Waterjetting 27e - Borehole Back Pressure Effects

In the earlier posts on this chapter of waterjet technology I have dealt with the changes in cutting performance when a waterjet stream cuts in to material that is either under pressure, or contains internal stresses that may not be obvious at first glance. In this post I will focus, instead, on the changes in performance when the borehole becomes filled with water under pressure.

Figure 1. 12-inch cores of sandstone that have been drilled by the same jet drill, at the same speed, but at borehole pressures of 0, 500 psi, 1,000 psi, 1,500 psi and 2,000 psi. (Jet pump pressure 10,000 psi; 970 rpm; 40 inches/min ROP)

The water used in the test also contained a small amount of polyethylene oxide (Polyox) that, at the time, was the only polymer readily available to enhance jet performance under water, although there are now liquids such as Superwater that similarly help.

It can be seen that even the change in pressure to 500 psi is sufficient to dramatically shorten the distance that the jet cuts through the material on a single pass, and the range then only shortens a little as pressure further increases. But the hole drilled at 2,000 psi is barely large enough to let the high pressure lance and nozzle assembly pass.

First an explanation of the equipment that we used to run the tests. A triaxial cell was used as the basic vessel to hold the core. This is so-called since it allows pressure to be applied around the rock core, and also since the cap can slide within seals, axial pressure given the third of the orthogonal directions for loading.

Figure 2. Triaxial cell used for the drilling experiments.

A valve was fitted on the flow line of water out of the chamber (just above the pressure dial) and this controlled the fluid pressure in the cell. The diameter of the outer (reaming) jet was 0.04 inches, and the rapid decay in range with the increase in pressure led to a second experiment, to see how changing the diameter changed the results. The equipment was modified for this test, the feed pipe to the nozzle was bent, so that, as it made a single circuit over the underlying rock, it would trace out a circular path rather cut a single hole. Then the top of the sample was cut at an angle so that, with the rotation the distance from the jet to the target would vary and the range of the jet could be seen. (Figure 4).

Figure 3. Modified equipment to find the effective jet range against back pressure.

A simplified factorial experiment was run with three nozzle diameters and five back pressures, measuring the depth of cut into the sandstone in each case.

Figure 4. The resulting cut when a 0.03 inch diameter jet was rotated over sandstone with a 1,000 psi back pressure in the cell. The 10,000 psi jet was brought up to pressure with the jet at the greatest standoff (hole at the bottom) and the back pressure was set before making a single pass over the sample. The depth of cut was averaged over several readings made along its length.

The data was then plotted (with the curve smoothed here for simplicity in discussion).

Figure 5. A plot of range of jet cutting ability as a function of hole back pressure for three different nozzle diameters.

The graph shows that, for this set of conditions, the larger the jet the better, and that the first 500 psi of back pressure has an immediate effect on jet cutting effectiveness. Jet size should be at least 0.064 inches when drilling against back pressure in the hole. There was a significant improvement in cutting ability when the polymer (at 300 ppm) was subsequently added to the water, in a later series of tests. The small number of tests carried out, however, were too small a sample to provide more than guidance as to concentration since all three levels tested (100, 200 and 300 ppm) all showed considerably improved depths of cut (increasing to a depth of almost 2 inches against a back pressure of 2,500 psi) when contrasted with the performance levels shown above. The polymer tests were carried out with a jet nozzle diameter of 0.064 inches.

There are two parts to the effect of the borehole pressure. The first is simply one of increasing the resistance of the water to jet penetration, and lowering the effective jet pressure (since that is effectively the jet pressure less the borehole pressure).

It is important to recognize that it is not just the drop in effective pressure that causes the effect. To check that this was the case a hole was drilled with the same conditions otherwise as the left-hand rock sample in Figure 1, except that the jet pressure was dropped to 5,000 psi. Thus the differential pressure of the jet across the nozzle was less than that in the case of the other four rock samples shown in Figure 1. Yet the hole was of the same approximate irregular geometry as that shown by the left-hand core of Figure 1 even with the lower differential pressure with the prominent cone cut ahead of the bit that is not evident in the other cases.

