Waterjet Technology-Milling and bas reliefs

This Waterjet Weekly is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

This Waterjet Weekly is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

In the last two posts I have been discussing how, either with the use of masks, or with an orbiting nozzle tool, it is possible to mill the material from a confined space within a surface, thereby creating a pocket.

There are a number of advantages to the latter technique, albeit it does require a special tool, rather than using masks that can be made from material already available at a shop.

Using a steel plate to provide a mask, while cutting a square pocket in glass (Courtesy of Dr. Cutler)

Using a steel plate to provide a mask, while cutting a square pocket in glass (Courtesy of Dr. Cutler)

Detail of the corner of the pocket

Detail of the corner of the pocket

With the oscillating tool, which can go deeper into the part to keep the distance from the nozzle to the work surface short, the corners can’t be as sharp as they are with the mask, since the outer radius of the focusing tube provides a limiting bound, once it moves into the cut. However, for shallower pockets where the nozzle can be further away, then the limiting corner radius sensibly becomes the orbiting radius of the nozzle.

A milled pocket in glass made using the Wobbler.

A milled pocket in glass made using the Wobbler.

Note that the floor is relatively even in both cases, though in the masked case the view is taken only after the first pass over the glass. With the orbiting head it is possible to slightly tilt the head (it only requires a couple of degrees – depending on the other operational parameters) to ensure that the walls are being cut to as tight a tolerance to spec as desired (give or take a thou).

Dr. Hashish has noted, from some of the early work that he carried out, that it is possible to mill materials so that very thin skins (around 0.02 inches) can be left at the bottom of the pocket. As I will note in more detail next time, it is also possible to mill using abrasive waterjets in such a way as to leave intervening walls between adjacent pockets that are only that thick. If you have never had to do this in a conventional machine shop, you should know that as the wall of the pocket gets this thin, particularly at significant milling tool depth, the heat from the milling process, and the forces on the metal under the cutter are such that the wall will likely have some permanent deformation after the milling is over. Such is not the case where an abrasive waterjet system, of either variety, is used to cut the pocket.

Depths of cut uniformity can be held to a thousandth of an inch, though this requires some careful selection of both the abrasive size, and feed rate as a function of the other operational parameters of the system. As I mentioned last time, and Dr. Hashish demonstrated, as increasing precision is required in creating the floor of the pocket, so the abrasive being used must become finer and more precisely sieved to keep the wear pattern consistent.

The effect of change in abrasive size on the smoothness of the pocket floor

The effect of change in abrasive size on the smoothness of the pocket floor. Dr. M. Hashish

There is an interesting niche market waiting to be developed in sculpting, I believe, based on putting some of these factors together. It was Professor Borkowski of the Unconventional HydroJetting Technology Center at Koszalin University of Technology* who first demonstrated that, by controlling the jet feed rate over the target, that the depth of cut into the material (and thus the depth to the floor of the pocket) could be controlled.

If now a photograph is scanned, so that the color of individual pixels along the photograph can be identified, then this color can be translated into a required depth. By then setting the speed of the nozzle over that point on the target surface to give the required depth, then the jet will profile, from the color changes along the scanned path, the depth of cut on the milling path over the target. The details of the process are specified in the paper cited above, and the result has been the transfer of a 2-D image from a photograph to a 3-D bas relief cut into metal or other material surface. The depth control was well achievable using the rotational frequency of a stepping motor to drive the motion of the nozzle.

Outline of the process turning pictures into bas-relief (Dr. Borkowski).

Outline of the process turning pictures into bas-relief (Dr. Borkowski).

The initial pictures that were obtained with the very first experiments were somewhat simple, though more than adequate to validate the concept. Where a smoother surface was required secondary passes could be made either in a parallel or orthogonal direction.

