Waterjet Cutting-Abrasive water jet and cut taper

Dr. Summers Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Series

I have spent some time in recent weeks discussing the use of abrasives in waterjet cutting, and particularly some of the issues that are involved in getting the abrasive distributed relatively evenly through the jet stream, and accelerated to as high a velocity as possible by the time the jet leaves the focusing tube. This issue has become more important as clients request more precise cuts, and edge quality and alignment become more critical. As the jet cuts along a surface the amount of material that is removed (i.e. the depth of cut simplistically) is controlled by the number of particles that impact along that axis. And that, to a degree, is controlled by where that axis lies, relative to the axial diameter of the jet that runs parallel to the direction of cut. Different conditions give different particle densities, but even within those conditions, the material under the center of the jet will see many more particle impacts than those on the side.

Particle distribution across two abrasive waterjet streams with the same focusing tube diameter, but different waterjet orifice diameters

Particle distribution across two abrasive waterjet streams with the same focusing tube diameter, but different waterjet orifice diameters

As the above figure shows, in order to achieve the best abrasive cutting the rate of abrasive feed must be tailored to the nozzle size and the jet parameters. The density of the abrasive in the resulting stream can be optimized for those conditions and, as discussed in earlier posts, adding too much abrasive to the system will end up being counter productive. A simple example can show this, in a test where we cut grooves in a block of granite, with the concentration of abrasive in the jet stream increasing with each pass.

Cuts made into a granite block, with abrasive feed rate increased as the cuts progressed from the left-side of the block to the right. Note that beyond a certain AFR the depth of cut begins to decrease.

Cuts made into a granite block, with abrasive feed rate increased as the cuts progressed from the left-side of the block to the right. Note that beyond a certain AFR the depth of cut begins to decrease.

Figure 2. Cuts made into a granite block, with abrasive feed rate increased as the cuts progressed from the left-side of the block to the right. Note that beyond a certain AFR the depth of cut begins to decrease. (Yazici, Sina, Abrasive Jet Cutting and Drilling of Rock, Ph.D. Dissertation in Mining Engineering, University of Missouri- Rolla, Rolla, Missouri, 1989, 203 pages.) There is, however, a second consequence to the concentration of particles across the jet, and that is that the material under the jet on either side of the center-line of the cut will see a smaller number of particles impacting the surface, than that at the center. As a result the material will not be cut as deeply, and as the slots shown in Figure 2 illustrate, the cut will, as a result taper in on both sides. In many applications, where the material to be cut is relatively thin, or where the exact alignment of the edge is not that critical this may not be important. However there are applications where edge alignment is required on the order of a thousandth of an inch or two over the part thickness, with the part being half-an-inch or more thick. One way to achieve that precision of cut is to slow the traverse speed down. If the jet is moving slowly enough then there will be enough particles hitting the material at the edge of the cut, that the edge will be cut vertically downwards.

The effect of traverse speed on the edge taper angle (in degrees) in cutting titanium.

The effect of traverse speed on the edge taper angle (in degrees) in cutting titanium.

Notice that, because the jet tends to flare out a little as it moves away from the nozzle, the taper angle goes negative if the speed falls to too low a value. In this particular case the nozzle was moving across the surface at a speed of about quarter-of-an-inch per minute. To get enough particles on the sides of the jet to cut a parallel slot edge, however, means that much of the abrasive in the center of the jet is not doing any work, but is rather being powered up and paid for to no real advantage. Thus, in most cases, (though not all) cutting very slowly to achieve precision on the residual edge of the cut is an overly expensive way of achieving the precision. Given the relatively small angle that the taper cuts it is usually more cost-effective (providing the table allows this) to slightly tilt the cutting head, so that at higher cutting speeds the taper is effectively removed on the edge that is left. Obviously the taper on the piece of material being removed is made worse, but if that removed piece is going to be cut later into a different shape for another purpose, then this excessive taper on the initial surface comes with no great cost. The taper angle and the speed relationship will vary both for the material being cut, as well as for the different parameters of the abrasive waterjet, and so – as with most cases where this sort of precision is required – a small test program to establish the best parameters for the cut will be needed. There are other ways of achieving this precision in cutting. One is to make multiple passes over the surface, with the jet removing only very small increments of material at one time. Again if this is carried out carefully and precisely the edge quality can be maintained, at the same time as the depth of cut can be well controlled allowing pockets of material to be removed from the work piece. However that gets into the whole issue of milling material from a target, and that is the topic for another day. It brings up the inter-relationship between traverse speed and depth of cut (which combine to give the area of cut surface, which can be used in some cases to optimize the cutting performance of a system, particularly where edge quality is not that rigid a requirement). And more particularly it brings up the quality of the walls and floor of the pockets created.