Mike Hood has shown the effect of loss in cutting range by using back-lit shadow images of a jet at different back pressures.

Figure 6. Illustration of the effect of fluid back pressure, the shadow image of the jet shows how back pressure reduces the range.

As mentioned above, the effects extend beyond reducing the jet range, and lowering the jet differential pressure. The increased confinement on the rock will compress the grains of the rock more tightly together, making it more difficult for the pressurized water to penetrate into the rock structure. This combines with the higher pressure required to grow the cracks to effectively reduce the ability of the jet to penetrate into the rock.

At the same time, if you listen as the back pressure is increased (we used a Lichtarowicz Cell the increasing pitch of the sound shows (as does the damage induced) that the collapse of the cavitation bubbles generated around the edges of the submerged jet is becoming more intense as the pressure increases. I have discussed how this can be used as a benefit in breaking up rock in an earlier post.

Waterjetting 27d: Drilling at a fixed diameter

In the last post I described how we initially came up with a simple design for drilling through material, using an axially aligned jet and a larger jet offset to one side at an optimal angle of around 20 degrees.

One of the problems with the use of this design is that the outer jet has to remove all the material in front of the nozzle during the time that it rotates around and advances the distance of the incremental feed rate. If it does not then there is a significant problem. Consider the case where the drill penetrates through a layer of limestone, while drilling otherwise in sandstone.

Figure 1. Sectioned waterjet drilled hole through a sandstone:limestone:sandstone sandwich of rock.

Note that although the hole does not deviate as it goes through the harder material since, unlike conventional drills, there is no mechanical contact between the high-angled rock and the nozzle assembly. But the hole reduces in size. If the hole reduces in size below the diameter of the nozzle holder, this will not contact the rock until it has passed behind the plane of the reaming jet. In other words the only way the blocking rock can be removed is to back the nozzle along the hole so that the reaming jet can hit the material blocking progress.

Figure 2. Drill passage blocked by protruding rock in the path of the nozzle body, but behind the cutting plane of the inclined jet.

One way to ensure that this is not a problem is to advance the drill at a slower rate, with the rate of penetration controlled by the ability to cut the hardest rock that the drill will pass through. The problem with that approach, and concurrently that of setting a fixed advance rate, is that, at the same advance rate and rotation speed, the drill will drill through different rocks at a different diameter. While this can be an advantage, in a limited number of cases that I will discuss in a later post, in most cases it is better if the hole is at a relatively constant diameter.

So how can we solve this problem?

One approach taken in Australia was to change the design and location of the cutting jets. Rather than have a single jet cutting out to the perimeter of the hole, two jets were used, but crossed over the axis and cut on the opposite side to their location. This had an additional advantage over the initial design in that, when drilling longer holes (and this went on to drill horizontal holes that ranged up to a kilometer in length IIRC) the head was balanced and so did not wobble and get out of alignment because of the force imbalance.

To overcome the problem of drilling at too small a diameter additional reaming jets were placed on the front of the nozzle assembly, so that he hole would be reamed to the diameter needed to allow the support hose access.

Figure 3. The addition of a pair of reaming jets. Note that offsetting the two front nozzles will also allow them to put a torque on the front part of the nozzle, which can therefore be self-rotating from the left hand of arrow A forward.

But the problem is not completely solved with these changes, since should any rock protrude into the hole in the distance A, so that it hits the larger diameter that follows, again it is not possible for the reaming jets to cut this rock without backing up the drill.

There is another problem, in drilling horizontal holes where the hole diameter can vary. Consider that if the drill goes into a softer material then, at constant advance (ROP), the hole diameter becomes larger. As the drill moves over this larger hole it will be riding on the floor of the hole, and thus the front of the drill will tip forward into the floor of the larger hole. This will incline the drill downwards, and so the hole will no longer be of constant alignment, but rather will gradually, over distance, tip increasingly downwards.

