What Does Drag Bit Mean?
A drag bit is a large circumference bit used to enlarge a bore. The bit, as its name implies, is dragged back and forth through the bore, grinding another bit off with each pass until the bore is the correct diameter.
Designed for use in soft formations such as sand, clay or some types of soft rock, they may not work well in coarse gravel or when forced into hard rock formations, although, some drag bits are now made with tungsten carbide teeth, which may help ease this problem
Trenchlesspedia Explains Drag Bit
The drag bit has a central core, with at least three, but as many as eight, triangular cutting surfaces. The cutting edges of all or at least half of the total number of the bit's teeth face the forward end of the bit. Any other cutting edges face the drill string, so that the bit may be used while moving in either direction.
Used to enlarge a small bore, such as that created by a pilot bit in the guided boring method, the drag bit is attached to the drill string and traverses the bore in a back-and-forth motion. As the bit turns, the bit's teeth grind away at the sides of the smaller bore until it reaches the diameter specified in the design parameters.