Factors to be considered in milling a pocket, illustrated by a multi-level pocket created in glass.

Factors to be considered in milling a pocket, illustrated by a multi-level pocket created in glass.

Figure 4. Factors to be considered in milling a pocket, illustrated by a multi-level pocket created in glass.

Labels:abrasive cutting,abrasive particles,cut taper,milling,particle distribution,traverse speed

Waterjet Technology – Higher pressure washing with power.

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.

Waterjet Technology – Higher pressure washing with power.

Happy New Year.

With record low temperatures covering most of The United States, 40 degree weather looks very appealing this week.

In the last post, on surface cleaning, I showed how the jet from a fan nozzle spread very quickly once the water left the orifice. With this spread the stream got thinner, to the point that, very rapidly the jet broke into droplets. These droplets decelerate very rapidly in the air, and disintegrate into mist which rapidly slows down. That mist has little capacity but to get a surface wet, and thus, within a very short few inches, the jet loses power and the ability to clean.

How can we overcome this? Obviously the jet would work better if it could carry the energy to a greater distance. And the jet that does that (as we know from trips to Disney) is a cylindrical stream. In some parts of the cleaning trade this is known as a zero degree jet, to distinguish it from the fifteen degree or other angular designation of the fan jet nozzles that it is often sold with.

But the problem with a single cylindrical jet is that it has a very narrow point of application. Depending on the standoff from the nozzle to the target this will increase a little as the distance grows, but is still likely to be less than a tenth of an inch. That, by itself, would make cleaning a bridge deck a long and laborious job. But consider that if we spun the jet so that it is tilted out to cover a 15 degree cone, the same angle as the best of the fan jets, the water would travel further. With a good nozzle it is possible to extend the range to 3 ft, rather than the typical 4 inches of a fan jet.

Figure 1. The gain in performance when a fan spray is changed to a rotating cylindrical jet. (initially proposed by Veltrup, these are our numbers).

Figure 1. The gain in performance when a fan spray is changed to a rotating cylindrical jet. (initially proposed by Veltrup, these are our numbers).

In both cases the water flows out of the orifice at the same volume and pressure. But with the rotating jet the water is able to carry the energy some 9 times as far. As a result the area covered is 9-times as wide, and the job is carried out faster.

You can also look at it another way. It takes only about 10% of the water and the power to clean the surface with the rotating jet, as opposed to the amount required to clean with the fan jet. This is even though the pump unit and the flow rates are the same in both cases. This is why, when you buy some of the smaller pressure washers, they include a nozzle that has a round orifice and which then oscillates within a holder. Not quite as efficient as a controlled movement, but at least it is a start.

Now, of course, life is never quite as simple as it at first appears. Because the jet is being rotated there is sometimes, if the jet is being spun fast enough, some breakup of the jet because of the speed of rotation. And so, in the above example, too high rotation speed would have a disadvantage. Doug Wright showed this in a paper he presented to the WJTA in 2007.

Figure 2. The effectiveness of a rotating jet, at two speeds and at different distances (Doug Wright 2007 WJTA Conference Houston).

Figure 2. The effectiveness of a rotating jet, at two speeds and at different distances (Doug Wright 2007 WJTA Conference Houston).

On the other hand because the jet has to make a complete rotation before it comes back to the same point on the coverage width, if the lance is moving too fast relative to that turning speed, then the jet will miss part of the surface that it is supposed to be cleaning.

