It is generally believed that, when excavating soft to medium strength, non-abrasive materials (i .e., when drag bits will survive economically), the kerf and break technique is one of the best means available. In this method, rather widely spaced, deep slots, or kerfs, are cut, deliberately leaving intervening material to be broken out in a subsequent operation. Growing interest in machine excavation of stronger materials (i .e., non-explosive excavation of rock) has renewed interest in this technique to minimize total energy consumption and, perhaps more importantly, to minimize cutting force required and to reduce cutting tool consumption. It can be argued, for example, that cutter consumption, a major operating cost item, must be roughly proportional to cutting energy consumption. More recently, deep slot cutting using drag bits in conjunction with moderate pressure water jets has demonstrated some rather outstanding performance in very hard and abrasive materials, no doubt adding to the current interest in applying drag bits in materials stronger than coal and potash.
Smaller, more flexible machine, roughly in proportion to installed power;
Lower cutter forces (and again, smaller machines);
Fewer cutters and/or more slowly moving cutters to provide the same total excavation rate (as compared to e typical continuous coal miner for instance);
Larger product size with lower percentage of fines and less airborn dust;
Quieter operation.
Borer type coal mining machines are perhaps the most familiar kerf and break type machines, but a variety of excavation geometries is possible. In general terms, and independent of the exact geometry of the machine involved, it is clear that direct and energetic attack on only a small fraction of the material being excavated has some advantages, at least in principle. These include:Note that the stated advantages usually involve overall machine and/or operating parameters and not just the cutting structure itself. For example, the observation that fewer and more slowly moving cutters will sweep out the same total volume, a simple geometric fact, influences cutter economics, machine forces and component sizes, tool temperature and wear rate, dust generation, and, in some cases, the tendency to cause ignition of any gas present. Thus the study of drag bit cutter performance should ultimately embrace the entire machine design, even though it may begin with an examination of cutter rock mechanics. As examples of the types of questions that need answering, is it better to deliberately preserve a deep slot geometry and aviod premature break-out of intervening material, thus minimizing volume directly attacked while perhaps expending extra energy per unit slot volume? Or is it better to cut so as to encourage random early breakage, as is presently the case with coal mining machines? From an energy and tool wear (or at least tool cost) point of view the choice is probably not critical in friable material such as coal * but the choice is not so clear in relatively tough materials such as oil shale.
