Saturday, July 12, 2014

Waterjetting 23b - From DIAjet to Abrasive Slurry Jetting (ASJ)

When the Direct Injection of Abrasive jet (DIAjet) was first introduced to the general public, back in 1986, there was some initial skepticism as to the overall market potential for the system. Certainly, as the next post will discuss, the ability to transfer higher levels of energy from the pressurized water to the entrained abrasive with no particle fragmentation in the mixing chamber, had a number of advantages, that I will spell out below, but the long-term problem was to develop a method of sustaining a constant abrasive feed in the mix, critical to high precision cutting, while concurrently having a system that can run continuously both day and night. To my own knowledge there have been at least two different designs of ASJ system that have solved this problem, but the market was damaged by the early problems in sustaining continuous flow to the point that customers shied away from this advanced technology, even where it showed some considerable commercial advantage.

The first, most obvious advantage to this new tool was in the smaller cut width that it generated, relative to conventional Abrasive Waterjet Cutting (AWJ). And because DIAjet became known as the original BHRA technology, and there were competitors over the years, let me re-name the technology (as the WJTA did some years ago) as Abrasive Slurry Jetting (ASJ).

Figure 1. Cut size difference between ASJ and AWJ as an illustration. The cuts are in Plexiglas, with the ASJ cut on the left.

The reason for the difference in cut width is, as explained in the last post, that the volume of the cutting jet is cut by 90% when the air carrier for the abrasive is no longer necessary or present in the jet stream. This increase in jet “delicacy” can be illustrated by a small example, and a humorous competition between Don Miller and ourselves back some years ago.

Don was, as the technology evolved, one of the master players in moving the technology toward the micro-cutting market that, to this day, remains remarkably under-exploited.

Because the abrasive particles accelerate to a large extent with the water that is both the cutting and carrier fluid, the cutting ability of the jet is significantly less sensitive to the diameter of the nozzle than is the case with the conventional AWJ. And, because the waterjet is not disrupted within the mixing chamber as a way of helping mix the abrasive with the water, so the jet stream can be kept convergent away from the nozzle, increasing the range, as I will discuss further in a later section.

The high precision cutting, using a finer abrasive since the nozzle diameter is smaller, can create very delicate pieces. We had been asked to use our system to cut jewelry out of silver, since while the ASJ could cut this easily and quickly to the desired shapes (matching necklace and earings) trials with laser cutting had been less successful because of the high conductivity of the metal.

This led on to a demonstration of the precision of the cut that can be achieved. One of the early models cut with an AWJ had been of a dragon, it seemed to be a good idea to match this with a knight on a trusty steed.

Figure 2. Knight vs dragon, in this case the knight was cut with an ASJ, while the dragon was cut with an AWJ system.

Don Miller had put together his precision system in his garage, and was able to control the cutting ability with an on-off switch located upstream of the nozzle, using sliding diamond coated plates to ensure a seal, without the wear problems which are common when trying to valve an abrasive laden flow.

Figure 3. Don Miller’s cutting equipment with precision table. Don Miller).

The need for the rapid on-off design comes where a series of holes must be punched into the target metal in order to effectively create a screen, or similar device. Pratt and Whitney engineers had used a somewhat different concept, and had held the abrasive in a polymer. This is sometimes necessary when using the ASJ system, when the cut-off in flow to the nozzle occurs with abrasive in the feed line from the reservoir to the nozzle itself. If there is any significant delay before flow restarts then the abrasive will settle to the bottom of the containing pipe. When flow then restarts this initial plug of abrasive is picked up by water flow and can block the nozzle when it reaches it. The use of the polymer holds the abrasive in suspension to prevent this happening. (We have seen abrasive held in suspension for over a week, using relatively low concentrations of polymers such as those used to suspend fracking sand for the oil industry).

Figure 4. The diamond-coated valve used by Don Miller to control flow in an ASJ system. (Don Miller)

The risk of abrasive build-up is increased when the shut-off valve is directly behind the nozzle, or where it is mounted vertically, when a polymer is not used, but where the jet is cycling rapidly to drill a precise series of holes in a rapid sequence, then the issue of nozzle blockage doesn’t arise as much.

Figure 5. A grid of 85-micron diameter holes drilled at 2.5 holes/sec at a jet pressure of 10,000 psi (Don Miller).

When we discussed the relative scaling that could be achieved with the technique, Don’s answer to the thickness of the lance that we had given the knight was to put scales on his dragon.

Figure 6. The scales on Don Miller’s dragon. The picture width is around 1 mm.

I had promised to go back and put eyelashes on the horse, but somehow we never managed to find the time.

The delicacy and accuracy of the technique is in marked contrast to manufacturing techniques other than those using abrasive-laden water as a cutting medium. Not only is it possible to cut through metals and other materials without distortion, even with very narrow webs left holding the pieces together, but since there is no heat involved in the cutting process, the precision is retained over the cut and part, after completion. This is illustrated with Don’s construction of a butterfly wing through 150 micron thick stainless steel. (The scale on the illustration shows mm).

Figure 7. Detail of a butterfly wing cut by Don Miller, using his ASJ system (Don Miller)


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