I can illustrate this with a sort of an example. To make it obvious the rotating jet has enough power to cut into the material that it is being spun, and moved over. If the rotation speed is too slow, relative to the speed that the head is moving over the surface, then the grooves cut into the surface won’t touch one another and small ribs of material are left in the surface. This is not a good thing, either from a cleaning or mining perspective. The material we were cutting in this case was a simulated radioactive waste, that an improved design later went on to extract as a “hot” material in a real world project. These materials tend to be unforgiving if they are not properly cleaned off.

Figure 3. Cutting path into simulant showing the grooves and ribs where the rotation speed is not properly matched to the speed of the head over the surface.

Figure 3. Cutting path into simulant showing the grooves and ribs where the rotation speed is not properly matched to the speed of the head over the surface.

There is another answer, which is becoming more popular for a couple of different reasons. If the pressure of the water is increased, then the jet will remain coherent for a greater distance, at a higher rotation speed. Going to a higher rotation speed, also brings in an additional change in the design of the cleaning head.

 Figure 4. Cleaning head concept sectioned to show vacuum capture of the debris through the suction line after the jet has removed the material and washed it into the blue cylinder.


Figure 4. Cleaning head concept sectioned to show vacuum capture of the debris through the suction line after the jet has removed the material and washed it into the blue cylinder.

As the pressure increases, so the energy of the water and the debris rebounding from the surface increase. To a point this is good, since once they are away from the surface it is relatively simple, if the cleaning operation is confined within a small space by a covering dome, to attach a vacuum line to the dome, and suck all the water and debris into a recovery line. The surface remains relatively dry, all the water and debris is captured, and the tool can be made small enough, and light enough, that it can be moved either by a man or on the end of a robotically controlled arm. (The arm we designed the head for was over 30-ft long, which means that the forces from the jets had to be quite small).

With the higher pressure also comes the advantage that the amount of water that is required, for example to remove a lead-bearing paint from a surface, is much lower. If the water becomes contaminated by the material being washed off, then not only has the total volume to be collected, which is an expense, but it also must be stored and then properly be disposed of. And that may cost several times the cost of the actual cleaning operation, if the contaminant is particularly nasty. So reducing the volume of the water is particularly useful.

A friend of mine called Andrew Conn came up with the idea, for removing asbestos coatings from buildings, of tailoring the pressure and the flow from the nozzles, so that the amount of water required was just enough that it was absorbed by the asbestos as it was removed. Simplified and reduced the costs of cleanup, where that was a significant part of the overall price.

And speaking of using higher-pressure water, this means that there is no need for the abrasive additive, when cleaning say a ship hull. And that means that there is no need to buy, collect, and dispose of the abrasive during the operation.

Figure 5. Spent cleaning abrasive at a shipyard.

Figure 5. Spent cleaning abrasive at a shipyard.

There are other advantages to the use of high pressure water over abrasive when cleaning metal, and I’ll talk about that subject a little next time.

 

Water Blasting – Higher pressure washing with power

Dr. Summers Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Series

Water Blasting – Higher pressure washing with power

In the last post, on surface cleaning, I showed how the jet from a fan nozzle spread very quickly once the water left the orifice. With this spread the stream got thinner, to the point that, very rapidly the jet broke into droplets. These droplets decelerate very rapidly in the air, and disintegrate into mist which rapidly slows down. That mist has little capacity but to get a surface wet, and thus, within a very short few inches, the jet loses power and the ability to clean.

How can we overcome this? Obviously the jet would work better if it could carry the energy to a greater distance. And the jet that does that (as we know from trips to Disney) is a cylindrical stream. In some parts of the cleaning trade this is known as a zero degree jet, to distinguish it from the fifteen degree or other angular designation of the fan jet nozzles that it is often sold with.

But the problem with a single cylindrical jet is that it has a very narrow point of application. Depending on the standoff from the nozzle to the target this will increase a little as the distance grows, but is still likely to be less than a tenth of an inch. That, by itself, would make cleaning a bridge deck a long and laborious job. But consider that if we spun the jet so that it is tilted out to cover a 15 degree cone, the same angle as the best of the fan jets, the water would travel further. With a good nozzle it is possible to extend the range to 3 ft, rather than the typical 4 inches of a fan jet.

Rotating water blasting waterjet cutting head spray graphic

Figure 1. The gain in performance when a fan spray is changed to a rotating cylindrical jet. (initially proposed by Veltrup, these are our numbers).

