Waterjetting 24c - angled jets in cutting concrete

In this short section of the series I have been discussing some of the issues that relate to cutting through concrete. In today’s piece the discussion will continue, focusing on the angles that the jets are set at, when making repeated passes over an area to deepen the cut. The basic premise of the discussion holds true regardless of jet pressure, provided that the concrete is being removed by a moving continuous jet stream, rather than a pulsed jet system, which I will discuss next time.

As was mentioned at the beginning, the main way in waterjets remove concrete most efficiently is by washing out the cement around the individual particles of the aggregate, which, in turn, causes the particles to fall out of the slot, since they are no longer supported.

If a cutting head is built with the jets pointing vertically downwards, so that, as the head moves, so the jets spin over the surface and wash out the cement over a wider path, any cement that underlies a particle is not removed, and the particle remains held in place by the underlying cement column.

Regardless of how the nozzle is moved over the surface, with only vertical jets the path of the assembly very rapidly becomes blocked, and the nozzles can no longer move into the slot to deepen it.

The obvious solution to this is to incline the nozzles so that as they rotate over the surface, they can reach under individual particles and wash out the cement beneath them, removing their support. This also has the advantage of cutting a path into the concrete that is wider than the cutting head itself, so that on later passes the head can be lowered into the cut, shortening the standoff distance to the fresh surface and improving cutting efficiency.

Moving the head down a little on each pass also has the advantage that it exposes fresh layers of cement to the jet action and makes it more likely that all the cement within the desired slot is removed (and the aggregate with it) leaving a clear path for the assembly to move deeper into the slot.

So the question then arises as to what the most efficient angle is to tilt the nozzles to, relative to the perpendicular axis of the target surface. (I use that awkward phrase because not all targets are going to be flat horizontal bridge or garage decks).

Very shallow angles don’t work very well. The best demonstration of this was when we started cutting slots in granite, with an initial divergent jet angle of around 8 degrees. After the first few passes we noted that the slot was developing walls that sloped into the cut. As a result the slot was getting narrower with depth, and the nozzle assembly would no longer be able to move into the cut.

Figure 1. Tapering cut into granite. The nozzle had been advanced about a third of an inch after each pass of a dual-nozzle rotating head. Nevertheless the cut tapered as the cutting continued.

We had chosen that initial angle because it worked well when cutting slots in coal, but clearly in harder, less jointed material that was not the case. And so we, and others, have carried out tests to find out what the best angle would be for the cutting tests.

And, before I show the results, let me emphasize that these only hold true for a certain concrete mix. Where aggregate particle sizes are larger, the jet angle may need to change to make it easier to get around. The pattern of the jets on the surface, (affected by the ratio of the rotation speed of the head relative to the movement of the entire assembly over the surface) and the jet parameters themselves (jet pressure, nozzle diameter and standoff distance) also play a part. In this latter regard remember that the effective range for many waterjet streams is not that much more than a hundred diameters from the orifice, so that expecting some of the smaller nozzle sizes (say 0.005 inches) to cut cement more than half-an-inch from the nozzle may be an exercise in futility – and raising the jet pressure in that circumstance is unlikely to fix the fact that the target is simply out of range.

So, with those caveats, here is the result that was obtained by Puchala, Lechem and Hawrylewicz*:

Figure 2. The effect of nozzle angle on cutting performance in removing concrete (*Puchala, R.J., Lechem, A.S., and Hawrylewicz, B.M., "Mass Concrete Removal by High Pressure Waterjet," Paper 22, 8th International Symposium on Jet Cutting Technology, Durham, UK. September, 1986, pp. 219 - 229.)

Nevertheless it is clear that there is much better performance where the jets are inclined at an angle between 25 and 35 degrees to the normal to the target surface. This is reflected in the improved efficiency of cutting (as shown by the second line in figure 2, showing a more significant change with angle than is evident from the depth of cut measurements). In all cases we have found that the jet angle needs to be 15 degrees or greater to make sure that wall taper does not occur.

Correlating the rotation speed against the traverse speed of the head over the surface to find the optimal cutting performance is a little more difficult, and should generally be assessed for given concrete targets with a short test run, before the major effort is undertaken. One reason for this is the wide range in performance that can be found with different cements. We have worked with cement that was sufficiently weathered that it could almost be removed using one’s hands, on the one hand, and the new cements that contain silica fume, or small wires or fibers pose a different and more difficult challenge on the other.

This also holds true over setting the advance rate of the nozzle assembly into the slot after each pass (where the head is cutting in a series of passes to penetrate the slab). Here the advance is going to be controlled in part by the size of the aggregate, though it should be noted that even with little apparent progress the nozzle assembly should be advanced after each pass, since this exposed a fresh layer of cement to attack, and this will lead to more aggregate release and help in clearing the cut.

Waterjetting 16c - Optimal AFR and cutting curves

The discussion on surface quality which forms this month’s topic has, to date, focused on linear cutting since this has been the simplest way of explaining some of the factors that go into choosing an optimum abrasive feed rate (AFR) for a system. Along the way, however, I have pointed out that the internal design of an abrasive nozzle has a considerable impact on the relative performance of different systems.

