into the leg.

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, that 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.

Labels:figure,foam,gifts,granite sculpture,high-pressure cutting,miner,Missouri Miner,sculptures

 

Waterjet Technology- Hoses and High Pressure tubing

David Summers WW

Waterjet Technology- Hoses and High Pressure tubing

One of the first decisions one makes in connecting a waterjet pump to a nozzle is to select the size of the high-pressure pipeline that will take the water from the pump to the cutting nozzle. This choice has become a little more involved as ultra-high pressure hose has come on the market since this can be used at pressures that once could only be served with high-pressure tubing. However, at higher pressures the flexibility of hoses becomes reduced, both because of that pressure, and also because of the layers of protection that are built into the hose structure.

Much of the original plumbing, in the earlier days of the technology, used 3/16th inch inner diameter, 9/16th inch outer diameter, steel tubing. One reason for this was that, at this diameter, the tubing could be quite easily bent and curved into spiral shapes. And that, in turn, made it possible to provide some flexibility into an assembly that would otherwise have been quite rigid.

Figure 1. Early cutting nozzle with spiral coils in the high-pressure waterjet feed line to the nozzle.

Figure 1. Early cutting nozzle with spiral coils in the high-pressure waterjet feed line to the nozzle.

When cutting nozzles were first introduced into industry, they were fixed in place, because of the rigid connection to the pump. The target material had, therefore, to be fed underneath the nozzle, since it was easier to move that than to add flexibility to the water supply line.

Early waterjet slitting operation (courtesy of KMT Waterjet Systems)

Early waterjet slitting operation (courtesy of KMT Waterjet Systems)

However, because feed stock can vary in geometry some flexibility in the positioning of the cutting nozzle above the cutting table would allow the jet to do more than cut straight lines. A way had to be found to allow the nozzle to move, and this led into the development of a series of spiral turns that high-pressure tubing can be turned through, as it brings the water to the nozzle. (See Figure 1). That, in turn, allowed a slight nozzle movement. By adding this flexibility to the nozzle, a very significant marriage could then take place between robotics and waterjet cutting.

The force required to hold a nozzle in a fixed location becomes quite small as the flow rate reduces and the pressure increases. (at 40,0000 psi and a flow rate of 1 gpm the thrust is about 10 lb). The first assembly robots that came into use were quite weak, and as their arms extended the amount of thrust they could hold without wobbling was small, but critically more than 10 lb. And this gave an initial impetus to adding jet cutting heads to industrial robots of both the pedestal and gantry type, to allow rapid cutting of shapes on a target material, such as a car carpet, where the ports for the various pedals and sticks need to be removed.

But this marriage between the robot and the jet required that the jet support pipeline be flexible, so that it could allow the nozzle to be moved over the target, and positioned to cut, for example, the holes for retaining bolts, without damaging the intervening material.

The pipe had to be able to turn and to extend and retract, within a reasonable range, so that it could carry out the needed tasks. Bending the pipe into a series of loops produced that flexibility.

A single full circular bend in the pipe will acquire sufficient flexibility that the end of the pipe (and thus the nozzle) can be moved over an arc of about 9 degrees.

Coils on a pedestal-mounted robot allowing 3-dimensional positioning of the cutting nozzle.

Coils on a pedestal-mounted robot allowing 3-dimensional positioning of the cutting nozzle.

A large number of coils were required, since the tubing has only a very limited amount of flexibility in every turn. For example, if one wanted to stretch the connection by lowering the nozzle, then the several coils would act in the same way that the steel in a spring would as it extended. The movement can perhaps be illustrated with the following representation of a set of spirals, with metric dimensions.

Schematic of a series of coils, arranged to allow the nozzle to feed laterally.

Schematic of a series of coils, arranged to allow the nozzle to feed laterally.

Each spiral will also allow a slight angular adjustment also, and these add up, as more spirals are added to the passage.

Angular movement allowed per spiral. This should not exceed 9 degrees per turn.

Angular movement allowed per spiral. This should not exceed 9 degrees per turn.