In both cases the water flows out of the orifice at the same volume and pressure. But with the rotating jet the water is able to carry the energy some 9 times as far. As a result the area covered is 9-times as wide, and the job is carried out faster.

You can also look at it another way. It takes only about 10% of the water and the power to clean the surface with the rotating jet, as opposed to the amount required to clean with the fan jet. This is even though the pump unit and the flow rates are the same in both cases. This is why, when you buy some of the smaller pressure washers, they include a nozzle that has a round orifice and which then oscillates within a holder. Not quite as efficient as a controlled movement, but at least it is a start.

Now, of course, life is never quite as simple as it at first appears. Because the jet is being rotated there is sometimes, if the jet is being spun fast enough, some breakup of the jet because of the speed of rotation. And so, in the above example, too high rotation speed would have a disadvantage. Doug Wright showed this in a paper he presented to the WJTA in 2007.

Water blasting rotating jet effectiveness

Figure 2. The effectiveness of a rotating jet, at two speeds and at different distances (Doug Wright 2007 WJTA Conference Houston).

On the other hand because the jet has to make a complete rotation before it comes back to the same point on the coverage width, if the lance is moving too fast relative to that turning speed, then the jet will miss part of the surface that it is supposed to be cleaning.

I can illustrate this with a sort of an example. To make it obvious the rotating jet has enough power to cut into the material that it is being spun, and moved over. If the rotation speed is too slow, relative to the speed that the head is moving over the surface, then the grooves cut into the surface won’t touch one another and small ribs of material are left in the surface. This is not a good thing, either from a cleaning or mining perspective. The material we were cutting in this case was a simulated radioactive waste, that an improved design later went on to extract as a “hot” material in a real world project. These materials tend to be unforgiving if they are not properly cleaned off.

Water blasting grooves in rotation speed

Figure 3. Cutting path into simulant showing the grooves and ribs where the rotation speed is not properly matched to the speed of the head over the surface.

There is another answer, which is becoming more popular for a couple of different reasons. If the pressure of the water is increased, then the jet will remain coherent for a greater distance, at a higher rotation speed. Going to a higher rotation speed, also brings in an additional change in the design of the cleaning head.

Water blasting cleaning head vacuum capture of debris

Figure 4. Cleaning head concept sectioned to show vacuum capture of the debris through the suction line after the jet has removed the material and washed it into the blue cylinder.

As the pressure increases, so the energy of the water and the debris rebounding from the surface increase. To a point this is good, since once they are away from the surface it is relatively simple, if the cleaning operation is confined within a small space by a covering dome, to attach a vacuum line to the dome, and suck all the water and debris into a recovery line. The surface remains relatively dry, all the water and debris is captured, and the tool can be made small enough, and light enough, that it can be moved either by a man or on the end of a robotically controlled arm. (The arm we designed the head for was over 30-ft long, which means that the forces from the jets had to be quite small).

With the higher pressure also comes the advantage that the amount of water that is required, for example to remove a lead-bearing paint from a surface, is much lower. If the water becomes contaminated by the material being washed off, then not only has the total volume to be collected, which is an expense, but it also must be stored and then properly be disposed of. And that may cost several times the cost of the actual cleaning operation, if the contaminant is particularly nasty. So reducing the volume of the water is particularly useful.

A friend of mine called Andrew Conn came up with the idea, for removing asbestos coatings from buildings, of tailoring the pressure and the flow from the nozzles, so that the amount of water required was just enough that it was absorbed by the asbestos as it was removed. Simplified and reduced the costs of cleanup, where that was a significant part of the overall price.

And speaking of using higher-pressure water, this means that there is no need for the abrasive additive, when cleaning say a ship hull. And that means that there is no need to buy, collect, and dispose of the abrasive during the operation.

Spent water blasting cleaning abrasive at a shipyard.

Figure 5. Spent cleaning abrasive at a shipyard.

There are other advantages to the use of high pressure water over abrasive when cleaning metal, and I’ll talk about that subject a little next time.

Labels:abrasive cleaning,cylindrical jets,fan jets,pressure washing,radioactive waste,range,rotating jets,rotational speed,traverse speed