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A drag bit is simply a series of blades (usually three or four in number) which produce a shearing action when rotated. They are comparatively inexpensive due to the lack of moving parts and they can penetrate softer formations such as stiff clays and mudstones very quickly.

The drag bit usually used in very soft to medium formation when the dth hammer as the tricone bit, lose of their effectiveness.

The cleaning of the hole is realized by the injection of air or water.

There are two forms of drag bits:

- "Step" type for soft grounds

- "Chevron" type for medium hard grounds

Drag bits come in two designs: step and chevron. Step drill bits are designed to drill soft to medium formations while chevron type bits are forharder formations.

A step drill bit is used for drilling overburdens withsands, clays and soft formations. These bits will produce larger cuttings than a Chevron style bit.

Because the step drill bit is more aggressive than the Chevron, ROP will be higher. However, it will be less durable as well.

Rotary table speed should be between 60 and 80 rpm with light weight on bit (WOB).

Chevron drill bits are used for drilling more abrasive formations like soft sandstones and shales.

The Chevron style bit is more durable than the step drill bit but, because of cutting structure, will penetrate more slowly.

Rotary table speed should be between 50 and 60 rpm with light to medium weight on bit (WOB). Harder formations require more WOB.

Simply put, 4 wing bits are the most durable AND have the lowest ROP.

When making a decision on which bit to buy, however, don't purchase a 3 wing just to be able to drill faster. Always remember that damaged carbide cutters slow penetration rates.

Additionally, 4 wing bits are more stable and tend toward drilling a straighter hole.

Crystal

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