Early ball shape cut into metal to demonstrate speed control effect (Dr. Borkowski)

Early ball shape cut into metal to demonstrate speed control effect (Dr. Borkowski)

Figure 6. Early ball shape cut into metal to demonstrate speed control effect (Dr. Borkowski)

The next trial was with a ladies photograph:

Early trial of the technique to validate the effectiveness of the computer control (Dr. Borkowski)

Early trial of the technique to validate the effectiveness of the computer control (Dr. Borkowski)

More recently, as the process has been refined, much more detailed profiles have been demonstrated, as was seen, for example at the 2010 BHRA meeting in Graz.

Lizard bas-relief as shown at the 2010 waterjet meeting.

Lizard bas-relief as shown at the 2010 waterjet meeting.

The concept of changing depth of cut, and thus being able to transfer photographs from the screen or paper onto metals or rock was an interesting academic challenge, that MS&T chose to address in a slightly different way.

Consider that the depth can be achieved by changing the speed of the nozzle on a single pass, so that the depth is controlled, or one can control the depth when only plain waterjets are used, by rapidly switching the jet on or off, as it makes sequential passes over the projected picture area.

The first image on steel led the subject in the first photo to mutter something along the lines of putting them on tombstones to remember those who had passed, so the next tests used photographs of my Grandfather and Dr. Clark, who founded the RMERC.

Images of my Grandfather and Dr. Clark transferred to basalt. (Dr. Zhao)

Images of my Grandfather and Dr. Clark transferred to basalt. (Dr. Zhao)

The technology advanced to the point that it was used to generate the plaque presented to me on my retirement from active academia.

My retirement plaque

My retirement plaque

Which seems to be a good point to close until next time.

*This University was kind enough to give me an honorary diploma.

Waterjetting-The Effect of Standoff Distance.

This Waterjet Weekly is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

This Waterjet Weekly is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

One of the problems with relying on photographs is that they are sometimes not of the quality that one would wish. This has happened with today’s topic, where the pictures are old, smaller and in poorer condition than I had remembered. However, with your indulgence, I am going to step through them. I do apologize for their poor quality, however.

The topic is the way in which a waterjet first attacks a target. I have mentioned different parts of this process in the past. But in this post I want to show that it matters where the target is, relative to the nozzle, because the structure of the jet itself changes with that distance, which I call the standoff distance between the jet orifice and the initial target surface.

The break-up pattern of a waterjet (Yanaida K. “Flow Characteristics of Waterjets,” 2nd BHRA Conf. 1974, paper A2.)

The break-up pattern of a waterjet (Yanaida K. “Flow Characteristics of Waterjets,” 2nd BHRA Conf. 1974, paper A2.)

As I mentioned last time when the target is close to the nozzle, then the erosion pattern can, in the first few seconds of contact, be seen to be like a butterfly in pattern. The central part of the target, under the jet, is not eroded, but there is severe erosion around the edges of the jet diameter, where a grain will see high differential pressures across its width, and will be subject to high lateral jet flows.

Damage pattern around the impact point of a 10,000 psi pressure, 0.04 inch diameter jet on aluminum, target close to the nozzle

Damage pattern around the impact point of a 10,000 psi pressure, 0.04 inch diameter jet on aluminum, target close to the nozzle

As the nozzle is moved away from the target surface, however, that pattern of erosion changes. As the jet structure picture shows, the central zone at the initial pressure reduces in radius, and there is an intermediate zone of rapidly diminishing pressure, with an outer shroud of fine droplets. The effect on the impacted target is that there continues to be a small zone with no erosion in the center, and that erosion is still concentrated around this zone, in that of high differential pressure, which now encroaches on that central sector.

Erosion of an aluminum target with the nozzle 2-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice

Erosion of an aluminum target with the nozzle 2-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice

That central small plateau is reduced to a very small point by the 3-inch standoff, which is where the jet reaches the end of the distance where the pressure remains constant over the central section. Thus, by a 4-inch standoff the central section, though still present, is being eroded.

Erosion of an aluminum target with the nozzle 4-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice

Erosion of an aluminum target with the nozzle 4-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice

As the nozzle is moved further back from the surface, that central promontory disappears at around a six-inch standoff. It is interesting to note that at this point the cavity is starting to get noticeably deeper.