While, in many modern assemblies this may seem to be a quaint way of solving the problem, back when these systems were first put together it was very had to find high-pressure swivels that would operate at pressure for any length of time. In those days we had one source that provided a swivel that would run for many hours provided that all the external forces could be removed from the swivel irself. But the moment an out-of-alignment force hit the swivel it was ruined. In another application we had tested every swivel we could find, that would fit down a six-inch diameter hole, and had found one that would run for ten minutes. To finish our field demonstration, where we had to drill out 50-ft horizontally from a vertical access well, we had to continuously pour water onto the joint to keep it cool, and the manufacturer stood by with a pocket full of bearing washers that we had to replace every time one started to gall.

But that was over thirty years ago. Now the connections from the pump to the nozzle can flow through ultra-high-pressure hose with a flexibility that we could barely imagine. And ultra-high pressure swivels will run for well over a hundred hours each without showing any loss in performance. It was, however, a gradual transition from one to the other.

Ultra-high-pressure feed to a nozzle, using coils and swivels

Ultra-high-pressure feed to a nozzle, using coils and swivels

There are a couple of additional cautions that should be born in mind, when laying these lines out. While hose is more flexible, it is liable to pulsing, and moving slightly on a bearing surface, under pump cycling. In most places this is not a problem, but if the hose is confined and bent, then it may cause the hose to rub against a nearby surface. Over time this can generate heat, and can even wear through the various hose layers.

Worn hose and the scuff mark where it was rubbing on a plate.

Worn hose and the scuff mark where it was rubbing on a plate.

There are other issues with hose, smaller high-pressure lines can kink, when used in cleaning operations and this is a seriously BAD thing to happen, and I will discuss that in a future article. Similarly one must consider the weight of hose, particularly in hand-held operations, where it is important to address hose handling as part of the procedure, but again this will be discussed later.

 

Waterjet Technology–High Pressure Line losses

Dr. Summers Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Series

Waterjet Technology–High Pressure Line losses

High-pressure water jet pumps are, as a general rule, quite efficient at bringing water up to the pressure required for a given task. And yet, time after time, the jet that reaches the target is no longer capable of achieving the work that was promised when the system was designed. More often than not this drop in performance can be traced to the way that the water travels through the delivery system, and out of the nozzle that forms the jet.

The water flows that are used in a broad range of operations are quite low. Ten gallons a minute (gpm) and flows below that volume are mainly used in cutting operations and higher-pressure cleaning. Further, there are few occasions where hand-held operations will use flows much above 20 gpm, because of the thrust levels involved. And low flow rates mean that there is little pressure loss between the pump and the nozzle, right? UM! Well not exactly.

The pressure losses due to overcoming friction in the feed lines (whether hose or tubing) from the pump to the nozzle can make a significant difference in the operation of the system, as I mentioned in one of the early posts of this series. In that post I pointed out that a well-known research team (not us) spent two weeks running a system with 45,000 psi water pressure going into a feed line, but with only around 10,000 psi being usefully available when the flow reached the far end. (And I will freely confess later in this piece to having made a similar mistake myself). So the question naturally arises as to how these losses can be avoided.

In a word – diameter! The smaller the diameter of the feed line through which the water must flow, then the higher the pressure that is required to drive the water through that line, regardless of the nozzle size at the delivery end. The diameter of concern is, further, the inner diameter of the hose or tubing, not the outer diameter (though the combination is important in ensuring that the line can contain the pressure that the water is carrying through the line).

There are concerns over the condition of the line, the fittings that join the different parts together and other factors that I will cover in the posts following this one, but this will deal just with the simple pressure drop that occurs along a tube at different flow volumes. There are formulae that can be used, but a reasonable estimate of the loss can be obtained, either with the design tables that most manufacturers supply with their product, or through a simple nomogram that I will place at the end of this piece.

To begin with consider the basic equations that govern the pressure drop:

The equation relating pressure drop to flow volume and pipe diameter.

The equation relating pressure drop to flow volume and pipe diameter.

Note that in the above equation the pressure drop is related to the fifth power of the diameter of the tube – such is the power that even a small change in flow channel diameter will have on the pressure drop in the line.

When flow begins through a channel it is initially going to occur with the flow being laminar, in other words the water moves in layers. (There is an interesting video of this here and a video of one of the designs used, for example, to give the “solid” jet slugs that you might see jumping around the hedges at one of the Amusement Parks.

The difference between laminar and turbulent flow.

Figure 2. The difference between laminar and turbulent flow. (Equipment explained)

As water speed increases, however, the flow will transition from laminar flow into turbulent flow, where the roughness of the flow channel wall becomes more important. The roughness, resulting friction factor and the flow volume all then combine to allow the calculation of the pressure required to overcome the friction in the pipe. This holds true whether the flow is at the one or two gpm used in cutting at high pressure, or the relatively low pressure, high volume flows used in fighting fires.

