The drag bit is the oldest rotary tool still used by the drilling industry. These bits are currently manufactured in several different designs with various cutler blade quantities and shapes (Fig, 7-3). The cutting blades arc integrally made with the bit body or are fixed to it and rotate as a unit with the drillstem. This bit is used primarily in soft and gummy formations. Optimum performance is attained by arranging the hydraulic equipment and selecting nozzle sizes so maximum available hydraulic horsepower is expended at (he bit.
Metallurgy has played an important role in improving current bit designs
Fig. 7-3 Drag bit over the early two-blade fishtail bit. Some of the improvements causing continued success in certain soft formations arc the following:
Rolling Cutter Bits
The emergence of rolling cutter, or cone, bits has virtually eliminated the use of drag bits due to overall versatility. The rolling bit can drill all types of formations and can be altered lo achieve special functions, such as directional control or coring. In addition, technological advances have extended bit operating lives and permitted the bit to drill long formation intervals.
The basic design of the rolling cutter bit consists of the body, cones, and internal flow paths to allow fluid circulation. The bit body generally consists of 1) the shank, 2) bit legfs), and 3) journal pins for cone attachment. Cones are the actual segments of the bit that contact the formation, causing rock failure. The cone may have steel teeth forged as an integral part of the cone or tungsten carbide teeth inserted into the cone for increased tooth life. Flow paths within the bit let the drilling fluid circulate to achieve special functions such as hole cleaning and bit cooling.
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