Erosion of an aluminum target with the nozzle 6-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice. (The lower of the two circular damage patterns was caused through experimental conditions and should be ignored). The presence of a central mound can barely be discerned.

Erosion of an aluminum target with the nozzle 6-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice. (The lower of the two circular damage patterns was caused through experimental conditions and should be ignored). The presence of a central mound can barely be discerned.

By this time the central section of the jet is beginning to break down into, initially short strings, that very rapidly break into droplets. The damage pattern that results shows a cavity that is slightly increasing both in diameter and depth.

Erosion of an aluminum target with the nozzle 8-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice.

Erosion of an aluminum target with the nozzle 8-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice.

By this time the jet is continuing as a series of relatively large droplets, still holding a central structure, though surrounded by a rapidly decelerating cloud of mist.

Erosion of an aluminum target with the nozzle 10-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice.

Erosion of an aluminum target with the nozzle 10-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice.

It is one of the interesting oddities of the jet cutting business that the amount of material that is eroded from the target is a maximum at this distance.

However, and this was the subject of great debate back at the time that it was first presented, the ability to control the droplet size, and condition as a function of distance, and the reality that in most applications the target must be cut to depth meant that this has a very limited application. It can be used, if the droplets are generated properly, and used within the relatively narrow window that they exist, to improve surface erosion of material.

However, as Mike Rochester found when he studied this, back at Cambridge in the early 1970’s, the presence of a layer of water on the surface, and as the hole deepens this is almost always there, rapidly diminishes the effect.

The effect of a layer of water in diminishing the “droplet impact” effect in erosion of a surface. (After M.C. Rochester, J.H.Brunton “High Speed Impact of Liquid Jets on Solids” First BHRA Symp Jet Cutting Tech, April `972, Coventry UK, paper A1.)

The effect of a layer of water in diminishing the “droplet impact” effect in erosion of a surface. (After M.C. Rochester, J.H.Brunton “High Speed Impact of Liquid Jets on Solids” First BHRA Symp Jet Cutting Tech, April `972, Coventry UK, paper A1.)

There are ways of getting around this problem, but the presence of water in the cavity that the jet has produced can also lead to problems, and these will be the topic of the next two posts.

Waterjet Hose Connections and Nozzle Feed

This Waterjet Weekly is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

This Waterjet Weekly Blog is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

Waterjet Hose Connections and Nozzle Feed

There is often not a lot of choice, when laying out a project, over the path or distance that water must travel from the time it leaves the initial pump to the point where it reaches the nozzle and is usefully applied. Yet, over this length, if the right choices are made, some considerable improvement in performance might be achieved.

As discussed earlier, the simplest improvement is often just to increase the size of the delivery line, although there have been occasions where it was cheaper for us to use a double line rather than a larger single, in order to lower the friction loss to the nozzle, and keep operating pressure in the range that we needed.

There are other concerns with the layout of the feed line to the nozzle, If it is a hose, then any connections between sections should be whip-checked, so that should a coupling fail (which has been known to happen) then the released sections of hose will not whip around and cause injuries or other damage.

Waterjet hose connections under pressure

Hose that separated from a coupling, while under pressure.

The major risk comes at the moment of separation, while the pressure in the line is higher, before it drops under the larger area through which the water can now flow. To stop the whipping of the hose ends, the two should be restrained by attaching a cable with two loops that fit over either hose end making the connection.

Waterjet hose and whip connnection

Hose connection covered with a pressure dissipating sleeve (the blue cover) and with a whip connected to the hose ends on either side of the connection.

Apart from this, and remembering the possible risk about avoiding chaffing points and that, over time, high pressure tubing does fatigue (after many years of operation most of the pipe segments connecting between our ultra-high pressure pump and cutting table failed in a relatively short time period – but they all came at the same time, and were installed together and saw the same loading cycles). (Which emphasizes that, over time, the pressure rating of the tubing should be reduced).

Connections, T-joints and other fittings that are used in the feed line should be sized appropriately. Any time that the diameter of the flow channel changes, then there is a cost in terms of the delivered pressure. This is best checked with the manufacturer to ensure that this is accurately assessed in the flow and pressure calculations.