But (outside of us academics) few actually calculate the numbers. There really is no need, since most of the manufacturers provide the information in their catalogs. There are two ways of presenting the information. The older convention was just to provide a graph, from which one could read off the pressure drop, as a function of the pipe internal diameter, and for a given pipe length.

Pressure drop along a tube, as a function of flow rate and tube internal diameter. Note that the scales are logarithmic.

Figure 3. Pressure drop along a tube, as a function of flow rate and tube internal diameter. Note that the scales are logarithmic.

Charts such as this are a little difficult to read, and being on a log plot small mistakes in reading the value can give significantly wrong estimates so that a more spread-out method is often more helpful. The one that I prefer to use is a nomogram, where it is possible to do comparisons between different options on a single figure with a slightly expanded scale.

Consider, for example, this nomogram from the Parker Catalog which shows the relationship between the volume flowing down through a line, the inner diameter through which it is flowing, and the resulting velocity of the flow.

A nomogram to determine the best pipe diameter, based on the allowable velocity of the flow. (Parker)

A nomogram to determine the best pipe diameter, based on the allowable velocity of the flow. (Parker)

While this is not generally a concern in feed lines to nozzles (because of the high levels of filtration of the water) in lines that carry away spent water and debris the velocity can be of concern, and also in abrasive slurry systems, where flow rates above 40 ft/sec can lead to erosion of the line.

The more useful nomogram, however, is one that I have adapted from the U.S. Bureau of Mines (a Government agencies that is now, sadly, defunct).

Nomogram to calculate pressure loss along a 10-ft length of tubing.

Nomogram to calculate pressure loss along a 10-ft length of tubing.

Knowing the flow rate through the line, and setting a straight-edge (usually a ruler) to mark the level, the ruler is then positioned so that it also crosses the inner diameter of the tubing. In the example above that would align the ruler along the line shown, that runs from 20 gpm to 0.1875 inch pipe diameter (3/16ths of an inch). The point at which the line crosses the pressure drop gives the friction loss in the line. In this case that reads at 3,600 psi per 10 ft of pipe.

The example was taken from a field trial where we were drilling holes into the side of a rock pillar. We had no problem drilling the first ten feet, but when we added a second length of 10-ft tubing to allow us to drill holes 20-ft deep the drill did not work. It was not until late in the afternoon that we realized that by adding that second length of pipe we had dropped the cutting pressure coming out of the nozzle so that while the gage pressure was 10,000 psi, the initial jet pressure had been only 6,400 psi and when the second pipe length was added, the pressure fell to 2,800 psi. This was below the pressure at which it was possible to effectively cut the rock. And so we learned!

 

Waterjet High Pressure Pumps – Pump pulsations

Dr. Summers Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Series

 

Waterjet High Pressure Pumps – Pump pulsations

High-pressure pumps generally draw water into a cylindrical cavity, and then expel it with a reciprocating piston. There are a number of different ways in which the piston can be driven. It can be connected eccentrically to a rotating shaft, so that, as the shaft rotates, the piston is pushed in and out. The pistons can be moved by the rotation of an inclined plate, so that as the plate rotates, so the pistons are displaced.

 

Figure 1. Basic Components of a Swash Plate Pump (after Sugino et al, 9th International Waterjet Symposium, Sendai, Japan, 1988)

Figure 1. Basic Components of a Swash Plate Pump (after Sugino et al, 9th International Waterjet Symposium, Sendai, Japan, 1988)

And, more commonly at higher pressures, the piston can be of a dual size, so that as a lower pressure fluid on one side of the piston pushes forward, so a higher pressure fluid on the smaller end of the piston is driven into the outlet manifold, and out of the pump. This latter pump design has become commonly known as an Intensifier Pump. The simplified basis for its operation might be shown using the line drawing that was used earlier.

Figure 2. Simplified Sketch showing the operation of an intensifier.

Figure 2. Simplified Sketch showing the operation of an intensifier.

When the intensifier is built, the simplified beauty of its construction is more evident.

