Diamond Compact Cutter Studies for Geothermal Bit Design

As a prerequisite to design of a geothermal drill bit using diamond compact cutters, the rock cutting characteristics of these compacts have been studied in a series of instrumented single-cutter continuous and interrupted turning tests on hard, high-compressive strength sandstone and other rocks of known properties. A range of compact rake angles was used and cutting forces were measured. Rake angles affected cutting efficiency (specific energy) at light cuts but were less significant at high penetration forces more characteristic of bit weights used in drilling. In continuous turning, cutter forces exhibit regular, repeating variations probably related to the alternate crushing and fracturing modes which the rock undergoes. In interrupted cutting, forces drop to zero, as expected, while passing through a void but on subsequent impact the forces are no greater than measured in continuous cutting. Thus, force fluctuations when cutting a composite of hard and soft rock should not be significantly greater than those generated in cutting each rock separately. Of considerable interest is the fact that cutting forces were found initially to be independent of speed over the tenfold speed range and low depths of cut covered. More sophisticated cutting experiments covering a wider range of conditions and rock types showed a significant effect of speed on force. However, the effects of changing speed were not as great as those of geometry and depth of cut. The results of these experiments have been used in generation of a preliminary bit design.