It is therefore critical that the hole be drilled at a relatively constant diameter (allowing for some hole roughness). How to achieve this? The answer is to put a gaging ring or collar of the required hole diameter, in the cutting plane of the rotating jets.

Figure 4. The use of a collar at the front of the nozzle to ensure the hole is cut to the right diameter.

It itself this isn’t sufficient to give the hole a constant diameter, since there is still the problem of drilling through materials of differing resistance. To overcome that problem we put a spring at the back of the drill, with a contact switch to a valve on the feed to the hydraulic motor powering the drill advance. Thus the drill would start to rotate, and the motor would increase the speed of advance until the collar bumped up against the rock. At that time the spring would compress, the contact switch would close, and the advance would momentarily stop. The drill would rotate around and remove the obstructing rock, the spring would expand opening the flow to the motor, and the drill would move forward. It may sound as though it would be a stuttering advance, but when we tried it in a mine you couldn’t tell that the mechanism was working, apart from the hole being of constant diameter, and by watching the spring. It drilled at between 7 and 12 ft a minute in an aggressive sandstone.

Figure 5. The drill assembly used underground. The hydraulic advance motor (it pulls the drill forward using the chain drive) can be seen under the drill sash (the red and grey bar – painted in 1 ft intervals).

In a normal drilling operation when a drill intersects a previously drilled hole at a shallow angle, then the second drill will follow the path of the first hole, and cannot drill through the opposing wall at that shallow angle. (We know this from experience having broken two drill steels trying while excavating the OmniMax Theater under the Arch in St. Louis). But with the waterjet drill we were able to make to second drill cross the intersection.

Figure 6. Photo down one drill hole, showing the point where the hole intersected a second, and crossed without deviation.

Hopefully there is now enough background so that next time I can talk a little more about the effects of borehole pressure on drilling performance.

Waterjetting 27c - Drilling nozzle design

In discussing how stress affects the ability of waterjets to drill rock, I have discussed the effect of the stress in the ground on drill performance, but before discussing the effect of the borehole pressure it is perhaps best to spend this post talking about the simplest drill bit design.

The diameter of the first jets we used to cut into rock were about 0.04 inches in size, with the nozzle holder used to hold the nozzle on the end of the supply pipe being at around an inch in diameter. As a result, if the jet was to cut a path into the rock, it would have to rotate around the face of the rock ahead of it, removing all the rock ahead of the assembly, and allowing the head to advance.

Figure 1. Original concept of a waterjet drill used to penetrate sandstone.

Of course, back when we first did this in the 1960’s the swivels weren’t available to allow us to rotate the high pressure line, and so we rotated the rock samples instead.

Figure 2. First holes drilled at the University of Leeds. Note the central cone.

Because the jet had to penetrate across the diameter of the hole, so as to remove the cone ahead of the tool, and since the jet would only cut around 2.5 times the jet diameter in width at any one time, the rate that the head could move forward was limited to a maximum of 0.1 x rotation speed (rpm) in inches/minute. And, because the rotation speed controlled the depth which the jet cut into the rock, the rpm had to be kept down to ensure that the jet cut to the full required diameter on each pass. The top speed we could achieve, even in relatively soft sandstone, was around 4 inches a minute.

One of those fortunate accidents that sometimes befalls research folk then occurred. I had asked Jim Blaine, our machinist, to make a new design, with one jet pointing forward and one off to the side, intending that the two be offset. However, due to a misunderstanding, he drilled the second, smaller hole along the jet axis, while offsetting the angled jet to cut further out from the diameter. Since the nozzle was built we proceeded to try it.

Figure 3. First dual jet nozzle design.

Because the axial jet removed the central core, we could offset the inclined jet so that it needed to cut a shorter distance in order to reach the required gage for the hole. That meant that we could rotate the nozzle faster, which in turn meant a faster drilling speed, much faster.

Figure 4. Hole diameter as a function of rate of penetration of the drill (in meters/minute), for two outer jet angles, and two rotation speeds.

Note that in the above figure, with a 30 degree outer jet, spinning at 970 rpm we were able to drill a hole at a speed of roughly 280 inches/minute instead of the previous 4 inches/min by adding only 25% more water to the bit with the second orifice.

