INTRODUCTION
The designer of a bit for percussive drilling in rock must take into account four general areas:
Cutting structure
Flushing
Cutter and body metallurgy
Connection to the drill rod.
Of these the cutting structure design is the most critical in determining bit performance in terms of penetration rate and is very important in terms of bit life as well. There are only two types of cutting elements ordinarily used in percussive drill bits, wedges and hemispheres. It is a bit paradoxical to note that while the designer of a wedge bit has very few degrees of freedom (the wedge angle within a relatively limited range and the number of wedges which is usually one or four) there is an abundance of literature giving data on wedge indentation of various rocks and the effects of such parameters as wedge angle, wedge length, and indexing angle. Conversely, the designer of the so-called "button bit" has an almost unlimited amount of freedom in the radius of curvature of the indentor (and more than one size can be used on a bit) the number of indentors, their placement on the bit face, and even their exposure or projection from the bit body, there exists very little in the literature to aid him with this task.
Penetration rate can be maximized in two ways, by improving the energy transfer from the bit to the rock and by enhancing the interaction between indentors and previous craters in the "indexing" effect. Energy transfer is a function of the shape of the stress wave generated in the drill steel by piston impact and of the impedance or relationship between force applied to the bit and displacement into the rock. This force displacement relationship is in turn a function of the rock properties and the indentor geometry. Thus for a given drill operating in a given rock the energy efficiency of each percussive blow is a function only of the geometry of the cutting elements of the bit.
BACKGROUND
Button bits for percussive drilling were introduced for large down-the-hole hammers in about 1960 and were first made available for drifters in hole sizes below 2 inches in 1964. In both down-the-hole and larger diameter drifter drilling they primarily replaced "x" bit (4 wedges on wings at unequal angles) for drilling in hard rock. Their advantage was in maintaining satisfactory penetration rate over a long period of use without the frequent resharpenings required by wedge bits. This advantage was traded for a higher initial cost, a lower total life (assuming several resharpenings for the wedge bit), and, often, a lower initial penetration rate. In the smaller sizes button bits replaced "cross" bits (4 wedges at equal angles) for underground blast hole drilling. Early designs, however, were simply scaled down from the larger bits, which made the inserts much too small and fragile for this service.
Acceptance of button bits by the mining industry has been a gradual process over the past 15 years. There have been considerable improvements in both cutting structure design and metallurgy.