If, for example, the internal geometry is such that there is not an optimal transition of energy between the high-pressure waterjet stream and the abrasive particles, then trying to draw conclusions over the influence of some of the operational parameters, such as pressure, can lead to false conclusions. The optimal AFR changes with the relative sizes of the waterjet orifice, the location of the abrasive feed line, the length of the mixing chamber and the geometry of the focusing tube. These parameters are generally held fixed since most folk buy only one cutting head design, and tend to stick with it once purchased. However, as I pointed out at the beginning of this blog, there is a considerable difference between the performance of different abrasive cutting heads.

Figure 1. Comparison of the relative cutting performance of twelve different abrasive nozzle designs, when operated otherwise at the same pressures, water flow rates and AFR.

The best design, for the particular waterjet and AFR parameters that were tested in generating Figure 1, was 24% more effective than the average performance of the nozzle designs tested. This is indicative that the design was more efficient in accelerating the abrasive to a higher velocity than the competing designs. Those designs were tested at a number of pressures and AFR values to ensure that the conclusions held within the range of test – and they did. But as the pressures and AFR values change so there is a change in the optimal design with consequences on the optimal AFR as it relates to the operating pressure of the system.

Without an awareness of these inter-related parameters it is possible to draw erroneous conclusions about the best choice of cutting parameters for a given operation. The situation becomes even more complex where the paths being cut are no longer straight but involve complex contour cutting, and where there are requirements for zero taper and high surface quality on the cut faces of the part being generated.

One solution to the problem is to accept the limitations of the system, and cut the part at a constant speed, slow enough that the jet cuts through the piece on first contact with the abrasive stream over the length of the cut. (In other words after the abrasive bounces away from the initial contact plane along the cut it does not meet any more material before it exits from the bottom of the cut). At a pressure of 40,000 psi the cutting speed to achieve these requirements over an half-inch thick titanium target lies at around 0.3 inches per minute.

Figure 2. Change in cut face taper angle with traverse speed at a cutting pressure of 40,000 psi.

However as the pressure of the jet is increased the cutting speed to sustain that quality cut goes up significantly, so that there is a significant benefit to the increased pressure. But the optimization to achieve this is geared to ensuring that the optimal abrasive feed rate has been selected, for a given nozzle design and waterjet pressures. Without a short series of tests to ensure that the system is being run at this optimal condition it is not possible to accurately state how a system can best be used.

I have described, in an earlier post, how such a simple test can be run. It should be stressed, however, that the selection of an optimal AFR for a nozzle is based on the nozzle geometry and the operating pressure of the system. That selection will provide the best cutting jet and this jet will have different capabilities in different target materials. Composite materials will cut at a different optimal speed depending on the material type and thickness, and these values will differ when metals, or ceramic materials are being cut. But, as a general rule, the selection of the best cutting conditions are first established by knowing the thickness and type of material to be cut. This should then produce, based on tested performance tables, recommendations for the cutting speeds at different pressures, where the cutting pressure in turn defines the optimal abrasive feed rate. Based on an assessment of the different categories of cost of an individual operation one can then decide which set of conditions would provide the most economical and acceptable answer to providing the quality of cut required.

In some cases it may be that the cutting head can be tilted so that, particularly with straight cuts, the part being isolated will have a perpendicular edge, while the scrap piece will have a tapered edge at twice the normal angle. For example under the conditions illustrated in figure 2 tilting the nozzle by only one degree will allow cutting at 4 ipm rather than 0.3 ipm, a 12-fold gain in performance, depending on the assurance of the quality of the surface being sustained.

As mentioned earlier this option becomes more difficult as the part being cut acquires contours. At higher pressures the angle of the cutting face curve is reduced, but in thicker parts there is often a slight displacement backwards (a rooster tail as it is sometimes called) from the top edge of the cut to the bottom. When the nozzle comes to cutting around a curve that backward projection at the bottom of the cut can pull the cut edge away from vertical unless the cutting head is adjusted to ensure that this difference is minimized to the levels acceptable to the customer. Most commonly this is achieved by slowing the head speed according to the radius of the curve, with sharper turns being made at slower speeds. Some adjustment in the angle of the head can also be made, but this requires a more advanced method of control and programming in developing the cutting path for the head.

Note: Because of the season this site will be Dark next week, so let me take the opportunity of wishes the readers of the waterjetting series all the Compliments of the Season, and with hopes that you have a Prosperous and Happy New Year.

Waterjetting 11d - AWJ and cut taper

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.

Figure 1. Particle distribution across two abrasive waterjet streams with the same focusing tube diameter, but different waterjet orifice diameters (Mazurkiewicz, M., Olko, P., Jordan, R., "Abrasive Particle Distribution in a High Pressure Hydroabrasive Jet," International Water Jet Symposium, Beijing, China, September, 1987, pp. 4-1 - 4-10.)

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.

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.

Figure 3. 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.

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