Wednesday, December 18, 2013

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.