Saturday, October 25, 2014

Waterjetting 26c - Cutting tool shape

When it was first discovered that high-pressure waterjets could significantly improve the performance of mechanical cutting tools, whether in machining metal, or in cutting rock, it was anticipated that this would have a broad-ranging application. This has not been the case, and the reasons are varied, depending on the application, but quite often they relate to the way in which the mechanical tool was expected to work. The examples will, again, come from rock cutting, but also apply when cutting or machining other materials.

Figure 1. Three common types of picks used in cutting into stone for driving tunnels, or for cutting and mining coal.

The initial work of Mike Hood, in cutting quartzite, had used a relatively simple flat-faced bit that was dragged across the rock surface at a known depth of cut and directing s single, or pair of jets to cut along the line of contact between the rock and the carbide was relatively straightforward.

Figure 2. Locations of the jets for Mike Hood’s initial tests on improving jet performance. (Dr. Hood)

Getting the jets to cover the full zone of contact and rock crushing was critical to achieving the best results for the tests, and proved also effective when the cutting tools were tried in the field.

Figure 3. Relative normal forces on a cutting bit with change in the position of the jets assisting in the cutting of rock (Dr. Hood). Note that the machine stalled at 4 mm penetration without the jet assist (the black line).

The cutting picks that are more commonly used in softer rock, shown in figure 1, are not quite the same shape, nor have quite the same purpose. Early trials were with the forward attack pick, which through the early 1980’s was the most common design used.

Figure 4. Laboratory trials with a jet added to a forward attack pick

Rather than having a flat face, this pick has a wedge-shaped front face. This is so that, as the pick cuts into the rock, so the wedge shape pressing into that groove will put a high lateral load on the rock on either side of the cut, causing it to shear off the solid. Those chips can be seen to the front right of figure 4.

Where the jet cuts into and removes the crushed rock under the front of the bit, this allows the bit to make a deeper bite, and this, in turn, makes it more likely that the tool will make larger chips. This is not an unflawed benefit, particularly if the jetted slot now extends a little deeper than the tool.

Figure 5. Illustrating the wedging action of the tool in creating lateral chips beside the tool.

As the chips get larger so the force required to break them from the solid increases, and the actions do not occur symmetrically on each side of the tool. As a result the lateral loading on the tool becomes more significant, and because the process of chip forming and breakage is cyclic so there greater fluctuating forces make their way through the drive train back to the driving shaft and motors.

With most machine designs these fluctuating loads are, however, reduced in overall magnitude, because of the reduced forces needed to move the pick forward, without having to deal with the crushed material under the pick, which the water jet has removed, providing it is within about 1/10th of an inch of the cutting tool.

Achieving that positioning becomes a little more difficult with the transition to a radial pick, however there were additional problems with that intermediate design, particularly in harder rocks, where they wore out at rates as high as 7 picks per foot of advance. This led to the development of the point attack pick, as shown in figure 1.

This pick design has become the most popular for use in mining machines over the past 20-years. The round shape of the tool and shaft are designed so that, as the pick wears it will rotate in the holder, and this will spread the wear evenly around the tool, making it last longer – and in the case mentioned in the last paragraph a change to this design reduced pick costs to around 1 pick per foot of advance. But there are a couple of problems with adding waterjets to this tool.

Figure 6. Point attack tool geometry (Goktan and Gunes)

This geometry makes it very difficult to bring in a waterjet to hit the right point at the rock:tool contact, because of the double cone at the end of the tool. While the nozzle can be positioned so that it can direct a jet into the right point (for example by being at the point where α is in the figure) the problem arises with the size of the nozzle mounting block, and the small size of the jet, where a large number are being used to cover all the picks on the cutting head and total flow volume is limited. To fit the nozzle block means that it must be at a greater standoff distance from the point (perhaps four or five inches), while the small orifice size means that the effective range of the jet may be no more than an inch or two.

The change in pick design and the difficulty in adding waterjets to the new tool therefore led to a discontinuation of the trials of the combined system. This was unfortunate since the forward attack picks initially cut better than the point attack, but wore out more rapidly – hence the change. But with the addition of the waterjets the tool lifetime, and sharpness, was increased in some cases more than five-fold times, while the other benefits – such as the ability to use smaller machines to carry out similar performance – made capital investment less. But these events occurred at the wrong time, as the coal market was entering one of its down cycles as the developments were being made, and the technology was therefore not adopted.

I will conclude this small chapter next time, by addressing one of the answers to the problem of getting water to the point attack tool.

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