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|Introductory remarks Dr N. G. W. Cook: This paper was published in your Journal in May of this year and you will, therefore, have had the opportunity of reading it over the past six months. I can state that experience has shown that the paper is substantially correct, but a number of questions have to be answered. The first question, Mr President, that I would be happy to answer, is how is it that one is able to cut hard, abrasive quartzite with a strength some six times greater than that of good concrete? The essence of the answer is that there is a tremendous difference between abrasion, or scratching, and cutting, or chipping. For example, it is well known that a diamond will scratch or abrade almost anything. It is also relatively easy to chip, crack or crush a diamond. As a demonstration I have here an ordinary bench grinder, and with the bench grinder I also have a piece of tool steel. The wheel on this bench grinder is a silicon carbide wheel, which is an extremely hard material, and the tool steel is much softer. In rock-cutting we use tungsten carbide for cutting the quartzite. The quartzite and the tool steel have roughly the same hardness. You can sharpen tungsten carbide with a silicon carbide wheel. In fact, this is done. It won't surprise any of you, therefore, when I show you that tool steel can be sharpened with silicon carbide. Now, what I propose to do is to change the configuration of this experiment and use the soft tool steel to cut silicon carbide, and if I can do that, I think you will agree that I have gone some way to being able to cut quartzite, which is about as hard as tool steel, with tungsten carbide, which is harder than quartzite. Let me do this. (Demonstrates.) That, I think, is a very clear demonstration of how it is possible, with a metal, to cut an extremely hard and brittle material. What, essentially, is the difference that we have here? In the first case, I held the tool with a relatively low force against the hard silicon carbide wheel. This generated a relatively low average stress, which was insufficient to fracture the carbide wheel. Nevertheless the protuberances on the silicon carbide wheel indented the tool steel and abraded it at a multiplicity of points, and quickly reduced the section of the tool steel that I held against the wheel. By changing the configuration and clamping my tool steel in a firm post, against the silicon carbide wheel, I changed these conditions very extensively. I now had a high force. I had to exert a considerable moment on that lever. The tool was anchored so that it could sustain these high forces, and they generated high stresses, which were adquate to fracture the silicon carbide. Now, what about wear? It so happens that, in these two experiments I have just done, the power I was using was about the same-a small fraction of a horse power.|