As mentioned above, the limit on the advance rate was the depth which the jet cut into the wall, and the amount of rib between adjacent passes that the jet cut would leave.

Figure 5. An early hole drilled into Berea sandstone, at a slow advance rate, using a 10 ksi jet pressure.

Figure 6. Hole drilled into Berea sandstone at 970 rpm, 225 ipm advance rate, with a 15-degree inclined jet. Note that the hole perimeter has the equivalent of a thread cut into it.

It is pertinent to make a small observation over the advantage of that slightly roughened outer wall to the borehole. One of the ways in which miners hold up the roof while they are working underground, is to insert rods (known as roofbolts or rockbolts) into drilled holes placed in the surrounding rock. To improve the grip between these bolts and the wall, miners will also often insert packages of glue into the hole to fill the gap between the bolts and the rock wall.

Unfortunately when the hole is drilled with a conventional mechanical drilling bit, the walls of the hole are left relatively smooth. This means that the bolt has a poorer grip on the wall, and is more easily pulled out of the hole. The US Bureau of Mines ran anchorage tests for different rock wall finishes.

Figure 7. Effect of hole roughness on the anchor strength (US Bureau of Mines)

Conventionally a larger hole, with greater bearing surface, would give a stronger anchorage. This is shown by the greater load carried by the hole drilled with the 1-3/8th bit, over that drilled by the 1-1/4 inch bit. But both of these were smooth walled, and the bit drilled at 1-inch, with a roughened wall had almost three-times the pull strength even though of smaller size.

The roughness of the hole can be controlled by adjusting the feed rate, relative to the rotation speed, both as a function of the jet pressure, nozzle diameter and outer jet angle. It turned out, through experiment, that the optimal angle for the jet was at around 22.5-degrees, depending on the type of rock in which the drill was working.

The effect of rock properties plays a very significant role in the performance of the drill. And it was very easy, early in the program, to show that the important rock parameter was not the compressive strength of the material. To show this we drilled through prepared samples of an Indiana limestone and a sandstone, both of which had approximately the same (uniaxial) compressive strength. The advance rate was kept constant, as was the rotation speed, as the drill penetrated from one rock into the other, and then the hole was cut in half (as were the samples shown above).

Figure 8. Hole drilled from limestone into sandstone.

Although the hole maintained alignment, drilling straight forward through the steep interface between the two rocks (a problem with some conventional drills) the hole diameter changed dramatically.

How we changed the design to maintain hole diameter, and, at the same time, adjusted for changing borehole depth will be discussed next time.

Waterjetting 27b - Drilling rock under stress

Last time I opened discussion on the topic of cutting a material that contained high levels of stress. This is a more common situation when working with rock, since – as a general rule of thumb – the vertical stress on a rock increases by 1 psi, for every foot deeper one goes into the earth. Thus, for example, if one goes down around 700 feet, the depth of a number of coal mines, then the background pressure on that rock is some 700 psi due to the weight of the rock that is pressing on it from above.

Now I should also mention that this is only a general rule, because, over the millennia, the rocks move, are split by earthquakes, overlain by volcanic eruptions and many other events that make that generalized statement less accurate for any given location. And one factor is that, if there weren’t such movements, then the natural horizontal stress on the undisturbed rock would be about a quarter of the vertical stress (the ratio is known as Poisson’s ratio, though usually derived for the resulting strain on the material, rather than the driving stress).

What one often finds, when these values are measured, is that the horizontal stress is higher than the above simple calculation would suggest. Which is a long way of saying that it is often difficult, without making a measurement, to know exactly what stress a rock is actually undergoing when found underground. But if some of the rock is removed (because it contains valuable ore) then the stress field redistributes, and some of the simpler assumptions come back into play. And we found that out when we drilled these holes:

Figure 1. Oval holes drilled into a lead-bearing sandstone;

You can see that we were drilling oval holes. The drill we were using used two high-pressure (10,000 psi) waterjets that were rotating at constant speed as we fed the drill into the rock. (And I’ll discuss the drill design and other stress effects in the next post). The small dark spots in the rock are galena, and as I will discuss in some future post, we were able to separate the galena from the sandstone at the drill, in part because of the way the waterjets penetrate, as I will discuss below.