Moving down the line, this brings us to the end of the feed line, and the entrance to the nozzle. In later posts I will cover different pieces of equipment that can be used, for a variety of different tasks in manipulating the nozzle, but, for now, I would like to consider just the flow from the feed line into the nozzle (without discussing nozzle shape at this time).

Of all the systems I have examined, this is the one point in the assembly of a feed system that was most commonly ignored or badly constructed.

All of us, from time to time, get caught up in traffic flows through road construction. When the lanes are controlled, and traffic feeds are properly directed, it is possible to get through these relatively quickly. But in most cases that is not what happens. There are always drivers who do not ease into the required lanes soon enough, but rather drive rapidly as far as they can and then force their way into the remaining lanes, thereby breaking the steady flow into a process of stop-start-stop-start. The process becomes much less efficient, and instead of the traffic moving at a steady, but slow rate it often can take over half-an-hour or more to get through the restriction.

So it is with water flow down a pipe. If the flow can remain in a laminar mode, with the flow channel slowly constricted to speed the water up to the required velocity, then the resulting flow into and through the nozzle can give jet streams that can throw over 2,000 jet diameters. Instead in most cases the throw is about 125 jet diameters, and I will discuss how to find that distance in a post or two before long.

In this particular post the water has not reached the nozzle yet, and it could still be in a poor condition. The best way to explain why is to use a comparative graph showing jet pressure measurements after the jet has passed through the nozzle, to show how the structure is affected. The work was carried out by the Bureau of Mines (Kovscek, P.D., Taylor, C.D. and Thimons, E.D., Techniques to Increase Water Pressure for Improved Water-Jet-Assisted Cutting, US Bureau of Mines RI 9201, Report of Investigations, 1988, pp 10) and the only difference between the two different plots is that in the upper one a 4-inch straight length of pipe was connected just upstream of the nozzle to allow the flow to stabilize before it entered the nozzle.

The effect of flow conditioning the water, prior to passage into the nozzle.(

The effect of flow conditioning the water, prior to passage into the nozzle.(Kovscek, P.D., Taylor, C.D. and Thimons, E.D., Techniques to Increase Water Pressure for Improved Water-Jet-Assisted Cutting, US Bureau of Mines RI 9201, Report of Investigations, 1988, pp 10)

Figure 3. The effect of flow conditioning the water, prior to passage into the nozzle.(Kovscek, P.D., Taylor, C.D. and Thimons, E.D., Techniques to Increase Water Pressure for Improved Water-Jet-Assisted Cutting, US Bureau of Mines RI 9201, Report of Investigations, 1988, pp 10)

There is a caveat to this plot, and this is the assumption that the internal diameter of the feed pipe and the entrance diameter to the nozzle are the same size. In virtually every system that I have examined in the field this has not been true, and as I will show, in a later post, the difference that this can make is very large. For the above example the length is 4-inches, but this was for a specific nozzle size, and the more general condition is that the length should be in the range of 100 – 125 pipe diameters.

By the same token, if the nozzle does not make up with the end of the feed line, so that there is a little eddy pocket created, the ensuing jet will be of poor quality.

Which brings us to the final part of this post, because there are many situations where it is not possible, because of space restrictions, to fit that particular length of pipe just before the nozzle. The most glaring example of this that we have had to deal with is where high-pressure waterjets are fed down a borehole, and then used to drill lateral excavations out from the bottom.

But if the borehole is say 10-inches in diameter, and the jet is an inch in diameter, because it is being used for mining out valuable pockets of uranium, then it is not possible to get the required straight section. Again the pioneering work on this was carried out, first in Russia, and then by the U.S. Bureau of Mines, under George Savanick. By placing a shorter length of a flow straightening device within the flow path, just before the nozzle, the flow can be straightened over a much shorter distance, and this will be the topic of the next post. And when George did this he was able to cut cavities that extended more than 30-ft from the borehole (with some unexpected consequences – but we’ll cover that later – grin).

 

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.