Figure 3. Partially sectioned 90,000 psi intensifier showing the components and the small end of the reciprocating piston (Courtesy of KMT Waterjet Systems)

Figure 3. Partially sectioned 90,000 psi intensifier showing the components and the small end of the reciprocating piston (Courtesy of KMT Waterjet Systems)

However, what I would like to discuss today is what happens when the pistons in these cylinders reaches the ends of their stroke, and it is a little easier to use an Intensifier as a starting point for this discussion, although (as I will show) it also relates to the other designs of high-pressure pumps that also use pistons.

Consider if there was only one side to the piston, rather than it producing high pressure in both directions. This design is known as a single acting Intensifier, and it might, schematically, look like this:

Figure 4. Simplified schematic of a single-acting Intensifier

Figure 4. Simplified schematic of a single-acting Intensifier

As the piston starts to move from the right-hand side of the cylinder toward the left, driven by the pressure on the large side of the piston, it displaces water from the smaller diameter cylinder on the left. Assume that the area ratio is 20:1 and that the low-pressure fluid is entering at 5,000 psi, then, simplistically, the fluid in the high pressure pump chamber will be discharged at 100,000 psi. But not immediately!

Because the outlet valve has been set, so that it will not open until the fluid has reached the required discharge pressure, and this will require a small initial movement of the piston (perhaps around 12%), to compress the water and raise it to that pressure, before the valve opens. And, with a single intensifier piston, when the piston has moved all the way to the left, and the high pressure end is emptied of water, then there will be no more flow from that cylinder, until the piston has been pushed back to the far end of the cylinder, and the process is ready to start again.

Some of that problem of continuous flow is overcome when the single-acting intensifier is made dual-acting, because at the end of the stroke to the left, fluid has entered the chamber on the right, and when the piston starts its return journey the cylinder on the right will discharge high pressure fluid. But again not immediately!

One way of overcoming this is to use two single-acting pistons, but with a drive that is timed (phased) so that the second piston starts to move just before the first piston reaches the end of its stroke. This takes out the dead time during the directional change. The two can be compared:

Figure 5. Difference in the pulsation between a phased set of single acting intensifiers, and a double-acting unit. (Singh et al 11th International Waterjet Conference, 1992)

Figure 5. Difference in the pulsation between a phased set of single acting intensifiers, and a double-acting unit. (Singh et al 11th International Waterjet Conference, 1992)

In cutting operations reducing the pulsation from the jet is often important in minimizing variations in cut quality, and thus, to dampen the pulsations with a dual-acting system a different approach is taken, and a small accumulator is put into the delivery line, so that the fluid in that volume can help maintain the pressure during the time of transition.

 Figure 6. Effect of Accumulator volume on pressure variations (Chalmers 7th American Waterjet Conference, Seattle 1993)


Figure 6. Effect of Accumulator volume on pressure variations (Chalmers 7th American Waterjet Conference, Seattle 1993)

A simplified schematic can again be used to show where an accumulator might be placed.

Figure 7. Location of the Accumulator in the waterjet intensifier line

Figure 7. Location of the Accumulator in the waterjet intensifier line

On the other hand, in cleaning applications particularly with water and no abrasive, there are occasions (which I will get to later) where a pulsation might improve the operation of the system. A three piston pump, without an accumulator, will see a variation in the pressure output that may see an instantaneous drop to 12% below average, and then a rise to 6% above average, during a cycle. One way of reducing this is to increase the number of pistons that are being driven in the pump.

When one changes, for example, from the three pistons (triplex) to five pistons (quintupled), then the variation in outlet pressure is significantly less.

Figure 8. The effect of changing number of pump pistons on the variation in delivery pressure. (De Santis 3rd American Waterjet Conference, Pittsburgh, 1985)

Figure 8. The effect of changing number of pump pistons on the variation in delivery pressure. (De Santis 3rd American Waterjet Conference, Pittsburgh, 1985)

Part of the reason that longer steadier pulses of water, which come from the slower stroke of the intensifier, can be of advantage is that the water is a jet comes out of the nozzle at a speed that is controlled by the driving pressure. A strong change in pressure means that there is a change in the velocity of the water stream along the jet. This means that slower sections of the jet are, at greater standoff distances, caught up with by the following faster slugs of the jet. This makes the jet more unstable. That can, however, be an advantage in some cases, and this will be discussed at some later time, when a better foundation has been established to explain what the effects are.

Labels:accumulator,high-pressure pumps,Intensifiers,pressure pulsations,