WorldWide Drilling Resource

Drag Bit Basics by Chuck Mills, Owner and President Mills Machine Company Drag bits for rotary drilling have many different names: wing bits, blade bits, slug bits, and kelly drag bits. I’m not exactly sure where the names originated, but assume it’s derived from the cutting action as the bit is rotated and the blades drag along the bottom of the hole creating a shearing and scraping action into soil and rock. There are many different shapes and configurations of drag bits that have been designed to cut different soil and rock formations more efficiently. Regardless of design, all drag bits feature a leading and trailing edge to facilitate efficient drilling and spoils removal. Initial designs incorporated anywhere from 2-6 blades on a bit with hardened steel blades. Later, a crushed carbide composite was applied with a torch onto the leading edge of the bit for increased durability. Modern technology has improved the drilling footage for drag bits with the advent of sintered tungsten carbide inserts (TCI) and now polycrystalline diamond compacts (PDC) which are torch brazed with nickel, silver, or brass. Most of the modern-day drag bits feature 3-4 blades fabricated to a bit shank and have TCI brazed onto the cutting edge. The primary style or shape of the blade is a step or chevron/apex design. The step design starts with an apex at the point, then tapers down and out towards the gauge, or outside cutting diameter of the bit in a stairstep fashion. Then, inserts are brazed down the edge to protect the gauge diameter. The steel blades are typically made from steel plate anywhere from ½-1- inch-thick steel, although some larger bits can go up to 2- or 3-inch-thick steel. The shape of most drag bits will fe ature a point and then taper back, which helps the bit stay centered in the hole. Some drill operators like to drill a small pilot hole, then stack o ne or more drag bit reamers on top of the pilot bit to drill a straighter hole. O r if their drilling rig lacks enough power, they can open the hole increme ntally using this method. The step-type drag bits are designe d primarily to cut soft to medium soil and rock including clay, sand rock, sandstone, and some shale formations. They featur e the step-type inserts which produce small cuttings in a chipping fashion and pene- trate very fast. The only downside is th e corners will break off in harder rock. Thicker tungsten carbide inserts and thicker steel blades can extend the life of these bits, but ultimately you will have to use a different design to cut harder formations. Another type of drag bit for harder formations is the chevron- or apex-style blade which has a rugged, thicker piece of carbide brazed onto the cutting edge and resembles the shape of the roof on a house or the shape of an army pri- vate’s stripe or chevron. This design has more of a crushing action and as a result, will not drill as fast as the step- type, but will hold up longer in harder soils and rock. The tungsten carbide inserts and the steel blades are usually thicker than the step-type design. Drag bits are used throughout the drilling industry including mining, con- struction, exploration drilling, and even in water well applications. 26 NOVEMBER 2018 WorldWide Drilling Resource ® Mills Machine Company got its start in the drilling industry by rebuilding drag bits for the seismograph drilling industry in the early 1930s. EXB