Figure 2. Waterjet drill penetrating sandstone at up to 12 ft/min.

The region of the mine we were working in was around 700 ft. deep, and had been previously mined. Roughly half the rock volume had been removed, over a relatively large surface area, so that the pillars that were remaining were carrying roughly twice the load that they were before mining took place. On the other hand, since the rock on either side of the pillars had been removed, the vertical load was all that was acting on the rock within the body of the pillar, where we were drilling the holes. So very crudely the vertical stress, before we started drilling was around 1,500 psi in the rock.

Now, to explain why the holes are oval rather than round, consider that a waterjet works by getting into the cracks that exist in the rock, pressurizing the fluid and causing the crack to grow until it meets other cracks that together free a small piece of the rock mass. In this case the rock is made up of grains of sandstone and galena which have boundary cracks around each particle. By growing the cracks using this process, the rock is broken out into the individual grains of sand and galena.

But when the rock puts pressure on the rock, so the cracks are squeezed closed, and the water finds it harder to penetrate into them. This happens to the rock on the sides of the hole. As it is being formed, the load that was being carried by the rock being removed transfers to the rock on either side of the hole. Because the load is vertical this means that the jets find it harder to penetrate the rock on either side of the hole, and the horizontal diameter of the hole is therefore less than it would be otherwise.

Figure 3. Lines showing equal stress magnitude around a hole drilled into a rock loaded vertically. (This is purely representative and does not carry a scale, the lines are of diminishing intensity as they move away from the hole.)

On the other hand, as the load from the overlying rock moves out to either side of the hole, it comes off the rock at the top and bottom of the hole, and those cracks get larger, and were no longer being squeezed shut. As a result the jets found it easier to penetrate into the rock, and the vertical diameter of the hole is thus larger than it would be otherwise.

Put these two together and the result was that the jet drilled holes that were oval in shape, as shown in Figure 1.

As one way of making sure that this was really the cause of the change in hole shape, we used the waterjets to cut a slot around the perimeter of a part of the rock in the pillar. By making a horizontal cut above the slab that this outlined, we removed the vertical loading that the rock was seeing due to the overlying rock.

With no external loads on the rock, from either direction, it was as easy for the jets to cut into the rock in all directions, and, as a result, the holes that the jet drilled were round.

Figure 4. Round holes drilled in unstressed rock near the block of holes shown in Figure 1.

Again, while the effects are much larger when shown in cutting and drilling rock, the effects would be similar if we were cutting material that was under other internal stresses and which were then cut by a jet in a shop or other surface facility.

In the above case we were working in a mine where there was free access to the rock, the situation changes if we had been trying to drill down from the surface, and that will be the topic of the next post. In passing it should be noted that the waterjet drill was not only quieter, but also less powerful and smaller than the existing mechanical drill, and it could drill the rock faster.

Figure 5. Comparison of mechanical drill (upper) and the waterjet drilling equivalent (lower) on a drilling rig underground in a lead mine.(You need to look closely to see the drilling rod that is shown in Figure 2.)

Tech Talk - Geothermal Plant Opens

The Missouri University of Science & Technology Geothermal system was officially opened last Thursday, some months after the coal and wood fired power plant that had previously warmed the campus had been shut down.

Figure 1. Chancellor Schrader cutting the ribbon to officially open the system.

The operation ended up being a little larger than originally anticipated, although the receipt of several grants kept the need for external funding bonds down to $30 million. Overall, as the old heating and cooling system was replaced around campus, deferred maintenance costs of some $60 million disappeared as the new system eliminated those needs, and is anticipated to generate fuel overall savings of some $1 million initially rising to $2.8 million a year as future fuel prices rise over the years.

