Rotary drilling with fixed blade (drag) bits has long been practiced by the mining and petroleum industries, and considerable study has been given to defining their cutting action in terms of the pertinent variables.
Fairhurst, and later Fairhurst and Lacabanne, made illuminating studies of drag bit drilling, which showed the effects of certain bit design variables on both percussion and drag bit devices. They employed photographic techniques quite similar to those of this work except at much lower speeds. It was shown in their work that drilling of rock in the brittle state was a cyclical process in which instantaneous loads on the bit varied from some maximum value to near zero.
Goodrich has presented further data on the subject as well as a qualitative description of this drilling process. Again the postulated failure mechanism is periodic with alternate chipping and crushing periods. According to Goodrich, the volumes removed by the grinding and chipping phases are approximately equal.
The same basic "drilling" problem occurs in the metal cutting industries. Here the literature is more voluminous but the state of knowledge is still not very advanced, as may be discovered from a recent and comprehensive survey of their literature. Metals, however, are more subject to analysis by the classical mechanics of elasticity and plasticity theories than are rocks. It is interesting to note, however, that Hill prefaces his discussion of machining with an admission that the solution to the real problem is not known. A photoelastic solution for the stress distribution ahead of a metal cutting tool appears in the classical book of Coker and Filon.
No fundamental analysis of drag bit drilling appears in the oil industry literature; however, several investigators have studied the hydraulic aspects of cutting removal. Recent developments in drag bit design have renewed oil industry interest in the problem. Furthermore, it is well known that roller bits also employ varying degrees of drag bit action; hence any improvements in basic knowledge can also be applied to their design.
This paper presents preliminary experimental results of two dimensional cutting of some sedimentary rocks by a fixed blade. The rock samples were unconfined in all tests. All "cuts" were horizontal; hence the true helical path of an actual drill bit was not simulated.