In the end some 645 wells were drilled to feed three different geothermal plants located around the campus. Well depths ranged from 420 to 440 ft., and with a background temperature around the wells averaging around 60 deg F.

The installed system is, to a large extent, computer controlled, so that it was necessary to find employment for the fifteen workers at the power plant who would otherwise have been laid off. Given that some took retirement, the University was able to absorb the rest into the workforce in various ways. But it does point out that, now that the system is installed, the number of jobs associated with this new sustainable energy system are significantly below that required at the power plant, and the coal mine and forestry products supplier that previously supplied the fuel. Maintenance of the system, which is largely built around pumps, pipes and valves can, in the main, be carried out by the normal trades staff at the campus.

Figure 2. Overview board for the individual geothermal flow loops

To illustrate the degree of control that the new system exerts on the Heating and Air Conditioning (HAC) network, consider a simplified circuit for one building.

Figure 3. Illustrated circuit for a single building

Hot water is fed into the building from the network (top left) at a temperature of 118.7 degF, and is mixed with a portion of the previously circulated fluid to give a starting temperature of 113.6 degF entering the building. (The values are in the small boxes over the sensing valve emulations). The hot water circulates around the building providing heat as needed. At the point where the water would exit back to the network for reheating the temperature of the returning water is measured (in this case 102.3 degF). Depending on that temperature a control valve opens or closes to send more (or less) water back for reheating, while the remainder stays in the circuit, with make-up from the main network. (with the valve 41.3% open some 3% of the returning water is being recycled). The computer also calculates the heating load being fed to the building (327.5 kBtu/hr).

Figure 4. Details of the control valve and instrumented values.

By using a similar circuit for cooling the components of the system are largely similar, reducing the inventory costs for maintenance supplies, and the two circuits are simply monitored through instrumentation around the circuit.

This is similarly true for the three geothermal plants, the status of each of which is also represented by a monitoring screen.

Figure 5. Control circuit monitoring the performance of the heat exchangers between the field circulation water and that being used in the building circuit.

The heat exchanges between the ground water and the heating/cooling circuits is through use of three screw type heat recovery chillers, the operation of which is described as:

A heat recovery chiller operates on the basis of a refrigeration cycle: the same basic cycle that is used for refrigerators, air conditioners, and heat pumps you find in your homes. It is designed to provide both useful cooling and useful heating energy from the machine. The work or energy put into the machine through the compressor is used to simply transfer heat from evaporator to the condenser, which makes it a more efficient use of energy than combusting fuel for heat.

As seen in the diagram below, the refrigerate, R-134a in our chiller, is first compressed using a screw-type compressor. This hot gas is then condensed to a liquid as it travels in a circuit through the condenser, and heat is transferred to the water flowing through the condenser tube bundle. The pressure and temperature of the refrigerant is reduced as it flows through the throttling valve. The refrigerant next passes through the evaporator where heat is transferred from the water flowing through the evaporator tube bundle back to the refrigerant. Then the cycle repeats as the refrigerant goes back to the compressor. The refrigerant is confined inside of the heat pump chiller for the entire process.

Figure 6. Operation of the heat exchanger.

Figure 7. Overview of the three chiller units in the McNutt plant

Manually readable gages provide back-up to the computer monitoring instruments.

Figure 8. Monitoring gages for the chilled water loop.

When additional heat is needed, this is provided by a bank of natural gas heaters for the water that can be engaged as needed, and that are similarly monitored.

Figure 9. Overall monitoring board for the natural gas boiler system

While the system may get an early test of effectiveness this week as a Polar Vortex brings an early taste of winter to town, with temperatures predicted to drop to a high of 34 and a low of 19 on Thursday.

Figure 10. Natural gas boiler to provide additional heat as needed.

Since I won't be able to take advantage of those boilers, I’m glad I have my wood stacked, and that I swept my chimney this morning.

Waterjetting 27a - Cutting materials with internal stress

Safety glass, or toughened glass is typically designed so that, when it fails it will break into small pieces with few of the relatively sharp, and thus dangerous, fragments formed by ordinary glass. It is used in making shower doors, and automobile windows. As such is differs from laminated glass (which I will discuss in a later post in this section). The toughened glass is formed by quickly cooling the glass, after it has been heated. And one way to check if a sheet has been treated this way is to look at it through polarized sunglasses. Tempered glass will show a pattern. The reason for that, and for the rapidity of the breakup of the glass, is that the lines show the internal stresses that the glass treatment deliberately leaves in the material. (There is an interesting variation on this way to tell the difference using an iPhone.)

Figure 1. Broken pieces of tempered glass, showing the small fragments that result. (Floydglass)

Because the treatment puts the outer parts of the glass into compression, while the inner part is in tension once cracks start to appear in the glass, then the glass is designed so that these stresses will cause the cracks to grow, bifurcate and join in patterns that cause the glass to shatter into less dangerous fragments. But this creates a considerable problem if there is a need to reshape the glass after it has been heat-treated.

Note that this treatment is the opposite of the result where glass is annealed, where – by cooling the glass at a slow rate – the internal stresses are much reduced, but as a result, when the glass breaks the fragments can be more damaging.

Figure 2. Sheets of annealed glass, showing how it may break from impact. (ADMglass)

Annealed glass is, as a general rule, relatively easy to cut with an abrasive waterjet system provided that certain simple precautions are taken. However, when it comes to cutting tempered glass, one of the suggestions is to anneal it first, so as to get rid of the internal stresses. Unfortunately, in the process this also removes the benefits of the tempered treatment.

When one tries, without other treatment, to cut tempered glass the results are not pretty. Edgar Hernandez has posted a video of what can be expected to happen.

The problem goes back to the basic way in which waterjets, and abrasive waterjets work in cutting through material. Simplistically waterjet impact will penetrate the cracks that exist in a target surface; the following slug of water then pressurizes the water within the crack, causing it to grow. As cracks get longer it takes less and less pressure, either internally within the crack, or in the surrounding material, for that crack to grow catastrophically to failure of the piece. Where there are relatively few natural cracks in the material – as happens with glass – then abrasive is introduced into the waterjet stream, so that the impact of the small particles will form small cracks when they hit the glass surface. Normally those cracks are relatively small, and when first cutting into or piercing the glass the pressure of the jet is often lowered so that the particle speed is also lower and the crack length that the particles create is also small and localized around the impact point, so that the integrity of the whole piece is not threatened.

Figure 3. Cracks around the impact of single particles of abrasive onto glass.

The problem, from a cutting aspect, with tempered glass is that the internal stresses that are deliberately placed into the glass are designed so that cracks do not have to be very long before the concentrated stress at the crack tip (which increases with crack length) reaches a point where it will continue to grow at an increasing rate to failure of the piece. The longest cut we have made in tempered glass before it shattered was about an inch-and-a-half.

Because the stress in the glass is an inherent part of the nature of that particular type of glass there is no really effective way of cutting the material, after it has been tempered. If a particular shape is required then the glass should be cut to final shape before it is tempered, and care should be taken to ensure that there aren’t any large cracks or chips along the edge of the glass before it is then tempered.

Stress problems aren’t restricted, however, to trying to cut tempered glass. When cutting larger pieces of metal one can also run into problems from stresses that were left in the material after it was initially formed. Perhaps the most common of these is found where a partial cut allows a stressed part to lift slightly above the plane of the rest of the material. If the part is being cut in steps, the raised piece can then move into the path of the cutting nozzle as it moves back over the piece. This can have some unfortunate consequences for the nozzle and focusing tube (there goes bitter experience speaking again).

Other problems that can crop up come from the shifting of the piece in the plane of the part, but where the stress relief moves the edges so that subsequent cuts into the part no longer comply with the blueprint for the cuts, since the material has shifted. This shift can be a relatively small movement – depending on the level of stress that was captured in the material, but it can be enough to take the final part out of tolerance, and thus it never hurts to be sure of the stress condition of the piece before starting to cut.

I’ll return to this theme, but with a different illustration of stress effects next time.